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Well, I used redis to hold a few sets for each user. You can atomically add items to sets in redis. Mongodb doesn't support this functionality (AFAIK). MongoDB is great for dumping and retrieving lots of data and is more durable than redis. Haven't built a site using these tools before so has all been a learning experience.

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Take a look at the $push and $addToSet[1] update operators in mongodb. They are both for atomically adding one or more elements to an array in an object. On the other hand, MongoDB doesn't support server-side set union or intersection so if you are using those features redis would still probably be a good use there.

[1] http://www.mongodb.org/display/DOCS/Updating#Updating-%24add...

Redis now uses the same journaling for durability that Mongo does. What you get with Redis is complex atomic ops and transactions, but at the cost of not having sharding.

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