MongoDB Documentation, Release 3.0.4 replica set has an even number of members, add an arbiter to obtain a majority of votes in an election for primary. Arbiters do not require dedicated hardware. For more information on arbiters, see Replica Set Arbiter (page 571). An arbiter (page 571) will always be an arbiter whereas a primary (page 564) may step down and become a secondary (page 567) and a secondary (page 567) may become the primary during an election. Secondaries apply operations from the primary asynchronously. By applying operations after the primary, sets can continue to function despite the failure of one or more members. For more information on replication mechanics, see Replica Set Oplog (page 593) and Replica Set Data Synchronization (page 594). When a primary does not communicate with the other members of the set for more than 10 seconds, the replica set will attempt to select another member to become the new primary. The first secondary that receives a majority of the votes becomes primary. See Replica Set Elections (page 580) and Rollbacks During Replica Set Failover (page 584) for more information. Asynchronous Replication Automatic Failover 9.1. Replication Introduction 561