In Mongo, are unique indexes supported for sharded environments?

Question

We're exploring Mongo for a new website and we want to ensure no two users sign up with the same email address, so, a unique index on the email field is necessary.

Is this supported in a sharded Mongo environment? Mongo documentation states unique indexes are supported, but makes no mention of shards.

http://www.mongodb.org/display/DOCS/Indexes#Indexes-UniqueIndexes

Answer 1

From the documentation:

You can use the {unique: true} option to ensure that the underlying index enforces uniqueness so long as the unique index is a prefix of the shard key. (note: prior to version 2.0 this worked only if the collection is empty).

http://www.mongodb.org/display/DOCS/Configuring+Sharding

This makes intuitive sense, as mongodb has no way of checking uniqueness across shards, unless the shard key gives you such cross-shard uniqueness guarantees "for free".