CouchDB

CouchDB is an open-source NoSQL document-oriented database that stores data as JSON documents, provides a RESTful HTTP API, and uses JavaScript for MapReduce-based indexing. It emphasizes a schemaless data model, multi-version concurrency control, and eventual consistency, with built-in replication to synchronize data between databases across servers or devices.

Data is organized into databases containing documents, each with a unique _id and a _rev revision tag. Documents can include attachments such as files. Indexes are created as views defined by MapReduce functions (typically in JavaScript) and are used to query the documents efficiently.

CouchDB offers bi-directional replication and synchronization, enabling master-master setups and offline operation. Replication is performed over HTTP and can handle conflicts, which are resolved by the application or via built-in merge policies.

The system runs on the Erlang virtual machine and uses an append-only, multi-version storage model to support MVCC and reliable replication. The REST API provides access to databases, documents, and views, enabling straightforward integration with web and mobile applications.

CouchDB originated with Damien Katz in the mid-2000s and was later donated to the Apache Software Foundation, where it is maintained as an open-source project under the Apache License 2.0. It remains a common option for applications requiring flexible data models, offline capabilities, and distributed synchronization.

See also: Apache CouchDB, NoSQL, MapReduce.