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I don't disagree with allowing that flexibility, but letting Redis instances do dual duty would greatly simplify things for some really common types of deployments. It's awfully nice with Mongo to just run three members and get a pretty decent failover feature for "free", versus having to worry about maintaining an entirely different set of processes.

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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...

I think that flexibility is absolutely needed for reliable failure detection.

Oh I agree with you, I was really just asking for the ability to run dual mode daemons. Companies who host these things for people might appreciate the option. :)

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