Merging changes

Parallel development

Once someone has their own branch of a project, they can make and commit changes in parallel to any development proceeding on the original branch. Pretty soon though, these independent lines of development will need to be combined again. This process is known as merging.

The merge command

To incorporate changes from another branch, use the merge command. Its syntax is:

bzr merge [URL]

If no URL is given, a default is used, initially the branch this branch originated from. For example, if Bill made a branch from Mary’s work, he can merge her subsequent changes by simply typing this:

bzr merge

On the other hand, Mary might want to merge into her branch the work Bill has done in his. In this case, she needs to explicitly give the URL the first time, e.g.:

bzr merge sftp://mary@bill-laptop/cool-repo/cool-trunk

This sets the default merge branch if one is not already set. To change the default after it is set, use the --remember option.

How does merging work?

A variety of algorithms exist for merging changes. Bazaar’s default algorithm is a variation of 3-way merging which works as follows. Given an ancestor A and two branches B and C, the following table provides the rules used.

A B C Result Comment
x x x x unchanged
x x y y line from C
x y x y line from B
x y z ? conflict

Note that some merges can only be completed with the assistance of a human. Details on how to resolve these are given in Resolving conflicts.

Recording a merge

After any conflicts are resolved, the merge needs to be committed. For example:

bzr commit -m "Merged Mary's changes"

Even if there are no conflicts, an explicit commit is still required. Unlike some other tools, this is considered a feature in Bazaar. A clean merge is not necessarily a good merge so making the commit a separate explicit step allows you to run your test suite first to verify all is good. If problems are found, you should correct them before committing the merge or throw the merge away using revert.

Merge tracking

One of the most important features of Bazaar is distributed, high quality merge tracking. In other words, Bazaar remembers what has been merged already and uses that information to intelligently choose the best ancestor for a merge, minimizing the number and size of conflicts.

If you are a refugee from many other VCS tools, it can be really hard to “unlearn” the please-let-me-avoid-merging-at-any-cost habit. Bazaar lets you safely merge as often as you like with other people. By working in a peer-to-peer manner when it makes sense to do so, you also avoid using a central branch as an “integration swamp”, keeping its quality higher. When the change you’re collaborating on is truly ready for wider sharing, that’s the time to merge and commit it to a central branch, not before.

Merging that Just Works truly can change how developers work together.

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