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This posting follows on from my previous postings about Bazaar, but is a bit more advanced. In most cases you don’t need to worry about this, since the tools should just work. However if problems occur (or if you’re just curious about how things work), it can be useful to know a bit about what’s going on inside.

Changesets vs. Tree Snapshots

A lot of the tutorials for Arch list “changeset orientation” as one of its benefits over other systems such as Subversion, which were said to be based on “tree snapshots”. At first this puzzled me, since from my mathematical background the relationship between these two concepts seemed the same as the relationship between integrals and derivatives:

• A changeset is just the difference between two tree snapshots.
• The state of a tree at a particular point in just the result of taking the initial tree state (which might be an empty tree), and applying all changesets on the line of development made before that point.

The distinction isn’t clear cut in the existing tools either — Subversion uses changesets to store the data in the repository while providing a “tree snapshot” style view, and Bazaar generates tree snapshots in its revision library to increase performance of some operations.

So the distinction people talk about isn’t a simple matter of the repository storage format. Instead the difference is in the metadata stored along with the changes that describes the ancestry of the code.

Changesets and Branching

In the simple case of a single branch, you end up with a simple series of changesets. The tree for each revision is constructed by taking the last revision’s tree and applying the relevant changeset. Alternatively, you can say that the tree for patch-3 contains the changesets base-0, patch-1, patch-2 and patch-3.

Branching fits into this model pretty well. As with other systems, a particular revision can have multiple children. In the diagram below, the trees for both patch-2 from the original branch and patch-1 from the new branch “contain” base-0 and patch-1 from the original branch. Any apparent asymmetry is just in the naming and storage locations — both revisions are branches are just patches against the same parent revision.

So far, there’s no rocket science. Nothing that Subversion doesn’t represent. Pretty much every version control system under the sun tracks this kind of linear revision ancestry (as can be seen using svn log or similar). The differences really only become apparent when merges are taken into consideration.

Merges

Just as a particular revision can have multiple child revisions (a.k.a. branching), a tree can have multiple parent revisions when merges occur. When you merge two revisions, the result should contain all the changes that exist in the parent revisions.

In the above diagram, we want to merge the changes made on the second branch back into the original one. The usual way to merge changes goes something like this:

1. Identify the most recent common ancestor of the two revisions.
2. Take the difference between one of the revisions to merge and apply those changes to the other revision.

If the changes on the two branches are to different parts of the tree, this process can be done without any extra user intervention. If the two branches touch the same bits of code, the conflicts will have to be resolved manually.

It is important to pick the most recent common ancestor, otherwise the real changes in the two branches will get mixed in with changes common to the two branches, which can result in spurious merge conflicts.

In this particular case, it is obvious which common ancestor to use: patch-1 from the original branch. In Arch, the result of the merge is represented as a changeset on the original branch that contains the changes found on the second branch. In addition to the changes, it adds some metadata (known as patch logs) that records that patch-1 and patch-2 from the second branch have been merged in. This becomes important when performing future merges between the two branches.

Repeated Merges

While it was possible to pick the correct merge ancestor in the previous example using just the linear revision ancestry of the two branches, that isn’t true for subsequent merges between the two branches. Consider the following merge that results in patch-6 on the original branch:

Here the best merge ancestor to use is patch-2 on the second branch. However, without the record of the previous merge, the same ancestor as the previous merge would be chosen (which is what CVS will do by default with repeated merges).

While the above ancestor could be selected by just recording when you last merged with a particular branch, that is not sufficient when there are merges between more than two branches.

More Than Two Branches

Below is a fairly simple example involving three branches, where some changes have been merged from the third branch (yellow) into the original branch (cyan) and the second branch (magenta). Finally, there is a merge between the magenta and cyan branches.

For this last merge, there are a number of possible merge ancestors. If we ignore the yellow branch, the latest common ancestor is the initial branch point. This would result in merging the changes in patch-1, patch-2, patch-3 and patch-4 from the second branch into the patch-5 tree on the original branch. However, this is likely to result in a number of conflicts, since both branches contain changes merged from the yellow branch, which are going to overlap.

The better common ancestor ancestor to choose in this case is patch-2 on the yellow branch, which avoids the common changes.

Bazaar’s merge command will handle this kind of merge ancestry just fine (something that isn’t true for of the older tla star-merge algorithm).

Conclusion

This article doesn’t cover all aspects of branching and merging with Bazaar. One aspect I have completely ignored is the concept of “cherry picking”. This refers to applying a particular change to a tree, without the other changesets that exist on that branch. Cherry picking is mostly orthogonal to standard merging — in fact, one of the complications in merge ancestor selection is that it needs to ignore cherry picked patches.

Network effects also come into play here — if you make your code available as an Arch branch, then Bazaar is more useful to others since they can branch and merge with your archive (and the reverse holds too). The Ubuntu Arch imports certainly help here, but to get the full advantage of the advanced merge capabilities both sides need to be tracking history.

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).