MongoDB Documentation, Release 3.0.4 However, a deployment with a primary, secondary, and an arbiter ensures that a replica set remains available if the primary or the secondary is unavailable. If the primary is unavailable, the replica set will elect the secondary to be primary. Deploy a Replica Set (page 603). Replica Sets with Four or More Members Overview Although the standard replica set configuration has three members, you can deploy larger sets. Add additional members to a set to increase redundancy or to add capacity for distributing secondary read operations. Considerations As you add new members to a replica set, consider the following: Odd Number of Voting Members Ensure that the replica set has an odd number of voting members. If you have an even number of voting members, deploy an arbiter (page ?? ) so that the set has an odd number. For example, the following replica set includes an arbiter to ensure an odd number of voting members. 576 Chapter 9. Replication See also: