Apache CouchDB

Document database with seamless multi-primary replication and an HTTP/JSON API, designed for reliability from single-node setups to large clusters. Uses JSON documents and JavaScript MapReduce for indexing and querying, with ACID guarantees and a crash-resistant append-only storage model.

Hosted under the Apache Software Foundation with contributions from IBM, DigitalOcean, and Neighbourhoodie. Replication is the main differentiator: the Couch Replication Protocol enables bi-directional sync between server clusters, mobile devices, and web browsers, supporting offline-first architectures. PouchDB implements the same protocol for browsers and mobile, so data can flow end-to-end without custom sync logic.

Key capabilities:

• Multi-primary replication across server clusters, mobile, and browsers via the Couch Replication Protocol
• HTTP/JSON API compatible with any client that supports standard web protocols
• Single-node or clustered deployment with high availability
• MapReduce views and indexing defined in JavaScript inside design documents
• Offline-first data sync for mobile and environments with unreliable networks

Use cases: Content management and document storage with built-in revision history, mobile and offline-capable apps where data must sync across devices and edge locations, distributed systems that need eventual consistency without manual conflict handling. Developers benefit from REST-style access without drivers, incremental replication with filtering, and design documents that replicate alongside data so app logic travels with the database.