Merge lp:~jamesh/thumbnailer/landing-20150622 into lp:thumbnailer

[James Henstridge]

Merge lp:thumbnailer/devel r240 into trunk.

Merge lp:thumbnailer/devel r231 into trunk.

I've used lp:~ci-train-bot/thumbnailer/thumbnailer-ubuntu-wily-proposed as a base for this branch, since the last landing doesn't seem to have made it to trunk. I've also integrated Adam Conrad's packaging fix that was made on top of wily-proposed outside of CI prior to the merge.

I've summarised the most visible changes in the Debian changelog (I can add more if you want).

[PS Jenkins bot (ps-jenkins)]

PASSED: Continuous integration, rev:126
http://jenkins.qa.ubuntu.com/job/thumbnailer-ci/171/
Executed test runs:
    SUCCESS: http://jenkins.qa.ubuntu.com/job/thumbnailer-wily-amd64-ci/3
    SUCCESS: http://jenkins.qa.ubuntu.com/job/thumbnailer-wily-armhf-ci/3
        deb: http://jenkins.qa.ubuntu.com/job/thumbnailer-wily-armhf-ci/3/artifact/work/output/*zip*/output.zip
    SUCCESS: http://jenkins.qa.ubuntu.com/job/thumbnailer-wily-i386-ci/3

Click here to trigger a rebuild:
http://s-jenkins.ubuntu-ci:8080/job/thumbnailer-ci/171/rebuild

review: Approve (continuous-integration)

[Michi Henning (michihenning)]

Looking good to me. Works well on the phone, aside from gstreamer problems. But, even when gstreamer does something stupid, we recover well.

review: Approve

[lp:~jamesh/thumbnailer/landing-20150622]

127. By James Henstridge on 2015-06-23

Merge from devel

128. By James Henstridge on 2015-06-23

Update Debian changelog.

[Victor Thompson (vthompson)]

I'm still seeing the same issue with regard to thumbnails timing out [1]

Music app log [2]

Syslog [3]

1 - https://www.youtube.com/watch?v=LBwUGe8n1Hs
2 - http://paste.ubuntu.com/11767442/
3 - http://paste.ubuntu.com/11767445/

[Victor Thompson (vthompson)]

Actually, disregard my last comment. Upon restarting the device the thumbnails are starting to show up without immediately timing out. Once the thumbnails are generated the first time, they appear to re-generate from cache (I assume) immediately--whereas the version in the device images would slowly generate them each time and seemingly block on something being processed.

[James Henstridge (jamesh)]

My guess is that your app was running with the new QML plugin but talking to the old thumbnailer-service. There were some changes to the API as part of the rewrite (to support requesting thumbnails of arbitrary size rather than the fixed set of sizes), so the placeholder images would be due to the broken D-Bus method calls. Rebooting would have made sure you were on the new code.

One of the main causes of the old serialised behaviour was due to the old QQuickImageProvider API, which requested thumbnails one by one on a single worker thread. With this branch, we're using the new QQuickAsyncImageProvider API so we can start multiple thumbnail jobs in parallel.

[Michi Henning (michihenning)]

Hi Victor, thanks heaps for helping to test this!

If you want to see what's going on with the cache, you can run a tail -f on ~/.cache/upstart/dbus.log. All the thumbnailer messages end up in there. If you want to test with an empty cache, you can run

thumbnailer-admin clear

This completely wipes the cached thumbnails. You can pull out to lots of statistics for the three internal caches with

thumbnailer-admin stats -v

[lp:~jamesh/thumbnailer/landing-20150622]

129. By James Henstridge on 2015-06-25

Merge from devel

[Michi Henning (michihenning)]

Yes, thank you!

review: Approve

[lp:~jamesh/thumbnailer/landing-20150622]

130. By James Henstridge on 2015-07-02

Merge lp:thumbnailer/devel r240

[Michi Henning (michihenning)]

Yes, definitely!

review: Approve

[lp:~jamesh/thumbnailer/landing-20150622]

131. By James Henstridge on 2015-07-14

Merge lp:thumbnailer/devel r241

[Michi Henning (michihenning)]

LGTM!

review: Approve

=== modified file 'CMakeLists.txt'
--- CMakeLists.txt	2014-09-25 09:47:05 +0000
+++ CMakeLists.txt	2015-07-14 06:30:34 +0000
@@ -1,27 +1,82 @@
 project(thumbnailer C CXX)
-cmake_minimum_required(VERSION 2.8.7)
-set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Wall -pedantic -Wextra")
-
-include(CheckCXXCompilerFlag)
-check_cxx_compiler_flag(-std=c++11 HAS_C11)
-# Precise's GCC is too old to have -std=c++11
-if(HAS_C11)
-set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -std=c++11")
-else()
-set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -std=c++0x -Dfinal=")
-endif()
-
-set(SO_VERSION_MAJOR "1")
-set(SO_VERSION_MINOR "1")
-set(SO_VERSION_PATCH "0")
-
-set(SO_VERSION "${SO_VERSION_MAJOR}.${SO_VERSION_MINOR}.${SO_VERSION_PATCH}")
-
-# Increment this manually whenever breaking ABI.
-# http://tldp.org/HOWTO/Program-Library-HOWTO/shared-libraries.html#AEN135
-set(ABI_VERSION 0)
-
-include(cmake/coverage.cmake)
+
+set(CMAKE_INCLUDE_CURRENT_DIR ON)
+
+cmake_minimum_required(VERSION 2.8.11)
+
+# Suppress complaints by cmake about COMPILE_DEFINITIONS_<Config> caused by qt5 cmake macros.
+cmake_policy(SET CMP0043 OLD)
+
+string(TOLOWER "${CMAKE_BUILD_TYPE}" cmake_build_type_lower) # Build types should always be lower case
+
+set(ACCEPTED_BUILD_TYPES "" none release debug relwithdebinfo coverage)
+list(FIND ACCEPTED_BUILD_TYPES "${cmake_build_type_lower}" IS_BUILD_TYPE_ACCEPTED)
+if(${IS_BUILD_TYPE_ACCEPTED} EQUAL -1)
+    message(FATAL_ERROR "Invalid CMAKE_BUILD_TYPE: ${CMAKE_BUILD_TYPE}\nValid types are: ${ACCEPTED_BUILD_TYPES}")
+endif()
+
+set(CMAKE_MODULE_PATH ${CMAKE_SOURCE_DIR}/cmake ${CMAKE_MODULE_PATH})
+
+set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} --std=c++11 -Wall -pedantic -Wextra")
+
+# Some additional warnings not included by the general flags set above.
+set(EXTRA_C_WARNINGS "-Wcast-align -Wcast-qual -Wformat -Wredundant-decls -Wswitch-default")
+set(EXTRA_CXX_WARNINGS "-Wnon-virtual-dtor -Wctor-dtor-privacy -Wold-style-cast")
+set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} ${EXTRA_C_WARNINGS} ${EXTRA_CXX_WARNINGS}")
+
+# By default, for release builds, warnings become hard errors.
+if ("${cmake_build_type_lower}" STREQUAL "release" OR "${cmake_build_type_lower}" STREQUAL "relwithdebinfo")
+    option(Werror "Treat warnings as errors" ON)
+else()
+    option(Werror "Treat warnings as errors" OFF)
+endif()
+
+# If warnings are errors, don't error on deprecated declarations.
+if (${Werror})
+    set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Werror")
+    if ("${cmake_build_type_lower}" STREQUAL "release" OR "${cmake_build_type_lower}" STREQUAL "relwithdebinfo")
+        set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Wno-error=deprecated-declarations")
+    endif()
+endif()
+
+add_definitions(-DQT_NO_KEYWORDS)
+
+# Flags for thread/address sanitizer
+set(SANITIZER "" CACHE STRING "Build with -fsanitize=<value> (legal values: thread, address)")
+
+if ("${SANITIZER}" STREQUAL "")
+    # Do nothing
+elseif (${SANITIZER} STREQUAL "thread")
+    set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -fsanitize=thread -fno-omit-frame-pointer -g")
+elseif (${SANITIZER} STREQUAL "address")
+    set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -fsanitize=address -fno-omit-frame-pointer -g")
+else()
+    message(FATAL_ERROR "Invalid SANITIZER setting: ${SANITIZER}")
+endif()
+
+# Some tests are slow, so make it possible not to run them
+# during day-to-day development.
+option(slowtests "Run slow tests" ON)
+
+# Definitions for testing with valgrind.
+
+configure_file(CTestCustom.cmake.in CTestCustom.cmake) # Tests in CTestCustom.cmake are skipped for valgrind
+
+find_program(MEMORYCHECK_COMMAND NAMES valgrind)
+if (MEMORYCHECK_COMMAND)
+    set(MEMORYCHECK_COMMAND_OPTIONS
+        "--suppressions=${CMAKE_SOURCE_DIR}/valgrind-suppress --errors-for-leak-kinds=definite --show-leak-kinds=definite --leak-check=full --num-callers=50 --error-exitcode=3"
+    )
+    add_custom_target(valgrind DEPENDS NightlyMemCheck)
+else()
+    message(WARNING "Cannot find valgrind: valgrind target will not be available")
+endif()
+
+include(CTest)
+enable_testing()
+
+include(EnableCoverageReport)
+
 include(cmake/UseGSettings.cmake)
 
 include(GNUInstallDirs)
@@ -29,27 +84,46 @@
 set(SHARE_PRIV_DIR ${CMAKE_INSTALL_LIBDIR}/thumbnailer)
 set(SHARE_PRIV_ABS ${CMAKE_INSTALL_PREFIX}/${SHARE_PRIV_DIR})
 
+find_package(Boost COMPONENTS filesystem regex system REQUIRED)
 find_package(Threads REQUIRED)
 find_package(Qt5Core REQUIRED)
+find_package(Qt5DBus REQUIRED)
+
 include(FindPkgConfig)
-pkg_check_modules(GST_DEPS REQUIRED gstreamer-1.0 gstreamer-plugins-base-1.0 gstreamer-pbutils-1.0 gstreamer-app-1.0)
+pkg_check_modules(GST_DEPS REQUIRED gstreamer-1.0 gstreamer-plugins-base-1.0 gstreamer-tag-1.0)
+pkg_check_modules(GOBJ_DEPS REQUIRED gobject-2.0)
 pkg_check_modules(GIO_DEPS REQUIRED gio-2.0 gio-unix-2.0)
 pkg_check_modules(IMG_DEPS REQUIRED gdk-pixbuf-2.0 libexif)
-pkg_check_modules(SOUP_DEPS REQUIRED libsoup-2.4)
-pkg_check_modules(XML_DEPS REQUIRED libxml-2.0)
+pkg_check_modules(UNITY_API_DEPS REQUIRED libunity-api)
+pkg_check_modules(APPARMOR_DEPS REQUIRED libapparmor)
 
 include_directories(${GST_DEPS_INCLUDE_DIRS})
+include_directories(${GOBJ_DEPS_INCLUDE_DIRS})
 include_directories(${GIO_DEPS_INCLUDE_DIRS})
 include_directories(${IMG_DEPS_INCLUDE_DIRS})
-include_directories(${SOUP_DEPS_INCLUDE_DIRS})
-include_directories(${XML_DEPS_INCLUDE_DIRS})
+include_directories(${UNITY_API_DEPS_INCLUDE_DIRS})
+include_directories(${APPARMOR_DEPS_INCLUDE_DIRS})
 include_directories(include)
 include_directories(${CMAKE_CURRENT_BINARY_DIR}/include)
 
-enable_testing()
 add_subdirectory(src)
 add_subdirectory(data)
-add_subdirectory(plugins/Ubuntu/Thumbnailer)
+add_subdirectory(plugins/Ubuntu/Thumbnailer.0.1)
 add_subdirectory(tests)
-add_subdirectory(tools)
 add_subdirectory(include)
+
+enable_coverage_report(
+    TARGETS
+        thumbnailer
+        thumbnailer-admin
+        thumbnailer-service
+        thumbnailer-qml
+        vs-thumb
+        testutils
+    FILTER
+        ${CMAKE_SOURCE_DIR}/include/core/*
+        ${CMAKE_SOURCE_DIR}/tests/*
+        ${CMAKE_BINARY_DIR}/*
+    TESTS
+        ${UNIT_TEST_TARGETS}
+)

=== added file 'COPYING.GPL'
--- COPYING.GPL	1970-01-01 00:00:00 +0000
+++ COPYING.GPL	2015-07-14 06:30:34 +0000
@@ -0,0 +1,674 @@
+                    GNU GENERAL PUBLIC LICENSE
+                       Version 3, 29 June 2007
+
+ Copyright (C) 2007 Free Software Foundation, Inc. <http://fsf.org/>
+ Everyone is permitted to copy and distribute verbatim copies
+ of this license document, but changing it is not allowed.
+
+                            Preamble
+
+  The GNU General Public License is a free, copyleft license for
+software and other kinds of works.
+
+  The licenses for most software and other practical works are designed
+to take away your freedom to share and change the works.  By contrast,
+the GNU General Public License is intended to guarantee your freedom to
+share and change all versions of a program--to make sure it remains free
+software for all its users.  We, the Free Software Foundation, use the
+GNU General Public License for most of our software; it applies also to
+any other work released this way by its authors.  You can apply it to
+your programs, too.
+
+  When we speak of free software, we are referring to freedom, not
+price.  Our General Public Licenses are designed to make sure that you
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+
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+by the Installation Information.  But this requirement does not apply
+if neither you nor any third party retains the ability to install
+modified object code on the User Product (for example, the work has
+been installed in ROM).
+
+  The requirement to provide Installation Information does not include a
+requirement to continue to provide support service, warranty, or updates
+for a work that has been modified or installed by the recipient, or for
+the User Product in which it has been modified or installed.  Access to a
+network may be denied when the modification itself materially and
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+protocols for communication across the network.
+
+  Corresponding Source conveyed, and Installation Information provided,
+in accord with this section must be in a format that is publicly
+documented (and with an implementation available to the public in
+source code form), and must require no special password or key for
+unpacking, reading or copying.
+
+  7. Additional Terms.
+
+  "Additional permissions" are terms that supplement the terms of this
+License by making exceptions from one or more of its conditions.
+Additional permissions that are applicable to the entire Program shall
+be treated as though they were included in this License, to the extent
+that they are valid under applicable law.  If additional permissions
+apply only to part of the Program, that part may be used separately
+under those permissions, but the entire Program remains governed by
+this License without regard to the additional permissions.
+
+  When you convey a copy of a covered work, you may at your option
+remove any additional permissions from that copy, or from any part of
+it.  (Additional permissions may be written to require their own
+removal in certain cases when you modify the work.)  You may place
+additional permissions on material, added by you to a covered work,
+for which you have or can give appropriate copyright permission.
+
+  Notwithstanding any other provision of this License, for material you
+add to a covered work, you may (if authorized by the copyright holders of
+that material) supplement the terms of this License with terms:
+
+    a) Disclaiming warranty or limiting liability differently from the
+    terms of sections 15 and 16 of this License; or
+
+    b) Requiring preservation of specified reasonable legal notices or
+    author attributions in that material or in the Appropriate Legal
+    Notices displayed by works containing it; or
+
+    c) Prohibiting misrepresentation of the origin of that material, or
+    requiring that modified versions of such material be marked in
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+
+    d) Limiting the use for publicity purposes of names of licensors or
+    authors of the material; or
+
+    e) Declining to grant rights under trademark law for use of some
+    trade names, trademarks, or service marks; or
+
+    f) Requiring indemnification of licensors and authors of that
+    material by anyone who conveys the material (or modified versions of
+    it) with contractual assumptions of liability to the recipient, for
+    any liability that these contractual assumptions directly impose on
+    those licensors and authors.
+
+  All other non-permissive additional terms are considered "further
+restrictions" within the meaning of section 10.  If the Program as you
+received it, or any part of it, contains a notice stating that it is
+governed by this License along with a term that is a further
+restriction, you may remove that term.  If a license document contains
+a further restriction but permits relicensing or conveying under this
+License, you may add to a covered work material governed by the terms
+of that license document, provided that the further restriction does
+not survive such relicensing or conveying.
+
+  If you add terms to a covered work in accord with this section, you
+must place, in the relevant source files, a statement of the
+additional terms that apply to those files, or a notice indicating
+where to find the applicable terms.
+
+  Additional terms, permissive or non-permissive, may be stated in the
+form of a separately written license, or stated as exceptions;
+the above requirements apply either way.
+
+  8. Termination.
+
+  You may not propagate or modify a covered work except as expressly
+provided under this License.  Any attempt otherwise to propagate or
+modify it is void, and will automatically terminate your rights under
+this License (including any patent licenses granted under the third
+paragraph of section 11).
+
+  However, if you cease all violation of this License, then your
+license from a particular copyright holder is reinstated (a)
+provisionally, unless and until the copyright holder explicitly and
+finally terminates your license, and (b) permanently, if the copyright
+holder fails to notify you of the violation by some reasonable means
+prior to 60 days after the cessation.
+
+  Moreover, your license from a particular copyright holder is
+reinstated permanently if the copyright holder notifies you of the
+violation by some reasonable means, this is the first time you have
+received notice of violation of this License (for any work) from that
+copyright holder, and you cure the violation prior to 30 days after
+your receipt of the notice.
+
+  Termination of your rights under this section does not terminate the
+licenses of parties who have received copies or rights from you under
+this License.  If your rights have been terminated and not permanently
+reinstated, you do not qualify to receive new licenses for the same
+material under section 10.
+
+  9. Acceptance Not Required for Having Copies.
+
+  You are not required to accept this License in order to receive or
+run a copy of the Program.  Ancillary propagation of a covered work
+occurring solely as a consequence of using peer-to-peer transmission
+to receive a copy likewise does not require acceptance.  However,
+nothing other than this License grants you permission to propagate or
+modify any covered work.  These actions infringe copyright if you do
+not accept this License.  Therefore, by modifying or propagating a
+covered work, you indicate your acceptance of this License to do so.
+
+  10. Automatic Licensing of Downstream Recipients.
+
+  Each time you convey a covered work, the recipient automatically
+receives a license from the original licensors, to run, modify and
+propagate that work, subject to this License.  You are not responsible
+for enforcing compliance by third parties with this License.
+
+  An "entity transaction" is a transaction transferring control of an
+organization, or substantially all assets of one, or subdividing an
+organization, or merging organizations.  If propagation of a covered
+work results from an entity transaction, each party to that
+transaction who receives a copy of the work also receives whatever
+licenses to the work the party's predecessor in interest had or could
+give under the previous paragraph, plus a right to possession of the
+Corresponding Source of the work from the predecessor in interest, if
+the predecessor has it or can get it with reasonable efforts.
+
+  You may not impose any further restrictions on the exercise of the
+rights granted or affirmed under this License.  For example, you may
+not impose a license fee, royalty, or other charge for exercise of
+rights granted under this License, and you may not initiate litigation
+(including a cross-claim or counterclaim in a lawsuit) alleging that
+any patent claim is infringed by making, using, selling, offering for
+sale, or importing the Program or any portion of it.
+
+  11. Patents.
+
+  A "contributor" is a copyright holder who authorizes use under this
+License of the Program or a work on which the Program is based.  The
+work thus licensed is called the contributor's "contributor version".
+
+  A contributor's "essential patent claims" are all patent claims
+owned or controlled by the contributor, whether already acquired or
+hereafter acquired, that would be infringed by some manner, permitted
+by this License, of making, using, or selling its contributor version,
+but do not include claims that would be infringed only as a
+consequence of further modification of the contributor version.  For
+purposes of this definition, "control" includes the right to grant
+patent sublicenses in a manner consistent with the requirements of
+this License.
+
+  Each contributor grants you a non-exclusive, worldwide, royalty-free
+patent license under the contributor's essential patent claims, to
+make, use, sell, offer for sale, import and otherwise run, modify and
+propagate the contents of its contributor version.
+
+  In the following three paragraphs, a "patent license" is any express
+agreement or commitment, however denominated, not to enforce a patent
+(such as an express permission to practice a patent or covenant not to
+sue for patent infringement).  To "grant" such a patent license to a
+party means to make such an agreement or commitment not to enforce a
+patent against the party.
+
+  If you convey a covered work, knowingly relying on a patent license,
+and the Corresponding Source of the work is not available for anyone
+to copy, free of charge and under the terms of this License, through a
+publicly available network server or other readily accessible means,
+then you must either (1) cause the Corresponding Source to be so
+available, or (2) arrange to deprive yourself of the benefit of the
+patent license for this particular work, or (3) arrange, in a manner
+consistent with the requirements of this License, to extend the patent
+license to downstream recipients.  "Knowingly relying" means you have
+actual knowledge that, but for the patent license, your conveying the
+covered work in a country, or your recipient's use of the covered work
+in a country, would infringe one or more identifiable patents in that
+country that you have reason to believe are valid.
+
+  If, pursuant to or in connection with a single transaction or
+arrangement, you convey, or propagate by procuring conveyance of, a
+covered work, and grant a patent license to some of the parties
+receiving the covered work authorizing them to use, propagate, modify
+or convey a specific copy of the covered work, then the patent license
+you grant is automatically extended to all recipients of the covered
+work and works based on it.
+
+  A patent license is "discriminatory" if it does not include within
+the scope of its coverage, prohibits the exercise of, or is
+conditioned on the non-exercise of one or more of the rights that are
+specifically granted under this License.  You may not convey a covered
+work if you are a party to an arrangement with a third party that is
+in the business of distributing software, under which you make payment
+to the third party based on the extent of your activity of conveying
+the work, and under which the third party grants, to any of the
+parties who would receive the covered work from you, a discriminatory
+patent license (a) in connection with copies of the covered work
+conveyed by you (or copies made from those copies), or (b) primarily
+for and in connection with specific products or compilations that
+contain the covered work, unless you entered into that arrangement,
+or that patent license was granted, prior to 28 March 2007.
+
+  Nothing in this License shall be construed as excluding or limiting
+any implied license or other defenses to infringement that may
+otherwise be available to you under applicable patent law.
+
+  12. No Surrender of Others' Freedom.
+
+  If conditions are imposed on you (whether by court order, agreement or
+otherwise) that contradict the conditions of this License, they do not
+excuse you from the conditions of this License.  If you cannot convey a
+covered work so as to satisfy simultaneously your obligations under this
+License and any other pertinent obligations, then as a consequence you may
+not convey it at all.  For example, if you agree to terms that obligate you
+to collect a royalty for further conveying from those to whom you convey
+the Program, the only way you could satisfy both those terms and this
+License would be to refrain entirely from conveying the Program.
+
+  13. Use with the GNU Affero General Public License.
+
+  Notwithstanding any other provision of this License, you have
+permission to link or combine any covered work with a work licensed
+under version 3 of the GNU Affero General Public License into a single
+combined work, and to convey the resulting work.  The terms of this
+License will continue to apply to the part which is the covered work,
+but the special requirements of the GNU Affero General Public License,
+section 13, concerning interaction through a network will apply to the
+combination as such.
+
+  14. Revised Versions of this License.
+
+  The Free Software Foundation may publish revised and/or new versions of
+the GNU General Public License from time to time.  Such new versions will
+be similar in spirit to the present version, but may differ in detail to
+address new problems or concerns.
+
+  Each version is given a distinguishing version number.  If the
+Program specifies that a certain numbered version of the GNU General
+Public License "or any later version" applies to it, you have the
+option of following the terms and conditions either of that numbered
+version or of any later version published by the Free Software
+Foundation.  If the Program does not specify a version number of the
+GNU General Public License, you may choose any version ever published
+by the Free Software Foundation.
+
+  If the Program specifies that a proxy can decide which future
+versions of the GNU General Public License can be used, that proxy's
+public statement of acceptance of a version permanently authorizes you
+to choose that version for the Program.
+
+  Later license versions may give you additional or different
+permissions.  However, no additional obligations are imposed on any
+author or copyright holder as a result of your choosing to follow a
+later version.
+
+  15. Disclaimer of Warranty.
+
+  THERE IS NO WARRANTY FOR THE PROGRAM, TO THE EXTENT PERMITTED BY
+APPLICABLE LAW.  EXCEPT WHEN OTHERWISE STATED IN WRITING THE COPYRIGHT
+HOLDERS AND/OR OTHER PARTIES PROVIDE THE PROGRAM "AS IS" WITHOUT WARRANTY
+OF ANY KIND, EITHER EXPRESSED OR IMPLIED, INCLUDING, BUT NOT LIMITED TO,
+THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
+PURPOSE.  THE ENTIRE RISK AS TO THE QUALITY AND PERFORMANCE OF THE PROGRAM
+IS WITH YOU.  SHOULD THE PROGRAM PROVE DEFECTIVE, YOU ASSUME THE COST OF
+ALL NECESSARY SERVICING, REPAIR OR CORRECTION.
+
+  16. Limitation of Liability.
+
+  IN NO EVENT UNLESS REQUIRED BY APPLICABLE LAW OR AGREED TO IN WRITING
+WILL ANY COPYRIGHT HOLDER, OR ANY OTHER PARTY WHO MODIFIES AND/OR CONVEYS
+THE PROGRAM AS PERMITTED ABOVE, BE LIABLE TO YOU FOR DAMAGES, INCLUDING ANY
+GENERAL, SPECIAL, INCIDENTAL OR CONSEQUENTIAL DAMAGES ARISING OUT OF THE
+USE OR INABILITY TO USE THE PROGRAM (INCLUDING BUT NOT LIMITED TO LOSS OF
+DATA OR DATA BEING RENDERED INACCURATE OR LOSSES SUSTAINED BY YOU OR THIRD
+PARTIES OR A FAILURE OF THE PROGRAM TO OPERATE WITH ANY OTHER PROGRAMS),
+EVEN IF SUCH HOLDER OR OTHER PARTY HAS BEEN ADVISED OF THE POSSIBILITY OF
+SUCH DAMAGES.
+
+  17. Interpretation of Sections 15 and 16.
+
+  If the disclaimer of warranty and limitation of liability provided
+above cannot be given local legal effect according to their terms,
+reviewing courts shall apply local law that most closely approximates
+an absolute waiver of all civil liability in connection with the
+Program, unless a warranty or assumption of liability accompanies a
+copy of the Program in return for a fee.
+
+                     END OF TERMS AND CONDITIONS
+
+            How to Apply These Terms to Your New Programs
+
+  If you develop a new program, and you want it to be of the greatest
+possible use to the public, the best way to achieve this is to make it
+free software which everyone can redistribute and change under these terms.
+
+  To do so, attach the following notices to the program.  It is safest
+to attach them to the start of each source file to most effectively
+state the exclusion of warranty; and each file should have at least
+the "copyright" line and a pointer to where the full notice is found.
+
+    <one line to give the program's name and a brief idea of what it does.>
+    Copyright (C) <year>  <name of author>
+
+    This program is free software: you can redistribute it and/or modify
+    it under the terms of the GNU General Public License as published by
+    the Free Software Foundation, either version 3 of the License, or
+    (at your option) any later version.
+
+    This program is distributed in the hope that it will be useful,
+    but WITHOUT ANY WARRANTY; without even the implied warranty of
+    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+    GNU General Public License for more details.
+
+    You should have received a copy of the GNU General Public License
+    along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+Also add information on how to contact you by electronic and paper mail.
+
+  If the program does terminal interaction, make it output a short
+notice like this when it starts in an interactive mode:
+
+    <program>  Copyright (C) <year>  <name of author>
+    This program comes with ABSOLUTELY NO WARRANTY; for details type `show w'.
+    This is free software, and you are welcome to redistribute it
+    under certain conditions; type `show c' for details.
+
+The hypothetical commands `show w' and `show c' should show the appropriate
+parts of the General Public License.  Of course, your program's commands
+might be different; for a GUI interface, you would use an "about box".
+
+  You should also get your employer (if you work as a programmer) or school,
+if any, to sign a "copyright disclaimer" for the program, if necessary.
+For more information on this, and how to apply and follow the GNU GPL, see
+<http://www.gnu.org/licenses/>.
+
+  The GNU General Public License does not permit incorporating your program
+into proprietary programs.  If your program is a subroutine library, you
+may consider it more useful to permit linking proprietary applications with
+the library.  If this is what you want to do, use the GNU Lesser General
+Public License instead of this License.  But first, please read
+<http://www.gnu.org/philosophy/why-not-lgpl.html>.

=== renamed file 'COPYING' => 'COPYING.LGPL'
=== added file 'CTestCustom.cmake.in'
--- CTestCustom.cmake.in	1970-01-01 00:00:00 +0000
+++ CTestCustom.cmake.in	2015-07-14 06:30:34 +0000
@@ -0,0 +1,17 @@
+#
+# Tests listed here will not be run by the valgrind target,
+# either because there is not point (we don't want to
+# test that a python script doesn't leak), or because,
+# under valgrind, the test runs too slowly to meet
+# its timing constraints (or crashes valgrind).
+#
+
+SET(CTEST_CUSTOM_MEMCHECK_IGNORE
+    test_qml  # Too many bugs in QML to suppress them selectively
+    speed
+    copyright
+    stand-alone-internal-headers
+    stand-alone-core-headers
+    stand-alone-core-internal-headers
+    whitespace
+)

=== added file 'HACKING'
--- HACKING	1970-01-01 00:00:00 +0000
+++ HACKING	2015-07-14 06:30:34 +0000
@@ -0,0 +1,121 @@
+Building the code
+-----------------
+
+By default, the code is built in release mode. To build a debug version, use
+
+    $ mkdir builddebug
+    $ cd builddebug
+    $ cmake -DCMAKE_BUILD_TYPE=debug ..
+    $ make
+
+For a release version, use -DCMAKE_BUILD_TYPE=release
+
+Running the tests
+-----------------
+
+    $ make
+    $ make test
+
+Note that "make test" alone is dangerous because it does not rebuild
+any tests if either the library or the test files themselves need
+rebuilding. It's not possible to fix this with cmake because cmake cannot
+add build dependencies to built-in targets. To make sure that everything
+is up-to-date, run "make" before running "make test"!
+
+To run the tests with valgrind:
+
+    $ make valgrind
+
+It doesn't make sense for some tests to run them with valgrind. For
+example, the header compilation tests don't need valgrind because
+we'd just be testing that Python doesn't leak. There are also some
+tests that run too slow and time out under valgrind and, occasionally,
+valgrind crashes for a particular test.
+
+There are two ways to suppress tests:
+
+You can add a test name to CTestCustom.cmake.in to suppress that
+test completely. That makes sense for the header compilation tests,
+for example.
+
+If a specific test case in a test program causes a valgrind problem,
+you can selectively disable a section of code like this:
+
+    #include <valgrind/valgrind.h>
+
+    if (!RUNNING_ON_VALGRIND)
+    {
+        // Code here crashes valgrind...
+    }
+
+That way, the test will still be run as part of the normal "make test"
+target, but will be ommitted when running "make valgrind".
+
+Coverage
+--------
+
+To build with the flags for coverage testing enabled and get coverage:
+
+    $ mkdir buildcoverage
+    $ cd buildcoverage
+    $ cmake -DCMAKE_BUILD_TYPE=coverage
+    $ make
+    $ make test
+    $ make coverage
+
+Unfortunately, it is not possible to get 100% coverage for some files,
+mainly due to gcc's generation of two destructors for dynamic and non-
+dynamic instances. For abstract base classes and for classes that
+prevent stack and static allocation, this causes one of the destructors
+to be reported as uncovered.
+
+There are also issues with some functions in header files that are
+incorrectly reported as uncovered due to inlining, as well as
+the impossibility of covering defensive assert(false) statements,
+such as an assert in the default branch of a switch, where the
+switch is meant to handle all possible cases explicitly.
+
+If you run a binary and get lots of warnings about a "merge mismatch for summaries",
+this is caused by having made changes to the source that add or remove code
+that was previously run, so the new coverage output cannot sensibly be merged
+into the old coverage output. You can get rid of this problem by running
+
+    $ make clean-coverage
+
+This deletes all the .gcda files, allowing the merge to (sometimes) succeed again.
+If this doesn't work either, the only remedy is to do a clean build.
+
+Code style
+----------
+
+Please maintain the existing coding style. For details
+on the style, see lp:canonical-client-development-guidelines.
+
+We use a format tool that fixes a whole lot of issues
+regarding code style. See the HACKING file of lp:unity-scopes-api
+for details on the tool.
+
+Thread and address sanitizer
+----------------------------
+
+Set SANITIZER to "thread" or "address" to build with the
+corresponding sanitizer enabled.
+
+Some libraries cause a cause a large number of warnings from
+thread sanitizer. The tsan-suppress file supresses the benign
+race conditions we currently know about. To run the tests
+(from the build directory) with these suppressions
+enabled, use:
+
+    $ TSAN_OPTIONS="suppressions=../tsan-suppress" make test
+
+If a test runs too slowly under address sanitizer, you can
+hide a section of code from address sanitzer with:
+
+#if defined(__has_feature)
+    #if !__has_feature(address_sanitizer)
+
+    // Code here takes forever under address sanitizer...
+
+    #endif
+#endif

=== removed file 'cmake/coverage.cmake'
--- cmake/coverage.cmake	2013-09-30 12:17:23 +0000
+++ cmake/coverage.cmake	1970-01-01 00:00:00 +0000
@@ -1,34 +0,0 @@
-if (CMAKE_BUILD_TYPE MATCHES coverage)
-  set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} --coverage")
-  set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} --coverage")
-  set(CMAKE_MODULE_LINKER_FLAGS "${CMAKE_MODULE_LINKER_FLAGS} --coverage")
-  set(CMAKE_SHARED_LINKER_FLAGS "${CMAKE_SHARED_LINKER_FLAGS} --coverage")
-
-  find_program(GCOVR_EXECUTABLE gcovr HINTS ${GCOVR_ROOT} "${GCOVR_ROOT}/bin")
-  if (NOT GCOVR_EXECUTABLE)
-    message(STATUS "Gcovr binary was not found, can not generate XML coverage info.")
-  else ()
-    message(STATUS "Gcovr found, can generate XML coverage info.")
-    add_custom_target (coverage-xml
-      WORKING_DIRECTORY ${CMAKE_BINARY_DIR}
-      COMMAND "${GCOVR_EXECUTABLE}" --exclude="test.*" -x -r "${CMAKE_SOURCE_DIR}" 
-      --object-directory=${CMAKE_BINARY_DIR} -o coverage.xml)
-  endif()
-
-  find_program(LCOV_EXECUTABLE lcov HINTS ${LCOV_ROOT} "${GCOVR_ROOT}/bin")
-  find_program(GENHTML_EXECUTABLE genhtml HINTS ${GENHTML_ROOT})
-  if (NOT LCOV_EXECUTABLE)
-    message(STATUS "Lcov binary was not found, can not generate HTML coverage info.")
-  else ()
-    if(NOT GENHTML_EXECUTABLE)
-      message(STATUS "Genthml binary not found, can not generate HTML coverage info.")
-    else()
-      message(STATUS "Lcov and genhtml found, can generate HTML coverage info.")
-      add_custom_target (coverage-html
-        WORKING_DIRECTORY ${CMAKE_BINARY_DIR}
-        COMMAND "${LCOV_EXECUTABLE}" --directory ${CMAKE_BINARY_DIR} --capture --output-file coverage.info --no-checksum
-        COMMAND "${GENHTML_EXECUTABLE}" --prefix ${CMAKE_BINARY_DIR} --output-directory coveragereport --title "Code Coverage" --legend --show-details coverage.info
-        )
-    endif()
-  endif()
-endif()

=== modified file 'data/CMakeLists.txt'
--- data/CMakeLists.txt	2014-09-26 11:24:27 +0000
+++ data/CMakeLists.txt	2015-07-14 06:30:34 +0000
@@ -22,4 +22,3 @@
   "${CMAKE_CURRENT_BINARY_DIR}/thumbnailer"
   DESTINATION "${CMAKE_INSTALL_SYSCONFDIR}/apport/blacklist.d"
 )
-

=== modified file 'data/com.canonical.Unity.Thumbnailer.gschema.xml'
--- data/com.canonical.Unity.Thumbnailer.gschema.xml	2014-09-16 13:00:17 +0000
+++ data/com.canonical.Unity.Thumbnailer.gschema.xml	2015-07-14 06:30:34 +0000
@@ -6,5 +6,72 @@
       <summary>API key for dash.ubuntu.com service</summary>
       <description>API key for dash.ubuntu.com service</description>
     </key>
+
+    <key type="i" name="full-size-cache-size">
+      <default>50</default>
+      <summary>Size of the full-size image cache in megabytes</summary>
+      <description>The full-size cache is used to store full-size extracted media artwork and downloaded album art.</description>
+    </key>
+
+    <key type="i" name="thumbnail-cache-size">
+      <default>100</default>
+      <summary>Size of the thumbnail image cache in megabytes</summary>
+      <description>The thumbnail image cache stores scaled thumbnail images.</description>
+    </key>
+
+    <key type="i" name="failure-cache-size">
+      <default>2</default>
+      <summary>Size of the failure cache in megabytes</summary>
+      <description>The failure cache records information about failed downloads or failed thumbnail extractions.</description>
+    </key>
+
+    <key type="i" name="max-thumbnail-size">
+      <default>1920</default>
+      <summary>Maximum size in pixels for a thumbnail</summary>
+      <description>
+        Requests for thumbnails larger than this will automatically reduce the thumbnail to max-thumbnail-size in the larger dimension. An exception are requests for local thumbnails with size 0x0, which are returned in their original size.
+     </description>
+    </key>
+
+    <key type="i" name="retry-not-found-hours">
+      <default>168</default>
+      <summary>Time to wait before re-trying for remote artwork that did not exist</summary>
+      <description>
+        If artwork cannot be retrieved because the remote server authoritavely confirmed that no artwork exists for an artist and album, this parameter defines how long the thumbnailer waits before trying to download the same image again. The default is 168 hours (one week).
+     </description>
+    </key>
+
+    <key type="i" name="retry-error-hours">
+      <default>2</default>
+      <summary>Time to wait before re-trying remote artwork that failed to download</summary>
+      <description>
+        If artwork cannot be retrieved from the remote server due to an unexpected error (such as the server not responding), this parameter defines how long the thumbnailer waits before trying to download the same image again. The default is two hours.
+     </description>
+    </key>
+
+    <key type="i" name="max-downloads">
+      <default>2</default>
+      <summary>Maximum number of concurrent downloads</summary>
+      <description>
+        This parameter sets the maximum number of concurrent downloads for remote artwork.
+     </description>
+    </key>
+
+    <key type="i" name="max-extractions">
+      <default>0</default>
+      <summary>Maximum number of concurrent image extractions</summary>
+      <description>
+        This parameter sets the maximum number of concurrent image extractions from local media files. The default value is zero, which sets the value according to the number of CPU cores.
+     </description>
+    </key>
+
+    <key type="i" name="extraction-timeout">
+      <default>10</default>
+      <summary>Maximum amount of time to wait for an image extraction or download (in seconds)</summary>
+      <description>
+        This parameter sets the amount of time (in seconds) to wait for a remote image download or
+        a thumbnail extraction before giving up.
+     </description>
+    </key>
   </schema>
 </schemalist>

=== modified file 'debian/changelog'
--- debian/changelog	2015-03-12 08:38:44 +0000
+++ debian/changelog	2015-07-14 06:30:34 +0000
@@ -1,3 +1,52 @@
+thumbnailer (2.1-0ubuntu1) UNRELEASED; urgency=medium
+
+  [ James Henstridge ]
+  * Expose more settings for the thumbnailer daemon via GSettings.
+  * Change the security policy implementation to use aa_query_label()
+    for file access checks rather than relying on file descriptor
+    passing.  This avoids the file descriptor exhaustion bugs with the
+    previous implementation. (LP: #1381713)
+
+  [ Michi Henning ]
+  * Add thumbnailer-admin tool, which can be used to check the status
+    of the cache and generate thumbnails via the command line.
+  * Add time stamps to log messages.
+  * Reduce the memory footprint of small caches, which in turn reduces
+    the footprint of thumbnailer-service.
+  * Don't implicitly compact the cache on service shutdown, as this
+    can take some time.
+  * Persist the cache stats over restarts of thumbnailer-service.
+  * Don't save JPEG thumbnails at quality=100, since the quality
+    improvement isn't justified by the extra storage.
+  * Don't retry downloads when we have a hit from the failure cache.
+  * Warn if an app requests an unsized thumbnail.
+
+ -- James Henstridge <[e-mail]>  Mon, 22 Jun 2015 17:31:39 +0800
+
+thumbnailer (2.0+15.10.20150527-0ubuntu2) wily; urgency=medium
+
+  * Switch to unversioned Conflicts/Replaces on libthumbnailer0 to force
+    removal of the obsolete package on upgrade from the previous version.
+
+ -- Adam Conrad <[e-mail]>  Wed, 17 Jun 2015 05:15:54 -0600
+
+thumbnailer (2.0+15.10.20150527-0ubuntu1) wily; urgency=medium
+
+  [ James Henstridge ]
+  * New upstream version with many changes:
+    - Switch to new cache implementation based on leveldb.
+    - Move local file thumbnailing to the central D-Bus daemon,
+      allowing for the cache to be shared between apps and its size
+      kept under control.
+    - The libthumbnailer library has been removed, since all clients
+      of the cache rely on the D-Bus interface now.
+    - The QML plugin now uses the QQuickAsyncImageProvider interface,
+      enabling multiple thumbnails to be loaded simultaneously.
+    - Improved test coverage.
+  * Remove the libthumbnailer0 and libthumbnailer-dev binary packages.
+
+ -- CI Train Bot <[e-mail]>  Wed, 27 May 2015 05:36:31 +0000
+
 thumbnailer (1.3+15.04.20150312-0ubuntu1) vivid; urgency=medium
 
   [ Albert Astals Cid ]

=== modified file 'debian/control'
--- debian/control	2014-09-25 07:23:54 +0000
+++ debian/control	2015-07-14 06:30:34 +0000
@@ -4,57 +4,43 @@
 Maintainer: Ubuntu Core Developers <[e-mail]>
 Standards-Version: 3.9.4
 Build-Depends: cmake,
+               cmake-extras (>= 0.4),
                debhelper (>= 9),
                gstreamer1.0-plugins-good,
+               libapparmor-dev,
+               libboost-filesystem-dev,
+               libboost-regex-dev,
                libexif-dev,
                libgdk-pixbuf2.0-dev,
                libgstreamer1.0-dev,
                libgstreamer-plugins-base1.0-dev,
                libgtest-dev,
-               libsoup2.4-dev,
-               libxml2-dev,
+               libleveldb-dev,
+               libqtdbustest1-dev,
+               libunity-api-dev,
+               python3-tornado,
+               qml-module-qtquick2,
+               qml-module-qttest,
                qt5-default,
                qtbase5-dev,
                qtbase5-dev-tools,
                qtdeclarative5-dev,
                shared-mime-info,
+               xvfb,
 Homepage: https://launchpad.net/thumbnailer
 # if you don't have have commit access to this branch but would like to upload
 # directly to Ubuntu, don't worry: your changes will be merged back into the
 # upstream branch
 Vcs-Bzr: lp:thumbnailer
 
-Package: libthumbnailer0
-Architecture: any
-Multi-Arch: same
-Pre-Depends: ${misc:Pre-Depends},
-Depends: ${misc:Depends},
-         ${shlibs:Depends},
-Description: generate thumbnails from files
- Thumbnailer is a shared library that generates
- thumbnail images for multiple file types such
- as video and audio files.
-
-Package: libthumbnailer-dev
-Architecture: any
-Multi-Arch: same
-Pre-Depends: ${misc:Pre-Depends},
-Depends: ${misc:Depends},
-         ${shlibs:Depends},
-         gstreamer1.0-plugins-base,
-         gstreamer1.0-plugins-good,
-         libthumbnailer0 (= ${binary:Version}),
-Description: development files for thumbnailer
- This package contains development files
- for the thumbnailer package.
-
 Package: thumbnailer-service
 Architecture: any
 Multi-Arch: foreign
 Pre-Depends: ${misc:Pre-Depends},
 Depends: ${misc:Depends},
          ${shlibs:Depends},
-         libthumbnailer0 (= ${binary:Version}),
+Conflicts: libthumbnailer0
+Replaces: libthumbnailer0
 Description: D-Bus service for out of process thumbnailing
  This package provides a D-Bus service that can provide thumbnails on
  behalf of another process.
@@ -66,7 +52,6 @@
 Depends: ${misc:Depends},
          ${shlibs:Depends},
          thumbnailer-common (= ${binary:Version}),
-         libthumbnailer0 (= ${binary:Version}),
 Provides: ubuntu-thumbnailer-impl,
           ubuntu-thumbnailer-impl-0,
 Recommends: thumbnailer-service (= ${binary:Version}),

=== modified file 'debian/copyright'
--- debian/copyright	2013-09-30 15:55:07 +0000
+++ debian/copyright	2015-07-14 06:30:34 +0000
@@ -3,7 +3,27 @@
 Source: https://launchpad.net/thumbnailer
 
 Files: *
-Copyright: 2013 Canonical ltd
+Copyright: 2015 Canonical Ltd.
+License: GPL-3
+ This program is free software: you can redistribute it and/or modify
+ it under the terms of the GNU General Public License version 3 as
+ published by the Free Software Foundation.
+ .
+ This program is distributed in the hope that it will be useful,
+ but WITHOUT ANY WARRANTY; without even the implied warranty of
+ MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ GNU General Public License for more details.
+ .
+ On Debian/Ubuntu systems, the full text of the GPL v3 can be found in
+ `/usr/share/common-licenses/GPL-3'
+
+Files: plugins/*
+       src/core/*
+       include/core/*
+       tests/core/* include/internal/gobj_memory.h
+       include/internal/safe_strerror.h
+       src/safe_strerror.cpp
+Copyright: 2015 Canonical ltd
 License: LGPL-3
  This program is free software: you can redistribute it and/or modify
  it under the terms of version 3 of the GNU Lesser General Public
@@ -12,7 +32,7 @@
  This program is distributed in the hope that it will be useful,
  but WITHOUT ANY WARRANTY; without even the implied warranty of
  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
- GNU General Public License for more details.
+ GNU Lesser General Public License for more details.
  .
  On Debian systems, the full text of the GNU Lesser General Public
  License version 3 can be found in the file

=== removed file 'debian/libthumbnailer-dev.install'
--- debian/libthumbnailer-dev.install	2013-09-25 12:45:32 +0000
+++ debian/libthumbnailer-dev.install	1970-01-01 00:00:00 +0000
@@ -1,3 +0,0 @@
-usr/include/*
-usr/lib/*/lib*.so
-usr/lib/*/pkgconfig/*

=== removed file 'debian/libthumbnailer0.install'
--- debian/libthumbnailer0.install	2014-09-26 11:24:27 +0000
+++ debian/libthumbnailer0.install	1970-01-01 00:00:00 +0000
@@ -1,2 +0,0 @@
-usr/lib/*/libthumb*.so.*
-usr/lib/*/thumbnailer/vs-thumb

=== removed file 'debian/libthumbnailer0.symbols'
--- debian/libthumbnailer0.symbols	2014-08-11 14:20:31 +0000
+++ debian/libthumbnailer0.symbols	1970-01-01 00:00:00 +0000
@@ -1,7 +0,0 @@
-libthumbnailer.so.0 libthumbnailer0 #MINVER#
- (c++)"Thumbnailer::get_thumbnail(std::basic_string<char, std::char_traits<char>, std::allocator<char> > const&, ThumbnailSize)@Base" 1.0
- (c++)"Thumbnailer::get_thumbnail(std::basic_string<char, std::char_traits<char>, std::allocator<char> > const&, ThumbnailSize, ThumbnailPolicy)@Base" 1.0
- (c++)"Thumbnailer::get_album_art(std::basic_string<char, std::char_traits<char>, std::allocator<char> > const&, std::basic_string<char, std::char_traits<char>, std::allocator<char> > const&, ThumbnailSize, ThumbnailPolicy)@Base" 1.0
- (c++)"Thumbnailer::Thumbnailer()@Base" 1.0
- (c++)"Thumbnailer::~Thumbnailer()@Base" 1.0
- (c++)"Thumbnailer::get_artist_art(std::basic_string<char, std::char_traits<char>, std::allocator<char> > const&, std::basic_string<char, std::char_traits<char>, std::allocator<char> > const&, ThumbnailSize, ThumbnailPolicy)@Base" 1.1+14.10.20140811

=== modified file 'debian/rules'
--- debian/rules	2014-09-25 08:54:10 +0000
+++ debian/rules	2015-07-14 06:30:34 +0000
@@ -15,3 +15,6 @@
 override_dh_auto_configure:
 	dh_auto_configure -- -DCMAKE_INSTALL_SYSCONFDIR=/etc
 
+# Tests are not written to be run in parallel.
+override_dh_auto_test:
+	dh_auto_test --max-parallel=1 -- ARGS="--verbose"

=== modified file 'debian/thumbnailer-service.install'
--- debian/thumbnailer-service.install	2014-09-16 12:12:21 +0000
+++ debian/thumbnailer-service.install	2015-07-14 06:30:34 +0000
@@ -1,3 +1,5 @@
+usr/bin/thumbnailer-admin
 usr/lib/*/thumbnailer/thumbnailer-service
+usr/lib/*/thumbnailer/vs-thumb
 usr/share/dbus-1/services/com.canonical.Thumbnailer.service
 usr/share/glib-2.0/schemas/*

=== modified file 'include/CMakeLists.txt'
--- include/CMakeLists.txt	2013-10-02 09:43:16 +0000
+++ include/CMakeLists.txt	2015-07-14 06:30:34 +0000
@@ -1,12 +1,1 @@
 add_subdirectory(internal)
-
-install(FILES thumbnailer.h DESTINATION include)
-
-# Build and install a pkg-config file
-set(prefix ${CMAKE_INSTALL_PREFIX})
-set(exec_prefix ${prefix}/bin)
-set(libdir ${prefix}/${LIBDIR})
-set(includedir ${prefix}/include)
-set(pkg-name "lib${COL_LIB_BASENAME}")
-configure_file(thumbnailer.pc.in thumbnailer.pc @ONLY)
-install(FILES ${CMAKE_CURRENT_BINARY_DIR}/thumbnailer.pc DESTINATION ${LIBDIR}/pkgconfig)

=== added directory 'include/core'
=== added file 'include/core/CMakeLists.txt'
=== added file 'include/core/cache_codec.h'
--- include/core/cache_codec.h	1970-01-01 00:00:00 +0000
+++ include/core/cache_codec.h	2015-07-14 06:30:34 +0000
@@ -0,0 +1,50 @@
+/*
+ * Copyright (C) 2015 Canonical Ltd.
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU Lesser General Public License version 3 as
+ * published by the Free Software Foundation.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ *
+ * Authored by: Michi Henning <[e-mail]>
+ */
+
+#pragma once
+
+#include <string>
+
+namespace core
+{
+
+/**
+Traits for serialization and deserialization of cache custom types.
+To use custom types, specialize this template
+for each custom type (other than string) in the `core` namespace.
+
+\warning Do _not_ specialize this struct for `std::string`!
+Doing so has no effect.
+
+\see PersistentCache
+*/
+
+template <typename T>
+struct CacheCodec
+{
+    /**
+    \brief Converts a value of custom type T into a string.
+    */
+    static std::string encode(T const& value);
+    /**
+    \brief Converts a string into a value of custom type T.
+    */
+    static T decode(std::string const& s);
+};
+
+}  // namespace core

=== added file 'include/core/cache_discard_policy.h'
--- include/core/cache_discard_policy.h	1970-01-01 00:00:00 +0000
+++ include/core/cache_discard_policy.h	2015-07-14 06:30:34 +0000
@@ -0,0 +1,41 @@
+/*
+ * Copyright (C) 2015 Canonical Ltd.
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU Lesser General Public License version 3 as
+ * published by the Free Software Foundation.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ *
+ * Authored by: Michi Henning <[e-mail]>
+ */
+
+#pragma once
+
+namespace core
+{
+
+/**
+\brief Indicates the discard policy to make room for entries when the cache is full.
+
+Once the cache is full and another entry is added,
+`lru_ttl` unconditionally deletes all entries that have expired and then,
+if deleting these entries did not create sufficient free space, deletes entries
+in least-recently-used order until enough space is available.
+
+If the discard policy is set to `lru_only`, entries do not maintain an expiry time and
+are therefore discarded strictly in LRU order.
+*/
+enum class CacheDiscardPolicy
+{
+    lru_ttl,  ///< Evict expired entries first, followed by eviction in LRU order
+    lru_only  ///< Evict in LRU order
+};
+
+}  // namespace core

=== added file 'include/core/cache_events.h'
--- include/core/cache_events.h	1970-01-01 00:00:00 +0000
+++ include/core/cache_events.h	2015-07-14 06:30:34 +0000
@@ -0,0 +1,103 @@
+/*
+ * Copyright (C) 2015 Canonical Ltd.
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU Lesser General Public License version 3 as
+ * published by the Free Software Foundation.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ *
+ * Authored by: Michi Henning <[e-mail]>
+ */
+
+#pragma once
+
+#include <cstdint>
+#include <type_traits>
+
+/**
+\brief Top-level namespace for core functionality.
+*/
+
+namespace core
+{
+
+/**
+\brief Event types that can be monitored.
+*/
+
+// Note: Any change here must have a corresponding change to
+//       CacheEventIndex in core/internal/cache_event_indexes.h!
+
+enum class CacheEvent : uint32_t
+{
+    get        = 1 << 0,  ///< An entry was returned by a call to `get()`, `get_or_put()`, `take()`, or `take_data()`.
+    put        = 1 << 1,  ///< An entry was added by a call to `put()` or `get_or_put()`.
+    invalidate = 1 << 2,  ///< An entry was removed by a call to `invalidate()`, `take()`, or `take_data()`.
+    touch      = 1 << 3,  ///< An entry was refreshed by a call to `touch()`.
+    miss       = 1 << 4,  ///< A call to `get()`, `get_or_put()`, `take()`, or `take_data()` failed to return an entry.
+    evict_ttl  = 1 << 5,  ///< An expired entry was evicted due to a call to `put()`,
+                          ///< `get_or_put()`, `trim_to()`, or `resize()`.
+    evict_lru  = 1 << 6,  ///< The oldest entry was evicted due to a call to `put()`,
+                          ///< `get_or_put()`, `trim_to()`, or `resize()`.
+    END_       = 1 << 7   ///< End marker
+};
+
+/**
+\brief Returns the bitwise OR of two event types.
+*/
+inline CacheEvent operator|(CacheEvent left, CacheEvent right)
+{
+    auto l = std::underlying_type<CacheEvent>::type(left);
+    auto r = std::underlying_type<CacheEvent>::type(right);
+    return CacheEvent(l | r);
+}
+
+/**
+\brief Assigns the bitwise OR of `left` and `right` to `left`.
+*/
+inline CacheEvent& operator|=(CacheEvent& left, CacheEvent right)
+{
+    return left = left | right;
+}
+
+/**
+\brief Returns the bitwise AND of two event types.
+*/
+inline CacheEvent operator&(CacheEvent left, CacheEvent right)
+{
+    auto l = std::underlying_type<CacheEvent>::type(left);
+    auto r = std::underlying_type<CacheEvent>::type(right);
+    return CacheEvent(l & r);
+}
+
+/**
+\brief Assigns the bitwise AND of `left` and `right` to `left`.
+*/
+inline CacheEvent& operator&=(CacheEvent& left, CacheEvent right)
+{
+    return left = left & right;
+}
+
+/**
+\brief Returns the bitwise NOT of `ev`. Unused bits are set to zero.
+*/
+inline CacheEvent operator~(CacheEvent ev)
+{
+    auto mask = std::underlying_type<CacheEvent>::type(CacheEvent::END_) - 1;
+    auto event = std::underlying_type<CacheEvent>::type(ev);
+    return CacheEvent(~event & mask);
+}
+
+/**
+\brief Convenience definition for all event types.
+*/
+static constexpr auto AllCacheEvents = CacheEvent(std::underlying_type<CacheEvent>::type(CacheEvent::END_) - 1);
+
+}  // namespace core

=== added directory 'include/core/internal'
=== added file 'include/core/internal/cache_event_indexes.h'
--- include/core/internal/cache_event_indexes.h	1970-01-01 00:00:00 +0000
+++ include/core/internal/cache_event_indexes.h	2015-07-14 06:30:34 +0000
@@ -0,0 +1,46 @@
+/*
+ * Copyright (C) 2015 Canonical Ltd.
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU Lesser General Public License version 3 as
+ * published by the Free Software Foundation.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ *
+ * Authored by: Michi Henning <[e-mail]>
+ */
+
+#pragma once
+
+#include <cstdint>
+
+namespace core
+{
+
+namespace internal
+{
+
+// Note: Any change here must have a corresponding change to
+//       CacheEvent in core/cache_events.h!
+
+enum class CacheEventIndex : uint32_t
+{
+    get        = 0,
+    put        = 1,
+    invalidate = 2,
+    touch      = 3,
+    miss       = 4,
+    evict_ttl  = 5,
+    evict_lru  = 6,
+    END_       = 7
+};
+
+}  // namespace internal
+
+}  // namespace core

=== added file 'include/core/internal/persistent_string_cache_impl.h'
--- include/core/internal/persistent_string_cache_impl.h	1970-01-01 00:00:00 +0000
+++ include/core/internal/persistent_string_cache_impl.h	2015-07-14 06:30:34 +0000
@@ -0,0 +1,191 @@
+/*
+ * Copyright (C) 2015 Canonical Ltd.
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU Lesser General Public License version 3 as
+ * published by the Free Software Foundation.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ *
+ * Authored by: Michi Henning <[e-mail]>
+ */
+
+#pragma once
+
+#include <core/internal/cache_event_indexes.h>
+#include <core/persistent_string_cache.h>
+
+#include <leveldb/db.h>
+
+#include <mutex>
+#include <sstream>
+
+namespace core
+{
+
+namespace internal
+{
+
+class PersistentStringCacheStats;
+
+class PersistentStringCacheImpl
+{
+public:
+    PersistentStringCacheImpl(std::string const& cache_path,
+                              int64_t max_size_in_bytes,
+                              core::CacheDiscardPolicy policy,
+                              PersistentStringCache* pimpl = nullptr);
+    PersistentStringCacheImpl(std::string const& cache_path, PersistentStringCache* pimpl = nullptr);
+
+    PersistentStringCacheImpl(PersistentStringCacheImpl const&) = delete;
+    PersistentStringCacheImpl& operator=(PersistentStringCacheImpl const&) = delete;
+
+    PersistentStringCacheImpl(PersistentStringCacheImpl&&) = delete;
+    PersistentStringCacheImpl& operator=(PersistentStringCacheImpl&&) = delete;
+
+    ~PersistentStringCacheImpl();
+
+    bool get(std::string const& key, std::string& value) const;
+    bool get(std::string const& key, std::string& value, std::string* metadata) const;
+    bool get_metadata(std::string const& key, std::string& metadata) const;
+    bool contains_key(std::string const& key) const;
+    int64_t size() const noexcept;
+    int64_t size_in_bytes() const noexcept;
+    int64_t max_size_in_bytes() const noexcept;
+    int64_t disk_size_in_bytes() const;
+    CacheDiscardPolicy discard_policy() const noexcept;
+    core::PersistentCacheStats stats() const;
+
+    bool put(std::string const& key,
+             std::string const& value,
+             std::chrono::time_point<std::chrono::system_clock> expiry_time = std::chrono::system_clock::time_point());
+    bool put(std::string const& key,
+             char const* value,
+             int64_t size,
+             std::chrono::time_point<std::chrono::system_clock> expiry_time = std::chrono::system_clock::time_point());
+    bool put(std::string const& key,
+             std::string const& value,
+             std::string const* metadata,
+             std::chrono::time_point<std::chrono::system_clock> expiry_time = std::chrono::system_clock::time_point());
+    bool put(std::string const& key,
+             char const* value_data,
+             int64_t value_size,
+             char const* metadata_data,
+             int64_t metadata_size,
+             std::chrono::time_point<std::chrono::system_clock> expiry_time = std::chrono::system_clock::time_point());
+    bool get_or_put(std::string const& key, std::string& value, PersistentStringCache::Loader load_func);
+    bool get_or_put(std::string const& key,
+                    std::string& value,
+                    std::string* metadata,
+                    PersistentStringCache::Loader load_func);
+    bool put_metadata(std::string const& key, std::string const& metadata);
+    bool put_metadata(std::string const& key, char const* metadata, int64_t metadata_size);
+    bool take(std::string const& key, std::string& value);
+    bool take(std::string const& key, std::string& value, std::string* metadata);
+    bool invalidate(std::string const& key);
+    void invalidate(std::vector<std::string> const& keys);
+    template<typename It>
+    void invalidate(It begin, It end);
+    void invalidate(std::initializer_list<std::string> const& keys);
+    void invalidate();
+    bool touch(
+        std::string const& key,
+        std::chrono::time_point<std::chrono::system_clock> expiry_time = std::chrono::system_clock::time_point());
+    void clear_stats() noexcept;
+    void resize(int64_t size_in_bytes);
+    void trim_to(int64_t used_size_in_bytes);
+    void compact();
+    void set_handler(CacheEvent events, PersistentStringCache::EventCallback cb);
+
+private:
+    // Simple struct to serialize/deserialize a data tuple.
+    // For the stringified representation, fields are separated
+    // by a space.
+
+    struct DataTuple
+    {
+        int64_t atime;  // Last access time, msec since the epoch
+        int64_t etime;  // Expiry time, msec since the epoch
+        int64_t size;   // Size in bytes
+
+        DataTuple(int64_t at, int64_t et, int64_t s) noexcept
+            : atime(at)
+            , etime(et)
+            , size(s)
+        {
+        }
+
+        DataTuple() noexcept
+            : DataTuple(0, 0, 0)
+        {
+        }
+
+        DataTuple(std::string const& s) noexcept
+        {
+            std::istringstream is(s);
+            is >> atime >> etime >> size;
+            assert(!is.bad());
+        }
+
+        DataTuple(DataTuple const&) = default;
+        DataTuple(DataTuple&&) = default;
+
+        DataTuple& operator=(DataTuple const&) = default;
+        DataTuple& operator=(DataTuple&&) = default;
+
+        std::string to_string() const
+        {
+            std::ostringstream os;
+            os << atime << " " << etime << " " << size;
+            return os.str();
+        }
+    };
+
+    void init_stats();
+    void init_db(leveldb::Options options);
+    bool cache_is_new() const;
+    void write_version();
+    void check_version();
+    void read_settings();
+    void write_settings();
+    void read_stats();
+    void write_stats();
+    bool read_dirty_flag() const;
+    void write_dirty_flag(bool is_dirty);
+    DataTuple get_data(std::string const& key, bool& found) const;
+    bool get_value_and_metadata(std::string const& key,
+                                DataTuple& data,
+                                std::string& value,
+                                std::string* metadata) const;
+    void batch_delete(std::string const& key, DataTuple const& data, leveldb::WriteBatch& batch);
+    void delete_entry(std::string const& key, DataTuple const& data);
+    void delete_at_least(int64_t bytes_needed, std::string const& skip_key = "");
+    void call_handler(std::string const& key, core::internal::CacheEventIndex event) const;
+
+    std::string make_message(leveldb::Status const& s, std::string const& msg) const;
+    std::string make_message(std::string const& msg) const;
+    void throw_if_error(leveldb::Status const& s, std::string const& msg) const;
+    void throw_logic_error(std::string const& msg) const;
+    void throw_invalid_argument(std::string const& msg) const;
+    void throw_corrupt_error(std::string const& msg) const;
+
+    PersistentStringCache* pimpl_;                 // Back-pointer to owning pimpl.
+    std::unique_ptr<leveldb::Cache> block_cache_;  // Must be defined *before* db_!
+    std::unique_ptr<leveldb::DB> db_;
+    std::shared_ptr<PersistentStringCacheStats> stats_;
+
+    std::array<PersistentStringCache::EventCallback, static_cast<unsigned>(CacheEventIndex::END_)>
+        handlers_;
+
+    mutable std::recursive_mutex mutex_;
+};
+
+}  // namespace internal
+
+}  // namespace core

=== added file 'include/core/internal/persistent_string_cache_stats.h'
--- include/core/internal/persistent_string_cache_stats.h	1970-01-01 00:00:00 +0000
+++ include/core/internal/persistent_string_cache_stats.h	2015-07-14 06:30:34 +0000
@@ -0,0 +1,237 @@
+/*
+ * Copyright (C) 2015 Canonical Ltd.
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU Lesser General Public License version 3 as
+ * published by the Free Software Foundation.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ *
+ * Authored by: Michi Henning <[e-mail]>
+ */
+
+#pragma once
+
+#include <core/cache_discard_policy.h>
+#include <core/persistent_cache_stats.h>
+
+#include <cassert>
+#include <cmath>
+#include <cstring>
+#include <sstream>
+
+namespace core
+{
+
+namespace internal
+{
+
+// Simple stats class to keep track of accesses.
+
+class PersistentStringCacheStats
+{
+public:
+    PersistentStringCacheStats() noexcept
+        : policy_(CacheDiscardPolicy::lru_only)
+        , max_cache_size_(0)
+        , num_entries_(0)
+        , cache_size_(0)
+        , state_(Initialized)
+    {
+        clear();
+        hist_.resize(PersistentCacheStats::NUM_BINS, 0);
+    }
+
+    PersistentStringCacheStats(PersistentStringCacheStats const&) = default;
+    PersistentStringCacheStats(PersistentStringCacheStats&&) = default;
+    PersistentStringCacheStats& operator=(PersistentStringCacheStats const&) = default;
+    PersistentStringCacheStats& operator=(PersistentStringCacheStats&&) = default;
+
+    std::string cache_path_;           // Immutable
+    core::CacheDiscardPolicy policy_;  // Immutable
+    int64_t max_cache_size_;
+
+    int64_t num_entries_;
+    int64_t cache_size_;
+    PersistentCacheStats::Histogram hist_;
+
+    // Values below are reset by a call to clear().
+    int64_t hits_;
+    int64_t misses_;
+    int64_t hits_since_last_miss_;
+    int64_t misses_since_last_hit_;
+    int64_t longest_hit_run_;
+    int64_t longest_miss_run_;
+    int64_t ttl_evictions_;
+    int64_t lru_evictions_;
+    std::chrono::system_clock::time_point most_recent_hit_time_;
+    std::chrono::system_clock::time_point most_recent_miss_time_;
+    std::chrono::system_clock::time_point longest_hit_run_time_;
+    std::chrono::system_clock::time_point longest_miss_run_time_;
+
+    enum State
+    {
+        Initialized,
+        LastAccessWasHit,
+        LastAccessWasMiss
+    };
+    State state_;
+
+    void inc_hits() noexcept
+    {
+        ++hits_since_last_miss_;
+        ++hits_;
+        most_recent_hit_time_ = std::chrono::system_clock::now();
+        misses_since_last_hit_ = 0;
+        if (state_ != LastAccessWasHit)
+        {
+            state_ = LastAccessWasHit;
+            misses_since_last_hit_ = 0;
+        }
+        if (state_ != LastAccessWasMiss && hits_since_last_miss_ > longest_hit_run_)
+        {
+            longest_hit_run_ = hits_since_last_miss_;
+            longest_hit_run_time_ = most_recent_hit_time_;
+        }
+    }
+
+    void inc_misses() noexcept
+    {
+        ++misses_since_last_hit_;
+        ++misses_;
+        most_recent_miss_time_ = std::chrono::system_clock::now();
+        hits_since_last_miss_ = 0;
+        if (state_ != LastAccessWasMiss)
+        {
+            state_ = LastAccessWasMiss;
+            hits_since_last_miss_ = 0;
+        }
+        if (state_ != LastAccessWasHit && misses_since_last_hit_ > longest_miss_run_)
+        {
+            longest_miss_run_ = misses_since_last_hit_;
+            longest_miss_run_time_ = most_recent_miss_time_;
+        }
+    }
+
+    void hist_decrement(int64_t size) noexcept
+    {
+        assert(size > 0);
+        --hist_[size_to_index(size)];
+    }
+
+    void hist_increment(int64_t size) noexcept
+    {
+        assert(size > 0);
+        ++hist_[size_to_index(size)];
+    }
+
+    void hist_clear() noexcept
+    {
+        memset(&hist_[0], 0, hist_.size() * sizeof(PersistentCacheStats::Histogram::value_type));
+    }
+
+    void clear() noexcept
+    {
+        hits_ = 0;
+        misses_ = 0;
+        hits_since_last_miss_ = 0;
+        misses_since_last_hit_ = 0;
+        longest_hit_run_ = 0;
+        longest_miss_run_ = 0;
+        ttl_evictions_ = 0;
+        lru_evictions_ = 0;
+        most_recent_hit_time_ = std::chrono::system_clock::time_point();
+        most_recent_miss_time_ = std::chrono::system_clock::time_point();
+        longest_hit_run_time_ = std::chrono::system_clock::time_point();
+        longest_miss_run_time_ = std::chrono::system_clock::time_point();
+    }
+
+    // Serialize the stats.
+
+    std::string serialize() const
+    {
+        using namespace std;
+        using namespace std::chrono;
+
+        ostringstream os;
+        os << num_entries_ << " "
+           << cache_size_ << " "
+           << hits_ << " "
+           << misses_ << " "
+           << hits_since_last_miss_ << " "
+           << misses_since_last_hit_ << " "
+           << longest_hit_run_ << " "
+           << longest_miss_run_ << " "
+           << ttl_evictions_ << " "
+           << lru_evictions_ << " "
+           << duration_cast<milliseconds>(most_recent_hit_time_.time_since_epoch()).count() << " "
+           << duration_cast<milliseconds>(most_recent_miss_time_.time_since_epoch()).count() << " "
+           << duration_cast<milliseconds>(longest_hit_run_time_.time_since_epoch()).count() << " "
+           << duration_cast<milliseconds>(longest_miss_run_time_.time_since_epoch()).count();
+        for (auto d : hist_)
+        {
+            os << " " << d;
+        }
+        return os.str();
+    }
+
+    // De-serialize the stats.
+
+    void deserialize(const std::string& s) noexcept
+    {
+        using namespace std;
+        using namespace std::chrono;
+
+        istringstream is(s);
+        int64_t mrht;
+        int64_t mrmt;
+        int64_t lhrt;
+        int64_t lmrt;
+        is >> num_entries_
+           >> cache_size_
+           >> hits_
+           >> misses_
+           >> hits_since_last_miss_
+           >> misses_since_last_hit_
+           >> longest_hit_run_
+           >> longest_miss_run_
+           >> ttl_evictions_
+           >> lru_evictions_
+           >> mrht
+           >> mrmt
+           >> lhrt
+           >> lmrt;
+        for (unsigned i = 0; i < PersistentCacheStats::NUM_BINS; ++i)
+        {
+            is >> hist_[i];
+        }
+        assert(!is.bad());
+        most_recent_hit_time_ = system_clock::time_point(milliseconds(mrht));
+        most_recent_miss_time_ = system_clock::time_point(milliseconds(mrmt));
+        longest_hit_run_time_ = system_clock::time_point(milliseconds(lhrt));
+        longest_miss_run_time_ = system_clock::time_point(milliseconds(lmrt));
+    }
+
+private:
+    unsigned size_to_index(int64_t size) const noexcept
+    {
+        using namespace std;
+        assert(size > 0);
+        unsigned log = floor(log10(size));     // 0..9 = 0, 10..99 = 1, 100..199 = 2, etc.
+        unsigned exp = pow(10, log);           // 0..9 = 1, 10..99 = 10, 100..199 = 100, etc.
+        unsigned div = size / exp;             // Extracts first decimal digit of size.
+        int index = log * 10 + div - log - 1;  // Partition each power of 10 into 9 bins.
+        index -= 8;                            // Sizes < 10 all go into bin 0;
+        return index < 0 ? 0 : (index > int(hist_.size()) - 1 ? hist_.size() - 1 : index);
+    }
+};
+
+}  // namespace internal
+
+}  // namespace core

=== added file 'include/core/optional.h'
--- include/core/optional.h	1970-01-01 00:00:00 +0000
+++ include/core/optional.h	2015-07-14 06:30:34 +0000
@@ -0,0 +1,35 @@
+/*
+ * Copyright (C) 2015 Canonical Ltd.
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU Lesser General Public License version 3 as
+ * published by the Free Software Foundation.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ *
+ * Authored by: Michi Henning <[e-mail]>
+ */
+
+#pragma once
+
+#include <boost/optional.hpp>
+
+namespace core
+{
+
+/**
+\brief Convenience typedef for nullable values.
+
+\note You should use `core::Optional` in preference to `boost::optional`
+in your code. This will ease an eventual transition to `std::optional`.
+*/
+template <typename T>
+using Optional = boost::optional<T>;
+
+}  // namespace core

=== added file 'include/core/persistent_cache.h'
--- include/core/persistent_cache.h	1970-01-01 00:00:00 +0000
+++ include/core/persistent_cache.h	2015-07-14 06:30:34 +0000
@@ -0,0 +1,3104 @@
+/*
+ * Copyright (C) 2015 Canonical Ltd.
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU Lesser General Public License version 3 as
+ * published by the Free Software Foundation.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ *
+ * Authored by: Michi Henning <[e-mail]>
+ */
+
+#pragma once
+
+#include <core/cache_codec.h>
+#include <core/persistent_string_cache.h>
+
+namespace core
+{
+
+/**
+\brief A persistent cache of key-value pairs and metadata of user-defined type.
+
+`K`, `V`, and `M` are the key type, value type, and metadata type, respectively.
+
+\note This template is a simple type adapter that forwards to
+core::PersistentStringCache. See the documentation there for details
+on cache operations and semantics.
+
+In order to use the cache with custom types (other than `std::string`),
+you must provide methods to encode the type to `string`, and decode
+from `string` back to the type.
+
+For example, suppose we have the following structure that we want to use
+as the key type of the cache:
+
+\code{.cpp}
+struct Person
+{
+    string name;
+    int    age;
+};
+\endcode
+
+In order to use the cache with the `Person` struct as the key, you must specialize
+the CacheCodec struct in namespace `core`:
+
+\code{.cpp}
+namespace core  // Specializations must be placed into namespace core.
+{
+
+template <>
+string CacheCodec<Person>::encode(Person const& p)
+{
+    ostringstream s;
+    s << p.age << ' ' << p.name;
+    return s.str();
+}
+
+template <>
+Person CacheCodec<Person>::decode(string const& str)
+{
+    istringstream s;
+    Person p;
+    s >> p.age >> p.name;
+    return p;
+}
+
+}  // namespace core
+\endcode
+
+For this example, it is convenient to stream the age first because this
+guarantees that `decode()` will work correctly even if the name contains
+a space. The order in which you stream the fields does not matter, only that
+(for custom _key_ types) the string representation of each value is unique.
+
+With these two methods defined, we can now use the cache with `Person` instances
+as the key. For example:
+
+\code{.cpp}
+// Custom cache using Person as the key, and string as the value and metadata.
+using PersonCache = core::PersistentCache<Person, string>;
+
+auto c = PersonCache::open("my_cache", 1024 * 1024 * 1024, CacheDiscardPolicy::LRU_only);
+
+Person bjarne{"Bjarne Stroustrup", 65};
+c->put(bjarne, "C++ inventor");
+auto value = c->get(bjarne);
+if (value)
+{
+    cout << bjarne.name << ": " << *value << endl;
+}
+Person person{"no such person", 0};
+value = c->get(person);
+assert(!value);
+\endcode
+
+Running this code produces the output:
+
+\code
+Bjarne Stroustrup: C++ inventor
+\endcode
+
+You can use a custom type for the cache's value and metadata as well by simply providing
+CacheCodec specializations as needed.
+
+\see core::CacheCodec
+\see core::PersistentStringCache
+*/
+
+template <typename K, typename V, typename M = std::string>
+class PersistentCache
+{
+public:
+    /**
+    Convenience typedef for the return type of open().
+    */
+    typedef std::unique_ptr<PersistentCache<K, V, M>> UPtr;
+
+    /**
+    \brief Simple pair of value and metadata.
+    */
+    struct Data
+    {
+        /**
+        \brief Stores the value of an entry.
+        */
+        V value;
+
+        /**
+        \brief Stores the metadata of an entry. If no metadata exists for an entry,
+        `metadata` is returned as the empty string when it is retrieved.
+        */
+        M metadata;
+    };
+
+    /** @name Typedefs for nullable keys, values, and metadata.
+    */
+
+    //{@
+
+    /**
+    \brief Convenience typedefs for returning nullable values.
+
+    \note You should use `OptionalKey`, `OptionalValue`, and `OptionalMetadata` in
+    your code in preference to `boost::optional`. This will ease an eventual
+    transition to `std::optional`.
+    */
+    typedef Optional<K> OptionalKey;
+    typedef Optional<V> OptionalValue;
+    typedef Optional<M> OptionalMetadata;
+    typedef Optional<Data> OptionalData;
+
+    //@}
+
+    /** @name Copy and Assignment
+    */
+
+    //{@
+    PersistentCache(PersistentCache const&) = delete;
+    PersistentCache& operator=(PersistentCache const&) = delete;
+
+    PersistentCache(PersistentCache&&) = default;
+    PersistentCache& operator=(PersistentCache&&) = default;
+    //@}
+
+    /**
+    Destroys the instance.
+
+    The destructor compacts the database. This ensures that, while a cache is
+    not in use, it comsumes as little disk space as possible.
+    */
+    ~PersistentCache() = default;
+
+    /** @name Creation Methods
+    */
+
+    //{@
+
+    /**
+    \brief Creates or opens a PersistentCache.
+    */
+    static UPtr open(std::string const& cache_path, int64_t max_size_in_bytes, CacheDiscardPolicy policy);
+
+    /**
+    \brief Opens an existing PersistentCache.
+    */
+    static UPtr open(std::string const& cache_path);
+
+    //@}
+
+    /** @name Accessors
+    */
+
+    //{@
+
+    /**
+    \brief Returns the value of an entry in the cache, provided the entry has not expired.
+    */
+    OptionalValue get(K const& key) const;
+
+    /**
+    \brief Returns the data for an entry in the cache, provided the entry has not expired.
+    */
+    OptionalData get_data(K const& key) const;
+
+    /**
+    \brief Returns the metadata for an entry in the cache, provided the entry has not expired.
+    */
+    OptionalMetadata get_metadata(K const& key) const;
+
+    /**
+    \brief Tests if an (unexpired) entry is in the cache.
+    */
+    bool contains_key(K const& key) const;
+
+    /**
+    \brief Returns the number of entries in the cache.
+    */
+    int64_t size() const noexcept;
+
+    /**
+    \brief Returns the number of bytes consumed by entries in the cache.
+    */
+    int64_t size_in_bytes() const noexcept;
+
+    /**
+    \brief Returns the maximum size of the cache in bytes.
+    */
+    int64_t max_size_in_bytes() const noexcept;
+
+    /**
+    \brief Returns an estimate of the disk space consumed by the cache.
+    */
+    int64_t disk_size_in_bytes() const;
+
+    /**
+    \brief Returns the discard policy of the cache.
+    */
+    CacheDiscardPolicy discard_policy() const noexcept;
+
+    /**
+    \brief Returns statistics for the cache.
+    */
+    PersistentCacheStats stats() const;
+
+    //@}
+
+    /** @name Modifiers
+    */
+
+    //{@
+
+    /**
+    \brief Adds or updates an entry. If `V` = `std::string`, the method is also overloaded to
+    to accept `char const*` and `size`.
+    */
+    bool put(K const& key,
+             V const& value,
+             std::chrono::time_point<std::chrono::system_clock> expiry_time = std::chrono::system_clock::time_point());
+
+    /**
+    \brief Adds or updates an entry and its metadata. If 'V' or `M` = `std::string`,
+    the method is also overloaded to accept `char const*` and `size`.
+    */
+    bool put(K const& key,
+             V const& value,
+             M const& metadata,
+             std::chrono::time_point<std::chrono::system_clock> expiry_time = std::chrono::system_clock::time_point());
+
+    /**
+    \brief Function called by the cache to load an entry after a cache miss.
+    */
+    typedef std::function<void(K const& key, PersistentCache<K, V, M>& cache)> Loader;
+
+    /**
+    \brief Atomically retrieves or stores a cache entry.
+    */
+    OptionalValue get_or_put(K const& key, Loader const& load_func);
+
+    /**
+    \brief Atomically retrieves or stores a cache entry.
+    */
+    OptionalData get_or_put_data(K const& key, Loader const& load_func);
+
+    /**
+    \brief Adds or replaces the metadata for an entry. If `M` = `std::string`, an overload that accepts
+    `const char*` and `size` is provided as well.
+    */
+    bool put_metadata(K const& key, M const& metadata);
+
+    /**
+    \brief Removes an entry and returns its value.
+    */
+    OptionalValue take(K const& key);
+
+    /**
+    \brief Removes an entry and returns its value and metadata.
+    */
+    OptionalData take_data(K const& key);
+
+    /**
+    \brief Removes an entry and its associated metadata (if any).
+    */
+    bool invalidate(K const& key);
+
+    /**
+    \brief Atomically removes the specified entries from the cache.
+    */
+    void invalidate(std::vector<K> const& keys);
+
+    /**
+    \brief Atomically removes the specified entries from the cache.
+    */
+    template <typename It>
+    void invalidate(It begin, It end);
+
+    /**
+    \brief Atomically removes the specified entries from the cache.
+    */
+    void invalidate(std::initializer_list<K> const& keys);
+
+    /**
+    \brief Deletes all entries from the cache.
+    */
+    void invalidate();
+
+    /**
+    \brief Updates the access time of an entry.
+    */
+    bool touch(
+        K const& key,
+        std::chrono::time_point<std::chrono::system_clock> expiry_time = std::chrono::system_clock::time_point());
+
+    /**
+    \brief Resets all statistics counters.
+    */
+    void clear_stats();
+
+    /**
+    \brief Changes the maximum size of the cache.
+    */
+    void resize(int64_t size_in_bytes);
+
+    /**
+    \brief Expires entries.
+    */
+    void trim_to(int64_t used_size_in_bytes);
+
+    /**
+    \brief Compacts the database.
+    */
+    void compact();
+
+    //@}
+
+    /** @name Monitoring cache activity
+    */
+
+    //{@
+
+    /**
+    \brief The type of a handler function.
+    */
+    typedef std::function<void(K const& key, CacheEvent ev, PersistentCacheStats const& stats)> EventCallback;
+
+    /**
+    \brief Installs a handler for one or more events.
+    */
+    void set_handler(CacheEvent events, EventCallback cb);
+
+    //@}
+
+private:
+    // @cond
+    PersistentCache(std::string const& cache_path, int64_t max_size_in_bytes, CacheDiscardPolicy policy);
+    PersistentCache(std::string const& cache_path);
+
+    std::unique_ptr<PersistentStringCache> p_;
+    // @endcond
+};
+
+// @cond
+
+// Method implementations. Out-of-line because, otherwise, things become completely unreadable.
+
+template <typename K, typename V, typename M>
+PersistentCache<K, V, M>::PersistentCache(std::string const& cache_path,
+                                          int64_t max_size_in_bytes,
+                                          CacheDiscardPolicy policy)
+    : p_(PersistentStringCache::open(cache_path, max_size_in_bytes, policy))
+{
+}
+
+template <typename K, typename V, typename M>
+PersistentCache<K, V, M>::PersistentCache(std::string const& cache_path)
+    : p_(PersistentStringCache::open(cache_path))
+{
+}
+
+template <typename K, typename V, typename M>
+typename PersistentCache<K, V, M>::UPtr PersistentCache<K, V, M>::open(std::string const& cache_path,
+                                                                       int64_t max_size_in_bytes,
+                                                                       CacheDiscardPolicy policy)
+{
+    return PersistentCache<K, V, M>::UPtr(new PersistentCache<K, V, M>(cache_path, max_size_in_bytes, policy));
+}
+
+template <typename K, typename V, typename M>
+typename PersistentCache<K, V, M>::UPtr PersistentCache<K, V, M>::open(std::string const& cache_path)
+{
+    return PersistentCache<K, V, M>::UPtr(new PersistentCache<K, V, M>(cache_path));
+}
+
+template <typename K, typename V, typename M>
+typename PersistentCache<K, V, M>::OptionalValue PersistentCache<K, V, M>::get(K const& key) const
+{
+    auto const& svalue = p_->get(CacheCodec<K>::encode(key));
+    return svalue ? OptionalValue(CacheCodec<V>::decode(*svalue)) : OptionalValue();
+}
+
+template <typename K, typename V, typename M>
+typename PersistentCache<K, V, M>::OptionalData PersistentCache<K, V, M>::get_data(K const& key) const
+{
+    auto sdata = p_->get_data(CacheCodec<K>::encode(key));
+    if (!sdata)
+    {
+        return OptionalData();
+    }
+    return OptionalData({CacheCodec<V>::decode(sdata->value), CacheCodec<M>::decode(sdata->metadata)});
+}
+
+template <typename K, typename V, typename M>
+typename PersistentCache<K, V, M>::OptionalMetadata PersistentCache<K, V, M>::get_metadata(K const& key) const
+{
+    auto smeta = p_->get_metadata(CacheCodec<K>::encode(key));
+    return smeta ? OptionalMetadata(CacheCodec<M>::decode(*smeta)) : OptionalMetadata();
+}
+
+template <typename K, typename V, typename M>
+bool PersistentCache<K, V, M>::contains_key(K const& key) const
+{
+    return p_->contains_key(CacheCodec<K>::encode(key));
+}
+
+template <typename K, typename V, typename M>
+int64_t PersistentCache<K, V, M>::size() const noexcept
+{
+    return p_->size();
+}
+
+template <typename K, typename V, typename M>
+int64_t PersistentCache<K, V, M>::size_in_bytes() const noexcept
+{
+    return p_->size_in_bytes();
+}
+
+template <typename K, typename V, typename M>
+int64_t PersistentCache<K, V, M>::max_size_in_bytes() const noexcept
+{
+    return p_->max_size_in_bytes();
+}
+
+template <typename K, typename V, typename M>
+int64_t PersistentCache<K, V, M>::disk_size_in_bytes() const
+{
+    return p_->disk_size_in_bytes();
+}
+
+template <typename K, typename V, typename M>
+CacheDiscardPolicy PersistentCache<K, V, M>::discard_policy() const noexcept
+{
+    return p_->discard_policy();
+}
+
+template <typename K, typename V, typename M>
+PersistentCacheStats PersistentCache<K, V, M>::stats() const
+{
+    return p_->stats();
+}
+
+template <typename K, typename V, typename M>
+bool PersistentCache<K, V, M>::put(K const& key,
+                                   V const& value,
+                                   std::chrono::time_point<std::chrono::system_clock> expiry_time)
+{
+    return p_->put(CacheCodec<K>::encode(key), CacheCodec<V>::encode(value), expiry_time);
+}
+
+template <typename K, typename V, typename M>
+bool PersistentCache<K, V, M>::put(K const& key,
+                                   V const& value,
+                                   M const& metadata,
+                                   std::chrono::time_point<std::chrono::system_clock> expiry_time)
+{
+    return p_->put(CacheCodec<K>::encode(key), CacheCodec<V>::encode(value), CacheCodec<M>::encode(metadata),
+                   expiry_time);
+}
+
+template <typename K, typename V, typename M>
+typename PersistentCache<K, V, M>::OptionalValue PersistentCache<K, V, M>::get_or_put(
+    K const& key, PersistentCache<K, V, M>::Loader const& load_func)
+{
+    std::string const& skey = CacheCodec<K>::encode(key);
+    auto sload_func = [&](std::string const&, PersistentStringCache const&)
+    {
+        load_func(key, *this);
+    };
+    auto svalue = p_->get_or_put(skey, sload_func);
+    return svalue ? OptionalValue(CacheCodec<V>::decode(*svalue)) : OptionalValue();
+}
+
+template <typename K, typename V, typename M>
+typename PersistentCache<K, V, M>::OptionalData PersistentCache<K, V, M>::get_or_put_data(
+    K const& key, PersistentCache<K, V, M>::Loader const& load_func)
+{
+    std::string const& skey = CacheCodec<K>::encode(key);
+    auto sload_func = [&](std::string const&, PersistentStringCache const&)
+    {
+        load_func(key, *this);
+    };
+    auto sdata = p_->get_or_put_data(skey, sload_func);
+    if (!sdata)
+    {
+        return OptionalData();
+    }
+    return OptionalData({CacheCodec<V>::decode(sdata->value), CacheCodec<M>::decode(sdata->metadata)});
+}
+
+template <typename K, typename V, typename M>
+bool PersistentCache<K, V, M>::put_metadata(K const& key, M const& metadata)
+{
+    return p_->put_metadata(CacheCodec<K>::encode(key), CacheCodec<M>::encode(metadata));
+}
+
+template <typename K, typename V, typename M>
+typename PersistentCache<K, V, M>::OptionalValue PersistentCache<K, V, M>::take(K const& key)
+{
+    auto svalue = p_->take(CacheCodec<K>::encode(key));
+    return svalue ? OptionalValue(CacheCodec<V>::decode(*svalue)) : OptionalValue();
+}
+
+template <typename K, typename V, typename M>
+typename PersistentCache<K, V, M>::OptionalData PersistentCache<K, V, M>::take_data(K const& key)
+{
+    auto sdata = p_->take_data(CacheCodec<K>::encode(key));
+    if (!sdata)
+    {
+        return OptionalData();
+    }
+    return OptionalData({CacheCodec<V>::decode(sdata->value), CacheCodec<M>::decode(sdata->metadata)});
+}
+
+template <typename K, typename V, typename M>
+bool PersistentCache<K, V, M>::invalidate(K const& key)
+{
+    return p_->invalidate(CacheCodec<K>::encode(key));
+}
+
+template <typename K, typename V, typename M>
+void PersistentCache<K, V, M>::invalidate(std::vector<K> const& keys)
+{
+    invalidate(keys.begin(), keys.end());
+}
+
+template <typename K, typename V, typename M>
+template <typename It>
+void PersistentCache<K, V, M>::invalidate(It begin, It end)
+{
+    std::vector<std::string> skeys;
+    for (auto&& it = begin; it < end; ++it)
+    {
+        skeys.push_back(CacheCodec<K>::encode(*it));
+    }
+    p_->invalidate(skeys.begin(), skeys.end());
+}
+
+template <typename K, typename V, typename M>
+void PersistentCache<K, V, M>::invalidate(std::initializer_list<K> const& keys)
+{
+    invalidate(keys.begin(), keys.end());
+}
+
+template <typename K, typename V, typename M>
+void PersistentCache<K, V, M>::invalidate()
+{
+    p_->invalidate();
+}
+
+template <typename K, typename V, typename M>
+bool PersistentCache<K, V, M>::touch(K const& key, std::chrono::time_point<std::chrono::system_clock> expiry_time)
+{
+    return p_->touch(CacheCodec<K>::encode(key), expiry_time);
+}
+
+template <typename K, typename V, typename M>
+void PersistentCache<K, V, M>::clear_stats()
+{
+    p_->clear_stats();
+}
+
+template <typename K, typename V, typename M>
+void PersistentCache<K, V, M>::resize(int64_t size_in_bytes)
+{
+    p_->resize(size_in_bytes);
+}
+
+template <typename K, typename V, typename M>
+void PersistentCache<K, V, M>::trim_to(int64_t used_size_in_bytes)
+{
+    p_->trim_to(used_size_in_bytes);
+}
+
+template <typename K, typename V, typename M>
+void PersistentCache<K, V, M>::compact()
+{
+    p_->compact();
+}
+
+template <typename K, typename V, typename M>
+void PersistentCache<K, V, M>::set_handler(CacheEvent events, EventCallback cb)
+{
+    auto scb = [cb](std::string const& key, CacheEvent ev, PersistentCacheStats const& c)
+    {
+        cb(CacheCodec<K>::decode(key), ev, c);
+    };
+    p_->set_handler(events, scb);
+}
+
+// Below are specializations for the various combinations of one or more of K, V, and M
+// being of type std::string. Without this, we would end up calling through a string-to-string
+// codec, which would force a copy for everything of type string, which is brutally inefficient.
+//
+// This is verbose because we must specialize the class template in order to avoid
+// calling the CacheCodec methods for type string. The combinations that follow are
+// <s,V,M>, <K,s,M>, <K,V,s>, <s,s,M>, <s,V,s>, <K,s,s>, and <s,s,s>.
+// The code is identical, except that, where we know that K, V, or M are std::string,
+// it avoids calling the encode()/decode() methods.
+
+// Specialization for K = std::string.
+
+template <typename V, typename M>
+class PersistentCache<std::string, V, M>
+{
+public:
+    typedef std::unique_ptr<PersistentCache<std::string, V, M>> UPtr;
+
+    typedef Optional<std::string> OptionalKey;
+    typedef Optional<V> OptionalValue;
+    typedef Optional<M> OptionalMetadata;
+
+    struct Data
+    {
+        V value;
+        M metadata;
+    };
+    typedef Optional<Data> OptionalData;
+
+    PersistentCache(PersistentCache const&) = delete;
+    PersistentCache& operator=(PersistentCache const&) = delete;
+
+    PersistentCache(PersistentCache&&) = default;
+    PersistentCache& operator=(PersistentCache&&) = default;
+
+    ~PersistentCache() = default;
+
+    static UPtr open(std::string const& cache_path, int64_t max_size_in_bytes, CacheDiscardPolicy policy);
+    static UPtr open(std::string const& cache_path);
+
+    OptionalValue get(std::string const& key) const;
+    OptionalData get_data(std::string const& key) const;
+    OptionalMetadata get_metadata(std::string const& key) const;
+    bool contains_key(std::string const& key) const;
+    int64_t size() const noexcept;
+    int64_t size_in_bytes() const noexcept;
+    int64_t max_size_in_bytes() const noexcept;
+    int64_t disk_size_in_bytes() const;
+    CacheDiscardPolicy discard_policy() const noexcept;
+    PersistentCacheStats stats() const;
+
+    bool put(std::string const& key,
+             V const& value,
+             std::chrono::time_point<std::chrono::system_clock> expiry_time = std::chrono::system_clock::time_point());
+    bool put(std::string const& key,
+             V const& value,
+             M const& metadata,
+             std::chrono::time_point<std::chrono::system_clock> expiry_time = std::chrono::system_clock::time_point());
+
+    typedef std::function<void(std::string const& key, PersistentCache<std::string, V, M>& cache)> Loader;
+
+    OptionalValue get_or_put(std::string const& key, Loader const& load_func);
+    OptionalData get_or_put_data(std::string const& key, Loader const& load_func);
+
+    bool put_metadata(std::string const& key, M const& metadata);
+    OptionalValue take(std::string const& key);
+    OptionalData take_data(std::string const& key);
+    bool invalidate(std::string const& key);
+    void invalidate(std::vector<std::string> const& keys);
+    template <typename It>
+    void invalidate(It begin, It end);
+    void invalidate(std::initializer_list<std::string> const& keys);
+    void invalidate();
+    bool touch(
+        std::string const& key,
+        std::chrono::time_point<std::chrono::system_clock> expiry_time = std::chrono::system_clock::time_point());
+    void clear_stats();
+    void resize(int64_t size_in_bytes);
+    void trim_to(int64_t used_size_in_bytes);
+    void compact();
+
+    typedef std::function<void(std::string const& key, CacheEvent ev, PersistentCacheStats const& stats)> EventCallback;
+
+    void set_handler(CacheEvent events, EventCallback cb);
+
+private:
+    PersistentCache(std::string const& cache_path, int64_t max_size_in_bytes, CacheDiscardPolicy policy);
+    PersistentCache(std::string const& cache_path);
+
+    std::unique_ptr<PersistentStringCache> p_;
+};
+
+template <typename V, typename M>
+PersistentCache<std::string, V, M>::PersistentCache(std::string const& cache_path,
+                                                    int64_t max_size_in_bytes,
+                                                    CacheDiscardPolicy policy)
+    : p_(PersistentStringCache::open(cache_path, max_size_in_bytes, policy))
+{
+}
+
+template <typename V, typename M>
+PersistentCache<std::string, V, M>::PersistentCache(std::string const& cache_path)
+    : p_(PersistentStringCache::open(cache_path))
+{
+}
+
+template <typename V, typename M>
+typename PersistentCache<std::string, V, M>::UPtr PersistentCache<std::string, V, M>::open(
+    std::string const& cache_path, int64_t max_size_in_bytes, CacheDiscardPolicy policy)
+{
+    return PersistentCache<std::string, V, M>::UPtr(
+        new PersistentCache<std::string, V, M>(cache_path, max_size_in_bytes, policy));
+}
+
+template <typename V, typename M>
+typename PersistentCache<std::string, V, M>::UPtr PersistentCache<std::string, V, M>::open(
+    std::string const& cache_path)
+{
+    return PersistentCache<std::string, V, M>::UPtr(new PersistentCache<std::string, V, M>(cache_path));
+}
+
+template <typename V, typename M>
+typename PersistentCache<std::string, V, M>::OptionalValue PersistentCache<std::string, V, M>::get(
+    std::string const& key) const
+{
+    auto const& svalue = p_->get(key);
+    return svalue ? OptionalValue(CacheCodec<V>::decode(*svalue)) : OptionalValue();
+}
+
+template <typename V, typename M>
+typename PersistentCache<std::string, V, M>::OptionalData PersistentCache<std::string, V, M>::get_data(
+    std::string const& key) const
+{
+    auto sdata = p_->get_data(key);
+    if (!sdata)
+    {
+        return OptionalData();
+    }
+    return OptionalData({CacheCodec<V>::decode(sdata->value), CacheCodec<M>::decode(sdata->metadata)});
+}
+
+template <typename V, typename M>
+typename PersistentCache<std::string, V, M>::OptionalMetadata PersistentCache<std::string, V, M>::get_metadata(
+    std::string const& key) const
+{
+    auto smeta = p_->get_metadata(key);
+    return smeta ? OptionalMetadata(CacheCodec<M>::decode(*smeta)) : OptionalMetadata();
+}
+
+template <typename V, typename M>
+bool PersistentCache<std::string, V, M>::contains_key(std::string const& key) const
+{
+    return p_->contains_key(key);
+}
+
+template <typename V, typename M>
+int64_t PersistentCache<std::string, V, M>::size() const noexcept
+{
+    return p_->size();
+}
+
+template <typename V, typename M>
+int64_t PersistentCache<std::string, V, M>::size_in_bytes() const noexcept
+{
+    return p_->size_in_bytes();
+}
+
+template <typename V, typename M>
+int64_t PersistentCache<std::string, V, M>::max_size_in_bytes() const noexcept
+{
+    return p_->max_size_in_bytes();
+}
+
+template <typename V, typename M>
+int64_t PersistentCache<std::string, V, M>::disk_size_in_bytes() const
+{
+    return p_->disk_size_in_bytes();
+}
+
+template <typename V, typename M>
+CacheDiscardPolicy PersistentCache<std::string, V, M>::discard_policy() const noexcept
+{
+    return p_->discard_policy();
+}
+
+template <typename V, typename M>
+PersistentCacheStats PersistentCache<std::string, V, M>::stats() const
+{
+    return p_->stats();
+}
+
+template <typename V, typename M>
+bool PersistentCache<std::string, V, M>::put(std::string const& key,
+                                             V const& value,
+                                             std::chrono::time_point<std::chrono::system_clock> expiry_time)
+{
+    return p_->put(key, CacheCodec<V>::encode(value), expiry_time);
+}
+
+template <typename V, typename M>
+bool PersistentCache<std::string, V, M>::put(std::string const& key,
+                                             V const& value,
+                                             M const& metadata,
+                                             std::chrono::time_point<std::chrono::system_clock> expiry_time)
+{
+    return p_->put(key, CacheCodec<V>::encode(value), CacheCodec<M>::encode(metadata), expiry_time);
+}
+
+template <typename V, typename M>
+typename PersistentCache<std::string, V, M>::OptionalValue PersistentCache<std::string, V, M>::get_or_put(
+    std::string const& key, PersistentCache<std::string, V, M>::Loader const& load_func)
+{
+    auto sload_func = [&](std::string const&, PersistentStringCache const&)
+    {
+        load_func(key, *this);
+    };
+    auto svalue = p_->get_or_put(key, sload_func);
+    return svalue ? OptionalValue(CacheCodec<V>::decode(*svalue)) : OptionalValue();
+}
+
+template <typename V, typename M>
+typename PersistentCache<std::string, V, M>::OptionalData PersistentCache<std::string, V, M>::get_or_put_data(
+    std::string const& key, PersistentCache<std::string, V, M>::Loader const& load_func)
+{
+    auto sload_func = [&](std::string const&, PersistentStringCache const&)
+    {
+        load_func(key, *this);
+    };
+    auto sdata = p_->get_or_put_data(key, sload_func);
+    if (!sdata)
+    {
+        return OptionalData();
+    }
+    return OptionalData({CacheCodec<V>::decode(sdata->value), CacheCodec<M>::decode(sdata->metadata)});
+}
+
+template <typename V, typename M>
+bool PersistentCache<std::string, V, M>::put_metadata(std::string const& key, M const& metadata)
+{
+    return p_->put_metadata(key, CacheCodec<M>::encode(metadata));
+}
+
+template <typename V, typename M>
+typename PersistentCache<std::string, V, M>::OptionalValue PersistentCache<std::string, V, M>::take(
+    std::string const& key)
+{
+    auto svalue = p_->take(key);
+    return svalue ? OptionalValue(CacheCodec<V>::decode(*svalue)) : OptionalValue();
+}
+
+template <typename V, typename M>
+typename PersistentCache<std::string, V, M>::OptionalData PersistentCache<std::string, V, M>::take_data(
+    std::string const& key)
+{
+    auto sdata = p_->take_data(key);
+    if (!sdata)
+    {
+        return OptionalData();
+    }
+    return OptionalData({CacheCodec<V>::decode(sdata->value), CacheCodec<M>::decode(sdata->metadata)});
+}
+
+template <typename V, typename M>
+bool PersistentCache<std::string, V, M>::invalidate(std::string const& key)
+{
+    return p_->invalidate(key);
+}
+
+template <typename V, typename M>
+void PersistentCache<std::string, V, M>::invalidate(std::vector<std::string> const& keys)
+{
+    invalidate(keys.begin(), keys.end());
+}
+
+template <typename V, typename M>
+template <typename It>
+void PersistentCache<std::string, V, M>::invalidate(It begin, It end)
+{
+    std::vector<std::string> skeys;
+    for (auto&& it = begin; it < end; ++it)
+    {
+        skeys.push_back(*it);
+    }
+    p_->invalidate(skeys.begin(), skeys.end());
+}
+
+template <typename V, typename M>
+void PersistentCache<std::string, V, M>::invalidate(std::initializer_list<std::string> const& keys)
+{
+    invalidate(keys.begin(), keys.end());
+}
+
+template <typename V, typename M>
+void PersistentCache<std::string, V, M>::invalidate()
+{
+    p_->invalidate();
+}
+
+template <typename V, typename M>
+bool PersistentCache<std::string, V, M>::touch(std::string const& key,
+                                               std::chrono::time_point<std::chrono::system_clock> expiry_time)
+{
+    return p_->touch(key, expiry_time);
+}
+
+template <typename V, typename M>
+void PersistentCache<std::string, V, M>::clear_stats()
+{
+    p_->clear_stats();
+}
+
+template <typename V, typename M>
+void PersistentCache<std::string, V, M>::resize(int64_t size_in_bytes)
+{
+    p_->resize(size_in_bytes);
+}
+
+template <typename V, typename M>
+void PersistentCache<std::string, V, M>::trim_to(int64_t used_size_in_bytes)
+{
+    p_->trim_to(used_size_in_bytes);
+}
+
+template <typename V, typename M>
+void PersistentCache<std::string, V, M>::compact()
+{
+    p_->compact();
+}
+
+template <typename V, typename M>
+void PersistentCache<std::string, V, M>::set_handler(CacheEvent events, EventCallback cb)
+{
+    p_->set_handler(events, cb);
+}
+
+// Specialization for V = std::string.
+
+template <typename K, typename M>
+class PersistentCache<K, std::string, M>
+{
+public:
+    typedef std::unique_ptr<PersistentCache<K, std::string, M>> UPtr;
+
+    typedef Optional<K> OptionalKey;
+    typedef Optional<std::string> OptionalValue;
+    typedef Optional<M> OptionalMetadata;
+
+    struct Data
+    {
+        std::string value;
+        M metadata;
+    };
+    typedef Optional<Data> OptionalData;
+
+    PersistentCache(PersistentCache const&) = delete;
+    PersistentCache& operator=(PersistentCache const&) = delete;
+
+    PersistentCache(PersistentCache&&) = default;
+    PersistentCache& operator=(PersistentCache&&) = default;
+
+    ~PersistentCache() = default;
+
+    static UPtr open(std::string const& cache_path, int64_t max_size_in_bytes, CacheDiscardPolicy policy);
+    static UPtr open(std::string const& cache_path);
+
+    OptionalValue get(K const& key) const;
+    OptionalData get_data(K const& key) const;
+    OptionalMetadata get_metadata(K const& key) const;
+    bool contains_key(K const& key) const;
+    int64_t size() const noexcept;
+    int64_t size_in_bytes() const noexcept;
+    int64_t max_size_in_bytes() const noexcept;
+    int64_t disk_size_in_bytes() const;
+    CacheDiscardPolicy discard_policy() const noexcept;
+    PersistentCacheStats stats() const;
+
+    bool put(K const& key,
+             std::string const& value,
+             std::chrono::time_point<std::chrono::system_clock> expiry_time = std::chrono::system_clock::time_point());
+    bool put(K const& key,
+             char const* value,
+             int64_t size,
+             std::chrono::time_point<std::chrono::system_clock> expiry_time = std::chrono::system_clock::time_point());
+    bool put(K const& key,
+             std::string const& value,
+             M const& metadata,
+             std::chrono::time_point<std::chrono::system_clock> expiry_time = std::chrono::system_clock::time_point());
+    bool put(K const& key,
+             char const* value,
+             int64_t value_size,
+             M const& metadata,
+             std::chrono::time_point<std::chrono::system_clock> expiry_time = std::chrono::system_clock::time_point());
+
+    typedef std::function<void(K const& key, PersistentCache<K, std::string, M>& cache)> Loader;
+
+    OptionalValue get_or_put(K const& key, Loader const& load_func);
+    OptionalData get_or_put_data(K const& key, Loader const& load_func);
+
+    bool put_metadata(K const& key, M const& metadata);
+    OptionalValue take(K const& key);
+    OptionalData take_data(K const& key);
+    bool invalidate(K const& key);
+    void invalidate(std::vector<K> const& keys);
+    template <typename It>
+    void invalidate(It begin, It end);
+    void invalidate(std::initializer_list<K> const& keys);
+    void invalidate();
+    bool touch(
+        K const& key,
+        std::chrono::time_point<std::chrono::system_clock> expiry_time = std::chrono::system_clock::time_point());
+    void clear_stats();
+    void resize(int64_t size_in_bytes);
+    void trim_to(int64_t used_size_in_bytes);
+    void compact();
+
+    typedef std::function<void(K const& key, CacheEvent ev, PersistentCacheStats const& stats)> EventCallback;
+
+    void set_handler(CacheEvent events, EventCallback cb);
+
+private:
+    PersistentCache(std::string const& cache_path, int64_t max_size_in_bytes, CacheDiscardPolicy policy);
+    PersistentCache(std::string const& cache_path);
+
+    std::unique_ptr<PersistentStringCache> p_;
+};
+
+template <typename K, typename M>
+PersistentCache<K, std::string, M>::PersistentCache(std::string const& cache_path,
+                                                    int64_t max_size_in_bytes,
+                                                    CacheDiscardPolicy policy)
+    : p_(PersistentStringCache::open(cache_path, max_size_in_bytes, policy))
+{
+}
+
+template <typename K, typename M>
+PersistentCache<K, std::string, M>::PersistentCache(std::string const& cache_path)
+    : p_(PersistentStringCache::open(cache_path))
+{
+}
+
+template <typename K, typename M>
+typename PersistentCache<K, std::string, M>::UPtr PersistentCache<K, std::string, M>::open(
+    std::string const& cache_path, int64_t max_size_in_bytes, CacheDiscardPolicy policy)
+{
+    return PersistentCache<K, std::string, M>::UPtr(
+        new PersistentCache<K, std::string, M>(cache_path, max_size_in_bytes, policy));
+}
+
+template <typename K, typename M>
+typename PersistentCache<K, std::string, M>::UPtr PersistentCache<K, std::string, M>::open(
+    std::string const& cache_path)
+{
+    return PersistentCache<K, std::string, M>::UPtr(new PersistentCache<K, std::string, M>(cache_path));
+}
+
+template <typename K, typename M>
+typename PersistentCache<K, std::string, M>::OptionalValue PersistentCache<K, std::string, M>::get(K const& key) const
+{
+    auto const& svalue = p_->get(CacheCodec<K>::encode(key));
+    return svalue ? OptionalValue(*svalue) : OptionalValue();
+}
+
+template <typename K, typename M>
+typename PersistentCache<K, std::string, M>::OptionalData PersistentCache<K, std::string, M>::get_data(
+    K const& key) const
+{
+    auto sdata = p_->get_data(CacheCodec<K>::encode(key));
+    if (!sdata)
+    {
+        return OptionalData();
+    }
+    return OptionalData({sdata->value, CacheCodec<M>::decode(sdata->metadata)});
+}
+
+template <typename K, typename M>
+typename PersistentCache<K, std::string, M>::OptionalMetadata PersistentCache<K, std::string, M>::get_metadata(
+    K const& key) const
+{
+    auto smeta = p_->get_metadata(CacheCodec<K>::encode(key));
+    return smeta ? OptionalMetadata(CacheCodec<M>::decode(*smeta)) : OptionalMetadata();
+}
+
+template <typename K, typename M>
+bool PersistentCache<K, std::string, M>::contains_key(K const& key) const
+{
+    return p_->contains_key(CacheCodec<K>::encode(key));
+}
+
+template <typename K, typename M>
+int64_t PersistentCache<K, std::string, M>::size() const noexcept
+{
+    return p_->size();
+}
+
+template <typename K, typename M>
+int64_t PersistentCache<K, std::string, M>::size_in_bytes() const noexcept
+{
+    return p_->size_in_bytes();
+}
+
+template <typename K, typename M>
+int64_t PersistentCache<K, std::string, M>::max_size_in_bytes() const noexcept
+{
+    return p_->max_size_in_bytes();
+}
+
+template <typename K, typename M>
+int64_t PersistentCache<K, std::string, M>::disk_size_in_bytes() const
+{
+    return p_->disk_size_in_bytes();
+}
+
+template <typename K, typename M>
+CacheDiscardPolicy PersistentCache<K, std::string, M>::discard_policy() const noexcept
+{
+    return p_->discard_policy();
+}
+
+template <typename K, typename M>
+PersistentCacheStats PersistentCache<K, std::string, M>::stats() const
+{
+    return p_->stats();
+}
+
+template <typename K, typename M>
+bool PersistentCache<K, std::string, M>::put(K const& key,
+                                             std::string const& value,
+                                             std::chrono::time_point<std::chrono::system_clock> expiry_time)
+{
+    return p_->put(CacheCodec<K>::encode(key), value, expiry_time);
+}
+
+template <typename K, typename M>
+bool PersistentCache<K, std::string, M>::put(K const& key,
+                                             char const* value,
+                                             int64_t size,
+                                             std::chrono::time_point<std::chrono::system_clock> expiry_time)
+{
+    return p_->put(CacheCodec<K>::encode(key), value, size, expiry_time);
+}
+
+template <typename K, typename M>
+bool PersistentCache<K, std::string, M>::put(K const& key,
+                                             std::string const& value,
+                                             M const& metadata,
+                                             std::chrono::time_point<std::chrono::system_clock> expiry_time)
+{
+    return p_->put(CacheCodec<K>::encode(key), value, CacheCodec<M>::encode(metadata), expiry_time);
+}
+
+template <typename K, typename M>
+bool PersistentCache<K, std::string, M>::put(K const& key,
+                                             char const* value,
+                                             int64_t size,
+                                             M const& metadata,
+                                             std::chrono::time_point<std::chrono::system_clock> expiry_time)
+{
+    std::string md = CacheCodec<M>::encode(metadata);
+    return p_->put(CacheCodec<K>::encode(key), value, size, md.data(), md.size(), expiry_time);
+}
+
+template <typename K, typename M>
+typename PersistentCache<K, std::string, M>::OptionalValue PersistentCache<K, std::string, M>::get_or_put(
+    K const& key, PersistentCache<K, std::string, M>::Loader const& load_func)
+{
+    std::string const& skey = CacheCodec<K>::encode(key);
+    auto sload_func = [&](std::string const&, PersistentStringCache const&)
+    {
+        load_func(key, *this);
+    };
+    auto svalue = p_->get_or_put(skey, sload_func);
+    return svalue ? OptionalValue(*svalue) : OptionalValue();
+}
+
+template <typename K, typename M>
+typename PersistentCache<K, std::string, M>::OptionalData PersistentCache<K, std::string, M>::get_or_put_data(
+    K const& key, PersistentCache<K, std::string, M>::Loader const& load_func)
+{
+    std::string const& skey = CacheCodec<K>::encode(key);
+    auto sload_func = [&](std::string const&, PersistentStringCache const&)
+    {
+        load_func(key, *this);
+    };
+    auto sdata = p_->get_or_put_data(skey, sload_func);
+    if (!sdata)
+    {
+        return OptionalData();
+    }
+    return OptionalData({sdata->value, CacheCodec<M>::decode(sdata->metadata)});
+}
+
+template <typename K, typename M>
+bool PersistentCache<K, std::string, M>::put_metadata(K const& key, M const& metadata)
+{
+    return p_->put_metadata(CacheCodec<K>::encode(key), CacheCodec<M>::encode(metadata));
+}
+
+template <typename K, typename M>
+typename PersistentCache<K, std::string, M>::OptionalValue PersistentCache<K, std::string, M>::take(K const& key)
+{
+    auto svalue = p_->take(CacheCodec<K>::encode(key));
+    return svalue ? OptionalValue(*svalue) : OptionalValue();
+}
+
+template <typename K, typename M>
+typename PersistentCache<K, std::string, M>::OptionalData PersistentCache<K, std::string, M>::take_data(
+    K const& key)
+{
+    auto sdata = p_->take_data(CacheCodec<K>::encode(key));
+    if (!sdata)
+    {
+        return OptionalData();
+    }
+    return OptionalData({sdata->value, CacheCodec<M>::decode(sdata->metadata)});
+}
+
+template <typename K, typename M>
+bool PersistentCache<K, std::string, M>::invalidate(K const& key)
+{
+    return p_->invalidate(CacheCodec<K>::encode(key));
+}
+
+template <typename K, typename M>
+void PersistentCache<K, std::string, M>::invalidate(std::vector<K> const& keys)
+{
+    invalidate(keys.begin(), keys.end());
+}
+
+template <typename K, typename M>
+template <typename It>
+void PersistentCache<K, std::string, M>::invalidate(It begin, It end)
+{
+    std::vector<std::string> skeys;
+    for (auto&& it = begin; it < end; ++it)
+    {
+        skeys.push_back(CacheCodec<K>::encode(*it));
+    }
+    p_->invalidate(skeys.begin(), skeys.end());
+}
+
+template <typename K, typename M>
+void PersistentCache<K, std::string, M>::invalidate(std::initializer_list<K> const& keys)
+{
+    invalidate(keys.begin(), keys.end());
+}
+
+template <typename K, typename M>
+void PersistentCache<K, std::string, M>::invalidate()
+{
+    p_->invalidate();
+}
+
+template <typename K, typename M>
+bool PersistentCache<K, std::string, M>::touch(K const& key,
+                                               std::chrono::time_point<std::chrono::system_clock> expiry_time)
+{
+    return p_->touch(CacheCodec<K>::encode(key), expiry_time);
+}
+
+template <typename K, typename M>
+void PersistentCache<K, std::string, M>::clear_stats()
+{
+    p_->clear_stats();
+}
+
+template <typename K, typename M>
+void PersistentCache<K, std::string, M>::resize(int64_t size_in_bytes)
+{
+    p_->resize(size_in_bytes);
+}
+
+template <typename K, typename M>
+void PersistentCache<K, std::string, M>::trim_to(int64_t used_size_in_bytes)
+{
+    p_->trim_to(used_size_in_bytes);
+}
+
+template <typename K, typename M>
+void PersistentCache<K, std::string, M>::compact()
+{
+    p_->compact();
+}
+
+template <typename K, typename M>
+void PersistentCache<K, std::string, M>::set_handler(CacheEvent events, EventCallback cb)
+{
+    auto scb = [cb](std::string const& key, CacheEvent ev, PersistentCacheStats const& c)
+    {
+        cb(CacheCodec<K>::decode(key), ev, c);
+    };
+    p_->set_handler(events, scb);
+}
+
+// Specialization for M = std::string.
+
+template <typename K, typename V>
+class PersistentCache<K, V, std::string>
+{
+public:
+    typedef std::unique_ptr<PersistentCache<K, V, std::string>> UPtr;
+
+    typedef Optional<K> OptionalKey;
+    typedef Optional<V> OptionalValue;
+    typedef Optional<std::string> OptionalMetadata;
+
+    struct Data
+    {
+        V value;
+        std::string metadata;
+    };
+    typedef Optional<Data> OptionalData;
+
+    PersistentCache(PersistentCache const&) = delete;
+    PersistentCache& operator=(PersistentCache const&) = delete;
+
+    PersistentCache(PersistentCache&&) = default;
+    PersistentCache& operator=(PersistentCache&&) = default;
+
+    ~PersistentCache() = default;
+
+    static UPtr open(std::string const& cache_path, int64_t max_size_in_bytes, CacheDiscardPolicy policy);
+    static UPtr open(std::string const& cache_path);
+
+    OptionalValue get(K const& key) const;
+    OptionalData get_data(K const& key) const;
+    OptionalMetadata get_metadata(K const& key) const;
+    bool contains_key(K const& key) const;
+    int64_t size() const noexcept;
+    int64_t size_in_bytes() const noexcept;
+    int64_t max_size_in_bytes() const noexcept;
+    int64_t disk_size_in_bytes() const;
+    CacheDiscardPolicy discard_policy() const noexcept;
+    PersistentCacheStats stats() const;
+
+    bool put(K const& key,
+             V const& value,
+             std::chrono::time_point<std::chrono::system_clock> expiry_time = std::chrono::system_clock::time_point());
+    bool put(K const& key,
+             V const& value,
+             std::string const& metadata,
+             std::chrono::time_point<std::chrono::system_clock> expiry_time = std::chrono::system_clock::time_point());
+    bool put(K const& key,
+             V const& value,
+             char const* metadata,
+             int64_t size,
+             std::chrono::time_point<std::chrono::system_clock> expiry_time = std::chrono::system_clock::time_point());
+
+    typedef std::function<void(K const& key, PersistentCache<K, V, std::string>& cache)> Loader;
+
+    OptionalValue get_or_put(K const& key, Loader const& load_func);
+    OptionalData get_or_put_data(K const& key, Loader const& load_func);
+
+    bool put_metadata(K const& key, std::string const& metadata);
+    bool put_metadata(K const& key, char const* metadata, int64_t size);
+    OptionalValue take(K const& key);
+    OptionalData take_data(K const& key);
+    bool invalidate(K const& key);
+    void invalidate(std::vector<K> const& keys);
+    template <typename It>
+    void invalidate(It begin, It end);
+    void invalidate(std::initializer_list<K> const& keys);
+    void invalidate();
+    bool touch(
+        K const& key,
+        std::chrono::time_point<std::chrono::system_clock> expiry_time = std::chrono::system_clock::time_point());
+    void clear_stats();
+    void resize(int64_t size_in_bytes);
+    void trim_to(int64_t used_size_in_bytes);
+    void compact();
+
+    typedef std::function<void(K const& key, CacheEvent ev, PersistentCacheStats const& stats)> EventCallback;
+
+    void set_handler(CacheEvent events, EventCallback cb);
+
+private:
+    PersistentCache(std::string const& cache_path, int64_t max_size_in_bytes, CacheDiscardPolicy policy);
+    PersistentCache(std::string const& cache_path);
+
+    std::unique_ptr<PersistentStringCache> p_;
+};
+
+template <typename K, typename V>
+PersistentCache<K, V, std::string>::PersistentCache(std::string const& cache_path,
+                                                    int64_t max_size_in_bytes,
+                                                    CacheDiscardPolicy policy)
+    : p_(PersistentStringCache::open(cache_path, max_size_in_bytes, policy))
+{
+}
+
+template <typename K, typename V>
+PersistentCache<K, V, std::string>::PersistentCache(std::string const& cache_path)
+    : p_(PersistentStringCache::open(cache_path))
+{
+}
+
+template <typename K, typename V>
+typename PersistentCache<K, V, std::string>::UPtr PersistentCache<K, V, std::string>::open(
+    std::string const& cache_path, int64_t max_size_in_bytes, CacheDiscardPolicy policy)
+{
+    return PersistentCache<K, V, std::string>::UPtr(
+        new PersistentCache<K, V, std::string>(cache_path, max_size_in_bytes, policy));
+}
+
+template <typename K, typename V>
+typename PersistentCache<K, V, std::string>::UPtr PersistentCache<K, V, std::string>::open(
+    std::string const& cache_path)
+{
+    return PersistentCache<K, V, std::string>::UPtr(new PersistentCache<K, V, std::string>(cache_path));
+}
+
+template <typename K, typename V>
+typename PersistentCache<K, V, std::string>::OptionalValue PersistentCache<K, V, std::string>::get(K const& key) const
+{
+    auto const& svalue = p_->get(CacheCodec<K>::encode(key));
+    return svalue ? OptionalValue(CacheCodec<V>::decode(*svalue)) : OptionalValue();
+}
+
+template <typename K, typename V>
+typename PersistentCache<K, V, std::string>::OptionalData PersistentCache<K, V, std::string>::get_data(
+    K const& key) const
+{
+    auto sdata = p_->get_data(CacheCodec<K>::encode(key));
+    if (!sdata)
+    {
+        return OptionalData();
+    }
+    return OptionalData({CacheCodec<V>::decode(sdata->value), sdata->metadata});
+}
+
+template <typename K, typename V>
+typename PersistentCache<K, V, std::string>::OptionalMetadata PersistentCache<K, V, std::string>::get_metadata(
+    K const& key) const
+{
+    auto smeta = p_->get_metadata(CacheCodec<K>::encode(key));
+    return smeta ? OptionalMetadata(*smeta) : OptionalMetadata();
+}
+
+template <typename K, typename V>
+bool PersistentCache<K, V, std::string>::contains_key(K const& key) const
+{
+    return p_->contains_key(CacheCodec<K>::encode(key));
+}
+
+template <typename K, typename V>
+int64_t PersistentCache<K, V, std::string>::size() const noexcept
+{
+    return p_->size();
+}
+
+template <typename K, typename V>
+int64_t PersistentCache<K, V, std::string>::size_in_bytes() const noexcept
+{
+    return p_->size_in_bytes();
+}
+
+template <typename K, typename V>
+int64_t PersistentCache<K, V, std::string>::max_size_in_bytes() const noexcept
+{
+    return p_->max_size_in_bytes();
+}
+
+template <typename K, typename V>
+int64_t PersistentCache<K, V, std::string>::disk_size_in_bytes() const
+{
+    return p_->disk_size_in_bytes();
+}
+
+template <typename K, typename V>
+CacheDiscardPolicy PersistentCache<K, V, std::string>::discard_policy() const noexcept
+{
+    return p_->discard_policy();
+}
+
+template <typename K, typename V>
+PersistentCacheStats PersistentCache<K, V, std::string>::stats() const
+{
+    return p_->stats();
+}
+
+template <typename K, typename V>
+bool PersistentCache<K, V, std::string>::put(K const& key,
+                                             V const& value,
+                                             std::chrono::time_point<std::chrono::system_clock> expiry_time)
+{
+    return p_->put(CacheCodec<K>::encode(key), CacheCodec<V>::encode(value), expiry_time);
+}
+
+template <typename K, typename V>
+bool PersistentCache<K, V, std::string>::put(K const& key,
+                                             V const& value,
+                                             std::string const& metadata,
+                                             std::chrono::time_point<std::chrono::system_clock> expiry_time)
+{
+    std::string v = CacheCodec<V>::encode(value);
+    return p_->put(CacheCodec<K>::encode(key), v.data(), v.size(), metadata.data(), metadata.size(), expiry_time);
+}
+
+template <typename K, typename V>
+bool PersistentCache<K, V, std::string>::put(K const& key,
+                                             V const& value,
+                                             char const* metadata,
+                                             int64_t size,
+                                             std::chrono::time_point<std::chrono::system_clock> expiry_time)
+{
+    std::string v = CacheCodec<V>::encode(value);
+    return p_->put(CacheCodec<K>::encode(key), v.data(), v.size(), metadata, size, expiry_time);
+}
+
+template <typename K, typename V>
+typename PersistentCache<K, V, std::string>::OptionalValue PersistentCache<K, V, std::string>::get_or_put(
+    K const& key, PersistentCache<K, V, std::string>::Loader const& load_func)
+{
+    std::string const& skey = CacheCodec<K>::encode(key);
+    auto sload_func = [&](std::string const&, PersistentStringCache const&)
+    {
+        load_func(key, *this);
+    };
+    auto svalue = p_->get_or_put(skey, sload_func);
+    return svalue ? OptionalValue(CacheCodec<V>::decode(*svalue)) : OptionalValue();
+}
+
+template <typename K, typename V>
+typename PersistentCache<K, V, std::string>::OptionalData PersistentCache<K, V, std::string>::get_or_put_data(
+    K const& key, PersistentCache<K, V, std::string>::Loader const& load_func)
+{
+    std::string const& skey = CacheCodec<K>::encode(key);
+    auto sload_func = [&](std::string const&, PersistentStringCache const&)
+    {
+        load_func(key, *this);
+    };
+    auto sdata = p_->get_or_put_data(skey, sload_func);
+    if (!sdata)
+    {
+        return OptionalData();
+    }
+    return OptionalData({CacheCodec<V>::decode(sdata->value), sdata->metadata});
+}
+
+template <typename K, typename V>
+bool PersistentCache<K, V, std::string>::put_metadata(K const& key, std::string const& metadata)
+{
+    return p_->put_metadata(CacheCodec<K>::encode(key), metadata);
+}
+
+template <typename K, typename V>
+bool PersistentCache<K, V, std::string>::put_metadata(K const& key, char const* metadata, int64_t size)
+{
+    return p_->put_metadata(CacheCodec<K>::encode(key), metadata, size);
+}
+
+template <typename K, typename V>
+typename PersistentCache<K, V, std::string>::OptionalValue PersistentCache<K, V, std::string>::take(K const& key)
+{
+    auto svalue = p_->take(CacheCodec<K>::encode(key));
+    return svalue ? OptionalValue(CacheCodec<V>::decode(*svalue)) : OptionalValue();
+}
+
+template <typename K, typename V>
+typename PersistentCache<K, V, std::string>::OptionalData PersistentCache<K, V, std::string>::take_data(
+    K const& key)
+{
+    auto sdata = p_->take_data(CacheCodec<K>::encode(key));
+    if (!sdata)
+    {
+        return OptionalData();
+    }
+    return OptionalData({CacheCodec<V>::decode(sdata->value), sdata->metadata});
+}
+
+template <typename K, typename V>
+bool PersistentCache<K, V, std::string>::invalidate(K const& key)
+{
+    return p_->invalidate(CacheCodec<K>::encode(key));
+}
+
+template <typename K, typename V>
+void PersistentCache<K, V, std::string>::invalidate(std::vector<K> const& keys)
+{
+    invalidate(keys.begin(), keys.end());
+}
+
+template <typename K, typename V>
+template <typename It>
+void PersistentCache<K, V, std::string>::invalidate(It begin, It end)
+{
+    std::vector<std::string> skeys;
+    for (auto&& it = begin; it < end; ++it)
+    {
+        skeys.push_back(CacheCodec<K>::encode(*it));
+    }
+    p_->invalidate(skeys.begin(), skeys.end());
+}
+
+template <typename K, typename V>
+void PersistentCache<K, V, std::string>::invalidate(std::initializer_list<K> const& keys)
+{
+    invalidate(keys.begin(), keys.end());
+}
+
+template <typename K, typename V>
+void PersistentCache<K, V, std::string>::invalidate()
+{
+    p_->invalidate();
+}
+
+template <typename K, typename V>
+bool PersistentCache<K, V, std::string>::touch(K const& key,
+                                               std::chrono::time_point<std::chrono::system_clock> expiry_time)
+{
+    return p_->touch(CacheCodec<K>::encode(key), expiry_time);
+}
+
+template <typename K, typename V>
+void PersistentCache<K, V, std::string>::clear_stats()
+{
+    p_->clear_stats();
+}
+
+template <typename K, typename V>
+void PersistentCache<K, V, std::string>::resize(int64_t size_in_bytes)
+{
+    p_->resize(size_in_bytes);
+}
+
+template <typename K, typename V>
+void PersistentCache<K, V, std::string>::trim_to(int64_t used_size_in_bytes)
+{
+    p_->trim_to(used_size_in_bytes);
+}
+
+template <typename K, typename V>
+void PersistentCache<K, V, std::string>::compact()
+{
+    p_->compact();
+}
+
+template <typename K, typename V>
+void PersistentCache<K, V, std::string>::set_handler(CacheEvent events, EventCallback cb)
+{
+    auto scb = [cb](std::string const& key, CacheEvent ev, PersistentCacheStats const& c)
+    {
+        cb(CacheCodec<K>::decode(key), ev, c);
+    };
+    p_->set_handler(events, scb);
+}
+
+// Specialization for K and V = std::string.
+
+template <typename M>
+class PersistentCache<std::string, std::string, M>
+{
+public:
+    typedef std::unique_ptr<PersistentCache<std::string, std::string, M>> UPtr;
+
+    typedef Optional<std::string> OptionalKey;
+    typedef Optional<std::string> OptionalValue;
+    typedef Optional<M> OptionalMetadata;
+
+    struct Data
+    {
+        std::string value;
+        M metadata;
+    };
+    typedef Optional<Data> OptionalData;
+
+    PersistentCache(PersistentCache const&) = delete;
+    PersistentCache& operator=(PersistentCache const&) = delete;
+
+    PersistentCache(PersistentCache&&) = default;
+    PersistentCache& operator=(PersistentCache&&) = default;
+
+    ~PersistentCache() = default;
+
+    static UPtr open(std::string const& cache_path, int64_t max_size_in_bytes, CacheDiscardPolicy policy);
+    static UPtr open(std::string const& cache_path);
+
+    OptionalValue get(std::string const& key) const;
+    OptionalData get_data(std::string const& key) const;
+    OptionalMetadata get_metadata(std::string const& key) const;
+    bool contains_key(std::string const& key) const;
+    int64_t size() const noexcept;
+    int64_t size_in_bytes() const noexcept;
+    int64_t max_size_in_bytes() const noexcept;
+    int64_t disk_size_in_bytes() const;
+    CacheDiscardPolicy discard_policy() const noexcept;
+    PersistentCacheStats stats() const;
+
+    bool put(std::string const& key,
+             std::string const& value,
+             std::chrono::time_point<std::chrono::system_clock> expiry_time = std::chrono::system_clock::time_point());
+    bool put(std::string const& key,
+             char const* value,
+             int64_t size,
+             std::chrono::time_point<std::chrono::system_clock> expiry_time = std::chrono::system_clock::time_point());
+    bool put(std::string const& key,
+             std::string const& value,
+             M const& metadata,
+             std::chrono::time_point<std::chrono::system_clock> expiry_time = std::chrono::system_clock::time_point());
+    bool put(std::string const& key,
+             char const* value,
+             int64_t value_size,
+             M const& metadata,
+             std::chrono::time_point<std::chrono::system_clock> expiry_time = std::chrono::system_clock::time_point());
+
+    typedef std::function<void(std::string const& key, PersistentCache<std::string, std::string, M>& cache)> Loader;
+
+    OptionalValue get_or_put(std::string const& key, Loader const& load_func);
+    OptionalData get_or_put_data(std::string const& key, Loader const& load_func);
+
+    bool put_metadata(std::string const& key, M const& metadata);
+    OptionalValue take(std::string const& key);
+    OptionalData take_data(std::string const& key);
+    bool invalidate(std::string const& key);
+    void invalidate(std::vector<std::string> const& keys);
+    template <typename It>
+    void invalidate(It begin, It end);
+    void invalidate(std::initializer_list<std::string> const& keys);
+    void invalidate();
+    bool touch(
+        std::string const& key,
+        std::chrono::time_point<std::chrono::system_clock> expiry_time = std::chrono::system_clock::time_point());
+    void clear_stats();
+    void resize(int64_t size_in_bytes);
+    void trim_to(int64_t used_size_in_bytes);
+    void compact();
+
+    typedef std::function<void(std::string const& key, CacheEvent ev, PersistentCacheStats const& stats)> EventCallback;
+
+    void set_handler(CacheEvent events, EventCallback cb);
+
+private:
+    PersistentCache(std::string const& cache_path, int64_t max_size_in_bytes, CacheDiscardPolicy policy);
+    PersistentCache(std::string const& cache_path);
+
+    std::unique_ptr<PersistentStringCache> p_;
+};
+
+template <typename M>
+PersistentCache<std::string, std::string, M>::PersistentCache(std::string const& cache_path,
+                                                              int64_t max_size_in_bytes,
+                                                              CacheDiscardPolicy policy)
+    : p_(PersistentStringCache::open(cache_path, max_size_in_bytes, policy))
+{
+}
+
+template <typename M>
+PersistentCache<std::string, std::string, M>::PersistentCache(std::string const& cache_path)
+    : p_(PersistentStringCache::open(cache_path))
+{
+}
+
+template <typename M>
+typename PersistentCache<std::string, std::string, M>::UPtr PersistentCache<std::string, std::string, M>::open(
+    std::string const& cache_path, int64_t max_size_in_bytes, CacheDiscardPolicy policy)
+{
+    return PersistentCache<std::string, std::string, M>::UPtr(
+        new PersistentCache<std::string, std::string, M>(cache_path, max_size_in_bytes, policy));
+}
+
+template <typename M>
+typename PersistentCache<std::string, std::string, M>::UPtr PersistentCache<std::string, std::string, M>::open(
+    std::string const& cache_path)
+{
+    return PersistentCache<std::string, std::string, M>::UPtr(
+        new PersistentCache<std::string, std::string, M>(cache_path));
+}
+
+template <typename M>
+typename PersistentCache<std::string, std::string, M>::OptionalValue PersistentCache<std::string, std::string, M>::get(
+    std::string const& key) const
+{
+    auto const& svalue = p_->get(key);
+    return svalue ? OptionalValue(*svalue) : OptionalValue();
+}
+
+template <typename M>
+typename PersistentCache<std::string, std::string, M>::OptionalData
+    PersistentCache<std::string, std::string, M>::get_data(std::string const& key) const
+{
+    auto sdata = p_->get_data(key);
+    if (!sdata)
+    {
+        return OptionalData();
+    }
+    return OptionalData({sdata->value, CacheCodec<M>::decode(sdata->metadata)});
+}
+
+template <typename M>
+typename PersistentCache<std::string, std::string, M>::OptionalMetadata
+    PersistentCache<std::string, std::string, M>::get_metadata(std::string const& key) const
+{
+    auto smeta = p_->get_metadata(key);
+    return smeta ? OptionalMetadata(CacheCodec<M>::decode(*smeta)) : OptionalMetadata();
+}
+
+template <typename M>
+bool PersistentCache<std::string, std::string, M>::contains_key(std::string const& key) const
+{
+    return p_->contains_key(key);
+}
+
+template <typename M>
+int64_t PersistentCache<std::string, std::string, M>::size() const noexcept
+{
+    return p_->size();
+}
+
+template <typename M>
+int64_t PersistentCache<std::string, std::string, M>::size_in_bytes() const noexcept
+{
+    return p_->size_in_bytes();
+}
+
+template <typename M>
+int64_t PersistentCache<std::string, std::string, M>::max_size_in_bytes() const noexcept
+{
+    return p_->max_size_in_bytes();
+}
+
+template <typename M>
+int64_t PersistentCache<std::string, std::string, M>::disk_size_in_bytes() const
+{
+    return p_->disk_size_in_bytes();
+}
+
+template <typename M>
+CacheDiscardPolicy PersistentCache<std::string, std::string, M>::discard_policy() const noexcept
+{
+    return p_->discard_policy();
+}
+
+template <typename M>
+PersistentCacheStats PersistentCache<std::string, std::string, M>::stats() const
+{
+    return p_->stats();
+}
+
+template <typename M>
+bool PersistentCache<std::string, std::string, M>::put(std::string const& key,
+                                                       std::string const& value,
+                                                       std::chrono::time_point<std::chrono::system_clock> expiry_time)
+{
+    return p_->put(key, value, expiry_time);
+}
+
+template <typename M>
+bool PersistentCache<std::string, std::string, M>::put(std::string const& key,
+                                                       char const* value,
+                                                       int64_t size,
+                                                       std::chrono::time_point<std::chrono::system_clock> expiry_time)
+{
+    return p_->put(key, value, size, nullptr, 0, expiry_time);
+}
+
+template <typename M>
+bool PersistentCache<std::string, std::string, M>::put(std::string const& key,
+                                                       std::string const& value,
+                                                       M const& metadata,
+                                                       std::chrono::time_point<std::chrono::system_clock> expiry_time)
+{
+    return p_->put(key, value, CacheCodec<M>::encode(metadata), expiry_time);
+}
+
+template <typename M>
+bool PersistentCache<std::string, std::string, M>::put(std::string const& key,
+                                                       char const* value,
+                                                       int64_t size,
+                                                       M const& metadata,
+                                                       std::chrono::time_point<std::chrono::system_clock> expiry_time)
+{
+    std::string md = CacheCodec<M>::encode(metadata);
+    return p_->put(key, value, size, md.data(), md.size(), expiry_time);
+}
+
+template <typename M>
+typename PersistentCache<std::string, std::string, M>::OptionalValue
+    PersistentCache<std::string, std::string, M>::get_or_put(
+        std::string const& key, PersistentCache<std::string, std::string, M>::Loader const& load_func)
+{
+    auto sload_func = [&](std::string const&, PersistentStringCache const&)
+    {
+        load_func(key, *this);
+    };
+    auto svalue = p_->get_or_put(key, sload_func);
+    return svalue ? OptionalValue(*svalue) : OptionalValue();
+}
+
+template <typename M>
+typename PersistentCache<std::string, std::string, M>::OptionalData
+    PersistentCache<std::string, std::string, M>::get_or_put_data(
+        std::string const& key, PersistentCache<std::string, std::string, M>::Loader const& load_func)
+{
+    auto sload_func = [&](std::string const&, PersistentStringCache const&)
+    {
+        load_func(key, *this);
+    };
+    auto sdata = p_->get_or_put_data(key, sload_func);
+    if (!sdata)
+    {
+        return OptionalData();
+    }
+    return OptionalData({sdata->value, CacheCodec<M>::decode(sdata->metadata)});
+}
+
+template <typename M>
+bool PersistentCache<std::string, std::string, M>::put_metadata(std::string const& key, M const& metadata)
+{
+    return p_->put_metadata(key, CacheCodec<M>::encode(metadata));
+}
+
+template <typename M>
+typename PersistentCache<std::string, std::string, M>::OptionalValue PersistentCache<std::string, std::string, M>::take(
+    std::string const& key)
+{
+    auto svalue = p_->take(key);
+    return svalue ? OptionalValue(*svalue) : OptionalValue();
+}
+
+template <typename M>
+typename PersistentCache<std::string, std::string, M>::OptionalData
+    PersistentCache<std::string, std::string, M>::take_data(std::string const& key)
+{
+    auto sdata = p_->take_data(key);
+    if (!sdata)
+    {
+        return OptionalData();
+    }
+    return OptionalData({sdata->value, CacheCodec<M>::decode(sdata->metadata)});
+}
+
+template <typename M>
+bool PersistentCache<std::string, std::string, M>::invalidate(std::string const& key)
+{
+    return p_->invalidate(key);
+}
+
+template <typename M>
+void PersistentCache<std::string, std::string, M>::invalidate(std::vector<std::string> const& keys)
+{
+    invalidate(keys.begin(), keys.end());
+}
+
+template <typename M>
+template <typename It>
+void PersistentCache<std::string, std::string, M>::invalidate(It begin, It end)
+{
+    std::vector<std::string> skeys;
+    for (auto&& it = begin; it < end; ++it)
+    {
+        skeys.push_back(*it);
+    }
+    p_->invalidate(skeys.begin(), skeys.end());
+}
+
+template <typename M>
+void PersistentCache<std::string, std::string, M>::invalidate(std::initializer_list<std::string> const& keys)
+{
+    invalidate(keys.begin(), keys.end());
+}
+
+template <typename M>
+void PersistentCache<std::string, std::string, M>::invalidate()
+{
+    p_->invalidate();
+}
+
+template <typename M>
+bool PersistentCache<std::string, std::string, M>::touch(std::string const& key,
+                                                         std::chrono::time_point<std::chrono::system_clock> expiry_time)
+{
+    return p_->touch(key, expiry_time);
+}
+
+template <typename M>
+void PersistentCache<std::string, std::string, M>::clear_stats()
+{
+    p_->clear_stats();
+}
+
+template <typename M>
+void PersistentCache<std::string, std::string, M>::resize(int64_t size_in_bytes)
+{
+    p_->resize(size_in_bytes);
+}
+
+template <typename M>
+void PersistentCache<std::string, std::string, M>::trim_to(int64_t used_size_in_bytes)
+{
+    p_->trim_to(used_size_in_bytes);
+}
+
+template <typename M>
+void PersistentCache<std::string, std::string, M>::compact()
+{
+    p_->compact();
+}
+
+template <typename M>
+void PersistentCache<std::string, std::string, M>::set_handler(CacheEvent events, EventCallback cb)
+{
+    p_->set_handler(events, cb);
+}
+
+// Specialization for K and M = std::string.
+
+template <typename V>
+class PersistentCache<std::string, V, std::string>
+{
+public:
+    typedef std::unique_ptr<PersistentCache<std::string, V, std::string>> UPtr;
+
+    typedef Optional<std::string> OptionalKey;
+    typedef Optional<V> OptionalValue;
+    typedef Optional<std::string> OptionalMetadata;
+
+    struct Data
+    {
+        V value;
+        std::string metadata;
+    };
+    typedef Optional<Data> OptionalData;
+
+    PersistentCache(PersistentCache const&) = delete;
+    PersistentCache& operator=(PersistentCache const&) = delete;
+
+    PersistentCache(PersistentCache&&) = default;
+    PersistentCache& operator=(PersistentCache&&) = default;
+
+    ~PersistentCache() = default;
+
+    static UPtr open(std::string const& cache_path, int64_t max_size_in_bytes, CacheDiscardPolicy policy);
+    static UPtr open(std::string const& cache_path);
+
+    OptionalValue get(std::string const& key) const;
+    OptionalData get_data(std::string const& key) const;
+    OptionalMetadata get_metadata(std::string const& key) const;
+    bool contains_key(std::string const& key) const;
+    int64_t size() const noexcept;
+    int64_t size_in_bytes() const noexcept;
+    int64_t max_size_in_bytes() const noexcept;
+    int64_t disk_size_in_bytes() const;
+    CacheDiscardPolicy discard_policy() const noexcept;
+    PersistentCacheStats stats() const;
+
+    bool put(std::string const& key,
+             V const& value,
+             std::chrono::time_point<std::chrono::system_clock> expiry_time = std::chrono::system_clock::time_point());
+    bool put(std::string const& key,
+             V const& value,
+             std::string const& metadata,
+             std::chrono::time_point<std::chrono::system_clock> expiry_time = std::chrono::system_clock::time_point());
+    bool put(std::string const& key,
+             V const& value,
+             char const* metadata,
+             int64_t size,
+             std::chrono::time_point<std::chrono::system_clock> expiry_time = std::chrono::system_clock::time_point());
+
+    typedef std::function<void(std::string const& key, PersistentCache<std::string, V, std::string>& cache)> Loader;
+
+    OptionalValue get_or_put(std::string const& key, Loader const& load_func);
+    OptionalData get_or_put_data(std::string const& key, Loader const& load_func);
+
+    bool put_metadata(std::string const& key, std::string const& metadata);
+    bool put_metadata(std::string const& key, char const* metadata, int64_t size);
+    OptionalValue take(std::string const& key);
+    OptionalData take_data(std::string const& key);
+    bool invalidate(std::string const& key);
+    void invalidate(std::vector<std::string> const& keys);
+    template <typename It>
+    void invalidate(It begin, It end);
+    void invalidate(std::initializer_list<std::string> const& keys);
+    void invalidate();
+    bool touch(
+        std::string const& key,
+        std::chrono::time_point<std::chrono::system_clock> expiry_time = std::chrono::system_clock::time_point());
+    void clear_stats();
+    void resize(int64_t size_in_bytes);
+    void trim_to(int64_t used_size_in_bytes);
+    void compact();
+
+    typedef std::function<void(std::string const& key, CacheEvent ev, PersistentCacheStats const& stats)> EventCallback;
+
+    void set_handler(CacheEvent events, EventCallback cb);
+
+private:
+    PersistentCache(std::string const& cache_path, int64_t max_size_in_bytes, CacheDiscardPolicy policy);
+    PersistentCache(std::string const& cache_path);
+
+    std::unique_ptr<PersistentStringCache> p_;
+};
+
+template <typename V>
+PersistentCache<std::string, V, std::string>::PersistentCache(std::string const& cache_path,
+                                                              int64_t max_size_in_bytes,
+                                                              CacheDiscardPolicy policy)
+    : p_(PersistentStringCache::open(cache_path, max_size_in_bytes, policy))
+{
+}
+
+template <typename V>
+PersistentCache<std::string, V, std::string>::PersistentCache(std::string const& cache_path)
+    : p_(PersistentStringCache::open(cache_path))
+{
+}
+
+template <typename V>
+typename PersistentCache<std::string, V, std::string>::UPtr PersistentCache<std::string, V, std::string>::open(
+    std::string const& cache_path, int64_t max_size_in_bytes, CacheDiscardPolicy policy)
+{
+    return PersistentCache<std::string, V, std::string>::UPtr(
+        new PersistentCache<std::string, V, std::string>(cache_path, max_size_in_bytes, policy));
+}
+
+template <typename V>
+typename PersistentCache<std::string, V, std::string>::UPtr PersistentCache<std::string, V, std::string>::open(
+    std::string const& cache_path)
+{
+    return PersistentCache<std::string, V, std::string>::UPtr(
+        new PersistentCache<std::string, V, std::string>(cache_path));
+}
+
+template <typename V>
+typename PersistentCache<std::string, V, std::string>::OptionalValue PersistentCache<std::string, V, std::string>::get(
+    std::string const& key) const
+{
+    auto const& svalue = p_->get(key);
+    return svalue ? OptionalValue(CacheCodec<V>::decode(*svalue)) : OptionalValue();
+}
+
+template <typename V>
+typename PersistentCache<std::string, V, std::string>::OptionalData
+    PersistentCache<std::string, V, std::string>::get_data(std::string const& key) const
+{
+    auto sdata = p_->get_data(key);
+    if (!sdata)
+    {
+        return OptionalData();
+    }
+    return OptionalData({CacheCodec<V>::decode(sdata->value), sdata->metadata});
+}
+
+template <typename V>
+typename PersistentCache<std::string, V, std::string>::OptionalMetadata
+    PersistentCache<std::string, V, std::string>::get_metadata(std::string const& key) const
+{
+    auto smeta = p_->get_metadata(key);
+    return smeta ? OptionalMetadata(*smeta) : OptionalMetadata();
+}
+
+template <typename V>
+bool PersistentCache<std::string, V, std::string>::contains_key(std::string const& key) const
+{
+    return p_->contains_key(key);
+}
+
+template <typename V>
+int64_t PersistentCache<std::string, V, std::string>::size() const noexcept
+{
+    return p_->size();
+}
+
+template <typename V>
+int64_t PersistentCache<std::string, V, std::string>::size_in_bytes() const noexcept
+{
+    return p_->size_in_bytes();
+}
+
+template <typename V>
+int64_t PersistentCache<std::string, V, std::string>::max_size_in_bytes() const noexcept
+{
+    return p_->max_size_in_bytes();
+}
+
+template <typename V>
+int64_t PersistentCache<std::string, V, std::string>::disk_size_in_bytes() const
+{
+    return p_->disk_size_in_bytes();
+}
+
+template <typename V>
+CacheDiscardPolicy PersistentCache<std::string, V, std::string>::discard_policy() const noexcept
+{
+    return p_->discard_policy();
+}
+
+template <typename V>
+PersistentCacheStats PersistentCache<std::string, V, std::string>::stats() const
+{
+    return p_->stats();
+}
+
+template <typename V>
+bool PersistentCache<std::string, V, std::string>::put(std::string const& key,
+                                                       V const& value,
+                                                       std::chrono::time_point<std::chrono::system_clock> expiry_time)
+{
+    return p_->put(key, CacheCodec<V>::encode(value), expiry_time);
+}
+
+template <typename V>
+bool PersistentCache<std::string, V, std::string>::put(std::string const& key,
+                                                       V const& value,
+                                                       std::string const& metadata,
+                                                       std::chrono::time_point<std::chrono::system_clock> expiry_time)
+{
+    std::string v = CacheCodec<V>::encode(value);
+    return p_->put(key, v.data(), v.size(), metadata.data(), metadata.size(), expiry_time);
+}
+
+template <typename V>
+bool PersistentCache<std::string, V, std::string>::put(std::string const& key,
+                                                       V const& value,
+                                                       char const* metadata,
+                                                       int64_t size,
+                                                       std::chrono::time_point<std::chrono::system_clock> expiry_time)
+{
+    std::string v = CacheCodec<V>::encode(value);
+    return p_->put(key, v.data(), v.size(), metadata, size, expiry_time);
+}
+
+template <typename V>
+typename PersistentCache<std::string, V, std::string>::OptionalValue
+    PersistentCache<std::string, V, std::string>::get_or_put(
+        std::string const& key, PersistentCache<std::string, V, std::string>::Loader const& load_func)
+{
+    auto sload_func = [&](std::string const&, PersistentStringCache const&)
+    {
+        load_func(key, *this);
+    };
+    auto svalue = p_->get_or_put(key, sload_func);
+    return svalue ? OptionalValue(CacheCodec<V>::decode(*svalue)) : OptionalValue();
+}
+
+template <typename V>
+typename PersistentCache<std::string, V, std::string>::OptionalData
+    PersistentCache<std::string, V, std::string>::get_or_put_data(
+        std::string const& key, PersistentCache<std::string, V, std::string>::Loader const& load_func)
+{
+    auto sload_func = [&](std::string const&, PersistentStringCache const&)
+    {
+        load_func(key, *this);
+    };
+    auto sdata = p_->get_or_put_data(key, sload_func);
+    if (!sdata)
+    {
+        return OptionalData();
+    }
+    return OptionalData({CacheCodec<V>::decode(sdata->value), sdata->metadata});
+}
+
+template <typename V>
+bool PersistentCache<std::string, V, std::string>::put_metadata(std::string const& key, std::string const& metadata)
+{
+    return p_->put_metadata(key, metadata);
+}
+
+template <typename V>
+bool PersistentCache<std::string, V, std::string>::put_metadata(std::string const& key,
+                                                                char const* metadata,
+                                                                int64_t size)
+{
+    return p_->put_metadata(key, metadata, size);
+}
+
+template <typename V>
+typename PersistentCache<std::string, V, std::string>::OptionalValue PersistentCache<std::string, V, std::string>::take(
+    std::string const& key)
+{
+    auto svalue = p_->take(key);
+    return svalue ? OptionalValue(CacheCodec<V>::decode(*svalue)) : OptionalValue();
+}
+
+template <typename V>
+typename PersistentCache<std::string, V, std::string>::OptionalData
+    PersistentCache<std::string, V, std::string>::take_data(std::string const& key)
+{
+    auto sdata = p_->take_data(key);
+    if (!sdata)
+    {
+        return OptionalData();
+    }
+    return OptionalData({CacheCodec<V>::decode(sdata->value), sdata->metadata});
+}
+
+template <typename V>
+bool PersistentCache<std::string, V, std::string>::invalidate(std::string const& key)
+{
+    return p_->invalidate(key);
+}
+
+template <typename V>
+void PersistentCache<std::string, V, std::string>::invalidate(std::vector<std::string> const& keys)
+{
+    invalidate(keys.begin(), keys.end());
+}
+
+template <typename V>
+template <typename It>
+void PersistentCache<std::string, V, std::string>::invalidate(It begin, It end)
+{
+    std::vector<std::string> skeys;
+    for (auto&& it = begin; it < end; ++it)
+    {
+        skeys.push_back(*it);
+    }
+    p_->invalidate(skeys.begin(), skeys.end());
+}
+
+template <typename V>
+void PersistentCache<std::string, V, std::string>::invalidate(std::initializer_list<std::string> const& keys)
+{
+    invalidate(keys.begin(), keys.end());
+}
+
+template <typename V>
+void PersistentCache<std::string, V, std::string>::invalidate()
+{
+    p_->invalidate();
+}
+
+template <typename V>
+bool PersistentCache<std::string, V, std::string>::touch(std::string const& key,
+                                                         std::chrono::time_point<std::chrono::system_clock> expiry_time)
+{
+    return p_->touch(key, expiry_time);
+}
+
+template <typename V>
+void PersistentCache<std::string, V, std::string>::clear_stats()
+{
+    p_->clear_stats();
+}
+
+template <typename V>
+void PersistentCache<std::string, V, std::string>::resize(int64_t size_in_bytes)
+{
+    p_->resize(size_in_bytes);
+}
+
+template <typename V>
+void PersistentCache<std::string, V, std::string>::trim_to(int64_t used_size_in_bytes)
+{
+    p_->trim_to(used_size_in_bytes);
+}
+
+template <typename V>
+void PersistentCache<std::string, V, std::string>::compact()
+{
+    p_->compact();
+}
+
+template <typename V>
+void PersistentCache<std::string, V, std::string>::set_handler(CacheEvent events, EventCallback cb)
+{
+    p_->set_handler(events, cb);
+}
+
+// Specialization for V and M = std::string.
+
+template <typename K>
+class PersistentCache<K, std::string, std::string>
+{
+public:
+    typedef std::unique_ptr<PersistentCache<K, std::string, std::string>> UPtr;
+
+    typedef Optional<K> OptionalKey;
+    typedef Optional<std::string> OptionalValue;
+    typedef Optional<std::string> OptionalMetadata;
+
+    struct Data
+    {
+        std::string value;
+        std::string metadata;
+    };
+    typedef Optional<Data> OptionalData;
+
+    PersistentCache(PersistentCache const&) = delete;
+    PersistentCache& operator=(PersistentCache const&) = delete;
+
+    PersistentCache(PersistentCache&&) = default;
+    PersistentCache& operator=(PersistentCache&&) = default;
+
+    ~PersistentCache() = default;
+
+    static UPtr open(std::string const& cache_path, int64_t max_size_in_bytes, CacheDiscardPolicy policy);
+    static UPtr open(std::string const& cache_path);
+
+    OptionalValue get(K const& key) const;
+    OptionalData get_data(K const& key) const;
+    OptionalMetadata get_metadata(K const& key) const;
+    bool contains_key(K const& key) const;
+    int64_t size() const noexcept;
+    int64_t size_in_bytes() const noexcept;
+    int64_t max_size_in_bytes() const noexcept;
+    int64_t disk_size_in_bytes() const;
+    CacheDiscardPolicy discard_policy() const noexcept;
+    PersistentCacheStats stats() const;
+
+    bool put(K const& key,
+             std::string const& value,
+             std::chrono::time_point<std::chrono::system_clock> expiry_time = std::chrono::system_clock::time_point());
+    bool put(K const& key,
+             char const* value,
+             int64_t size,
+             std::chrono::time_point<std::chrono::system_clock> expiry_time = std::chrono::system_clock::time_point());
+    bool put(K const& key,
+             std::string const& value,
+             std::string const& metadata,
+             std::chrono::time_point<std::chrono::system_clock> expiry_time = std::chrono::system_clock::time_point());
+    bool put(K const& key,
+             char const* value,
+             int64_t value_size,
+             char const* metadata,
+             int64_t metadata_size,
+             std::chrono::time_point<std::chrono::system_clock> expiry_time = std::chrono::system_clock::time_point());
+
+    typedef std::function<void(K const& key, PersistentCache<K, std::string, std::string>& cache)> Loader;
+
+    OptionalValue get_or_put(K const& key, Loader const& load_func);
+    OptionalData get_or_put_data(K const& key, Loader const& load_func);
+
+    bool put_metadata(K const& key, std::string const& metadata);
+    bool put_metadata(K const& key, char const* metadata, int64_t size);
+    OptionalValue take(K const& key);
+    OptionalData take_data(K const& key);
+    bool invalidate(K const& key);
+    void invalidate(std::vector<K> const& keys);
+    template <typename It>
+    void invalidate(It begin, It end);
+    void invalidate(std::initializer_list<K> const& keys);
+    void invalidate();
+    bool touch(
+        K const& key,
+        std::chrono::time_point<std::chrono::system_clock> expiry_time = std::chrono::system_clock::time_point());
+    void clear_stats();
+    void resize(int64_t size_in_bytes);
+    void trim_to(int64_t used_size_in_bytes);
+    void compact();
+
+    typedef std::function<void(K const& key, CacheEvent ev, PersistentCacheStats const& stats)> EventCallback;
+
+    void set_handler(CacheEvent events, EventCallback cb);
+
+private:
+    PersistentCache(std::string const& cache_path, int64_t max_size_in_bytes, CacheDiscardPolicy policy);
+    PersistentCache(std::string const& cache_path);
+
+    std::unique_ptr<PersistentStringCache> p_;
+};
+
+template <typename K>
+PersistentCache<K, std::string, std::string>::PersistentCache(std::string const& cache_path,
+                                                              int64_t max_size_in_bytes,
+                                                              CacheDiscardPolicy policy)
+    : p_(PersistentStringCache::open(cache_path, max_size_in_bytes, policy))
+{
+}
+
+template <typename K>
+PersistentCache<K, std::string, std::string>::PersistentCache(std::string const& cache_path)
+    : p_(PersistentStringCache::open(cache_path))
+{
+}
+
+template <typename K>
+typename PersistentCache<K, std::string, std::string>::UPtr PersistentCache<K, std::string, std::string>::open(
+    std::string const& cache_path, int64_t max_size_in_bytes, CacheDiscardPolicy policy)
+{
+    return PersistentCache<K, std::string, std::string>::UPtr(
+        new PersistentCache<K, std::string, std::string>(cache_path, max_size_in_bytes, policy));
+}
+
+template <typename K>
+typename PersistentCache<K, std::string, std::string>::UPtr PersistentCache<K, std::string, std::string>::open(
+    std::string const& cache_path)
+{
+    return PersistentCache<K, std::string, std::string>::UPtr(
+        new PersistentCache<K, std::string, std::string>(cache_path));
+}
+
+template <typename K>
+typename PersistentCache<K, std::string, std::string>::OptionalValue PersistentCache<K, std::string, std::string>::get(
+    K const& key) const
+{
+    auto const& svalue = p_->get(CacheCodec<K>::encode(key));
+    return svalue ? OptionalValue(*svalue) : OptionalValue();
+}
+
+template <typename K>
+typename PersistentCache<K, std::string, std::string>::OptionalData
+    PersistentCache<K, std::string, std::string>::get_data(K const& key) const
+{
+    auto sdata = p_->get_data(CacheCodec<K>::encode(key));
+    if (!sdata)
+    {
+        return OptionalData();
+    }
+    return OptionalData({sdata->value, sdata->metadata});
+}
+
+template <typename K>
+typename PersistentCache<K, std::string, std::string>::OptionalMetadata
+    PersistentCache<K, std::string, std::string>::get_metadata(K const& key) const
+{
+    auto smeta = p_->get_metadata(CacheCodec<K>::encode(key));
+    return smeta ? OptionalMetadata(*smeta) : OptionalMetadata();
+}
+
+template <typename K>
+bool PersistentCache<K, std::string, std::string>::contains_key(K const& key) const
+{
+    return p_->contains_key(CacheCodec<K>::encode(key));
+}
+
+template <typename K>
+int64_t PersistentCache<K, std::string, std::string>::size() const noexcept
+{
+    return p_->size();
+}
+
+template <typename K>
+int64_t PersistentCache<K, std::string, std::string>::size_in_bytes() const noexcept
+{
+    return p_->size_in_bytes();
+}
+
+template <typename K>
+int64_t PersistentCache<K, std::string, std::string>::max_size_in_bytes() const noexcept
+{
+    return p_->max_size_in_bytes();
+}
+
+template <typename K>
+int64_t PersistentCache<K, std::string, std::string>::disk_size_in_bytes() const
+{
+    return p_->disk_size_in_bytes();
+}
+
+template <typename K>
+CacheDiscardPolicy PersistentCache<K, std::string, std::string>::discard_policy() const noexcept
+{
+    return p_->discard_policy();
+}
+
+template <typename K>
+PersistentCacheStats PersistentCache<K, std::string, std::string>::stats() const
+{
+    return p_->stats();
+}
+
+template <typename K>
+bool PersistentCache<K, std::string, std::string>::put(K const& key,
+                                                       std::string const& value,
+                                                       std::chrono::time_point<std::chrono::system_clock> expiry_time)
+{
+    return p_->put(CacheCodec<K>::encode(key), value, expiry_time);
+}
+
+template <typename K>
+bool PersistentCache<K, std::string, std::string>::put(K const& key,
+                                                       char const* value,
+                                                       int64_t size,
+                                                       std::chrono::time_point<std::chrono::system_clock> expiry_time)
+{
+    return p_->put(CacheCodec<K>::encode(key), value, size, expiry_time);
+}
+
+template <typename K>
+bool PersistentCache<K, std::string, std::string>::put(K const& key,
+                                                       std::string const& value,
+                                                       std::string const& metadata,
+                                                       std::chrono::time_point<std::chrono::system_clock> expiry_time)
+{
+    return p_->put(CacheCodec<K>::encode(key), value, metadata, expiry_time);
+}
+
+template <typename K>
+bool PersistentCache<K, std::string, std::string>::put(K const& key,
+                                                       char const* value,
+                                                       int64_t value_size,
+                                                       char const* metadata,
+                                                       int64_t metadata_size,
+                                                       std::chrono::time_point<std::chrono::system_clock> expiry_time)
+{
+    return p_->put(CacheCodec<K>::encode(key), value, value_size, metadata, metadata_size, expiry_time);
+}
+
+template <typename K>
+typename PersistentCache<K, std::string, std::string>::OptionalValue
+    PersistentCache<K, std::string, std::string>::get_or_put(
+        K const& key, PersistentCache<K, std::string, std::string>::Loader const& load_func)
+{
+    auto skey = CacheCodec<K>::encode(key);
+    auto sload_func = [&](std::string const&, PersistentStringCache const&)
+    {
+        load_func(key, *this);
+    };
+    auto svalue = p_->get_or_put(skey, sload_func);
+    return svalue ? OptionalValue(*svalue) : OptionalValue();
+}
+
+template <typename K>
+typename PersistentCache<K, std::string, std::string>::OptionalData
+    PersistentCache<K, std::string, std::string>::get_or_put_data(
+        K const& key, PersistentCache<K, std::string, std::string>::Loader const& load_func)
+{
+    auto skey = CacheCodec<K>::encode(key);
+    auto sload_func = [&](std::string const&, PersistentStringCache const&)
+    {
+        load_func(key, *this);
+    };
+    auto sdata = p_->get_or_put_data(skey, sload_func);
+    if (!sdata)
+    {
+        return OptionalData();
+    }
+    return OptionalData({sdata->value, sdata->metadata});
+}
+
+template <typename K>
+bool PersistentCache<K, std::string, std::string>::put_metadata(K const& key, std::string const& metadata)
+{
+    return p_->put_metadata(CacheCodec<K>::encode(key), metadata);
+}
+
+template <typename K>
+bool PersistentCache<K, std::string, std::string>::put_metadata(K const& key, char const* metadata, int64_t size)
+{
+    return p_->put_metadata(CacheCodec<K>::encode(key), metadata, size);
+}
+
+template <typename K>
+typename PersistentCache<K, std::string, std::string>::OptionalValue PersistentCache<K, std::string, std::string>::take(
+    K const& key)
+{
+    auto svalue = p_->take(CacheCodec<K>::encode(key));
+    return svalue ? OptionalValue(*svalue) : OptionalValue();
+}
+
+template <typename K>
+typename PersistentCache<K, std::string, std::string>::OptionalData
+    PersistentCache<K, std::string, std::string>::take_data(K const& key)
+{
+    auto sdata = p_->take_data(CacheCodec<K>::encode(key));
+    if (!sdata)
+    {
+        return OptionalData();
+    }
+    return OptionalData({sdata->value, sdata->metadata});
+}
+
+template <typename K>
+bool PersistentCache<K, std::string, std::string>::invalidate(K const& key)
+{
+    return p_->invalidate(CacheCodec<K>::encode(key));
+}
+
+template <typename K>
+void PersistentCache<K, std::string, std::string>::invalidate(std::vector<K> const& keys)
+{
+    invalidate(keys.begin(), keys.end());
+}
+
+template <typename K>
+template <typename It>
+void PersistentCache<K, std::string, std::string>::invalidate(It begin, It end)
+{
+    std::vector<std::string> skeys;
+    for (auto&& it = begin; it < end; ++it)
+    {
+        skeys.push_back(CacheCodec<K>::encode(*it));
+    }
+    p_->invalidate(skeys.begin(), skeys.end());
+}
+
+template <typename K>
+void PersistentCache<K, std::string, std::string>::invalidate(std::initializer_list<K> const& keys)
+{
+    invalidate(keys.begin(), keys.end());
+}
+
+template <typename K>
+void PersistentCache<K, std::string, std::string>::invalidate()
+{
+    p_->invalidate();
+}
+
+template <typename K>
+bool PersistentCache<K, std::string, std::string>::touch(K const& key,
+                                                         std::chrono::time_point<std::chrono::system_clock> expiry_time)
+{
+    return p_->touch(CacheCodec<K>::encode(key), expiry_time);
+}
+
+template <typename K>
+void PersistentCache<K, std::string, std::string>::clear_stats()
+{
+    p_->clear_stats();
+}
+
+template <typename K>
+void PersistentCache<K, std::string, std::string>::resize(int64_t size_in_bytes)
+{
+    p_->resize(size_in_bytes);
+}
+
+template <typename K>
+void PersistentCache<K, std::string, std::string>::trim_to(int64_t used_size_in_bytes)
+{
+    p_->trim_to(used_size_in_bytes);
+}
+
+template <typename K>
+void PersistentCache<K, std::string, std::string>::compact()
+{
+    p_->compact();
+}
+
+template <typename K>
+void PersistentCache<K, std::string, std::string>::set_handler(CacheEvent events, EventCallback cb)
+{
+    auto scb = [cb](std::string const& key, CacheEvent ev, PersistentCacheStats const& c)
+    {
+        cb(CacheCodec<K>::decode(key), ev, c);
+    };
+    p_->set_handler(events, scb);
+}
+
+// Specialization for K, V, and M = std::string.
+
+template <>
+class PersistentCache<std::string, std::string, std::string>
+{
+public:
+    typedef std::unique_ptr<PersistentCache<std::string, std::string, std::string>> UPtr;
+
+    typedef Optional<std::string> OptionalKey;
+    typedef Optional<std::string> OptionalValue;
+    typedef Optional<std::string> OptionalMetadata;
+
+    struct Data
+    {
+        std::string value;
+        std::string metadata;
+    };
+    typedef Optional<Data> OptionalData;
+
+    PersistentCache(PersistentCache const&) = delete;
+    PersistentCache& operator=(PersistentCache const&) = delete;
+
+    PersistentCache(PersistentCache&&) = default;
+    PersistentCache& operator=(PersistentCache&&) = default;
+
+    ~PersistentCache() = default;
+
+    static UPtr open(std::string const& cache_path, int64_t max_size_in_bytes, CacheDiscardPolicy policy);
+    static UPtr open(std::string const& cache_path);
+
+    OptionalValue get(std::string const& key) const;
+    OptionalData get_data(std::string const& key) const;
+    OptionalMetadata get_metadata(std::string const& key) const;
+    bool contains_key(std::string const& key) const;
+    int64_t size() const noexcept;
+    int64_t size_in_bytes() const noexcept;
+    int64_t max_size_in_bytes() const noexcept;
+    int64_t disk_size_in_bytes() const;
+    CacheDiscardPolicy discard_policy() const noexcept;
+    PersistentCacheStats stats() const;
+
+    bool put(std::string const& key,
+             std::string const& value,
+             std::chrono::time_point<std::chrono::system_clock> expiry_time = std::chrono::system_clock::time_point());
+    bool put(std::string const& key,
+             char const* value,
+             int64_t size,
+             std::chrono::time_point<std::chrono::system_clock> expiry_time = std::chrono::system_clock::time_point());
+    bool put(std::string const& key,
+             std::string const& value,
+             std::string const& metadata,
+             std::chrono::time_point<std::chrono::system_clock> expiry_time = std::chrono::system_clock::time_point());
+    bool put(std::string const& key,
+             char const* value,
+             int64_t value_size,
+             char const* metadata,
+             int64_t metadata_size,
+             std::chrono::time_point<std::chrono::system_clock> expiry_time = std::chrono::system_clock::time_point());
+
+    typedef std::function<void(std::string const& key, PersistentCache<std::string, std::string, std::string>& cache)>
+        Loader;
+
+    OptionalValue get_or_put(std::string const& key, Loader const& load_func);
+    OptionalData get_or_put_data(std::string const& key, Loader const& load_func);
+
+    bool put_metadata(std::string const& key, std::string const& metadata);
+    bool put_metadata(std::string const& key, char const* metadata, int64_t size);
+    OptionalValue take(std::string const& key);
+    OptionalData take_data(std::string const& key);
+    bool invalidate(std::string const& key);
+    void invalidate(std::vector<std::string> const& keys);
+    template <typename It>
+    void invalidate(It begin, It end);
+    void invalidate(std::initializer_list<std::string> const& keys);
+    void invalidate();
+    bool touch(
+        std::string const& key,
+        std::chrono::time_point<std::chrono::system_clock> expiry_time = std::chrono::system_clock::time_point());
+    void clear_stats();
+    void resize(int64_t size_in_bytes);
+    void trim_to(int64_t used_size_in_bytes);
+    void compact();
+
+    typedef std::function<void(std::string const& key, CacheEvent ev, PersistentCacheStats const& stats)> EventCallback;
+
+    void set_handler(CacheEvent events, EventCallback cb);
+
+private:
+    PersistentCache(std::string const& cache_path, int64_t max_size_in_bytes, CacheDiscardPolicy policy);
+    PersistentCache(std::string const& cache_path);
+
+    std::unique_ptr<PersistentStringCache> p_;
+};
+
+PersistentCache<std::string, std::string, std::string>::PersistentCache(std::string const& cache_path,
+                                                                        int64_t max_size_in_bytes,
+                                                                        CacheDiscardPolicy policy)
+    : p_(PersistentStringCache::open(cache_path, max_size_in_bytes, policy))
+{
+}
+
+PersistentCache<std::string, std::string, std::string>::PersistentCache(std::string const& cache_path)
+    : p_(PersistentStringCache::open(cache_path))
+{
+}
+
+typename PersistentCache<std::string, std::string, std::string>::UPtr
+    PersistentCache<std::string, std::string, std::string>::open(std::string const& cache_path,
+                                                                 int64_t max_size_in_bytes,
+                                                                 CacheDiscardPolicy policy)
+{
+    return PersistentCache<std::string, std::string, std::string>::UPtr(
+        new PersistentCache<std::string, std::string, std::string>(cache_path, max_size_in_bytes, policy));
+}
+
+typename PersistentCache<std::string, std::string, std::string>::UPtr
+    PersistentCache<std::string, std::string, std::string>::open(std::string const& cache_path)
+{
+    return PersistentCache<std::string, std::string, std::string>::UPtr(
+        new PersistentCache<std::string, std::string, std::string>(cache_path));
+}
+
+typename PersistentCache<std::string, std::string, std::string>::OptionalValue
+    PersistentCache<std::string, std::string, std::string>::get(std::string const& key) const
+{
+    auto const& svalue = p_->get(key);
+    return svalue ? OptionalValue(*svalue) : OptionalValue();