Submerged, the CollabNet Subversion blog, just posted a nice article describing the new merge-tracking features that will be included in Subversion 1.5, so I thought I’d post1 some more about one of the ‘new in 1.5’[not-yet-out] features (and there’s a lot to choose from!).
I’ve already mentioned some improvements we’ve made to FSFS (and
a little bit about svnadmin), but those are both
behind-the-scenes changes, so this time I thought I’d cover one of the new
Apache configuration directives for
mod_dav_svn — one that allows
administrators to control the location of the activities database.
mod_dav_svn provides a bridge between the WebDAV world — which works in
terms of ‘activities’ and ‘version resources’ — and the Subversion world —
which works with ‘transactions’ and ‘nodes’. One of the many differences
between these worlds is the way an activity (or transaction) is named: in
WebDAV, the client names the activity, while in Subversion, the server names
So one of the things
mod_dav_svn needs to track is the relationship
between WebDAV activity names and the corresponding Subversion transaction.
Before 1.5, the repository contained a file called
stored this mapping, implemented using APR-util’s simple database format.
One problem with this approach was that the precise format would be whatever
one of many supported dbm formats was chosen as the default when APR-util
was compiled. This made it hard to reason about the safety of storing a
Subversion repository on NFS, for example.
For 1.5, we switched to a simpler scheme where we create one file for each
activity (storing it under the directory
dav/activities.d/); each file
contains the name of the corresponding Subversion transaction. This is much
simpler, and shouldn’t cause any scaling problems — there are typically
only a small number of transactions open at any one time.
So, to finally get to the point, one of the new configuration directives is
SVNActivitiesDB. It allows the administrator to override the location
of this activities ‘database’.
This directive behaves differently depending on whether repositories are being
served with an
SVNParentPath directive. If the relevant section
of the configuration file looks like this:
/tmp/activities/ will be the directory used to store the files mapping
activities to transactions for the repository at
/home/svn/myrepo/. On the
other hand, if the configuration file looks like the following (where the
repository name is inferred from the client’s URL):
then the path supplied to
SVNActivitiesDB will be used to store the
activities databases for all repositories available through the parent path,
and so the database for the
myrepo repository will be at
Why would you want to move the activity database away from the main
repository? The main reason is in cases where the repository is stored on
a network filesystem, where you ideally want to be able to move all the
transient and non-shared file operations onto local storage. While the
activities database isn’t updated that much compared to the transaction
data itself, it’s a start (and we’d like to be able to move the transaction
data to local storage as well; we just haven’t done it yet
Of course, if you have more than one Apache server active at the same time, you’ll also need to ensure some form of session or host affinity so that the client returns to the same server that originally established the activity ↦ transaction mapping.
There are more new features in
mod_dav_svn than just this, but they’ll have
to wait until later.