<?xml version="1.0" encoding="UTF-8" standalone="no"?>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml"><head><meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /><title>Using Branches</title><link rel="stylesheet" href="styles.css" type="text/css" /><meta name="generator" content="DocBook XSL Stylesheets V1.73.2" /><link rel="start" href="index.html" title="Version Control with Subversion" /><link rel="up" href="svn.branchmerge.html" title="Chapter 4. Branching and Merging" /><link rel="prev" href="svn.branchmerge.whatis.html" title="What's a Branch?" /><link rel="next" href="svn.branchmerge.basicmerging.html" title="Basic Merging" /></head><body><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="3" align="center">Using Branches</th></tr><tr><td width="20%" align="left"><a accesskey="p" href="svn.branchmerge.whatis.html">Prev</a> </td><th width="60%" align="center">Chapter 4. Branching and Merging</th><td width="20%" align="right"> <a accesskey="n" href="svn.branchmerge.basicmerging.html">Next</a></td></tr></table><hr /></div><div class="sect1" lang="en" xml:lang="en"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a id="svn.branchmerge.using"></a>Using Branches</h2></div></div></div><p>At this point, you should understand how each commit creates
an entire new filesystem tree (called a “<span class="quote">revision</span>”)
in the repository. If not, go back and read about revisions in
<a class="xref" href="svn.basic.in-action.html#svn.basic.in-action.revs" title="Revisions">the section called “Revisions”</a>.</p><p>For this chapter, we'll go back to the same example from
<a class="xref" href="svn.basic.html" title="Chapter 1. Fundamental Concepts">Chapter 1, <i>Fundamental Concepts</i></a>. Remember that you and your
collaborator, Sally, are sharing a repository that contains two
projects, <code class="filename">paint</code> and
<code class="filename">calc</code>. Notice that in <a class="xref" href="svn.branchmerge.using.html#svn.branchmerge.using.dia-1" title="Figure 4.2. Starting repository layout">Figure 4.2, “Starting repository layout”</a>, however, each project
directory now contains subdirectories named
<code class="filename">trunk</code> and <code class="filename">branches</code>.
The reason for this will soon become clear.</p><div class="figure"><a id="svn.branchmerge.using.dia-1"></a><p class="title"><b>Figure 4.2. Starting repository layout</b></p><div class="figure-contents"><div><img src="images/ch04dia2.png" alt="Starting repository layout" /></div></div></div><br class="figure-break" /><p>As before, assume that Sally and you both have working
copies of the “<span class="quote">calc</span>” project. Specifically, you
each have a working copy of <code class="filename">/calc/trunk</code>.
All the files for the project are in this subdirectory rather
than in <code class="filename">/calc</code> itself, because your team has
decided that <code class="filename">/calc/trunk</code> is where the
“<span class="quote">main line</span>” of development is going to take
place.</p><p>Let's say that you've been given the task of implementing a
large software feature. It will take a long time to write, and
will affect all the files in the project. The immediate problem
is that you don't want to interfere with Sally, who is in the
process of fixing small bugs here and there. She's depending on
the fact that the latest version of the project (in
<code class="filename">/calc/trunk</code>) is always usable. If you
start committing your changes bit-by-bit, you'll surely break
things for Sally (and other team members as well.)</p><p>One strategy is to crawl into a hole: you and Sally can stop
sharing information for a week or two. That is, start gutting
and reorganizing all the files in your working copy, but don't
commit or update until you're completely finished with the task.
There are a number of problems with this, though. First, it's
not very safe. Most people like to save their work to the
repository frequently, should something bad accidentally happen
to their working copy. Second, it's not very flexible. If you
do your work on different computers (perhaps you have a working
copy of <code class="filename">/calc/trunk</code> on two different
machines), you'll need to manually copy your changes back and
forth, or just do all the work on a single computer. By that
same token, it's difficult to share your changes-in-progress
with anyone else. A common software development “<span class="quote">best
practice</span>” is to allow your peers to review your work as
you go. If nobody sees your intermediate commits, you lose
potential feedback, and may end up going down the wrong path for
weeks before another person on your team notices. Finally, when
you're finished with all your changes, you might find it very
difficult to re-merge your final work with the rest of the
company's main body of code. Sally (or others) may have made
many other changes in the repository that are difficult to
incorporate into your working copy—especially if you
run <span class="command"><strong>svn update</strong></span> after weeks of
isolation.</p><p>The better solution is to create your own branch, or line of
development, in the repository. This allows you to save your
half-broken work frequently without interfering with others, yet
you can still selectively share information with your
collaborators. You'll see exactly how this works as we go.
</p><div class="sect2" lang="en" xml:lang="en"><div class="titlepage"><div><div><h3 class="title"><a id="svn.branchmerge.using.create"></a>Creating a Branch</h3></div></div></div><p>Creating a branch is very simple—you make a copy of
the project in the repository using the <span class="command"><strong>svn
copy</strong></span> command. Subversion is not only able to copy
single files, but whole directories as well. In this case,
you want to make a copy of the
<code class="filename">/calc/trunk</code> directory. Where should the
new copy live? Wherever you wish—it's a matter of
project policy. Let's say that your team has a policy of
creating branches in the <code class="filename">/calc/branches</code>
area of the repository, and you want to name your branch
<code class="literal">my-calc-branch</code>. You'll want to create a
new directory,
<code class="filename">/calc/branches/my-calc-branch</code>, which
begins its life as a copy of
<code class="filename">/calc/trunk</code>.</p><p>At this point, you might have seen <span class="command"><strong>svn
copy</strong></span> used to copy one file to another within a
working copy. But it can also be used to do
a “<span class="quote">remote</span>” copy entirely within the repository.
Just copy one URL to another:</p><pre class="screen">
$ svn copy http://svn.example.com/repos/calc/trunk \
http://svn.example.com/repos/calc/branches/my-calc-branch \
-m "Creating a private branch of /calc/trunk."
Committed revision 341.
</pre><p>This command causes a near-instantaneous commit in the
repository, creating a new directory in revision 341. The new
directory is a copy of <code class="filename">/calc/trunk</code>. This
is shown in
<a class="xref" href="svn.branchmerge.using.html#svn.branchmerge.using.create.dia-1" title="Figure 4.3. Repository with new copy">Figure 4.3, “Repository with new copy”</a>.
<sup>[<a id="id370827" href="#ftn.id370827" class="footnote">22</a>]</sup>
While it's also possible to create a branch by
using <span class="command"><strong>svn copy</strong></span> to duplicate a directory
within the working copy, this technique isn't recommended. It
can be quite slow, in fact! Copying a directory on the
client-side is a linear-time operation, in that it actually
has to duplicate every file and subdirectory on local disk.
Copying a directory on the server, however, is a constant-time
operation, and it's the way most people create
branches.</p><div class="figure"><a id="svn.branchmerge.using.create.dia-1"></a><p class="title"><b>Figure 4.3. Repository with new copy</b></p><div class="figure-contents"><div><img src="images/ch04dia3.png" alt="Repository with new copy" /></div></div></div><br class="figure-break" /><div class="sidebar"><p class="title"><b>Cheap Copies</b></p><p>Subversion's repository has a special design. When you
copy a directory, you don't need to worry about the
repository growing huge—Subversion doesn't actually
duplicate any data. Instead, it creates a new directory
entry that points to an <span class="emphasis"><em>existing</em></span> tree.
If you're an experienced Unix user, you'll recognize this as
the same concept behind a hard-link. As further changes are
made to files and directories beneath the copied directory,
Subversion continues to employ this hard-link concept where
it can. It only duplicates data when it is necessary to
disambiguate different versions of objects.</p><p>This is why you'll often hear Subversion users talk
about “<span class="quote">cheap copies</span>”. It doesn't matter how
large the directory is—it takes a very tiny, constant
amount of time and space to make a copy of it. In fact,
this feature is the basis of how commits work in Subversion:
each revision is a “<span class="quote">cheap copy</span>” of the previous
revision, with a few items lazily changed within. (To read
more about this, visit Subversion's website and read about
the “<span class="quote">bubble up</span>” method in Subversion's design
documents.)</p><p>Of course, these internal mechanics of copying and
sharing data are hidden from the user, who simply sees
copies of trees. The main point here is that copies are
cheap, both in time and space. If you create a branch
entirely within the repository (by running <span class="command"><strong>svn copy
URL1 URL2</strong></span>), it's a quick, constant-time operation.
Make branches as often as you want.</p></div></div><div class="sect2" lang="en" xml:lang="en"><div class="titlepage"><div><div><h3 class="title"><a id="svn.branchmerge.using.work"></a>Working with Your Branch</h3></div></div></div><p>Now that you've created a branch of the project, you can
check out a new working copy to start using it:</p><pre class="screen">
$ svn checkout http://svn.example.com/repos/calc/branches/my-calc-branch
A my-calc-branch/Makefile
A my-calc-branch/integer.c
A my-calc-branch/button.c
Checked out revision 341.
</pre><p>There's nothing special about this working copy; it simply
mirrors a different directory in the repository. When you
commit changes, however, Sally won't see them when she
updates, because her working copy is of
<code class="filename">/calc/trunk</code>. (Be sure to read <a class="xref" href="svn.branchmerge.switchwc.html" title="Traversing Branches">the section called “Traversing Branches”</a> later in this chapter: the
<span class="command"><strong>svn switch</strong></span> command is an alternate way of
creating a working copy of a branch.)</p><p>Let's pretend that a week goes by, and the following
commits happen:</p><div class="itemizedlist"><ul type="disc"><li><p>
You make a change to
<code class="filename">/calc/branches/my-calc-branch/button.c</code>,
which creates revision 342.</p></li><li><p>
You make a change to
<code class="filename">/calc/branches/my-calc-branch/integer.c</code>,
which creates revision 343.</p></li><li><p>
Sally makes a change to
<code class="filename">/calc/trunk/integer.c</code>, which creates
revision 344.</p></li></ul></div><p>There are now two independent lines of development, shown
in <a class="xref" href="svn.branchmerge.using.html#svn.branchmerge.using.work.dia-1" title="Figure 4.4. The branching of one file's history">Figure 4.4, “The branching of one file's history”</a>, happening on
<code class="filename">integer.c</code>.</p><div class="figure"><a id="svn.branchmerge.using.work.dia-1"></a><p class="title"><b>Figure 4.4. The branching of one file's history</b></p><div class="figure-contents"><div><img src="images/ch04dia4.png" alt="The branching of one file's history" /></div></div></div><br class="figure-break" /><p>Things get interesting when you look at the history of
changes made to your copy of
<code class="filename">integer.c</code>:</p><pre class="screen">
$ pwd
/home/user/my-calc-branch
$ svn log -v integer.c
------------------------------------------------------------------------
r343 | user | 2002-11-07 15:27:56 -0600 (Thu, 07 Nov 2002) | 2 lines
Changed paths:
M /calc/branches/my-calc-branch/integer.c
* integer.c: frozzled the wazjub.
------------------------------------------------------------------------
r341 | user | 2002-11-03 15:27:56 -0600 (Thu, 07 Nov 2002) | 2 lines
Changed paths:
A /calc/branches/my-calc-branch (from /calc/trunk:340)
Creating a private branch of /calc/trunk.
------------------------------------------------------------------------
r303 | sally | 2002-10-29 21:14:35 -0600 (Tue, 29 Oct 2002) | 2 lines
Changed paths:
M /calc/trunk/integer.c
* integer.c: changed a docstring.
------------------------------------------------------------------------
r98 | sally | 2002-02-22 15:35:29 -0600 (Fri, 22 Feb 2002) | 2 lines
Changed paths:
M /calc/trunk/integer.c
* integer.c: adding this file to the project.
------------------------------------------------------------------------
</pre><p>Notice that Subversion is tracing the history of your
branch's <code class="filename">integer.c</code> all the way back
through time, even traversing the point where it was copied.
It shows the creation of the branch as an event in the
history, because <code class="filename">integer.c</code> was implicitly
copied when all of <code class="filename">/calc/trunk/</code> was
copied. Now look what happens when Sally runs the same
command on her copy of the file:</p><pre class="screen">
$ pwd
/home/sally/calc
$ svn log -v integer.c
------------------------------------------------------------------------
r344 | sally | 2002-11-07 15:27:56 -0600 (Thu, 07 Nov 2002) | 2 lines
Changed paths:
M /calc/trunk/integer.c
* integer.c: fix a bunch of spelling errors.
------------------------------------------------------------------------
r303 | sally | 2002-10-29 21:14:35 -0600 (Tue, 29 Oct 2002) | 2 lines
Changed paths:
M /calc/trunk/integer.c
* integer.c: changed a docstring.
------------------------------------------------------------------------
r98 | sally | 2002-02-22 15:35:29 -0600 (Fri, 22 Feb 2002) | 2 lines
Changed paths:
M /calc/trunk/integer.c
* integer.c: adding this file to the project.
------------------------------------------------------------------------
</pre><p>Sally sees her own revision 344 change, but not the change
you made in revision 343. As far as Subversion is concerned,
these two commits affected different files in different
repository locations. However, Subversion
<span class="emphasis"><em>does</em></span> show that the two files share a
common history. Before the branch-copy was made in revision
341, they used to be the same file. That's why you and Sally
both see the changes made in revisions 303 and 98.</p></div><div class="sect2" lang="en" xml:lang="en"><div class="titlepage"><div><div><h3 class="title"><a id="svn.branchmerge.using.concepts"></a>The Key Concepts Behind Branching</h3></div></div></div><p>There are two important lessons that you should remember
from this section. First, Subversion has no internal concept
of a branch—it only knows how to make copies. When you
copy a directory, the resulting directory is only
a “<span class="quote">branch</span>” because <span class="emphasis"><em>you</em></span>
attach that meaning to it. You may think of the directory
differently, or treat it differently, but to Subversion it's
just an ordinary directory that happens to carry some extra
historical information.</p><p>Second, because of this copy mechanism, Subversion's
branches exist as <span class="emphasis"><em>normal filesystem
directories</em></span> in the repository. This is different
from other version control systems, where branches are
typically defined by adding
extra-dimensional “<span class="quote">labels</span>” to collections of
files. The location of your branch directory doesn't matter
to Subversion. Most teams follow a convention of putting all
branches into a <code class="filename">/branches</code> directory, but
you're free to invent any policy you wish.</p></div><div class="footnotes"><br /><hr width="100" align="left" /><div class="footnote"><p><sup>[<a id="ftn.id370827" href="#id370827" class="para">22</a>] </sup>Subversion does not support copying between different
repositories. When using URLs with <span class="command"><strong>svn
copy</strong></span> or <span class="command"><strong>svn move</strong></span>, you can only
copy items within the same repository.</p></div></div></div><div class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="svn.branchmerge.whatis.html">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="svn.branchmerge.html">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="svn.branchmerge.basicmerging.html">Next</a></td></tr><tr><td width="40%" align="left" valign="top">What's a Branch? </td><td width="20%" align="center"><a accesskey="h" href="index.html">Home</a></td><td width="40%" align="right" valign="top"> Basic Merging</td></tr></table></div></body></html>
