Replication

MongoDB supports asynchronous replication of data between servers for failover and redundancy. Only one server (in the set/shard) is active for writes (the primary, or master) at a given time – this is to allow strong consistent (atomic) operations. One can optionally send read operations to the secondaries when eventual consistency semantics are acceptable.

Two forms of replication are available, Replica Sets and Master-Slave. Use Replica Sets – replica sets are a functional superset of master/slave and are handled by much newer, more robust code.

Each shard of a Mongo cluster is a replica set.

• Replica Set Documentation

Verifying propagation of writes with getlasterror

A client can block until a write operation has been replicated to N servers -- read more here .

See Also

• Master Slave Replication

Video

A MongoDB Replication Primer: Replica Sets in Practice - Presentation at MongoSV (Dec 2011)