Apache CouchDB Document Database

Apache CouchDB is an open-source NoSQL document-oriented database that uses JSON to store data, JavaScript for MapReduce queries, and HTTP as an API. Originally developed by Damien Katz at IBM in 2005 and donated to the Apache Software Foundation in 2008, CouchDB is designed for reliability, ease of use, and offline-first synchronization. Key features: document storage with JSON-based dynamic schemas allowing flexible data models without predefined table structures. ACID semantics via Multi-Version Concurrency Control (MVCC) providing eventual consistency with conflict detection and resolution for concurrent writes. Entire database accessible via standard HTTP REST API with JSON responses, enabling direct browser and mobile access. JavaScript-based MapReduce functions for creating secondary indexes and querying documents by arbitrary keys with incremental index updates. Mango queries provide declarative JSON-based query language with MongoDB-like syntax for finding and filtering documents. Bidirectional replication between instances enabling distributed databases and offline-first mobile apps via PouchDB. Automatic conflict detection during replication with application-level conflict resolution via revision tree. Store binary files as document attachments alongside JSON data. Continuous _changes feed for real-time notifications of database modifications enabling reactive applications. Role-based access control per database, validation functions for server-side data validation, and show and list functions for server-side document transformation. Sharding clustering for horizontal scalability across multiple nodes with automatic failover.