Bazaar (continued)

I got a few responses to the comparison between CVS, Subversion and Bazaar command line interfaces I posted earlier from Elijah, Mikael and David. As I stated in that post, I was looking at areas where the three systems could be compared. Of course, most people would choose Arch because of the things it can do with it that Subversion and CVS can't. Below I'll discuss two of those things: disconnected development and distributed development. I'll follow on from the examples in the previous post.

Disconnected Development

Disconnected development allows you to continue working on some code while not having access to the main repository. I hinted at how to do this in the previous post, but left out most of the details. The basic steps are:

  1. Create an archive on your machine
  2. Branch the module you want to work on into your local archive.
  3. Perform your development as normal
  4. When you connect again, switch back to the mainline, merge your local branch and commit the changes.

To create the local archive, you follow the same procedure as for creating the original archive. Something like this:

mkdir ~/archives
baz make-archive --signed joe@example.com ~/archives/joe@example.com

This creates an archive named joe@example.com (archive names are required to be an email address, optionally followed by some extra info) stored in the user's home directory.

Now we can create a branch in the local archive. From a working copy of the mainline branch, run the following command:

baz branch joe@example.com/modulename--devel--0

It was necessary to specify an archive name in this call to baz branch, because the branch was being created in a different archive to the one the working copy was pointing at. This leaves the working copy pointing at the new branch, so you can start working on it immediately.

You can commit as many revisions as you want, and compare to other revisions on the branch.

When you have access to the main repository again, it is trivial to merge your changes back into the mainline:

baz switch arch@example.org/modulename--devel--0
baz merge joe@example.com/modulename--devel--0
fix conflicts, if any exist, and mark them resolved
baz commit -s 'merge changes from joe@example.com/modulename--devel--0'

You can then ignore the branch in the joe@example.com archive, or continue to use it. If you want to continue working on the branch in that module, it is a simple matter to merge from the arch@example.org archive first to pick up the changes made while you were disconnected.

Distributed Development

In a distributed development environment, there is no main branch. Instead, each developer maintains their own branch, and pulls changes from other developers' archives. A few things fall out from this model:

  • To start working on a distributed project, you need to branch off from another developer's archive. This can be achieved using the same instructions as found in the "disconnected development" section above.
  • In order for other developers to pull changes from your archive, they will need to be able to access it. This isn't possible if it only exists in your home directory.
  • If you never merge from a particular developer, you don't even need to know they exist.
  • Conversely, you don't need to ask for permission to work on a module (however, if you want your changes to appear in the other developers' archives, you'll need to ask them to merge from you).

So assuming you've branched off an existing developer's branch of a module, and want other developers to merge your changes. Assuming they can't access your local computer, it will be necessary to create a mirror of the archive. To make the archive most widely available, you should mirror it to a directory that is published by a web server. The following command will create a mirror of the local archive:

baz make-archive --signed --listing --mirror joe@example.com \
        sftp://hostname/home/joe/public_html/joe@example.com

Once the archive is created, you can mirror all the changes in the local archive to the remote one using the following command:

baz archive-mirror joe@example.com

If you always have access to the mirror host, it is possible to set up a hook script that mirrors after every commit. However, if you often make changes while offline you might decide to mirror manually.

Now that the archive has been mirrored, other developers can merge your changes into their working copy using the following command:

baz merge http://hostname/~joe/joe@example.com/modulename--devel--0

(after they've used your archive once, they can use the short name for the archive, and it will use the same location as last time).

Conclusion

While Arch allows full distributed development, most projects don't use it in a fully distributed manner. Often there will be a central archive that is the "official" one, which tarball releases are made from. The exact policies can differ from project to project. Some possible policies are:

  • A core of developers have commit access to an "official" archive, which releases are made from. Developers generally commit directly to this archive (this is the default CVS/Subversion model). External developers follow the distributed development model, and get core developers to merge their changes.
  • As above, but the core developers usually develop their changes on separate branches (usually in their own archives), and only merge them when ready. This is how some projects currently use CVS, but has the benefit of allowing disconnected development.
  • Control of the official archive is managed by arch-pqm. Authorized developers can send merge requests to PQM (using PGP for authentication). When a merge request is received, the branch is merged into the mainline. If there are no conflicts and the test suite runs successfully, the changes are committed.

I'm not sure which model would work best for Gnome. At least initially, one of the first two models would probably be a good choice. If you have good test coverage, PQM can help ensure that the mainline stays buildable, and changes don't destabilise things.

As has been mentioned elsewhere, regularly updated mirrors of various CVS repositories are being set up at arch.ubuntu.com. You can find out whether a mirror has been created for a module by looking it up on Launchpad. If a branch exists, you can check it out or branch it by prepending "http://arch.ubuntu.com/" to the full branch name (e.g. http://arch.ubuntu.com/‌imendio@projects.ubuntu.com/‌gossip--MAIN--0).