CouchDB (V1.2)
Written in: Erlang
Main point: DB consistency, ease of use
License: Apache
Protocol: HTTP/REST
Bi-directional (!) replication,
continuous or ad-hoc,
with conflict detection,
thus, master-master replication. (!)
MVCC - write operations do not block reads
Previous versions of documents are available
Crash-only (reliable) design
Needs compacting from time to time
Views: embedded map/reduce
Formatting views: lists & shows
Server-side document validation possible
Authentication possible
Real-time updates via '_changes' (!)
Attachment handling
thus, CouchApps (standalone js apps)
Best used:
For accumulating, occasionally changing data, on which pre-defined queries are to be run. Places where versioning is important.
For example:
CRM, CMS systems. Master-master replication is an especially interesting feature, allowing easy multi-site deployments.
MongoDB (3.2)
Written in: C++
Main point: JSON document store
License: AGPL (Drivers: Apache)
Protocol: Custom, binary (BSON)
Master/slave replication (auto failover with replica sets)
Sharding built-in
Queries are javascript expressions
Run arbitrary javascript functions server-side
Geospatial queries
Multiple storage engines with different performance characteristics
Performance over features
Document validation
Journaling
Powerful aggregation framework
On 32bit systems, limited to ~2.5Gb
Text search integrated
GridFS to store big data + metadata (not actually an FS)
Has geospatial indexing
Data center aware
Best used:
If you need dynamic queries. If you prefer to define indexes, not map/reduce functions. If you need good performance on a big DB. If you wanted CouchDB, but your data changes too much, filling up disks.
For example:
For most things that you would do with MySQL or PostgreSQL, but having predefined columns really holds you back.
