Six thousand years ago, the Sumerians invented writing for transaction processing - Gray & Reuter
By any measure, MongoDB is a popular document-oriented JSON database. In the last dozen years, it has grown from its humble beginnings of a single lock per database to a modern multi-document transaction with snapshot isolation. MongoDB University has trained a large number of developers to develop on the MongoDB database.
There are many JSON databases now. While it's easy to start with MongoDB to learn NoSQL and flexible JSON schema, many customers choose Couchbase for performance, scale, and SQL. As you progress in your database evaluation and evolution, you should learn about other JSON databases. We're working on an online training course for MongoDB experts to learn Couchbase easily. Until we publish that, you'll have to read this article. :)