content_image/721080.jpg
## See also: 

Deploy a Replica Set (page 603), Add an Arbiter to Replica Set (page 614), and Add Members to a Replica Set (page 616). 

## Geographically Distributed Replica Sets 

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: 

• 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. 

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. 

<img src='content_image/721080.jpg'>

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). 

## See also: 

Deploy a Geographically Redundant Replica Set (page 608). 

## Additional Resource 

• Whitepaper: MongoDB Multi-Data Center Deployments 7 

