notes/webdav-proxy

Purpose: A master/slave server replication model for Subversion WebDAV-based
transactions.

All clients interact with a slave server, but the slave transparently
passes all of the write-oriented activites to the master (rewriting the
content as necessary).  The slaves are essentially read-only, but they
do have a complete copy of the repository locally.  This serves to
alleviate read traffic from the master server which may be desirable
in certain circumstances.

This model has several advantages to using a straight HTTP DAV-aware
caching proxy, in that each slave can respond to all read-only requests
without ever having to relay them to the master backend.

Requirements: Subversion 1.5 or newer (merged to trunk as of r22606)
              Apache HTTP Server 2.2.0 or higher with mod_proxy enabled
              (Several fixes to mod_proxy were committed for this patch that
               were not backported to the 2.0 branch of httpd.)

Example configuration:

Participants:
  Slave  -> slave.example.com  (there can be many!)
  Master -> master.example.com (there can only be one!)
  A WebDAV client (ra_neon, ra_serf, other WebDAV clients)

Each client does:

 % svn co http://slave.example.com/repos/slave/
 ...
 % svn ci
 % ...etc...

 (The client can perform all operations as normal.)

Each slave has:

<Location /repos/slave>
  DAV svn
  SVNPath /my/local/copy/of/repos
  SVNMasterURI http://master.example.com/repos/master
</Location>

The master:

The master MUST have a post-commit hook that updates all of the slaves.  An
example that does this using 'svnadmin dump'/'svnadmin load' and ssh is
provided below.  svnsync can probably do the same thing, but is left as an
exercise to the reader.

----
#!/bin/sh
REPOS="$1"
REV="$2"
SLAVE_HOST=slave.example.com
SLAVE_PATH=/my/local/copy/of/repos

# Ensure svnadmin is in your PATH on both this machine and the remote server!
svnadmin dump --incremental -r$2 $1 > /tmp/$2.dump
scp /tmp/$2.dump $SLAVE_HOST:$SLAVE_PATH
ssh $SLAVE_HOST "svnadmin load $SLAVE_PATH < $SLAVE_PATH/$2.dump"
ssh $SLAVE_HOST "rm $SLAVE_PATH/$2.dump"
rm /tmp/$2.dump
----

Issues/Thoughts:
- The master maybe should update the slaves using a DAV commit of its own.
  (essentially replay the commit once it is approved).  This requires
  a way to inject commits/user to the slave.  But, this would eliminate the
  reliance on post-commit hooks.
- This isn't multi-master replication.  The slave won't accept commits
  on its own.  If the master can't be contacted for a write operation, it
  will return a proxy error.  (Multi-master == distributed repositories.)
- Remove the location_filter for the header.  I believe mod_proxy does this
  for us already.  We may just be duplicating things.  We will still have
  to rewrite the bodies of the requests/responses though.
- Determine a better way to handle the MERGE call.  It's the only operation
  that doesn't occur on the activity URL.