Implicit Branching and Merging

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branches Scott Allen writes about a Branching and Merging primer (doc) written by Chris Birmele. It is a short but useful tool agnostic look at branching and merging in the abstract. This is a nice complement to my favorite tutorial on source control, Eric Sink’s Source Control HOWTO.

Another useful resource on branching strategies is Steve Harman’s guide to branching with CVS.

The primer takes a tool agnostic look at branching and points out several branching models. One thing to keep in mind is that not every model makes use of your source control tool’s branching feature. In particular, let’s take a closer look at the Branch-per-task model. This model is almost universally in use via what I call implicit branches, which are private and not shared among other team members.

Using a pessimistic locking source control system like Visual Source Safe (VSS), every time you check out a file (which grants you an exclusive lock on that file), you are implicitly making a branch as soon as you edit that file. However, this is not a branch that VSS recognizes. It is merely a branch by fact that the code on your system is not the same as the code in the repository. Also consider that other team members may be making changes to other files in the same codebase. Perhaps files that contain classes that the file you are working on are dependent. So when you check that file back in, you are performing an implicit merge.

This type of implicit branching pretty much maps to the primer’s Branch-per-Task model of branching. Optimistic locking source control systems such as CVS and Subversion make this implicit branching and merging a bit more explicit. Rather than checking out a file, you typically update your local desktop with the latest version from the repository and just start working on files. There is no need to exclusively lock files by checking them out which only gives you the illusion of safety.

When you are ready to commit your changes back into the system, you typically get latest again and merge in any changes that may have been committed by other team members into your local workspace. Finally, you commit your local changes (assuming everything builds) and resolve any automatic merge conflicts (which is may not be very likely since you just pulled all changes from the repository into your local workspace unless there is a lot of repository activity going on).

The point here is to recognize that the implicit branching model (branch-per-task) is almost certainly already in use in your day to day work. It is not necessary to employ your source control’s branching feature to employ this branching model, unless you need multiple developers working on that single task. In that case, you would create an explicit branch for that task so that it can be shared. However, keep in mind that when multiple developers work on an explicit branch, the branching and merging model for that individual branch will look like the implicit branch-per-task model as I described.