Thursday, May 22, 2008

This Week in Bazaar

This is the third in an amazingly regular weekly series of posts about current topics in the development world of the Bazaar distributed version control system. The series is co-authored by John Arbash Meinel, one of the primary developers on Bazaar, and Elliot Murphy, Launchpad developer and relentless agitator. This week we have a special guest, Jelmer Vernooij, Samba developer, and author of the Bazaar Subversion plugin.

In last week's episode, our fearless explorers braved the new world of plugins. Today we will focus on a specific plugin, and talk about how you can use Bazaar with Subversion. Earlier this week there was a very nice blog post about using Git with the Subversion servers on Google Code Hosting, and plenty of interesting discussion afterwards.


If you have Bazaar installed, why would you want to work with Subversion? Well, it's nice not to have to force the whole world to change at once. Bazaar-Subversion integration allows you to use Bazaar without any changes required from the project administrators to the central Subversion server.

There are three general cases, where you would want to use bzr-svn:
  1. Upstream uses Subversion, and you don't yet have commit access. With bzr-svn, you are able to still make your improvements with all the benefits of a great VCS.
  2. Project has chosen to use Subversion, you want something better, but still want to play nice with your fellow developers. You can commit to your local Bazaar branch, and push those changes back into Subversion. You can even do "bzr commit" in your Subversion checkout and have it commit those changes to the Subversion server.
  3. Migration from Subversion to Bazaar. Often when migrating from once VCS to another, there is a period of time where people are adjusting to the new system. bzr-svn allows you to continue allowing people to commit to Subversion, it's just another branch with changes to be merged.

Currently the bzr-svn dependencies can be a bit tricky to install on some platforms, but that should be much easier once Subversion 1.5 is released. Once you get things installed, it's pretty amazing what you can do. On most debian based systems, it is a simple "apt-get install bzr-svn" away.

Once you have bzr-svn installed, you can start using Subversion branches as though they were regular Bazaar branches.

General usage

Now that you have bzr-svn installed, how do you get a local copy of your Subversion project? Generally, it is just a "bzr checkout URL" away.

$ bzr checkout svn+

This will create a local checkout of your project that contains a local copy of the history present remotely.

You should now be able to use this branch like any regular Bazaar branch. Since this is a bound branch, any commits you make will also be show up in the Subversion repository.

It is possible to create new local branches from this branch, for example for feature branches::

$ bzr branch trunk feature-1

And to merge the branch back into Subversion once it is finished, you can use merge like you would with any ordinary Bazaar branch

$ bzr merge ../feature-1
$ bzr ci -m "Merge feature-1"

In addition to the code changes, bzr-svn will write metadata about the history of the new commit into Subversion. This means that your merge history is available, so when someone else comes along and grabs a copy of the branch using Bazaar, they can see what happened. To a normal Subversion client this is transparent, the custom properties are simply ignored.

It is also possible to push directly from the feature branch into Subversion::

$ bzr push http://subversion/project

This will preserve all of the history from the branch you are pushing - there is no need to rebase your local branch after pushing.

Since bzr-svn allows access to Subversion protocols and file formats using the standard Bazaar API, it is possible to use most standard Bazaar commands directly on Subversion formats and URLs. Commands like "bzr missing", "bzr log", or even "bzr viz" work out of the box.


Some bits and pieces to pique your interest in bzr-svn.
  • Subversion 1.5 introduces custom revision properties - this should allow bzr-svn to hide the properties used to store merge information. (At the moment, the file properties used show up in commit emails.)
  • Bazaar will soon be introducing shallow (stacked) branches. This will allow you to have a fully functioning local branch (including offline commits, etc), without needing to download the complete history to your local machine.
  • Bzr for GNOME developers is a quick guide for people who want to use Bazaar for developing with the Subversion Gnome repository.
  • Bazaar branches of Python are available. They are currently using bzr-svn to mirror the Subversion branches, allowing their developers to see what life is like developing with Bazaar.
For more information, check out the bzr-svn home page, FAQ, bug tracker, or join us on the Bazaar mailing list.

Next week: how to print money with Bazaar.

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