When should you branch?

Question

When working with a SCM system, when should you branch?

Answer 1

It also depends on the SCM tool you are using. Modern SCMs (git, mercurial, etc.) make it increasingly easy to create and destroy branches whenever needed. This allows you to, for example, make one branch per bug that you are working on. Once you merge your results into the trunk, you discard the branch.

Other SCMs, for example subversion and CVS, have a much "heavier" branching paradigm. That means, a branch is considered appropriate only for something bigger than a twenty-something-line patch. There, branches are classically used to track entire development tracks, like a previous or future product version.

Answer 2

In general term, the main purpose of branching (a VCS - Version Control System - feature) is to achieve code isolation.

You have at least one branch, which can be enough for sequential development, and is used for many tasks being recording (committed) on that same unique branch.

But that model shows quickly its limit:

When you have a development effort (refactoring, evolution, bug-fixes, ...) and you realize you cannot safely make those changes in the same branch than your current development branch (because you would break API, or introduce code that would break everything), then you need a another branch.
(To isolate that new code for the legacy one, even though the two code sets will be merge later on)

So that is your answer right there:
You should branch whenever you cannot pursue and record two development efforts in one branch.
(without having an horribly complicated history to maintain).

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A branch can be useful even if you are the only one working on the source code, of if you are many.
But you should not make "one branch per developer":
the "isolation" purpose is made to isolate a development effort (a task which can be as general as "let's develop the next version of our software" or as specific as "let's fix bug 23"),
not to isolate a "resource".

(a branch called "VonC" means nothing to another developer: What if "VonC" leaves the project? What are you supposed to do with it?
a branch called "bugfix_212" can be interpreted in the context of a bug tracking system for instance, and any developer can use it with at least some idea about what he is supposed to do with it)

A branch is not a tag (SVN is a Revision System which tries to propose versioning features like branching and tagging through directories with cheap file copy: that does not mean a tag is a branch)

To define a branch means also defining a merge workflow: you need to know where to merge your branch when you are done with it.
For that, the chapter 7 of Practical Perforce (Laura WINGERD - O'Reilly) is a good introduction (VCS agnostic) to merge workflow between different kind of branches: " "How Software Evolves" (pdf)

It defines the term codeline (branch which records significant evolution steps of the code, either through tags at certain points, or through important merge back to the branch)

It introduce the mainline model (a central codeline to record releases), and describes various purposes for branching:

• Active development streams: an persistent codeline when sequential various developments take place
• tasks branches: short-lived branches for more specific task (bug-fix is a classic one, but you can also define a branch for a merge effort you know to be complex to complete: you can merge, commit and test in that task branch without introducing problem for the main current development branch)
• staging branch: for preparing a release, with some pre-production specific data or config files.
• Private branches, ad hoc branches, and sparse branches: for very small tasks, just to be able to commit some work in progress without waiting for formal completion or test review.
  That allows to "commit early, commit often".

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_{Other interesting concepts around VCS: Basics concepts
(about ClearCase originally, but also valid for any VCS)}

Answer 3

When you need to make changes, based on your current branch, not destined for the next release from that branch (and not before).

For example, we work on trunk usually. Around the time of release, someone's going to need to make a change that we don't want in the current release (it may be before release, at the moment it's usually after release). This is when we branch, to put the release on its own branch and continue development for the next release on trunk.

Answer 4

Leaving all the technicalities aside.....

Branch when you know its easier to merge back!

Keeping in mind that merging will always be effected with how the work is carried out in a project.

Once this achieved all the other tertiary issues will come in to play.

Answer 5

All the 21th century SCMs are telling you:

Branch for every task you've to work on, no matter whether this is a new feature, a bugfix, a test, whatever. This is called topic branch, and it changes the way you work with your SCM.

You get:

• Better isolation
• Better traceability -> you associate tasks with branches, not individual changesets, which makes you free to commit as many times as you want and doesn't impose a limit like "one checkin per task".
• Tasks are independent (normally starting from a stable baseline, so you only focus on your code, not on fixing bugs from your folks), and you can choose whether you want to integrate them at some point or later, but they're always under version control
• You can review code easily (from the version control, not pre-commit bullshit) before hitting the main line

Tools that can do it:

• Git
• Mercurial
• Plastic SCM

Tools that CAN'T do it:

• SVN
• CVS
• VSS
• TFS
• Perforce

Answer 6

I find the advice from Laura Wingerd & Christopher Seiwald at Perforce is really concise and useful:

* Branch only when necessary.
* Don't copy when you mean to branch.
* Branch on incompatible policy.
* Branch late.
* Branch, instead of freeze.

See http://www.perforce.com/sites/default/files/pdf/perforce-best-practices.pdf for a detailed explanation of each of them and other best practice.