At the time of writing, mongos cannot definitively say whether or not a given database exists. Consider the following sequence (credit to max.hirschhorn@mongodb.com ):

• Client sends insert through mongos1
• Mongos1 runs enableSharding and assigns database to shard_A
• Mongos1 runs insert command and creates collection and inserts the document
• Client sends find through mongos2 (drivers round-robin through mongos processes with similar latencies and so this would be done without explicit application knowledge)
• Mongos2 hasn't learned of the database exist and returns empty result set
• This violates read-your-writes

This ticket tracks the work to make mongos2 definitively know of the created database upon its creation.