The Old Monk

CouchDB is a very cool document store which uses JavaScript map/reduce views. It is schema free and the docs are JSON. It also has an HTTP API which makes it possible to write purely JavaScript apps with persistence via CouchDB.

Couch has a concept of DB update notifications. A process is launched (and relaunched if it crashes, via an Erlang/OTP supervisor) and is notified of any updates (adds, deletes and changes) to the DBs. That's nice, but not sufficiently useful in all cases.

So I added a "document update notification" mechanism on the lines of db update notifications, which not only tells which document changed, but also what changed in the document. This makes it possible to write various kinds of "triggers" (or even "stored procedures" if you squint just right). The document update notification process gets the type of change (add/delete/update), the db name, the doc id and the bodies of the old and the new doc on each update. It can then use these to determine if something interesting changed and decide whether to take action or not.

The patch was offered upstream (annoucement) but wouldn't land up in the trunk because a feature superseding this functionality is already planned (comet notifications). However, I will maintain the patches out-of-tree till the superseding functionality lands in the trunk. The patches are fairly simple and non intrusive, and are available for both 0.8.1 and 0.9.

DocUpdateNotifications patch for CouchDB 0.9

DocUpdateNotifications patch for CouchDB 0.8.1

These patches were developed as part of my work for BaseCase. BaseCase thinks open source is cool and has gladly agreed to the release of these patches.