Mongo's sort of a weird halfway point between "totally horizontal" stores like Riak or Cassandra, and "Hope you have a ton of RAM" stores like your traditional

Mongo's sort of a weird halfway point between "totally horizontal" stores like Riak or Cassandra, and "Hope you have a ton of RAM" stores like your traditional RDBMS setup. I think it's hard to look at it and say that it wasn't designed with horizontal scaling in mind, but it's totally fair to say that it wasn't designed purely for horizontal scaling, either.