Have you thought about how much extra overhead there is in storing the checkins using MongoDB's BSON structures? Considering how often you do this, you might co

Have you thought about how much extra overhead there is in storing the checkins using MongoDB's BSON structures? Considering how often you do this, you might consider packing a specialized version of a user's history (at least for this purpose -- sort of a denormalization) into a array of 64-bit placeIDs? This means a checkin would only occupy 8 bytes of space and there would be no overhead. 1 billion checkins = 8GB of data. You could even keep these in memcached and use a sort of write-through strategy. EDIT: I thought about this a bit more, and the checkins are probably time-sensitive, so another 32-bit timestamp (Foursquare Epoch) would need to be stored for each checkin.