MongoDB vs CouchDB

MongoDB and CouchDB are both document databases but serve very different architectural patterns. MongoDB optimizes for query power and throughput; CouchDB optimizes for replication and offline access.

Architecture Differences

MongoDB uses the WiredTiger storage engine with document-level locking and compression. It follows a primary-secondary replication model within replica sets, with optional sharding across sets. Writes go to a single primary per shard.

CouchDB uses an append-only B-tree storage model and a unique multi-master replication protocol. Every node can accept writes, and conflicts are resolved deterministically using document revisions. This architecture enables offline-first applications where devices sync when connectivity is restored.

Performance Characteristics

MongoDB delivers higher throughput for most workloads due to its optimized storage engine and rich index types (compound, multikey, text, geospatial, wildcard). CouchDB's append-only design means writes are fast (always sequential) but compaction is needed periodically to reclaim space.

MongoDB's aggregation pipeline can perform complex server-side data transformations. CouchDB's MapReduce views are powerful but require more development effort for complex queries.

Trade-offs

CouchDB's killer feature is its replication protocol. With PouchDB on the client side, you can build applications that work offline and sync automatically when online. This is invaluable for mobile and edge computing scenarios.

MongoDB's trade-off is more complex operations for better performance and flexibility. CouchDB trades query power for replication simplicity and offline capabilities.

Conflict Resolution

CouchDB handles write conflicts through document revisions, automatically picking a deterministic winner while preserving conflicting versions for application-level resolution. MongoDB avoids this complexity by directing all writes through a single primary.

Real-World Usage

MongoDB powers eBay, Forbes, and Toyota. CouchDB is used by NPM (for package registry replication), Meebo, and various offline-first mobile applications.

Learn about document databases and replication patterns. Check our system design guide and pricing.