MongoDB Documentation, Release 3.0.4 Deployment Patterns The following documents describe common replica set deployment patterns. Other patterns are possible and effective depending on the application’s requirements. If needed, combine features of each architecture in your own deployment: The minimum architecture of a replica set has three members. A three member replica set can have either three members that hold data, or two members that hold data and an arbiter. Primary with Two Secondary Members A replica set with three members that store data has: These deployments provide two complete copies of the data set at all times in addition to the primary. These replica sets provide additional fault tolerance and high availability (page 580). If the primary is unavailable, the replica set elects a secondary to be primary and continues normal operation. The old primary rejoins the set when available. Primary with a Secondary and an Arbiter A three member replica set with a two members that store data has: Since the arbiter does not hold a copy of the data, these deployments provides only one complete copy of the data. Arbiters require fewer resources, at the expense of more limited redundancy and fault tolerance. Chapter 9. Replication 574 • One primary (page 564). • One secondary (page 567) member. The secondary can become primary in an election (page 580). • One arbiter (page 571). The arbiter only votes in elections. • Two secondary (page 567) members. Both secondaries can become the primary in an election (page 580). • One primary (page 564). Geographically Distributed Replica Sets (page 579) Geographically distributed sets include members in multiple lo- cations to protect against facility-specific failures, such as power outages. Replica Sets with Four or More Members (page 576) Four or more member replica sets provide greater redundancy and can support greater distribution of read operations and dedicated functionality. Three Member Replica Sets (page 574) Three-member replica sets provide the minimum recommended architecture for a replica set. Three Member Replica Sets