MongoDB Documentation, Release 3.0.4 See also: Geographically Distributed Replica Sets Additional Resource See also: 579 9.2. Replication Concepts $^{7}$http://www.mongodb.com/lp/white-paper/multi-dc?jmp=docs • One primary in the main data center. • One secondary member in the main data center. This member can become primary at any time. • One priority 0 (page 567) member in a second data center. This member cannot become primary. Deploy a Replica Set (page 603), Add an Arbiter to Replica Set (page 614), and Add Members to a Replica Set (page 616). Adding members to a replica set in multiple data centers adds redundancy and provides fault tolerance if one data center is unavailable. Members in additional data centers should have a priority of 0 (page 567) to prevent them from becoming primary. For example: the architecture of a geographically distributed replica set may be: In the following replica set, the primary and one secondary are in Data Center 1 , while Data Center 2 has a priority 0 (page 567) secondary that cannot become a primary. If the primary is unavailable, the replica set will elect a new primary from Data Center 1 . If the data centers cannot connect to each other, the member in Data Center 2 will not become the primary. If Data Center 1 becomes unavailable, you can manually recover the data set from Data Center 2 with minimal downtime. With sufficient write concern (page 78), there will be no data loss. To facilitate elections, the main data center should hold a majority of members. Also ensure that the set has an odd number of members. If adding a member in another data center results in a set with an even number of members, deploy an arbiter (page ?? ). For more information on elections, see Replica Set Elections (page 580). Deploy a Geographically Redundant Replica Set (page 608). • Whitepaper: MongoDB Multi-Data Center Deployments 7