Versions & Branches

One key reason ClearCase is an exceptional CM system is because of its efficient support for branching. What does that mean?

A branch is a named collection of sequential versions of a file. A given file may have many different branches, each with its own sequence of versions.

A branch name is first defined and then files and directories may begin to accumulate versions on that branch. Not all files in the ClearCase directory structure will have versions on each defined branch. We say a file "has a branchname branch" when there are versions "on branch branchname". You can think of a branch as a sparse copy of the entire ClearCase directory structure.

A simple branching structure for one file is depicted below. It shows that there is a central branch called /main and two side branches called /bugfixes and /release2.

The branching structure of a file looks like a hierarchical filesystem, where branches are directories and versions are files.

NT Note: Branch names use backslashes on NT. For example: \main, \bugfixes, and \release2.

/main

In fact, /main is a very special branch called the trunk. Like an oak tree, the ClearCase branches all sprout from the trunk, or from other branches. The /main branch is always defined for every file, but other branches may or may not be defined for any given file.

Automatic Branching

Traditional Version Control systems like RCS, SCCS and PVCS primarily operate on their own notion of a trunk. They also support branching from that trunk, but they do not support branches off of branches. Even though they support branches, it is up to the engineer to manually branch files and check files in on the correct branch. Any mistake has terrible consequences. In ClearCase, all branching is done automatically, so there is no opportunity for an engineer to make a mistake.

Version Identifiers

Until now we have referred to versions abstractly, without actually naming them. In ClearCase, a version identifier is a list of branch names followed by a numeric version number which applies to the last branch in the list, all separated by forward slashes. It looks suspiciously like a unix-style pathname. The version number could instead be a label.

Here are some examples of version identifiers. UNIX versions are in black, NT versions are in brown:

/main/4 \main\4	The fourth version on the trunk.
/main/release2/1 \main\release2\1	The first version on the release2 branch, which sprouts from the trunk.
/main/bugfixes/LATEST \main\bugfixes\LATEST	The latest (most recently checked-in version) on the bugfixes branch, which sprouts from the trunk.

The examples above show pairs of UNIX and NT versions which refer to the same versions. Versions created on UNIX appear with backslashes on NT, and versions created on NT appear with forward slashes on UNIX.

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