[merlynn]

Michael from MongoDB here. Amazon, Microsoft, and Google are certainly big players in this field, and common wisdom is that they are best qualified to scale, but in this case, we have a clear counterexample. DocumentDB clusters are limited to a single region of AWS. MongoDB Atlas clusters can span the entire globe with low-latency reads and writes in multiple regions. In terms of pricing, DocumentDB is cheaper when you choose to go with 2 instances. AWS achieves this by shifting high availability to the storage layer which in turn means that any failover could require between 1 and 2 minutes to happen - this compares to seconds that this would take with MongoDB Atlas. If you go with the default DocumentDB configuration and use 3 instances you end up paying a premium between 20-35%. With this in mind, it really depends on your use case whether DocumentDB ends up being cheaper for you. In terms of reliability and the hosting service is global, Atlas currently outperforms DocumentDB.