Merge lp:~jelmer/bzr/nested-trees-spec into lp:bzr

[Jelmer Vernooij]

Add a specification for nested trees.

Add the nested tree specification to the tree.

This is the document that Aaron initially committed to lp:~bzr-core/bzr/devnotes. I've updated it to reflect the recent discussions.

[Vincent Ladeuil (vila)]

On top of what we discuss on IRC, I now think we should land this and see it
as a work in progress.

Things I think we should do:

- turn the described scenarios into failing script tests (the syntax used
  should be almost copy'n'pastable) ,

- cleanup the document so that all references to stuff (subbranches format, composite tree) is not yet
  implemented is marked as such, better documented or removed if it's now
  obsolete

- keep working on the text itself to remove all advocacies by reaching a
  consensus for each point,

- better describe the data supporting the implementation (indexes,
  locations, revisions),

- better describe the known bugs/limitations,

- keep an eye on instrumenting *during* the implementation so we can better
  track the performance variations,

Instead of disussing further on this proposal, I'll try to address some of
the above (probably the tests and then some specific points as I run into
them) instead so we can the split the discussions.

review: Approve

[Marius Kruger (amanica)]

I also read this again now. I'm ecstatic to see any movement on this!

I saw some more trivial things which I proposed here:
https://code.launchpad.net/~amanica/bzr/nested-trees-typos/+merge/100066

2 questions below:

Case 7
+~~~~~~
+Curtis has made changes to one of the libraries. Barry wants to merge Curtis'
+changes into his copy.
+
+Example commands::
+
+ $ bzr merge -d project http://curtis.org/trunk#library2
+
+ Or alternatively:
+ $ bzr merge -d project/library2 http://curtis.org/trunk#library2

Should trunk#library2 not just be trunk/library2 ?
(just wondering, not sure what you guys decided)

+Case 15
+~~~~~~~
+Barry commits some changes to a library and to the main project, and then
+discovers the changes are not appropriate. He has not yet pushed his changes
+anywhere.
+
+Example commands::
+
+ $ bzr merge -d project http://library2

should this be the following?: $ bzr merge -d project/library2 http://library2

review: Approve

[Jelmer Vernooij (jelmer)]

sent to pqm by email

=== added file 'doc/developers/nested-trees.txt'
--- doc/developers/nested-trees.txt	1970-01-01 00:00:00 +0000
+++ doc/developers/nested-trees.txt	2012-04-01 23:40:26 +0000
@@ -0,0 +1,1023 @@
+************
+Nested Trees
+************
+
+:status: 2012-03-17: Draft spec
+
+.. sectnum::
+
+.. contents::
+
+Principles
+**********
+
+- Never store a location in versioned data.
+
+- Implementation of nested trees shall not make operations observably slower
+  for those not using nested trees.
+
+- A repository that holds a revision R should be able to reconstruct the
+  whole contents of that revision, including any nested trees.  Corolary:
+  if I fetch that revision, even into a branch that has no working tree, it
+  should bring across any referenced revisions, or (implicitly) add fallback
+  repositories.
+
+- The introduction or possible support for nested trees should not
+  have an impact on performance.
+
+Core Concepts
+*************
+
+**Subtree** A tree which is inside another tree, which bzr has been asked to
+treat as part of the outer tree.
+
+**Subbranch** The branch associated with a subtree.
+
+**Containing tree** A tree which has another tree inside it
+
+**Tree reference** A directory in a containing tree which contains a subtree.
+
+
+Basic design approach
+*********************
+(see "Design decisions" for extended rationale)
+By default, APIs and commands for containing trees should behave as though the
+subtrees were plain directories.  By default, commands in subtrees should not
+affect the containing trees.
+
+Downwards recursion
+~~~~~~~~~~~~~~~~~~~
+One of the objectives of nested trees is to provide ways of reproducing
+historical combinations of different codebases.  The dependency chain points
+downwards, such that trees are affected by the revision of their subtrees, but
+subtrees are oblivious to their containing trees.  Just as bazaar doesn't
+entice people to commit inconsistent trees, it should not entice people to
+commit inconsistent combinations of containing tree and subtree.  Therefore,
+commit should recurse downwards by default.
+
+Status and diff should reflect what will happen when commit is used, so they
+should also recurse downward by default.  Add almost does this already.  With
+status, diff, commit and add recursing downwards, it would be confusing to
+users if other operations did not.  Therefore, all operations should recurse
+downwards by default.
+
+No upwards recursion by default
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+One of the reasons for using nested trees is to gain performance by only
+committing in a subtree.  Therefore, operations should not recurse upwards by
+default.  However, some users do want to have upwards recursion, so it should
+be provided as an option.
+
+Modelling nested trees as a composite tree
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+The idea that a set of nested trees behaves like a single, larger tree seems
+relatively easy to grasp.  Both for users and for developers, it provides a
+clear expectation for the behaviour of nested trees.  There are no obvious
+drawbacks in terms of code clarity or performance.  Therefore, it seems like a
+good model to start with.
+
+Using root file-ids for tree-references
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+The idea that tree-reference file-ids are the same as the file-ids of the
+corresponding root directories has a nice symmetry.  It is one way of ensuring
+that "bzr split" is deterministic, and "bzr join" is deterministic.  When
+performing operations involving a tree and a split version of that tree, using
+the same file-id makes it easy to ensure that operations such as moves and
+renames are applied appropriately to the tree-reference.  Providing mechanisms
+whereby a tree-reference can be treated as it would if it had its old file-id
+encroaches on the territory of path tokens or file-id aliases.  Having "split"
+cause file-id changes means that in comparing these revisions, it would be seen
+as a deleting a directory and creating a new tree-reference with the same name.
+Handling this correctly in operations such as merge and revert would be more
+complicated than if it were treated as a kind change, especially when
+unversioned files are present in the subtree.
+
+Sub-branches
+~~~~~~~~~~~~
+The branches associated with subtrees shall be called "subbranches".
+
+The branch for the top tree will be in a special format, whose last_revision
+file lists all the last_revision info for all of the branches associated with
+the nested tree.  The .bzr directories of subtrees will have a "branch" that
+simply indicates that the top tree's branch should be used.
+
+In the top tree's last_revision file, the revision id and revno will be
+provided, indexed by the tree-reference file-id.
+
+The repository used by the top-tree's branch must be a shared repository, and
+will be used by the sub-branches.
+
+Only the top branch will have a branch.conf.  When an operation on a subbranch
+would normally use values from branch.conf it will look them up in the top
+branch's branch.conf and adjust for the sub-location if appropriate.  e.g. "bzr
+push" in a subtree will push just that subbranch to the corresponding subbranch
+in the configured push location of the top branch.
+
+Rationale
+.........
+If the branches were not local, the local subtrees might not be committable,
+and commits to the remote branch would make the local subtree out of date.
+They should not be in a separate location from the containing branch, because
+they might share history with the tree-reference's branch.  However, those
+local branches should not be at the same location as their tree, because the
+tree might be deleted or moved.  Indeed, they should not be anywhere within a
+working tree.
+
+subtree branches should not be above or beside their containing branch, because
+it could cause terrible confusion if subtrees from two different trees were
+updating the same branches with every push, pull, commit and uncommit.
+
+Subtree branches could be plain branches stored somewhere in the top tree's
+branch, but then a lookup mechanism would be needed to translate from file_id
+to location, and performance with large numbers of subbranches would be poor.
+
+Pull and non-initial push
+~~~~~~~~~~~~~~~~~~~~~~~~~
+When a pull involves updates to tree references, pull will always pull into the
+reference branch.  For all new revisions in the upper branch, it will determine
+the revision values of tree references, and fetch them into the repository.
+
+When new tree references are encountered, pull should create a corresponding
+subbranch in the top branch.
+
+Pulls will update the subtrees whose tree-references change, including creating
+trees for new sub-branches.
+
+Implementation strategies
+*************************
+
+Data storage
+************
+
+Trees
+~~~~~
+The root-ids of trees must be unique, so that the same file-id can be used in
+both the containing tree and the subtree, to simplify access within trees.
+Tree references are an inventory type that is distinct from a directory, and
+has a revision-id associated with it.
+All modern working trees support tree references.  Indices may be provided to
+ensure fast access to the list of subtrees.
+
+The various methods on ``Tree`` need to be updated to handle nested trees.
+
+Tree file ids are tuples containing inventory file ids, describing a path
+to the file. This means that if a file is in a nested tree with the root 
+fileid ``file_id_a`` and the file itself has the inventory file id ``file_id_b``
+then the tree file id is (file_id_a, file_id_b). This makes it easy to look up file ids
+without having to load and scan all nested trees for ``file_id_b``.
+
+Branches
+~~~~~~~~
+A new branch format, "subbranches", is introduced which provides multiple
+sub-branches, with their data referenced by file-id.  A new branch refrerence
+format, "subbranch-reference", is introduced which refers to sub-branches in a
+"subbranches" branch.
+
+Repositories
+~~~~~~~~~~~~
+Some repository formats have 'subtree' variants, e.g. pack-0.92-subtree,
+development-subtree.  These are hidden, experimental formats that support
+storing tree-references in their inventory formats.
+
+Repository indexing might be extended to provide fast access to
+tree-references.
+
+Commands
+********
+
+The following new options are introduced:
+
+``join --reference`` Cause an inner tree to be treated as a subtree.  The outer
+tree's branch must be in the new "subbranches" format.  The inner tree's branch
+will be cloned into the "subbranches" branch, and the local branch will be
+replaced with a "subbranch-reference".  Finally, a tree-reference will be
+added to the containing tree.
+
+(this is already implemented)
+
+API Changes
+***********
+
+Tree file ids as tuples
+~~~~~~~~~~~~~~~~~~~~~~~
+
+Implementation Changes
+**********************
+
+Branch changes
+~~~~~~~~~~~~~~
+
+ pull recurses into reference branches, and pulls *from* the source's reference
+ branches.
+
+Repository changes
+~~~~~~~~~~~~~~~~~~
+
+ fetch provides a list of tree-reference revision ids/file-id pairs for the
+ revisions that were fetched.  Fetch automatically fetches all revisions
+ associted with tree-references that were fetched.
+
+Use Cases
+*********
+
+Case 1
+~~~~~~
+Barry works on a project with three libraries.  He wants to keep up to date
+with the tip of those libraries, but he doesn't want them to be part of his
+source tree.
+
+Example commands::
+
+ Set up the tree:
+ $ bzr branch --nested http://library1 project
+ $ bzr branch --nested http://library2 project
+ $ bzr branch --nested http://library3 project
+ $ bzr commit project -m "Added three libraries"
+
+ Update a library to tip:
+ $ bzr pull -d project/library1 http://library1
+
+Case 2
+~~~~~~
+Now, Barry wants to add a fourth library.
+
+Example commands::
+
+ $ bzr branch --nested http://library4 project
+
+Case 3
+~~~~~~
+Barry wants to publish his project.
+
+Example commands::
+
+ $ bzr push -d project bzr+ssh://project/trunk
+
+Case 4
+~~~~~~
+Barry decides to make part of his project into another library
+
+Example commands::
+
+ $ bzr split --nested project/newlibrary
+
+Case 5
+~~~~~~
+Curtis wants to hack on Barry's project
+
+Example commands::
+
+ $ bzr branch http://project/trunk
+
+Case 6
+~~~~~~
+Barry wants to drop one of the libraries he was using
+
+Example commands::
+
+ $ rm project/library1
+ $ bzr commit project -m "Removed library1"
+
+Case 7
+~~~~~~
+Curtis has made changes to one of the libraries.  Barry wants to merge Curtis'
+changes into his copy.
+
+Example commands::
+
+ $ bzr merge -d project http://curtis.org/trunk/library2
+
+ Or alternatively:
+ $ bzr merge -d project/library2 http://curtis.org/trunk/library2
+
+Case 8
+~~~~~~
+Curtis has made changes to Barry's main project.  Barry wants to merge Curtis'
+changes into his copy.
+
+Example commands::
+
+ $ bzr merge -d project http://curtis.org/trunk
+
+
+Case 9
+~~~~~~
+Barry makes changes in his project and in a library, and he runs status
+
+Example commands::
+
+ $ echo bar > project/foo
+ $ echo qux > project/library2/baz
+ $ bzr status project
+  M foo
+  M library2/baz
+
+Case 10
+~~~~~~~
+Barry wants to upgrade the bazaar format of his project
+
+Example commands::
+
+ $ bzr upgrade project
+
+Case 11
+~~~~~~~
+Curtis wants to apply Barry's latest changes.
+
+Example commands::
+
+ $ bzr merge -d project http://project/trunk
+
+Case 12
+~~~~~~~
+Danilo wants to start a project with two libraries using nested trees from
+scratch.
+
+Example commands::
+
+ $ bzr init project
+ $ bzr branch --nested http://library4 project
+ $ bzr branch --nested http://library5 project
+ $ bzr commit project -m "Created new project."
+
+Case 13
+~~~~~~~
+Edwin has a project that doesn't use nested trees and he wants to start using
+nested trees.
+
+Example commands::
+ $ bzr split --nested project/subdir
+
+Case 14
+~~~~~~~
+Françis has a project with nested trees where the containing tree uses one
+Bazaar format and the subtree uses a different Bazaar format.
+
+Not supported.
+
+Case 15
+~~~~~~~
+Barry commits some changes to a library and to the main project, and then
+discovers the changes are not appropriate.  He has not yet pushed his changes
+anywhere.
+
+Example commands::
+
+ $ bzr merge -d project http://library2
+ $ bzr commit project -m "Updated library2"
+ $ bzr uncommit project --force
+
+Case 16
+~~~~~~~
+Barry commits some changes to a library and to the main project, publishes his
+branch, and then discovers the changes are not appropriate.
+
+Example commands::
+
+ $ bzr merge -d project http://library2
+ $ bzr commit project -m "Updated library2"
+ $ bzr push -d project
+ $ bzr revert -r-2 project
+ $ bzr commit project -m "Reverted inappropriate changes."
+ $ bzr push -d project
+
+Case 17
+~~~~~~~
+Gary is writing a project.  Henninge wants to split a library out of it.
+
+Example commands::
+
+ $ bzr branch project
+ $ bzr split project/library6
+ $ mv project/library6 .
+ $ rm project
+ $ bzr commit -m "split library6 into its own library."
+
+Case 18
+~~~~~~~
+Henning wants to update to receive Gary's latest changes.
+
+Example commands::
+
+ $ bzr merge -d library6
+
+Case 19
+~~~~~~~
+Gary wants to update to receive Henninge's changes, including splitting a
+library out.
+
+Example commands::
+
+ $ bzr split --nested project/library6
+ $ bzr commit project -m "Turned library6 into a library"
+ $ bzr merge -d project/library6 http://library6
+ $ bzr commit project -m "Merge Henninge's changes."
+
+
+Case 20
+~~~~~~~
+Gary wants to update to receive Henninge's changes, without splitting a library
+out.
+
+ $ bzr split --nested project/library6
+ $ bzr commit project -m "Turned library6 into a library"
+ # i.e. a cherrypick that skips the revision where library6 became a library.
+ $ bzr merge -d project/library6 http://library6 -r 5..-1
+ $ bzr commit project -m "Merge Henninge's changes."
+
+Case 21
+~~~~~~~
+
+John works on a project called FooBar, but has decided that it would be better
+structured as two projects, where Bar is a library that may be of general use
+outside of Foo.  As it happens, bar already has its own subdirectory.
+
+He runs:
+::
+
+    # Convert into two trees: foobar and foobar/bar.
+    # In each tree, files will be removed and deleted.  In foobar/bar, "bar"
+    # will have been moved to become the tree root.
+    # These changes will be committed later.
+    $ bzr upgrade foobar --format=subbranches
+    $ bzr split foobar/bar
+
+    # Add a tree-reference from foobar to foobar/bar, change bar's branch
+    # to a reference to subbranch in foobar's branch.
+    $ bzr join --nested foobar/bar
+
+    # This recurses into foobar/bar and commits the deletion of the containing
+    # tree.  In foobar, it commits a kind change for 'bar' from directory to
+    # tree-reference, and the deletion of the contents of bar.
+    $ bzr commit foobar
+
+This commits new revisions to foobar and bar, and foobar's tree-reference bar
+refers to the revision-id of bar.
+
+Next, he adds two new files: foobar/baz and foobar/bar/qux::
+
+    $ vi foobar/baz
+    $ vi foobar/bar/qux
+    # This adds qux to foobar/bar and adds baz to foobar.
+    $ bzr add foobar
+
+Since foobar/bar/qux is in a commitable state and foobar/baz is not, he invokes
+::
+
+    $ bzr commit foobar/bar
+
+This commits foobar/bar/baz/qux to the subtree and commits foobar/bar to the
+containing tree.
+
+(Had he wanted to commit to just the subtree, or just the containing tree, he
+could have specified an option.)
+
+
+Case 21
+~~~~~~~
+
+Robert wants to hack on a project, Baz, that is structured as a nested tree,
+which uses the library "quxlib", from quxlib.org.
+
+He runs:
+::
+
+    $ bzr branch http://baz.org/dev baz
+
+This creates a "subbranches" branch and working tree for baz, as normal.  Since
+tree-references were encountered, it adds subbranches for them to the baz
+branch.  All data is retrieved from baz.org, not quxlib.org.
+
+It creates a working tree for quxlib with a subbranch-reference.  It uses the
+revision-id from the tree-reference in the containing tree, not the head
+revision at baz.org.  This allows Robert to get a known-good nested tree.
+
+Later, Robert decides to update the version of quxlib being used to the latest
+from quxlib.org.  He runs::
+
+    $ bzr pull -d http://quxlib.org
+
+This updates the version of quxlib in the working tree, which mean that baz is
+now out-of-date with its last-committed tree.  Unfortunately, the new rev on
+quxlib is not completely compatible with the old one, and Robert must tweak a
+few files before Baz runs properly.  Once he has done so, he runs::
+
+    $ bzr commit baz
+
+Now he has committed a known-good nested tree, and the baz working tree is once
+again up-to-date.
+
+
+User documentation
+******************
+
+For many large projects, it is often useful to incorporate libraries
+maintained elsewhere or to construct them from multiple subprojects.
+While it is easy for a single user to set up a particular layout of
+multiple branches by hand, the different branches really need to be
+linked together if others are to reproduce the desired layout, and
+if the relationships are going to be managed over time.
+
+Bazaar has good support for building and managing external libraries
+and subprojects via a feature known as *nested trees*. In particular,
+nearly all of Bazaar's commonly used commands understand nested trees
+and Do The Right Thing as explained below. The relationship is hierarchical:
+the containing tree knows about its nested trees, but nested trees are unaware
+of the tree (or trees) containing them.
+
+At the moment, *nested trees* are the only type of nested item
+supported though *nested files* may be supported in the future.
+Nested trees may contain other nested trees as required.
+
+Note: This feature requires a recent branch format such as ``2.0``
+or later.
+
+
+Nesting an external project
+~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+To link an external project into a branch, use the ``branch`` command
+with the ``--nested`` option like this::
+
+  bzr branch --nested SOURCE-URL TARGET-DIR
+
+For example, assuming you already have a ``src/lib`` directory where
+libraries are kept::
+
+  bzr branch --nested http://example.com/xmlsaxlib src/lib/sax
+
+This will create a nested branch in the ``src/lib/sax`` directory,
+join it into the containing branch and save the source location.
+
+If you now run ``bzr status``, it will show the nested branch as
+uncommitted changes like this::
+
+  +  src/lib/sax
+  +  src/lib/sax/README
+  +  src/lib/sax/parser.py
+  ...
+
+To record this change, use the ``commit`` command as you normally would::
+
+  bzr commit -m "added SAX parsing library"
+
+Note that Bazaar stores the tip revision of each nested branch. This
+is an important feature in that it's then easy to reproduce the exact
+combination of libraries used for historical revisions. It also means
+that other developers pulling or merging your changes will get nested
+branches created for them at the right revisions of each.
+
+
+Refreshing a nested branch
+~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+As bugs are fixed and enhancements are made to nested projects, you
+will want to update the version being used. To do this, ``pull`` the
+latest version of the nested branch. For example::
+
+  bzr pull -d src/lib/sax
+
+If the latest revision is too unstable, you can always use the ``-r``
+option on the ``pull`` command to nominate a particular revision or tag.
+
+Now that you have the required version of the code, you can make
+any required adjustments (e.g. API changes), run your automated tests
+and commit something like this::
+
+  view src/lib/sax/README
+  (hack, hack, hack)
+  make test
+  bzr commit -m "upgraded SAX library to version 2.1.3"
+
+
+Changing a nested tree
+~~~~~~~~~~~~~~~~~~~~~~
+
+As well as keeping track of which revisions of external libraries
+are used over time, one of the reasons for nesting projects is to
+make minor changes. You may want to do this in order to fix and
+track particular bugs you need addressed. In other cases, you may want
+to make various local enhancements that aren't valuable outside
+the context of your project.
+
+As support for nested branches is integrated into most commonly
+used commands, this is actually quite easy to do: simply make
+the change to the required files as you normally would! For example::
+
+  edit src/lib/sax/parser.py
+  bzr commit -m "fix bug #42 in sax parser"
+
+Note that Bazaar is smart enough to recurse by default into nested
+branches, commit changes there, and commit the new nested branch tips
+in the current branch. Both commits get the same commit message.
+
+If you want to only commit the change to a nested branch for now, you
+can change into the nested branch before running commit like this::
+
+  cd src/lib/sax
+  bzr commit -m "fix bug #42 in sax parser"
+
+Alternatively, you can use a selective commit like this::
+
+  bzr commit -m "fix bug #42 in sax parser" src/lib/sax
+
+
+Reviewing nested tree changes
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Just like ``commit``, the ``status`` and ``diff`` commands implicitly
+recurse into nested trees. In the case of ``status``, it shows both the
+nested tree as having a pending change as well as the items within it that have
+changed. For example::
+
+   M src/lib/sax
+   M src/lib/sax/parser.py
+
+Once again, if you change into a nested tree though, ``status`` and
+``diff`` will operate just on that tree and not recurse upwards by
+default.
+
+
+Browsing nested tree history
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+As the branches of nested trees have their own history, the ``log`` command
+shows just the history of the containing branch. To see the history for
+a nested branch, nominate the branch explicitly like this::
+
+  bzr log src/lib/sax
+
+Note however that ``log -v`` and ``log -p`` on the containing branch
+will show what files in nested branches were changed in each revision.
+
+
+Splitting out a project
+~~~~~~~~~~~~~~~~~~~~~~~
+
+If you already have a large project and wish to partition it into
+reusable subprojects, use the ``split`` command. This takes an existing
+directory and makes it a separate branch. For example, imagine you have
+a directory holding UI widgets that another project would like to
+leverage. You can make it a separate branch like this::
+
+  bzr split src/uiwidgets
+
+To make the new project available to others, push it to a shared location
+like this::
+
+  cd src/uiwidgets
+  bzr push bzr://example.com/uiwidgets
+
+You also need to link it back into the original project as a nested branch
+using the ``join`` command like this (assuming the current directory is
+``src/uiwidgets``)::
+
+  bzr join --nested .
+  bzr commit -m "uiwidgets is now a nested project"
+
+Similar to ``branch --nested``, ``join --nested`` joins the nominated directory
+(which must hold a branch) into the containing tree.  In order to make sure
+that all versions of a tree can be reproduced, the branches of nested trees
+share a repository with their containing tree.
+
+
+Virtual projects
+~~~~~~~~~~~~~~~~
+
+By design, Bazaar is strict about tracking the actual revisions used of
+nested branches over time. Without this, projects cannot accurately
+reproduce exactly what was used to make a given build. There are
+isolated use cases though where is advantageous to say "give me the
+latest tip of these loosely coupled branches". To do this, create a
+small 'virtual project' which is just a bunch of *unpegged* nested
+branches. To mark nested branches as unpegged, use the ``--no-pegged``
+option of the ``join`` command like this::
+
+  bzr join --nested --no-pegged [DIR]
+
+To stop the nested branch tips from floating and to begin recording
+the tip revisions again, use the ``pegged`` option::
+
+  bzr join --nested --pegged [DIR]
+
+After changing whether one or more nested branches are pegged or not, you
+need to ``commit`` the branch to record that metadata. (The pegged state
+is recorded over time.)
+
+For example, you may be managing a company intranet site as a project
+which is nothing more than a list of unrelated departmental websites
+bundled together. You can set this up like this::
+
+  bzr init intranet-site
+  cd intranet-site
+  bzr branch bzr://ourserver/websites/research
+  bzr branch bzr://ourserver/websites/development
+  bzr branch bzr://ourserver/websites/support
+  bzr branch bzr://ourserver/websites/hr
+  bzr join --nested --no-pegged research
+  bzr join --nested --no-pegged development
+  bzr join --nested --no-pegged support
+  bzr join --nested --no-pegged hr
+  bzr commit -m "initial configuration of intranet-site"
+
+Publishing the overall site is then as easy as going to the server
+hosting your intranet and running something like::
+
+  bzr branch http://mymachine//projects/intranet-site
+
+Refreshing the overall site is as easy as::
+
+  bzr pull
+
+Virtual projects are also useful for providing a partial 'view' over
+a large project containing a large number of subprojects. For example,
+you may be working on an office suite and have a bunch of developers
+that only care about the word processor. You can create a virtual
+project for them like this::
+
+  bzr init wp-modules
+  cd wp-modules
+  bzr branch ../common
+  bzr branch ../printing
+  bzr branch ../spellchecker
+  bzr branch ../wordprocessor
+  bzr join --nested --no-pegged common
+  bzr join --nested --no-pegged printing
+  bzr join --nested --no-pegged spellchecker
+  bzr join --nested --no-pegged wordprocessor
+  bzr commit -m "initial configuration of wp-modules"
+
+Those developers can then get bootstrapped faster and have *just* the
+subprojects they care about by branching from ``wp-modules``.
+
+
+Nested branch tips & tricks
+~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+As explained above, most of Bazaar's commonly used commands recurse
+downwards into nested branches by default. To prevent this recursion,
+use the ``--no-recurse-nested`` option on various commands (including
+``commit``, ``status`` and ``diff``) that support it.
+
+Thanks to plugins like bzr-svn and bzr-git, Bazaar has strong support
+for transparently accessing branches managed by foreign VCS tools. This
+means that Bazaar can support projects where nested branches are hosted
+in supported foreign systems. For example, to nest a library maintained
+in Subversion::
+
+  bzr branch --nested svn://example.com/xmlhelpers src/lib/xmlhelpers
+
+If you want revisions to be committed both to a remote location and a
+local location, make the top-level branch a bound branch.  (Nested branches
+have no configuration of their own.)
+
+Most likely, you will have some branches that are identical to their upstream
+version and can be pulled, and some that have local changes and must be merged.
+You can update all of them at once using ``merge --pull``.  This will pull
+into the trees with no local changes, and merge into the ones with local
+changes.  Afterward, you should commit, which will commit only into the
+trees that were merged into.
+
+As you'd expect, a nested branch can be moved or deleted using the
+normal commands. For example, after splitting out a subproject, you
+may want to change its location like this::
+
+  bzr mv src/uiwidgets src/lib/uiwidgets
+  bzr commit -m "move uiwidgets into src/lib"
+
+Things to be aware of
+~~~~~~~~~~~~~~~~~~~~~
+
+Commands like ``commit`` and ``push`` need online access to the locations
+for nested branches which have updated their tip. In particular, ``commit``
+will update any changed nested branches first and only commit to the
+containing branch if all nested branch commits succeed. If you are working
+offline, you may want to ensure you have a local mirror location defined
+for nested branches you are likely to tweak. Alternatively, the
+``no-recurse-nested`` option to the ``commit`` command might to useful to
+commit some changes, leaving the nested branch commits until you are back
+online.
+
+At the moment, nested trees need to be incorporated as a whole.
+Filtered views can be used to restrict the set of files and directories
+logically seen. Currently though, filtered views are a lens onto a tree:
+they do not delete other files and the exposed files/directories must
+have the same paths as they do in the original branch. In the future,
+we may add support for nesting and moving selected files from a
+(read-only) nested branch something like this::
+
+  bzr nested DIR --file LICENSE --file doc/README::README
+  bzr commit -m "change which files are nested from project DIR"
+
+If you require this feature, please contact us with your needs.
+
+Design decisions
+****************
+
+The branches of subtrees shall either share a repository with the containing tree,
+or the containing tree's repository will be (implicitly) added as a fallback
+repository.
+
+The branches of subtrees shall be in a special format that shares a single
+last_revision file that is stored in the containing branch.
+
+The subtree branches shall be referenced in the last_revision file by file-id.
+
+Subtree branches shall not support individual configuration.
+
+Fetch shall automatically fetch the revisions mentioned by tree-references,
+recursively.
+
+The reserved revision-id "head:" shall be used in tree-references to refer to
+the tip revision of a branch.
+
+bzr-svn repositories with externals shall behave as though the multiple
+repositories were a single Bazaar repository with multiple branches.
+
+Shall commands recurse downwards by default?
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Yes.
+
+Pros:
+
+ - It is hard to accidentally produce inconsistent trees
+ - Inconsistent trees are hard for remote users to handle
+ - Accidentally committing too many things at once is easy to resolve
+ - It is hard to accidentally commit too many things at once
+
+Cons:
+
+ - Accidentally committing nuclear launch codes is easier to do
+ - A commit message that makes sense for the top may not make sense lower down.
+
+
+Shall commands recurse upwards by default?
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+No.
+
+
+Shall subtree branches be addressable?
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Ideally, yes. We might want to use the path segment parameters syntax here too.
+
+
+Shall we model nested trees as a composite tree?
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Yes.  Users will see recurse-downwards behaviour that allows operations that
+cross subtree boundaries, e.g. a merge in the top tree can move a file between
+subtrees.
+
+The downside is that we can't have cheap support for subtrees that are copies
+of one another, because we wouldn't know which copy to apply sets of changes
+to.
+
+
+Shall we use root-ids for tree references?
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Yes.  This fits well with our current lack of support of file copies.  If we do
+support file copies in future it will be possible to change this in a future
+format, and perform deterministic upgrades to that format.
+
+
+What about locking?
+~~~~~~~~~~~~~~~~~~~
+
+We should lock recursively.  It matches existing behaviour by failing earlier,
+and the extra cost does not seem onerous.  (To be fully efficient this requires
+an index of the subtrees, otherwise we need to scan the fully
+inventory/dirstate.)
+
+(Also, this decision can be changed later with no compatibility concerns.)
+
+
+How do we handle merge when the subtree hasn't diverged?
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+"bzr merge --pull" will be changed so that it will merge (not pull) when the
+local last revision's revno would change (i.e. is a non-lhs parent in the merge
+source).  This is expected to be the most common way to update nested trees.
+
+The existing "bzr merge --pull" behaviour will be renamed to "bzr merge
+--pull-renumber".
+
+"bzr merge" (with no "--pull") will do a merge in all trees.  "bzr pull" will
+do a pull in all trees.
+
+The rationale is that a very common use-case is that the top tree is a project
+the user is actively committing to, and the subtrees are mainly libraries that
+are being mirrored.  So a behaviour that forced every update to be a merge
+would be undesirable for the mirrored subtrees, but an update that is a pull
+wouldn't suit the changing top tree.  And the existing "merge --pull" (that can
+renumber revisions) isn't desireable for either the top tree or subtrees in
+this case.
+
+
+What should uncommit do?
+~~~~~~~~~~~~~~~~~~~~~~~~
+
+It will recurse, and subtrees will be uncommitted back to the revision recorded
+by the revision the top tree is uncommitting to.
+
+This means that operations like::
+
+   $ echo foo > versioned-file-in-top-tree.txt
+   $ bzr ci -m "Change file"
+   $ bzr uncommit
+
+will not cause a change in subtrees, since the top-level commit did not affect
+them.  But on the other hand:
+
+   $ echo foo > subtree/versioned-file-in-subtree.txt
+   $ bzr ci -m "Change file"
+   $ bzr uncommit
+
+will first uncommit to the subtree, then to the top tree.  The uncommit will
+restore both trees to their previous state.
+
+
+Some subtrees should have commits and some should not.  How?
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+We will not provide special support for this initially.  We might later support
+flagging some sub-trees as mirror-only or something similar, but this seems like
+it could be a general feature not specific to nesting.  (and it may only require
+a working tree format bump to add).
+
+Comparison with other systems
+*****************************
+
+Git submodules
+~~~~~~~~~~~~~~
+
+This allows separate repositories to be used for submodules.
+
+Mercurial Forests
+~~~~~~~~~~~~~~~~~
+
+The wiki page does not give confidence that this is a well-maintained project.
+It seems similar to config-manager-- its 'snapshot' files are like
+config-manager's config files, describing what branches to get and where to put
+them, optionally specifying a revision.  No metadata about nesting is stored in
+the tree.  Optionally, a 'snapshot.txt' file may be stored in the containing
+tree, but it can also be stored somewhere else.
+
+There is no attempt to integrate subtree support into the core commands.
+
+Mercurial Nested Repositories
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+This design was for an integrated feature, but there are apparently 4
+implementations as extensions.  While it does integrate subtree support into
+core commands, this may be off by default: "The alternative that I lean towards
+is to not recurse unless explicitly instructed to. Most probably, only a few
+commands should arguably even be aware of modules."
+
+Command comparison:
+
+- hg module add ~= bzr join --nested
+- hg module remove ~= bzr remove --keep
+- hg module record's functionality is part of nested commits in nested trees.
+
+Like submodules, this stores location information in versioned data: a
+.hgmodules directory.
+Like submodules and nested trees, particular revisions are recorded.
+
+Subversion "svn:externals"
+~~~~~~~~~~~~~~~~~~~~~~~~~~
+Like bzr, uses per-file metadata.  Like submodules and nested repositories,
+locations are versioned data.  Like Forests, revisions are optional.  Like
+nested repositories, there is limited integration into core commands; checkout
+and update support externals, and commit may support them in the future.
+However, there is no UI specific to creating and updating svn:externals
+references.
+
+Unlike all other alternatives, supports partial checkouts.  This is because svn
+natively supports partial checkouts.  Also, supports checkouts of tags, because
+tags are merely a convention in svn.
+
+Supports pulling in "head" subtrees too, not just specific ("known-good")
+revisions.  Nested trees supports this in order to provide high-fidelity
+imports.
+
+Some support for single-file svn:externals (see
+http://subversion.tigris.org/svn_1.6_releasenotes.html#externals), whereas bzr
+subtrees must be directories.
+
+Has a --ignore-externals option for checking out without pulling in the
+svn:externals items (svn checkout is more or less equivalent to bzr branch).
+You can also use that option on update (more or less like bzr merge).
+
+
+Comments on differences
+~~~~~~~~~~~~~~~~~~~~~~~
+externals support partial checkouts.  Nested trees could gain support for this
+once Bazaar itself supports partial checkouts.  Supporting a single file as a
+"subtree" would also depend on native bzr support.  On platforms that support
+symlinks, using symlinks to portions of a subtree can be an effective
+substitute.
+
+.. vim: ft=rst

=== modified file 'doc/developers/plans.txt'
--- doc/developers/plans.txt	2011-12-01 11:41:07 +0000
+++ doc/developers/plans.txt	2012-04-01 23:40:26 +0000
@@ -5,9 +5,7 @@
    :maxdepth: 1
 
    Performance roadmap <performance-roadmap>
-   colocated-branches
-   feature-flags
    new-config-rationale
    tortoise-strategy
    improved_chk_index
-
+   nested-trees