CouchDB vs. MongoDB

Comparison Feature	CouchDB	MongoDB
Data Model	It follows the document-oriented model and data is presented in JSON format.	It follows the document-oriented model but data is presented in BSON format.
Interface	CouchDB uses HTTP/REST based interface. It is very intuitive and very well designed.	MongoDB uses binary protocol and custom protocol over TCP/IP.
Object Storage	In CouchDB, database contains documents.	In MongoDB, database contains collections and collection contains documents.
Query Method	CouchDB follows Map/Reduce query method. (JavaScript+others)	MongoDB follows Map/Reduce (JavaScript) creating collection + object-based query language.
Replication	CouchDB supports master-master replication with custom conflict resolution functions.	MongoDB supports master-slave replication.
Concurrency	It follows MVCC (Multi Version Concurrency Control).	Update in-place.
Preferences	CouchDB favors availability.	MongoDB favors consistency.
Performance Consistency	In CouchDB is safer than MongoDB	In MongoDB, database contains collections and collection contains documents.
Consistency	CouchDB is eventually consistent.	MongoDB is strongly consistent.
Written in	it is written in Erlang.	it is written in C++.