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Replica Sets Distributed Across Two or More Data Centers

Overview

While replica sets provide basic protection against single-instance failure, replica sets whose members are all located in a single data center are susceptible to data center failures. Power outages, network interruptions, and natural disasters are all issues that can affect replica sets whose members are located in a single facility.

Distributing replica set members across geographically distincts data centers adds redundancy and provides fault tolerance if one of the data centers is unavailable.

Distribution of the Members

To protect your data in case of a data center failure, keep at least one member in an alternate data center. If possible, use an odd number of data centers, and choose a distribution of members that maximizes the likelihood that even with a loss of a data center, the remaining replica set members can form a majority or at minimum, provide a copy of your data.

Examples

Three-member Replica Set

For example, for a three-member replica set, some possible distributions of members include:

  • Two data centers: two members to Data Center 1 and one member to Data Center 2. If one of the members of the replica set is an arbiter, distribute the arbiter to Data Center 1 with a data-bearing member.
    • If Data Center 1 goes down, the replica set becomes read-only.
    • If Data Center 2 goes down, the replica set remains writeable as the members in Data Center 1 can hold an election.
  • Three data centers: one members to Data Center 1, one member to Data Center 2, and one member to Data Center 3.
    • If any Data Center goes down, the replica set remains writeable as the remaining members can hold an election.

Five-member Replica Set

For a replica set with 5 members, some possible distributions of members include:

  • Two data centers: three members to Data Center 1 and two members to Data Center 2.
    • If Data Center 1 goes down, the replica set becomes read-only.
    • If Data Center 2 goes down, the replica set remains writeable as the members in Data Center 1 can create a majority.
  • Three data centers: two member to Data Center 1, two members to Data Center 2, and one member to site Data Center 3.
    • If any Data Center goes down, the replica set remains writeable as the remaining members can hold an election.

For example, the following 5 member replica set distributes its members across three data centers.

Diagram of a 5 member replica set distributed across three data centers.

Electability of Members

Some members of the replica set, such as members that have networking restraint or limited resources, should not be able to become primary in a failover. Configure members that should not become primary to have priority 0.

In some cases, you may prefer that the members in one data center be elected primary before the members in the other data centers. You can modify the priority of the members such that the members in the one data center has higher priority than the members in the other data centers.

In the following example, the replica set members in Data Center 1 have a higher priority than the members in Data Center 2 and 3; the members in Data Center 2 have a higher priority than the member in Data Center 3:

Diagram of a 5 member replica set distributed across three data centers. Replica set includes members with priority 0.5 and priority 0.

Connectivity

Verify that your network configuration allows communication among all members; i.e. each member must be able to connect to every other member.