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<div class="sect1" title="Common Branching Patterns">
<div class="titlepage">
<div>
<div>
<h2 class="title" style="clear: both"><a id="svn.branchmerge.commonpatterns"></a>Common Branching Patterns</h2>
</div>
</div>
</div>
<p>There are many different uses for branching and <span class="command"><strong>svn
merge</strong></span>, and this section describes the most
common.</p>
<p>Version control is most often used for software
development, so here's a quick peek at two of the most common
branching/merging patterns used by teams of programmers. If
you're not using Subversion for software development, feel
free to skip this section. If you're a software developer
using version control for the first time, pay close attention,
as these patterns are often considered best practices by
experienced folk. These processes aren't specific to
Subversion; they're applicable to any version control system.
Still, it may help to see them described in Subversion
terms.</p>
<div class="sect2" title="Release Branches">
<div class="titlepage">
<div>
<div>
<h3 class="title"><a id="svn.branchmerge.commonpatterns.release"></a>Release Branches</h3>
</div>
</div>
</div>
<p>Most software has a typical life cycle: code, test,
release, repeat. There are two problems with this process.
First, developers need to keep writing new features while
quality assurance teams take time to test supposedly stable
versions of the software. New work cannot halt while the
software is tested. Second, the team almost always needs to
support older, released versions of software; if a bug is
discovered in the latest code, it most likely exists in
released versions as well, and customers will want to get
that bug fix without having to wait for a major new
release.</p>
<p>Here's where version control can help. The typical
procedure looks like this:</p>
<div class="orderedlist">
<ol class="orderedlist" type="1">
<li class="listitem">
<p><span class="emphasis"><em>Developers commit all new work to the
trunk.</em></span> Day-to-day changes are committed to
<code class="filename">/trunk</code>: new features, bug fixes, and
so on.</p>
</li>
<li class="listitem">
<p><span class="emphasis"><em>The trunk is copied to a
<span class="quote">“<span class="quote">release</span>”</span> branch.</em></span> When the team
thinks the software is ready for release (say, a 1.0
release), <code class="filename">/trunk</code> might be copied to
<code class="filename">/branches/1.0</code>.</p>
</li>
<li class="listitem">
<p><span class="emphasis"><em>Teams continue to work in
parallel.</em></span> One team begins rigorous testing of
the release branch, while another team continues new work
(say, for version 2.0) on <code class="filename">/trunk</code>. If
bugs are discovered in either location, fixes are cherrypicked
back and forth as necessary. At some point, however, even
that process stops. The branch is <span class="quote">“<span class="quote">frozen</span>”</span>
for final testing right before a release.</p>
</li>
<li class="listitem">
<p><span class="emphasis"><em>The branch is tagged and
released.</em></span> When testing is complete,
<code class="filename">/branches/1.0</code> is copied to
<code class="filename">/tags/1.0.0</code> as a reference
snapshot. The tag is packaged and released to
customers.</p>
</li>
<li class="listitem">
<p><span class="emphasis"><em>The branch is maintained over
time.</em></span> While work continues
on <code class="filename">/trunk</code> for version 2.0, bug fixes
continue to be ported from <code class="filename">/trunk</code> to
<code class="filename">/branches/1.0</code>. When enough
bug fixes have accumulated, management may decide to do a
1.0.1 release: <code class="filename">/branches/1.0</code> is
copied to <code class="filename">/tags/1.0.1</code>, and the tag
is packaged and released.</p>
</li>
</ol>
</div>
<p>This entire process repeats as the software matures:
when the 2.0 work is complete, a new 2.0 release branch is
created, tested, tagged, and eventually released. After
some years, the repository ends up with a number of release
branches in <span class="quote">“<span class="quote">maintenance</span>”</span> mode, and a number
of tags representing final shipped versions.</p>
</div>
<div class="sect2" title="Feature Branches">
<div class="titlepage">
<div>
<div>
<h3 class="title"><a id="svn.branchmerge.commonpatterns.feature"></a>Feature Branches</h3>
</div>
</div>
</div>
<p>
<a id="idp12941040" class="indexterm"></a>A <em class="firstterm">feature branch</em> is the sort of
branch that's been the dominant example in this chapter (the
one you've been working on while Sally continues to work on
<code class="filename">/trunk</code>). It's a temporary branch created
to work on a complex change without interfering with the
stability of <code class="filename">/trunk</code>. Unlike release
branches (which may need to be supported forever), feature
branches are born, used for a while, merged back to the trunk,
and then ultimately deleted. They have a finite span of
usefulness.</p>
<p>Again, project policies vary widely concerning exactly
when it's appropriate to create a feature branch. Some
projects never use feature branches at all: commits to
<code class="filename">/trunk</code> are a free-for-all. The
advantage to this system is that it's simple—nobody
needs to learn about branching or merging. The disadvantage
is that the trunk code is often unstable or unusable. Other
projects use branches to an extreme: no change is
<span class="emphasis"><em>ever</em></span> committed to the trunk directly.
Even the most trivial changes are created on a short-lived
branch, carefully reviewed, and merged to the trunk. Then
the branch is deleted. This system guarantees an
exceptionally stable and usable trunk at all times, but at
the cost of tremendous process overhead.</p>
<p>Most projects take a middle-of-the-road approach. They
commonly insist that <code class="filename">/trunk</code> compile and
pass regression tests at all times. A feature branch is
required only when a change requires a large number of
destabilizing commits. A good rule of thumb is to ask this
question: if the developer worked for days in isolation and
then committed the large change all at once (so that
<code class="filename">/trunk</code> were never destabilized), would it
be too large a change to review? If the answer to that
question is <span class="quote">“<span class="quote">yes,</span>”</span> the change should be
developed on a feature branch. As the developer commits
incremental changes to the branch, they can be easily reviewed
by peers.</p>
<p>Finally, there's the issue of how to best keep a feature
branch in <span class="quote">“<span class="quote">sync</span>”</span> with the trunk as work
progresses. As we mentioned earlier, there's a great risk to
working on a branch for weeks or months; trunk changes may
continue to pour in, to the point where the two lines of
development differ so greatly that it may become a nightmare
trying to merge the branch back to the trunk.</p>
<p>This situation is best avoided by regularly running an
automatic merge from trunk to the branch. Make up a policy:
once a week, merge the last week's worth of trunk changes to
the branch.</p>
<p>When you are eventually ready to merge the
<span class="quote">“<span class="quote">synchronized</span>”</span> feature branch back to the trunk,
begin by doing a final automatic merge of the latest trunk
changes to the branch. When that's done, the latest versions
of branch and trunk are absolutely identical except for
your branch changes. You can then run an automatic reintegrate
merge from the branch back to the trunk:</p>
<div class="informalexample">
<pre class="screen">
$ cd trunk-working-copy
$ svn update
Updating '.':
At revision 1910.
$ svn merge ^/calc/branches/mybranch
--- Merging differences between repository URLs into '.':
U real.c
U integer.c
A newdirectory
A newdirectory/newfile
U .
…
</pre>
</div>
<p>Another way of thinking about this pattern is that your
weekly sync of trunk to branch is analogous to running
<span class="command"><strong>svn update</strong></span> in a working copy, while the
final merge step is analogous to running <span class="command"><strong>svn
commit</strong></span> from a working copy. After all, what else
<span class="emphasis"><em>is</em></span> a working copy but a very shallow
private branch? It's a branch that's capable of
storing only one change at a time.</p>
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