"Microsoft Azure DocumentDB databases can now be used as the data store for apps written for MongoDB. Using existing drivers for MongoDB and this preview functionality, applications written for MongoDB can easily and transparently communicate with DocumentDB. Developers can use their current tools and skill set for MongoDB to build applications that leverage DocumentDB's fully managed and scalable NoSQL databases."
This is the same thing that Amazon did with MySQL: adjusting existing open source interface to connect to high-performance "cloud" based service instead of standard MySQL server engine. Clever.
Amazon Aurora – New Cost-Effective MySQL-Compatible Database Engine for Amazon RDS | AWS Blog
Amazon Aurora – MySQL Compatible Relational Database
AWS | Amazon Relational Database Service (RDS)