If you are thinking of them like they are interchangeable, you need to stop, use a SQL database and quit messing. Seriously, they are not at all the same catego

If you are thinking of them like they are interchangeable, you need to stop, use a SQL database and quit messing. Seriously, they are not at all the same category of thing. Mongo is an update-in-place structure store with powerful atomic operations. Mongo will not grow storage if you are just changing the value of existing fields. Couch is a compare-and-swap document snapshot store with multi-master replication. In replicated mode it's eventually-consistent with manual conflict resolution. Riak is an indexable, eventually consistent key-value store with tuneable tradeoffs for speed versus safety, and no central point of failure. Cassandra is a horizontally scalable, eventually consistent key-value column hybrid designed for huge data. And so on... they are not "databases", they are not something where you replace one with the other, they are specific tools for specific tasks, usually much more so than unsexy but adaptable SQL.