Branching, tagging, and merging are concepts common to
almost all version control systems. If you're not familiar with
these ideas, we provide a good introduction in this chapter. If
you are familiar, then hopefully you'll find it interesting to
see how Subversion implements these ideas.
Branching is a fundamental part of version control. If
you're going to allow Subversion to manage your data, then this
is a feature you'll eventually come to depend on. This chapter
assumes that you're already familiar with Subversion's basic
concepts (
Chapter 2, Basic Concepts
).
Suppose it's your job to maintain a document for a division
in your company, a handbook of some sort. One day a different
division asks you for the same handbook, but with a few parts
“tweaked” for them, since they do things slightly
differently.
What do you do in this situation? You do the obvious thing:
you make a second copy of your document, and begin maintaining
the two copies separately. As each department asks you to make
small changes, you incorporate them into one copy or the
other.
You often want to make the same change to both copies. For
example, if you discover a typo in the first copy, it's very
likely that the same typo exists in the second copy. The two
documents are almost the same, after all; they only differ in
small, specific ways.
This is the basic concept of a
branch—namely, a line of
development that exists independently of another line, yet still
shares a common history if you look far enough back in time. A
branch always begins life as a copy of something, and moves on
from there, generating its own history (see
Figure 4.1, “Branches of development”).
Subversion has commands to help you maintain parallel
branches of your files and directories. It allows you to create
branches by copying your data, and remembers that the copies are
related to one another. It also helps you duplicate changes
from one branch to another. Finally, it can make portions of
your working copy reflect different branches, so that you can
“mix and match” different lines of development in
your daily work.
