When browsing a Subversion repository, the web browser gets a clue about how to render a file’s contents by looking at the Content-Type: header returned in Apache’s response to the HTTP GET request. The value of this header is some sort of MIME type. By default, Apache will tell the web browsers that all repository files are of the “default” MIME type, typically text/plain. This can be frustrating, however, if a user wishes repository files to render as something more meaningful—for example, it might be nice to have a foo.html file in the repository actually render as HTML when browsing.

To make this happen, you need only make sure that your files have the proper svn:mime-type set. We discuss this in more detail in File Content Type, and you can even configure your client to automatically attach proper svn:mime-type properties to files entering the repository for the first time; see Automatic Property Setting.

So, in our example, if one were to set the svn:mime-type property to text/html on file foo.html, Apache would properly tell your web browser to render the file as HTML. One could also attach proper image/* MIME-type properties to image files and ultimately get an entire web site to be viewable directly from a repository! There’s generally no problem with this, as long as the web site doesn’t contain any dynamically generated content.

You generally will get more use out of URLs to versioned files—after all, that’s where the interesting content tends to lie. But you might have occasion to browse a Subversion directory listing, where you’ll quickly note that the generated HTML used to display that listing is very basic, and certainly not intended to be aesthetically pleasing (or even interesting). To enable customization of these directory displays, Subversion provides an XML index feature. A single SVNIndexXSLT directive in your repository’s Location block of httpd.conf will instruct mod_dav_svn to generate XML output when displaying a directory listing, and to reference the XSLT stylesheet of your choice:

<Location /svn>
  DAV svn
  SVNParentPath /var/svn
  SVNIndexXSLT "/svnindex.xsl"
  ...
</Location>

Using the SVNIndexXSLT directive and a creative XSLT stylesheet, you can make your directory listings match the color schemes and imagery used in other parts of your web site. Or, if you’d prefer, you can use the sample stylesheets provided in the Subversion source distribution’s tools/xslt/ directory. Keep in mind that the path provided to the SVNIndexXSLT directory is actually a URL path—browsers need to be able to read your stylesheets to make use of them!

If you’re serving a collection of repositories from a single URL via the SVNParentPath directive, then it’s also possible to have Apache display all available repositories to a web browser. Just activate the SVNListParentPath directive:

<Location /svn>
  DAV svn
  SVNParentPath /var/svn
  SVNListParentPath on
  ...
</Location>

If a user now points her web browser to the URL http://host.example.com/svn/, she’ll see a list of all Subversion repositories sitting in /var/svn. Obviously, this can be a security problem, so this feature is turned off by default.

First, configure your master server’s httpd.conf file in the usual way. Make the repository available at a certain URI location, and configure authentication and authorization however you’d like. After that’s done, configure each of your “slave” servers in the exact same way, but add the special SVNMasterURI directive to the block:

<Location /svn>
  DAV svn
  SVNPath /var/svn/repos
  SVNMasterURI http://master.example.com/svn
  ...
</Location>

This new directive tells a slave server to redirect all write requests to the master. (This is done automatically via Apache’s mod_proxy module.) Ordinary read requests, however, are still serviced by the slaves. Be sure that your master and slave servers all have matching authentication and authorization configurations; if they fall out of sync, it can lead to big headaches.

Next, we need to deal with the problem of infinite recursion. With the current configuration, imagine what will happen when a Subversion client performs a commit to the master server. After the commit completes, the server uses svnsync to replicate the new revision to each slave. But because svnsync appears to be just another Subversion client performing a commit, the slave will immediately attempt to proxy the incoming write request back to the master! Hilarity ensues.

The solution to this problem is to have the master push revisions to a different Location on the slaves. This location is configured to not proxy write requests at all, but to accept normal commits from (and only from) the master’s IP address:

<Location /svn-proxy-sync>
  DAV svn
  SVNPath /var/svn/repos
  Order deny,allow
  Deny from all
  # Only let the server's IP address access this Location:
  Allow from 10.20.30.40
  ...
</Location>

Now that you’ve configured your Location blocks on master and slaves, you need to configure the master to replicate to the slaves. This is done the usual way—using svnsync. If you’re not familiar with this tool, see Repository Replication for details.

First, make sure that each slave repository has a pre-revprop-change hook script that allows remote revision property changes. (This is standard procedure for being on the receiving end of svnsync.) Then, log into the master server and configure each of the slave repository URIs to receive data from the master repository on the local disk:

$ svnsync init http://slave1.example.com/svn-proxy-sync file://var/svn/repos
Copied properties for revision 0.
$ svnsync init http://slave2.example.com/svn-proxy-sync file://var/svn/repos
Copied properties for revision 0.
$ svnsync init http://slave3.example.com/svn-proxy-sync file://var/svn/repos
Copied properties for revision 0.

# Perform the initial replication

$ svnsync sync http://slave1.example.com/svn-proxy-sync
Transmitting file data ....
Committed revision 1.
Copied properties for revision 1.
Transmitting file data .......
Committed revision 2.
Copied properties for revision 2.
...

$ svnsync sync http://slave2.example.com/svn-proxy-sync
Transmitting file data ....
Committed revision 1.
Copied properties for revision 1.
Transmitting file data .......
Committed revision 2.
Copied properties for revision 2.
...

$ svnsync sync http://slave3.example.com/svn-proxy-sync
Transmitting file data ....
Committed revision 1.
Copied properties for revision 1.
Transmitting file data .......
Committed revision 2.
Copied properties for revision 2.
...

After this is done, configure the master server’s post-commit hook script to invoke svnsync on each slave server:

#!/bin/sh
# Post-commit script to replicate newly committed revision to slaves

svnsync sync http://slave1.example.com/svn-proxy-sync > /dev/null 2>&1
svnsync sync http://slave2.example.com/svn-proxy-sync > /dev/null 2>&1
svnsync sync http://slave3.example.com/svn-proxy-sync > /dev/null 2>&1

The extra bits on the end of each line aren’t necessary, but they’re a sneaky way to allow the sync commands to run in the background so that the Subversion client isn’t left waiting forever for the commit to finish. In addition to this post-commit hook, you’ll need a post-revprop-change hook as well so that when a user, say, modifies a log message, the slave servers get that change also:

#!/bin/sh
# Post-revprop-change script to replicate revprop-changes to slaves

REV=${2}
svnsync copy-revprops http://slave1.example.com/svn-proxy-sync ${REV} > /dev/null 2>&1
svnsync copy-revprops http://slave2.example.com/svn-proxy-sync ${REV} > /dev/null 2>&1
svnsync copy-revprops http://slave3.example.com/svn-proxy-sync ${REV} > /dev/null 2>&1

The only thing we’ve left out here is what to do about locks. Because locks are strictly enforced by the master server (the only place where commits happen), we don’t technically need to do anything. Many teams don’t use Subversion’s locking features at all, so it may be a nonissue for you. However, if lock changes aren’t replicated from master to slaves, it means that clients won’t be able to query the status of locks (e.g., svn status -u will show no information about repository locks). If this bothers you, you can write post-lock and post-unlock hook scripts that run svn lock and svn unlock on each slave machine, presumably through a remote shell method such as SSH. That’s left as an exercise for the reader!

Your master/slave replication system should now be ready to use. However, a couple of words of warning are in order. Remember that this replication isn’t entirely robust in the face of computer or network crashes. For example, if one of the automated svnsync commands fails to complete for some reason, the slaves will begin to fall behind. For example, your remote users will see that they’ve committed revision 100, but then when they run svn update, their local server will tell them that revision 100 doesn’t yet exist! Of course, the problem will be automatically fixed the next time another commit happens and the subsequent svnsync is successful—the sync will replicate all waiting revisions. But still, you may want to set up some sort of out-of-band monitoring to notice synchronization failures and force svnsync to run when things go wrong.