bzr-svn

Overview

bzr-svn lets developers use Bazaar as their VCS client on projects still using a central Subversion repository. Access to Subversion repositories is largely transparent, i.e. you can use most bzr commands directly on Subversion repositories exactly the same as if you were using bzr on native Bazaar branches.

Many bzr-svn users create a local mirror of the central Subversion trunk, work in local feature branches, and submit their overall change back to Subversion when it is ready to go. This lets them gain many of the advantages of distributed VCS tools without interrupting existing team-wide processes and tool integration hooks currently built on top of Subversion. Indeed, this is a common interim step for teams looking to adopt Bazaar but who are unable to do so yet for timing or non-technical reasons.

For installation instructions, see the bzr-svn home page: http://bazaar-vcs.org/BzrForeignBranches/Subversion.

A simple example

Here’s a simple example of how you can use bzr-svn to hack on a GNOME project like beagle. Firstly, setup a local shared repository for storing your branches in and checkout the trunk:

bzr init-repo --default-rich-root beagle-repo
cd beagle-repo
bzr checkout svn+ssh://svn.gnome.org/svn/beagle/trunk beagle-trunk

Note that using the default-rich-root option to init-repo is important as bzr-svn requires some extra metadata not yet supported in Bazaar’s default repository format. Next, create a feature branch and hack away:

bzr branch beagle-trunk beagle-feature1
cd beagle-feature1
(hack, hack, hack)
bzr commit -m "blah blah blah"
(hack, hack, hack)
bzr commit -m "blah blah blah"

When the feature is cooked, refresh your trunk mirror and merge your change:

cd ../beagle-trunk
bzr update
bzr merge ../beagle-feature1
bzr commit -m "Complete comment for SVN commit"

As your trunk mirror is a checkout, committing to it implicitly commits to the real Subversion trunk. That’s it!

Using a central repository mirror

For large projects, it often makes sense to tweak the recipe given above. In particular, the initial checkout can get quite slow so you may wish to import the Subversion repository into a Bazaar one once and for all for your project, and then branch from that native Bazaar repository instead. bzr-svn provides the svn-import command for doing this repository-to-repository conversion. Here’s an example of how to use it:

bzr svn-import svn+ssh://svn.gnome.org/svn/beagle

Here’s the recipe from above updated to use a central Bazaar mirror:

bzr init-repo --default-rich-root beagle-repo
cd beagle-repo
bzr branch bzr+ssh://bzr.gnome.org/beagle.bzr/trunk beagle-trunk
bzr branch beagle-trunk beagle-feature1
cd beagle-feature1
(hack, hack, hack)
bzr commit -m "blah blah blah"
(hack, hack, hack)
bzr commit -m "blah blah blah"
cd ../beagle-trunk
bzr pull
bzr merge ../beagle-feature1
bzr commit -m "Complete comment for SVN commit"
bzr push

In this case, committing to the trunk only commits the merge locally. To commit back to the master Subversion trunk, an additional command (bzr push) is required.

Note: You’ll need to give pull and push the relevant URLs the first time you use those commands in the trunk branch. After that, bzr remembers them.

The final piece of the puzzle in this setup is to put scripts in place to keep the central Bazaar mirror synchronized with the Subversion one. This can be done by adding a cron job, using a Subversion hook, or whatever makes sense in your environment.

Limitations of bzr-svn

Bazaar and Subversion are different tools with different capabilities so there will always be some limited interoperability issues. Here are some examples current as of bzr-svn 0.5.4:

  • Bazaar doesn’t support versioned properties
  • Bazaar doesn’t support tracking of file copies.

See the bzr-svn web page, http://bazaar-vcs.org/BzrForeignBranches/Subversion, for the current list of constraints.

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