Document databases may be the most popular NoSQL database variant of them all. Their great flexibility -- schemas can be grown or changed with remarkable ease -- makes them suitable for a wide range of applications, and their object nature fits in well with current programming practices. In turn, Couchbase Server and MongoDB have become two of the more popular representatives of open source document databases, though Couchbase Server is a recent arrival among the ranks of document databases.
In this context, the word "document" does not mean a word processing file or a PDF. Rather, a document is a data structure defined as a collection of named fields. JSON (JavaScript Object Notation) is currently the most widely used notation for defining documents within document-oriented databases. JSON's advantage as an object notation is that, once you comprehend its syntax -- and JSON is remarkably easy to grasp -- then you have all you need to define what amounts to the schema of a document database. That's because, in a document database, each document carries its own schema -- unlike an RDBMS, in which every row in a given table must have the same columns.
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The latest versions of Couchbase Server and MongoDB are both newly arrived. In December 2012, Couchbase released Couchbase Server 2.0, a version that makes Couchbase Server a full-fledged document database. Prior to that release, users could store JSON data into Couchbase, but the database wrote JSON data as a blob. Couchbase was, effectively, a key/value database.
10gen released MongoDB 2.4 just this week. MongoDB has been a document database from the get-go. This latest release incorporates numerous performance and usability enhancements.
Both databases are designed to run on commodity hardware, as well as for horizontal scaling via sharding (in Couchbase, the rough equivalent to a shard is called a partition). Both databases employ JSON as the document definition notation, though in MongoDB, the notation is BSON (Binary JSON), a binary-encoded superset of JSON that defines useful data types not found in JSON. While both databases employ JavaScript as the primary data manipulation language, both provide APIs for all the most popular programming languages to allow applications direct access to database operations.
Key differences
Of course there are differences. First, MongoDB's handling of documents is better developed. This becomes most obvious in the mongo shell, which serves the dual purpose of providing a management and development window into a MongoDB database. Database, collections, and documents are first-class entities in the shell. Collections are actually properties on database objects.
This is not to say that Couchbase is hobbled. You can easily manage your Couchbase cluster -- adding, deleting, and fetching documents -- from the Couchbase Management GUI, for which MongoDB has no counterpart. Indeed, if you prefer management via GUI consoles, score one for Couchbase Server. If, however, you favor life at the command line, you will be tipped in MongoDB's direction.
The cloud-based MongoDB Monitoring Service (MMS), which gathers statistics, is not meant to be a full-blown database management interface. But MongoDB's environment provides a near seamless connection between the data objects abstracted in the mongo shell and the database entities they model. This is particularly apparent when you discover that MongoDB allows you to create indexes on a specific document field using a single function call, whereas indexes in Couchbase must be created by more complex mapreduce operations.
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