svn-best-practices.html [plain text]
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<h1>Subversion Best Practices</h1>
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<p>This is a quick set of guidelines for making the best use of
Subversion in your day-to-day software development work.</p>
<h2>Use a sane repository layout</h2>
<p>There are many ways to lay out your repository. Because branches
and tags are ordinary directories, you'll need to account for them in
your repository structure.</p>
<p>The Subversion project officially recommends the idea of a "project
root", which represents an anchoring point for a project. A "project
root" contains exactly three subdirectories: <tt>/trunk</tt>,
<tt>/branches</tt>, and <tt>/tags</tt>. A repository may contain
only one project root, or it may contain a number of them.</p>
<p><em>Book reference:</em> <a
href="http://svnbook.red-bean.com/svnbook/ch05s04.html#svn-ch-5-sect-6.1">Choosing
a Repository Layout</a>.</p>
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<h2>Commit logical changesets</h2>
<p>When you commit a change to the repository, make sure your change
reflects a single purpose: the fixing of a specific bug, the addition
of a new feature, or some particular task. Your commit will create a
new revision number which can forever be used as a "name" for the
change. You can mention this revision number in bug databases, or use
it as an argument to <tt>svn merge</tt> should you want to undo the
change or port it to another branch.</p>
<p><em>Book reference:</em> "Subversion and Changesets" sidebar,
within <a
href="http://svnbook.red-bean.com/svnbook/ch04s03.html">chapter
4</a>.</p>
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<h2>Use the issue-tracker wisely</h2>
<p>Try to create as many two-way links between Subversion changesets
and your issue-tracking database as possible:</p>
<ul>
<li>If possible, refer to a specific issue ID in every commit log message.</li>
<li>When appending information to an issue (to describe progress, or
to close the issue) name the revision number(s) responsible
for the change.</li>
</ul>
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<h2>Track merges manually</h2>
<p>When committing the result of a merge, be sure to write a
descriptive log message that explains what was merged, something
like:</p>
<pre>Merged revisions 3490:4120 of /branches/foobranch to /trunk.</pre>
<p><em>Book reference:</em> <a
href="http://svnbook.red-bean.com/svnbook/ch04s03.html#svn-ch-4-sect-3.2">Tracking
merges manually</a>, and <a
href="http://svnbook.red-bean.com/svnbook/ch04s04.html#svn-ch-4-sect-4.1">Merging a whole branch to another</a>.</p>
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<h2>Understand mixed-revision working copies</h2>
<p>Your working copy's directories and files can be at different
"working" revisions: this is a deliberate feature which allows you to
mix and match older versions of things with newer ones. But there are
few facts you must be aware of:</p>
<ol>
<li>After every <tt>svn commit</tt>, your working copy has mixed
revisions. The things you just committed are now at the HEAD
revision, and everything else is at an older revision.</li>
<li>Certain commits are disallowed:
<ul>
<li>You cannot commit the deletion of a file or directory which
doesn't have a working revision of HEAD.</li>
<li>You cannot commit a property change to a directory which
doesn't have a working revision of HEAD.</li>
</ul>
</li>
<li><tt>svn update</tt> will bring your entire working copy to one
working revision, and is the typical solution to the
problems mentioned in point #2.</li>
</ol>
<p><em>Book reference:</em> <a
href="http://svnbook.red-bean.com/svnbook/ch02s03.html#svn-ch-2-sect-3.4">The
limitation of mixed revisions</a>.</p>
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<h2>Be patient with large files</h2>
<p>A nice feature of Subversion is that by design, there is no limit
to the size of files it can handle. Files are sent "streamily" in
both directions between Subversion client and server, using a small,
constant amount of memory on each side of the network.</p>
<p>Of course, there are a number of practical issues to consider.
While there's no need to worry about files in the kilobyte-sized range
(e.g. typical source-code files), committing larger files can take a
tremendous amount of both time and space (e.g. files that are dozens
or hundreds of megabytes large.)</p>
<p>To begin with, remember that your Subversion working copy stores
pristine copies of all version-controlled files in the
<tt>.svn/text-base/</tt> area. This means that your working copy
takes up at least twice as much disk space as the original dataset.
Beyond that, the Subversion client follows a (currently unadjustable)
algorithm for committing files:</p>
<ul>
<li>Copies the file to <tt>.svn/tmp/</tt> <em>(can take a while,
and temporarily uses extra disk space)</em>)</li>
<li>Performs a binary diff between the tmpfile and the pristine
copy, or between the tmpfile and an empty-file if newly
added. <em>(can take a very long time to compute, even
though only a small amount of data might ultimately be sent
over the network)</em></li>
<li>Sends the diff to the server, then moves the tmpfile into
<tt>.svn/text-base/</tt></li>
</ul>
<p>So while there's no theoretical limit to the size of your files,
you'll need to be aware that very large files may require quite a bit
of patient waiting while your client chugs away. You can rest
assured, however, that unlike CVS, your large files won't incapacitate
the server or affect other users.</p>
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<h2>Work around commands that don't understand copies/renames</h2>
<p>When a file or directory is copied or renamed, the Subversion repository
tracks that history. Unfortunately in Subversion 1.0, the only client
subcommand which actually takes advantage of this feature is <tt>svn
log</tt>. A number of other commands (such as <tt>svn diff</tt> and
<tt>svn cat</tt>) ought to be automatically following rename-history,
but aren't doing so yet.</p>
<p>In all of these cases, a basic workaround is to use <tt>'svn log
-v'</tt> to discover the proper path within the older revision.</p>
<p>For example, suppose you copied <tt>/trunk</tt> to
<tt>/branches/mybranch</tt> in revision 200, and then committed some
changes to <tt>/branches/mybranch/foo.c</tt> in subsequent revisions.
Now you'd like to compare revisions 80 and 250 of the file.</p>
<p>If you have a working copy of the branch and run <tt>svn diff
-r80:250 foo.c</tt>, you'll see an error about
<tt>/branches/mybranch/foo.c</tt> not existing in revision 80. To
remedy, you would run <tt>svn log -v</tt> on your branch or file to
discover that it was named <tt>/trunk/foo.c</tt> prior to revision 200,
and then compare the two URLs directly:</p>
<pre>
$ svn diff http://.../trunk/foo.c@80 \
http://.../branches/mybranch/foo.c@200
</pre>
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<h2>Know when to create branches</h2>
<p>This is a hotly debated question, and it really depends on the
culture of your software project. Rather than prescribe a universal
policy, we'll describe three common ones here.</p>
<h3>The Never-Branch system</h3>
<p>(Often used by nascent projects that don't yet have runnable code.)</p>
<ul>
<li>Users commit their day-to-day work on <tt>/trunk</tt>.</li>
<li>Occasionally <tt>/trunk</tt> "breaks" (doesn't compile, or fails
functional tests) when a user begins to commit a series of complicated
changes.</li>
</ul>
<p><em>Pros:</em> Very easy policy to follow. New developers have low
barrier to entry. Nobody needs to learn how to branch or merge.</p>
<p><em>Cons:</em> Chaotic development, code could be unstable at any
time.</p>
<p>A side note: this sort of development is a bit less risky in
Subversion than in CVS. Because Subversion commits are atomic, it's
not possible for a checkout or update to receive a "partial" commit
while somebody else is in the process of committing.</p>
<h3>The Always-Branch system</h3>
<p>(Often used by projects that favor heavy management and supervision.)</p>
<ul>
<li>Each user creates/works on a private branch for <em>every</em> coding task.
</li>
<li>When coding is complete, someone (original coder, peer, or
manager) reviews all private branch changes and merges them to
<tt>/trunk</tt>.</li>
</ul>
<p><em>Pros:</em> <tt>/trunk</tt> is guaranteed to be
<em>extremely</em> stable at all times. </p>
<p><em>Cons:</em> Coders are artificially isolated from each other,
possibly creating more merge conflicts than necessary.
Requires users to do lots of extra merging.</p>
<h3>The Branch-When-Needed system</h3>
<p>(This is the system used by the Subversion project.)
<ul>
<li>Users commit their day-to-day work on <tt>/trunk</tt>.</li>
<li>Rule #1: <tt>/trunk</tt> must compile and pass regression tests at
all times. Committers who violate this rule are publically
humiliated.</li>
<li>Rule #2: a single commit (changeset) must not be so large
so as to discourage peer-review.</li>
<li>Rule #3: if rules #1 and #2 come into conflict (i.e. it's
impossible to make a series of small commits without disrupting the
trunk), then the user should create a branch and commit a series of
smaller changesets there. This allows peer-review without disrupting
the stability of <tt>/trunk</tt>.</li>
</ul>
<p><em>Pros:</em> <tt>/trunk</tt> is guaranteed to be stable at all
times. The hassle of branching/merging is somewhat rare.</p>
<p><em>Cons:</em> Adds a bit of burden to users' daily work:
they must compile and test before every commit.</p>
<!--
Mapping CVS tasks to SVN tasks
==============================
This section is just a re-indexing of topics covered in the book,
for people who prefer to learn from the "bottom up" rather than "top down".
It shows some common CVS operations, and then the equivalent SVN operation,
followed by a link to the book which explains more.
* Importing data.
* Checking out a working copy.
* Seeing your changes.
* Undoing your changes.
* Resolving a conflict.
* Adding binary files.
* Activating keyword expansion and/or EOL translation.
TAGS:
* Creating a tag from a working copy
* Creating a remote tag
* Seeing all of a project's tags
* Comparing two tags
* Seeing the logs between two tags
* Tweaking a tag
BRANCHES:
* Creating a branch and switching to it
* Finding the beginning of a branch
* Merging a branch to trunk, or vice versa
-->
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