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Synchronize object data with a version control system

Project Description

Version Control Synchronization

This package contains code that helps with handling synchronization of persistent content with a version control system.

This can be useful in software that needs to be able to work offline. The web application runs on a user’s laptop that may be away from an internet connection. When connected again, the user syncs with a version control server, receiving updates that may have been made by others, and committing their own changes.

Another advantage is that the version control system always contains a history of how content developed over time. The version-control based content can also be used for other purposes independent of the application.

While this package has been written with other version control systems in mind, it has only been developed to work with SVN so far. Examples below are all given with SVN.

The synchronization sequence is as follows:

  1. save persistent state (IState) to svn checkout (ICheckout) on the same machine as the Zope application.
  2. svn up. Subversion merges in changed made by others users that were checked into the svn server.
  3. Any svn conflicts are automatically resolved.
  4. reload changes in svn checkout into persistent Python objects
  5. svn commit.

This is all happening in a single step. It can happen over and over again in a reasonably safe manner, as after the synchronization has concluded, the state of the persistent objects and that of the local SVN checkout will always be in sync.

During synchronisation, the system tries to take care only to synchronize those objects and files that have changed. That is, in step 1) only applies those objects that have been modified, added or removed will have an effect on the checkout. In step 4) only those files that have been changed, added or removed on the filesystem due to the up action will change the persistent object state.


Content to synchronize is represented by an object that provides IState. A state represents a container object, which should contain a data object (a container that contains the actual data to be synchronized) and a found object (a container that contains objects that would otherwise be lost during conflict resolution).

The following methods need to be implemented:

  • get_revision_nr(): return the last revision number that the

    application was synchronized with. The state typically stores this the application object.

  • set_revision_nr(nr): store the last revision number that the application was synchronized with.

  • objects(revision_nr): any object that has been modified (or added) since the synchronization for revision_nr. Returning ‘too many’ objects (objects that weren’t modified) is safe, though less efficient as they will then be re-exported.

    Typically in your application this would be implemented by doing a catalog search, so that they can be looked up quickly.

  • removed(revision_nr): any path that has had an object removed from it since revision_nr. It is safe to return paths that have been removed and have since been replaced by a different object with the same name. It is also safe to return ‘too many’ paths, though less efficient as the objects in these paths may be re-exported unnecessarily.

    Typically in your application you would maintain a list of removed objects by hooking into IObjectMovedEvent and IObjectRemovedEvent and recording the paths of all objects that were moved or removed. After an export it is safe to purge this list.

In this example, we will use a simpler, less efficient, implementation that goes through a content to find changes. It tracks the revision number as a special attribute of the root object:

>>> from z3c.vcsync.tests import TestState

The content

Now that we have something that can synchronize a tree of content in containers, let’s actually build ourselves a tree of content.

An item contains some payload data, and maintains the SVN revision after which it was changed. In a real application you would typically maintain the revision number of objects by using an annotation and listening to IObjectModifiedEvent, but we will use a property here:

>>> from z3c.vcsync.tests import Item

This code needs a get_revision_nr method available to get access to the revision number of last synchronization. For now we’ll just define this to return 0, but we will change this later:

>>> def get_revision_nr(self):
...    return 0
>>> Item.get_revision_nr = get_revision_nr

Besides the Item class, we also have a Container class:

>>> from z3c.vcsync.tests import Container

It is a class that implements enough of the dictionary API and implements the IContainer interface. A normal Zope 3 folder or Grok container will also work.

Let’s create a container now:

>>> root = Container()
>>> root.__name__ = 'root'

The container has two subcontainers (data and found).

>>> root['data'] = data = Container()
>>> root['found'] = Container()
>>> data['foo'] = Item(payload=1)
>>> data['bar'] = Item(payload=2)
>>> data['sub'] = Container()
>>> data['sub']['qux'] = Item(payload=3)

As part of the synchronization procedure we need the ability to export persistent python objects to the version control checkout directory in the form of files and directories.

Now that we have an implementation of IState that works for our state, let’s create our state object:

>>> state = TestState(root)

Reading from and writing to the filesystem

To integrate with the synchronization machinery, we need a way to dump a Python object to the filesystem (to an SVN working copy), and to parse it back to an object again.

Let’s grok this package first, as it provides some of the required infrastructure:

>>> import grokcore.component as grok
>>> grok.testing.grok('z3c.vcsync')

We need to provide a serializer for the Item class that takes an item and writes it to the filesystem to a file with a particular extension (.test):

>>> from z3c.vcsync.tests import ItemSerializer

We also need to provide a parser to load an object from the filesystem back into Python, overwriting the previously existing object:

>>> from z3c.vcsync.tests import ItemParser

Sometimes there is no previously existing object in the Python tree, and we need to add it. To do this we implement a factory (where we use the parser for the real work):

>>> from z3c.vcsync.tests import ItemFactory

Both parser and factory are registered per extension, in this case .test. This is the name of the utility.

We register these components:

>>> grok.testing.grok_component('ItemSerializer', ItemSerializer)
>>> grok.testing.grok_component('ItemParser', ItemParser)
>>> grok.testing.grok_component('ItemFactory', ItemFactory)

We also need a parser and factory for containers, registered for the empty extension (thus no special utility name). These can be very simple:

>>> from z3c.vcsync.tests import ContainerParser, ContainerFactory
>>> grok.testing.grok_component('ContainerParser', ContainerParser)
>>> grok.testing.grok_component('ContainerFactory', ContainerFactory)

Setting up the SVN repository

Now we need an SVN repository to synchronize with. We create a test SVN repository now and create a svn path to a checkout:

>>> from z3c.vcsync.tests import svn_repo_wc
>>> repo, wc = svn_repo_wc()

We can now initialize the SvnCheckout object with the SVN path to the checkout we just created:

>>> from z3c.vcsync.svn import SvnCheckout
>>> checkout = SvnCheckout(wc)

The root directory of the working copy will be synchronized with the root container of the state. The checkout will therefore contain data and a found sub-directories.

Constructing the synchronizer

Now that we have the checkout and the state, we can set up a synchronizer:

>>> from z3c.vcsync import Synchronizer
>>> s = Synchronizer(checkout, state)

Let’s make s the current synchronizer as well. We need this in this example to get back to the last revision number:

>>> current_synchronizer = s

It’s now time to set up our get_revision_nr method a bit better, making use of the information in the current synchronizer. In actual applications we’d probably get the revision number directly from the content, and there would be no need to get back to the synchronizer (it doesn’t need to be persistent but can be constructed on demand):

>>> def get_revision_nr(self):
...    return current_synchronizer.state.get_revision_nr()
>>> Item.get_revision_nr = get_revision_nr


We’ll synchronize for the first time now:

>>> info = s.sync("synchronize")

We will now examine the SVN checkout to see whether the synchronization was successful.

To do this we’ll introduce some helper functions that help us present the paths in a more readable form, relative to the base of the checkout:

>>> def pretty_path(path):
...     return path.relto(wc)
>>> def pretty_paths(paths):
...     return sorted([pretty_path(path) for path in paths])

We see that the Python object structure of containers and items has been translated to the same structure of directories and .test files on the filesystem:

>>> pretty_paths(wc.listdir())
>>> pretty_paths(wc.join('data').listdir())
['data/bar.test', 'data/foo.test', 'data/sub']
>>> pretty_paths(wc.join('data').join('sub').listdir())

The .test files have the payload data we expect:

>>> print wc.join('data').join('foo.test').read()
>>> print wc.join('data').join('bar.test').read()
>>> print wc.join('data').join('sub').join('qux.test').read()

Synchronization back into objects

Let’s now try the reverse: we will change the SVN content from another checkout, and synchronize the changes back into the object tree.

We have a second, empty tree that we will load objects into:

>>> root2 = Container()
>>> root2.__name__ = 'root'
>>> state2 = TestState(root2)

We make another checkout of the repository:

>>> import py
>>> wc2 = py.test.ensuretemp('wc2')
>>> wc2 = py.path.svnwc(wc2)
>>> wc2.checkout(repo)
>>> checkout2 = SvnCheckout(wc2)

Let’s make a synchronizer for this new checkout and state:

>>> s2 = Synchronizer(checkout2, state2)

This is now the current synchronizer (so that our get_revision_nr works properly):

>>> current_synchronizer = s2

Now we’ll synchronize:

>>> info = s2.sync("synchronize")

The state of objects in the tree must now mirror that of the original state:

>>> sorted(root2.keys())

>>> sorted(root2['data'].keys())
['bar', 'foo', 'sub']

Now we will change some of these objects, and synchronize again:

>>> root2['data']['bar'].payload = 20
>>> root2['data']['sub']['qux'].payload = 30
>>> info2 = s2.sync("synchronize")

We can now synchronize the original tree again:

>>> current_synchronizer = s
>>> info = s.sync("synchronize")

We should see the changes reflected into the original tree:

>>> root2['data']['bar'].payload
>>> root2['data']['sub']['qux'].payload

More information

To learn more about the APIs you can use and need to implement, see

To learn about using z3c.vcsync to import and export content, see importexport.txt.

More low-level information may be gleaned from conflicts.txt and internal.txt.

z3c.vcsync changes

0.17 (2009-06-05)

  • Depend only on grokcore.component and directly, not on grok itself. We want instead to depend only on zope.container, but we cannot do this yet as we need compatibility with Grok 0.14.
  • Use a somewhat less verbose way to set up the tests.
  • An issue existed when a common filesystem representation was used that could be translated (by a single factory) into a number of different classes. If an object was added, synchronized, then removed and an object of the name but different class was added, an error would occur when synchronizing this. This bug has been fixed at the (small) cost of a few-reparses here and there.

0.16 (2009-06-02)

  • Change the method by which zip files are created for export to a less filesystem-intensive method; files are directly added to the zip file. This hopefully brings performance benefits on platforms where accessing many small files is slow.

0.15 (2008-08-19)

  • Fix a bug where the SVN “R” status was not recognized by the py lib. Monkey-patch py for now, though a fix should be released with py 0.9.2 eventually. Strictly depend on py 0.9.1 for now to make sure monkey-patch applies cleanly.
  • A bit of refactoring of duplicated code in retrieving the objects modified and objects removed lists. Still not happy that this gets called twice per synchronization, but it was already doing this so doesn’t get worse either.

0.14 (2008-07-04)

  • Fixed a bug where too many path fragments were returned in case of conflicts. Now only paths that have in fact changed should be returned. Added some tests for this.
  • There was a case where two users would add a file with the same name to their own states independently. This used to result in an SVN error, but now also generates a conflict. This conflict is a bit different in its behavior unfortunately, as it prefers the version already in SVN as opposed to the one last added.

0.13 (2008-06-02)

  • The root directory of the checkout is now truly equivalent to the root object of the state. This means that if the SVN checkout content is to remain the same, the state root to synchronize should be one level higher (the parent of the current state root).
  • Conflict resolution has been cleaned up. When a conflict occurs, the other half of the conflict (the one not resolved) is moved into the found directory, which is created in the checkout root. This is also represented in the state container object and is synchronized like any other content.

0.12 (2008-05-16)

Features added

  • The API has been cleaned up and revised. This will break code that uses this library, but so far I don’t think that is many people yet. :)
  • A major refactoring of the tests, including real SVN tests. This requires SVN to be installed on the system where the tests are being run, including the svn-admin command.
  • IState objects now need to implement methods to access and maintain the revision number of the last revision.
  • The developer must now also implement IParser utilities for files that can be synchronized (besides IFactory, which used to be called IVcFactory. The IParser utility overwrites the existing object instead of creating a new one. This allows synchronization to be a bit nicer and not remove and recreate objects unnecessarily, which makes it harder to implement things like references between objects.
  • Add a facility to pass a special function along that is called for all objects created or modified during the synchronization (or during import). This function is called at the end when all objects that are going to exist already exist, so can be used in situations where the state of an object relies on the existence of another one.

0.11 (2008-03-10)

Bugs fixed

  • Do not try to remove non-existent files during synchronization. A file might have been removed in SVN and there is no more need to re-remove it if it was also removed locally.
  • There was an off-by one error during the “up” phase of synchronization with SVN, and as a result a log entry that was already processed could be re-processed during this next synchronisation. This could in some cases revive folders as unknown directories on the filesystem, leading to errors and inconsistencies.

0.10 (2008-01-08)

Features added

  • The .sync() method now does not return the revision number, but an ISynchronizationInfo object. This has a revision_nr attribute and also contains some information on what happened during the synchronization process.

Bugs fixed

  • revision number after synchronization was not always updated properly to the latest number of the repository. Now retrieve this number from commit() where possible.

0.9.1 (2007-11-29)

Bugs fixed

  • When resolving objects in the ZODB, a path was generated that has separators that are actually dependent on the operating system in use (/ for Unices, but \ for windows). This caused synchronization to fail on Windows, completely flattening hierarchies. Now use os.path.sep to be platform-independent.

0.9 (2007-11-25)

Features added

  • The importing logic now allows the user to import new content over existing content. In this case any existing content is left alone, but new objects are added. Any attempt to overwrite existing content is ignored.

Bugs fixed

  • In some cases a containing directory is referenced which does not exist anymore when removing files. In this case we do not need to remove the file anymore, as the directory itself is gone.
  • SVN doesn’t actually remove directories, just mark them for removal. This could confuse the system during synchronization: removed directories might reappear again as they were still on the filesystem during loading. Make sure now that any directories marked for removal are also properly removed in the filesystem before load starts, but after up (as rm-ing a directory marked for removal before svn up will actually re-add this directory!).


  • Previously the datetime of last synchronization was used to determine what to synchronize both in the ZODB as well as in the checkout. This has a significant drawback if the datetime setting of the computer the synchronization code is running on is ahead of the datetime setting of the version control server: updates could be lost.

    Changed the code to use a revision_nr instead. This is a number that increments with each synchronization, and the number can be used to determine both what changes have been made since last synchronization in the ZODB as well as in the version control system. This is a more robust approach.

0.8.1 (2007-11-07)

Bugs fixed

  • Fix a bug in conversion of SVN timestamps to datetimes. Previous code worked in DST, but not during winter time. The new code might of course break under DST - the mysterious of datetime conversion are legion.
  • A cleaner way to cache the files listing from SVN.
  • Work around a bug in the Py library. The Py library doesn’t support the R status code from SVN and raises a NotImplementedError when it encounters it. Evilly catch these NotImplementedErrors for now. The bug has been reported upstream and should be fixed in the next release of Py.
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