MongoDB Documentation, Release 3.0.4 11.6.4 How long does replica set failover take? 11.6.5 Does replication work over the Internet and WAN connections? 11.6.6 Can MongoDB replicate over a “noisy” connection? See also: 11.6.7 What is the preferred replication method: master/slave or replica sets? 11.6.8 What is the preferred replication method: replica sets or replica pairs? primary. But usually, the gap between the primary and secondary nodes is just few milliseconds on a local network connection. It varies, but a replica set will select a new primary within a minute. It may take 10-30 seconds for the members of a replica set to declare a primary inaccessible. This triggers an election . During the election, the cluster is unavailable for writes. The election itself may take another 10-30 seconds. Note: Eventually consistent reads, like the ones that will return from a replica set are only possible with a write concern that permits reads from secondary members. For example, a deployment may maintain a primary and secondary in an East-coast data center along with a secondary member for disaster recovery in a West-coast data center. Yes. Deploy a Geographically Redundant Replica Set (page 608) Yes, but not without connection failures and the obvious latency. Members of the set will attempt to reconnect to the other members of the set in response to networking flaps. This does not require administrator intervention. However, if the network connections among the nodes in the replica set are very slow, it might not be possible for the members of the node to keep up with the replication. If the TCP connection between the secondaries and the primary instance breaks, a replica set will automatically elect one of the secondary members of the set as primary. Changed in version 3.0.0: In MongoDB 3.0.0, replica sets can have up to 50 nodes (page 800). Previous versions limited the maximum number of replica set members to 12. Replica sets are the preferred replication mechanism in MongoDB. However, if your deployment requires more than 50 nodes, you must use master/slave replication. Deprecated since version 1.6. Replica sets replaced replica pairs in version 1.6. Replica sets are the preferred replication mechanism in MongoDB. 769 11.6. FAQ: Replication and Replica Sets