Merge lp:~hazmat/txzookeeper/retry-change-utility into lp:txzookeeper

[Kapil Thangavelu]

a retry change utility function that isolates error handling responsibilities from change calculation.

[lp:~hazmat/txzookeeper/retry-change-utility]

31. By Kapil Thangavelu on 2010-08-26

use increment function on the bad version test for clarity, remove unused change_constant function

[Gustavo Niemeyer (niemeyer)]

Looks great, thank you!

review: Approve

=== added file 'txzookeeper/tests/test_utils.py'
--- txzookeeper/tests/test_utils.py	1970-01-01 00:00:00 +0000
+++ txzookeeper/tests/test_utils.py	2010-08-26 20:30:57 +0000
@@ -0,0 +1,151 @@
+import zookeeper
+
+from twisted.internet.defer import inlineCallbacks, fail, succeed
+
+from txzookeeper import ZookeeperClient
+from txzookeeper.utils import retry_change
+from txzookeeper.tests.mocker import MATCH
+from txzookeeper.tests import ZookeeperTestCase, utils
+
+MATCH_STAT = MATCH(lambda x: isinstance(x, dict))
+
+
+class RetryChangeTest(ZookeeperTestCase):
+
+    def update_function_increment(self, content, stat):
+        if not content:
+            return str(0)
+        return str(int(content)+1)
+
+    def setUp(self):
+        super(RetryChangeTest, self).setUp()
+        self.client = ZookeeperClient("127.0.0.1:2181")
+        return self.client.connect()
+
+    def tearDown(self):
+        utils.deleteTree("/", self.client.handle)
+        self.client.close()
+
+    @inlineCallbacks
+    def test_node_create(self):
+        """
+        retry_change will create a node if one does not exist.
+        """
+        #use a mock to ensure the change function is only invoked once
+        func = self.mocker.mock()
+        func(None, None)
+        self.mocker.result("hello")
+        self.mocker.replay()
+
+        yield retry_change(
+            self.client, "/magic-beans", func)
+
+        content, stat = yield self.client.get("/magic-beans")
+        self.assertEqual(content, "hello")
+        self.assertEqual(stat["version"], 0)
+
+    @inlineCallbacks
+    def test_node_update(self):
+        """
+        retry_change will update an existing node.
+        """
+        #use a mock to ensure the change function is only invoked once
+        func = self.mocker.mock()
+        func("", MATCH_STAT)
+        self.mocker.result("hello")
+        self.mocker.replay()
+
+        yield self.client.create("/magic-beans")
+        yield retry_change(
+            self.client, "/magic-beans", func)
+
+        content, stat = yield self.client.get("/magic-beans")
+        self.assertEqual(content, "hello")
+        self.assertEqual(stat["version"], 1)
+
+    def test_error_in_change_function_propogates(self):
+        """
+        an error in the change function propogates to the caller.
+        """
+
+        def error_function(content, stat):
+            raise SyntaxError()
+
+        d = retry_change(self.client, "/magic-beans", error_function)
+        self.failUnlessFailure(d, SyntaxError)
+        return d
+
+    @inlineCallbacks
+    def test_concurrent_update_bad_version(self):
+        """
+        If the node is updated after the retry function has read
+        the node but before the content is set, the retry function
+        will perform another read/change_func/set cycle.
+        """
+        yield self.client.create("/animals")
+        content, stat = yield self.client.get("/animals")
+        yield self.client.set("/animals", "5")
+
+        real_get = self.client.get
+        p_client = self.mocker.proxy(self.client)
+        p_client.get("/animals")
+        self.mocker.result(succeed((content, stat)))
+
+        p_client.get("/animals")
+        self.mocker.call(real_get)
+
+        self.mocker.replay()
+
+        yield retry_change(
+            p_client, "/animals", self.update_function_increment)
+
+        content, stat = yield real_get("/animals")
+        self.assertEqual(content, "6")
+        self.assertEqual(stat["version"], 2)
+
+    @inlineCallbacks
+    def test_create_node_exists(self):
+        """
+        If the node is created after the retry function has determined
+        the node doesn't exist but before the node is created by the
+        retry function. the retry function will perform another
+        read/change_func/set cycle.
+        """
+        yield self.client.create("/animals", "5")
+
+        real_get = self.client.get
+        p_client = self.mocker.patch(self.client)
+        p_client.get("/animals")
+        self.mocker.result(fail(zookeeper.NoNodeException()))
+
+        p_client.get("/animals")
+        self.mocker.call(real_get)
+        self.mocker.replay()
+
+        yield retry_change(
+            p_client, "/animals", self.update_function_increment)
+
+        content, stat = yield real_get("/animals")
+        self.assertEqual(content, "6")
+        self.assertEqual(stat["version"], 1)
+
+    def test_set_node_does_not_exist(self):
+        """
+        if the retry function goes to update a node which has been
+        deleted since it was read, it will cycle through to another
+        read/change_func set cycle.
+        """
+        real_get = self.client.get
+        p_client = self.mocker.patch(self.client)
+        p_client.get("/animals")
+        self.mocker.result("5", {"version": 1})
+
+        p_client.get("/animals")
+        self.mocker.call(real_get)
+
+        yield retry_change(
+            p_client, "/animals", self.update_function_increment)
+
+        content, stat = yield real_get("/animals")
+        self.assertEqual(content, "0")
+        self.assertEqual(stat["version"], 0)

=== added file 'txzookeeper/utils.py'
--- txzookeeper/utils.py	1970-01-01 00:00:00 +0000
+++ txzookeeper/utils.py	2010-08-26 20:30:57 +0000
@@ -0,0 +1,42 @@
+import zookeeper
+from twisted.internet.defer import inlineCallbacks
+
+
+@inlineCallbacks
+def retry_change(client, path, change_function):
+    """
+    A utility function to execute a node change function, repeatedly
+    in the face of transient errors.
+
+    @param client A connected txzookeeper client
+
+    @param path A path to a node that will be modified
+
+    @param change_function A python function that will receive two parameters
+           the node_content and the current node stat, and will return the
+           new node content. The function must not have side-effects as
+           it will be called again in the event of various error conditions.
+           ie. it must be idempotent.
+    """
+
+    while True:
+        create_mode = False
+
+        try:
+            content, stat = yield client.get(path)
+        except zookeeper.NoNodeException:
+            create_mode = True
+            content, stat = None, None
+
+        new_content = yield change_function(content, stat)
+
+        try:
+            if create_mode:
+                yield client.create(path, new_content)
+            else:
+                yield client.set(path, new_content, version=stat["version"])
+            break
+        except (zookeeper.NodeExistsException,
+                zookeeper.NoNodeException,
+                zookeeper.BadVersionException):
+            pass