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Depending on what your goal is: Did you consider elasticsearch?

Reolication/clustering is real easy to set up.

Sharding is easy.

Scaling is easy.

I cant comment on query language vs mongo or rethink, since my experience with them is too limited.

For one thing, it isn't really a database. You can use it as one, but where elasticsearch shines is mostly write once and read often.

Correctness isn't the primary focus, so unless you are willing to lose some stuff, don't use elasticsearch as your primary database.

A couple more references regarding Elasticsearch as a primary datastore: