Apache CouchDB, commonly referred to as CouchDB, is an open source database that focuses on ease of use and on being "a database that completely embraces the web". It is a NoSQL database that uses JSON to store data, JavaScript as its query language using MapReduce and HTTP for an API. One of its distinguishing features is easy replication.

Unlike in a relational database, CouchDB does not store data and relationships in tables. Instead, each database is a collection of independent documents. Each document maintains its own data and self-contained schema. An application may access multiple databases, such as one stored on a user's mobile phone and another on a server. Document metadata contains revision information, making it possible to merge any differences that may have occurred while the databases were disconnected.

CouchDB implements a form of Multi-Version Concurrency Control (MVCC) in order to avoid the need to lock the database file during writes. Conflicts are left to the application to resolve. Resolving a conflict generally involves first merging data into one of the documents, then deleting the stale one.

Other features are ACID semantics with eventual consistency, MapReduce, incremental replication and fault-tolerance. Administration is supported with a built-in web application called Futon.

(from Wikipedia)

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