CouchDBs

CouchDBs is not a standard term in database literature; it usually refers to multiple Apache CouchDB deployments or instances in a distributed system. This article concentrates on Apache CouchDB, the open-source document-oriented database that underpins such deployments. Apache CouchDB stores data as JSON documents and uses a schema-free model, making it easy to evolve data structures over time. Documents can contain attachments and are accessed via a simple HTTP REST API. The database uses Multi-Version Concurrency Control to allow concurrent writes and to minimize locking.

CouchDB's queries are defined by map-reduce views stored in design documents, typically authored in JavaScript. In recent years a declarative query layer called Mango provides a more familiar query interface for ad hoc searches. The core storage engine is append-only, with periodic compaction to reclaim space.

A key feature is built-in replication and synchronization. CouchDB supports asynchronous, incremental replication between peers, including continuous replication for ongoing syncing. This peer-to-peer replication enables offline work and multi-master topologies, though conflicts can arise and must be resolved by application logic or via built-in conflict handling.

Clustering and scaling: CouchDB 2.x introduced clustering that allows horizontal scaling through data sharding and coordinated replication across nodes, while maintaining a single logical database name per cluster. The ecosystem includes tools such as Cloudant (now part of IBM) and community projects that extend clustering, monitoring, and management. PouchDB, a browser-side database, can mirror data to and from CouchDB instances, enabling offline-capable web and mobile apps.

History and status: CouchDB was originally created by Damien Katz and is maintained as an Apache Software Foundation project under the Apache License 2.0. It remains in active development with a focus on reliability, replication, and ease of use.