Realtime Analytics Using MongoDB, Python, Gevent, and ZeroMQ (slides and video)

They mentioned the "global javascript interpreter lock", which may be slightly inaccurate, but it's true that you can only run one m/r job per mongod instance at a time. It's relatively easy to get around that, though, since you can have replica sets and run jobs on the slave members. You can even have small replica set instances that only do m/r.

There are a couple of other problems, one is that Mongo writes map output into a temporary collection. There's also entirely too much BSON <-> Javascript converting going on. Mongo 2 addresses some of this with the JS Only flag, but it's limited to something like half a million inputs at a time: http://www.mongodb.org/display/DOCS/MapReduce#MapReduce-jsMo...

If I ran into intractable map reduce problems with MongoDB, I'd probably just jump to Riak or something. My guess is that they'll continue to improve things in Mongo land at roughly the same pace as I run into problems, though.

Unfortunately this prevented me from checking the content as it's just too annoying.

The video: http://blip.tv/rick-copeland/realtime-analytics-using-mongod...