[kenn]

As to MongoDB, in case you only have two members in a replica set, you add an arbiter, not strictly three members.

An arbiter is indeed a lightweight mongod process that doesn't have storage, and should be added to break the tie when you have even number of members. I like the MongoDB's design, and it is probably optimal for 99% of use cases.

There are three key concepts: Split-brain, Quorum, and Fencing. I find this article fascinating.

http://techthoughts.typepad.com/managing_computers/2007/10/s...