Grokking Bazaar

While Bazaar is similar to other VCS tools in many ways, there are some important differences that are not necessarily obvious at first glance. This section attempts to explain some of the things users need to know in order to “grok” Bazaar, i.e. to deeply understand it.

Note: It isn’t necessary to fully understand this section to use Bazaar. You may wish to skim this section now and come back to it at a later time.

Understanding revision numbers

All revisions in the mainline of a branch have a simple increasing integer. (First commit gets 1, 10th commit gets 10, etc.) This makes them fairly natural to use when you want to say “grab the 10th revision from my branch”, or “fixed in revision 3050”.

For revisions which have been merged into a branch, a dotted notation is used (e.g., 3112.1.5). Dotted revision numbers have three numbers [1]. The first number indicates what mainline revision change is derived from. The second number is the branch counter. There can be many branches derived from the same revision, so they all get a unique number. The third number is the number of revisions since the branch started. For example, 3112.1.5 is the first branch from revision 3112, the fifth revision on that branch.

Hierarchical history is good

Imagine a project with multiple developers contributing changes where many changes consist of a series of commits. To give a concrete example, consider the case where:

• The tip of the project’s trunk is revision 100.
• Mary makes 3 changes to deliver feature X.
• Bill makes 4 changes to deliver feature Y.

If the developers are working in parallel and using a traditional centralized VCS approach, the project history will most likely be linear with Mary’s changes and Bill’s changes interleaved. It might look like this:

107: Add documentation for Y
106: Fix bug found in testing Y
105: Fix bug found in testing X
104: Add code for Y
103: Add documentation for X
102: Add code and tests for X
101: Add tests for Y
100: ...

Many teams use this approach because their tools make branching and merging difficult. As a consequence, developers update from and commit to the trunk frequently, minimizing integration pain by spreading it over every commit. If you wish, you can use Bazaar exactly like this. Bazaar does offer other ways though that you ought to consider.

An alternative approach encouraged by distributed VCS tools is to create feature branches and to integrate those when they are ready. In this case, Mary’s feature branch would look like this:

103: Fix bug found in testing X
102: Add documentation for X
101: Add code and tests for X
100: ...

And Bill’s would look like this:

104: Add documentation for Y
103: Fix bug found in testing Y
102: Add code for Y
101: Add tests for Y
100: ...

If the features were independent and you wanted to keep linear history, the changes could be pushed back into the trunk in batches. (Technically, there are several ways of doing that but that’s beyond the scope of this discussion.) The resulting history might look like this:

107: Fix bug found in testing X
106: Add documentation for X
105: Add code and tests for X
104: Add documentation for Y
103: Fix bug found in testing Y
102: Add code for Y
101: Add tests for Y
100: ...

While this takes a bit more effort to achieve, it has some advantages over having revisions randomly intermixed. Better still though, branches can be merged together forming a non-linear history. The result might look like this:

102: Merge feature X
     100.2.3: Fix bug found in testing X
     100.2.2: Add documentation for X
     100.2.1: Add code and tests for X
101: Merge feature Y
     100.1.4: Add documentation for Y
     100.1.3: Fix bug found in testing Y
     100.1.2: Add code for Y
     100.1.1: Add tests for Y
100: ...

Or more likely this:

102: Merge feature X
     100.2.3: Fix bug
     100.2.2: Add documentation
     100.2.1: Add code and tests
101: Merge feature Y
     100.1.4: Add documentation
     100.1.3: Fix bug found in testing
     100.1.2: Add code
     100.1.1: Add tests
100: ...

This is considered good for many reasons:

• It makes it easier to understand the history of a project. Related changes are clustered together and clearly partitioned.
• You can easily collapse history to see just the commits on the mainline of a branch. When viewing the trunk history like this, you only see high level commits (instead of a large number of commits uninteresting at this level).
• If required, it makes backing out a feature much easier.
• Continuous integration tools can be used to ensure that all tests still pass before committing a merge to the mainline. (In many cases, it isn’t appropriate to trigger CI tools after every single commit as some tests will fail during development. In fact, adding the tests first - TDD style - will guarantee it!)

In summary, the important points are:

Organize your work using branches.

Integrate changes using merge.

Ordered revision numbers and hierarchy make history easier to follow.

Each branch has its own view of history

As explained above, Bazaar makes the distinction between:

• mainline revisions, i.e. ones you committed in your branch, and
• merged revisions, i.e. ones added as ancestors by committing a merge.

Each branch effectively has its own view of history, i.e. different branches can give the same revision a different “local” revision number. Mainline revisions always get allocated single number revision numbers while merged revisions always get allocated dotted revision numbers.

To extend the example above, here’s what the revision history of Mary’s branch would look like had she decided to merge the project trunk into her branch after completing her changes:

104: Merge mainline
     100.2.1: Merge feature Y
     100.1.4: Add documentation
     100.1.3: Fix bug found in testing
     100.1.2: Add code
     100.1.1: Add tests
103: Fix bug found in testing X
102: Add documentation for X
101: Add code and tests for X
100: ...

Once again, it’s easy for Mary to look at just her top level of history to see the steps she has taken to develop this change. In this context, merging the trunk (and resolving any conflicts caused by doing that) is just one step as far as the history of this branch is concerned.

It’s important to remember that Bazaar is not changing history here, nor is it changing the global revision identifiers. You can always use the latter if you really want to. In fact, you can use the branch specific revision numbers when communicating as long as you provide the branch URL as context. (In many Bazaar projects, developers imply the central trunk branch if they exchange a revision number without a branch URL.)

Merges do not change revision numbers in a branch, though they do allocate local revision numbers to newly merged revisions. The only time Bazaar will change revision numbers in a branch is when you explicitly ask it to mirror another branch.

Note: Revisions are numbered in a stable way: if two branches have the same revision in their mainline, all revisions in the ancestry of that revision will have the same revision numbers. For example, if Alice and Bob’s branches agree on revision 10, they will agree on all revisions before that.

Summary

In general, if you follow the advice given earlier - organise your work in branches and use merge to collaborate - you’ll find Bazaar generally does what you expect.

In coming chapters, we examine various ways of using Bazaar beginning with the simplest: using Bazaar for personal projects.