Thursday, June 19, 2008

This Week in Bazaar

This is the seventh in a 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, who is sentimental today.

MySQL Switches to Bazaar

Very big news for the Bazaar team today, as MySQL announces switching from Bitkeeper to Bazaar.

One of the things that was important in doing this conversion was doing a very high quality import of all the existing history. John did a great job working on that, and even added a new feature to Bazaar and bzr-gtk to enable this: per-file commit messages. Since per-file commit messages had been used for years in the MySQL code base, it was not acceptable to lose them, and none of the DVCS systems under consideration supported these messages. Although this feature is debated by some, it was important to preserve that history, and so support for per-file commit messages was added to Bazaar in a non-invasive way, where projects who wanted to use them could, but existing projects were not forced to adopt them. At the moment, to enter per-file commit messages you need to use the bzr-gtk GUI commit tool, but we'd love it if someone came up with a clean way to enable this in the standard CLI also.

It was also important to have a smooth transition period that did not interrupt delivering MySQL releases. This meant we needed a stable importer where the imports could be periodically refreshed without causing all of the developers around the world working on the project to re-download all their trees. At one point we were doing continuous imports of over 30 trees.

It's been a fun and challenging project providing support to MySQL during this time. Although we're really excited about this milestone, we still have plenty of work to do. Here are a few things we've learned, where we are working to make Bazaar even better.

Stacked Branches. We've talked previously about stacked branches, and for a project like MySQL this new feature will make uploading a new branch to Launchpad much faster.

Merging - Bazaar has several good merge algorithms, but we still have some ideas to make merging go even smoother, particularly for some of the complicated ancestries that MySQL has. All merge algorithms have their own set of trade offs, edge cases that they handle better or worse than other algorithms.

We also need to continue to add GUI tools, and make further enhancements to existing tools. If you are looking for a valuable way to contribute to Bazaar, try lending a hand to one of the Bazaar GUI projects.

Last week we asked about bzr screencasts, and James Westby told me about a screencast that he recorded - if anyone else is interested in getting involved in producing a series of screencasts, please do let us know.


bialix said...

Is there a technical document with explanation how per-file-commit messages are stored by bzr-gtk and used later? How usual bzr log shows them? It's unclear from your post what kind of support exists in bzr core for them.

Anonymous said...

Also, is there documentation/code for the migration available, so other BK users can migrate? :)