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 distinct data centers adds redundancy and provides fault tolerance if one of the data centers is unavailable.
Some cloud providers use regions and availability zones. Regions are separate geographic areas. Availability zones are the same as data centers and are separate physical locations within a region. A replica set has high availability if the nodes are in different availability zones because it is unlikely that all zones will fail at the same time.
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 Data Center 1 goes down, the replica set becomes read-only.
If Data Center 2 goes down, the replica set remains writable as the members in Data Center 1 can hold an election.
Three data centers: one member 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 writable as the remaining members can hold an election.
Note
Distributing replica set members across two data centers provides benefit over a single data center. In a two data center distribution,
If one of the data centers goes down, the data is still available for reads unlike a single data center distribution.
If the data center with a minority of the members goes down, the replica set can still serve write operations as well as read operations.
However, if the data center with the majority of the members goes down, the replica set becomes read-only.
If possible, distribute members across at least three data centers. For config server replica sets (CSRS), the best practice is to distribute across three (or more depending on the number of members) centers. If the cost of the third data center is prohibitive, one distribution possibility is to evenly distribute the data bearing members across the two data centers and store the remaining member in the cloud if your company policy allows.
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.
Note
Distributing replica set members across two data centers provides benefit over a single data center. In a two data center distribution,
If one of the data centers goes down, the data is still available for reads unlike a single data center distribution.
If the data center with a minority of the members goes down, the replica set can still serve write operations as well as read operations.
However, if the data center with the majority of the members goes down, the replica set becomes read-only.
If possible, distribute members across at least three data centers. For config server replica sets (CSRS), the best practice is to distribute across three (or more depending on the number of members) centers. If the cost of the third data center is prohibitive, one distribution possibility is to evenly distribute the data bearing members across the two data centers and store the remaining member in the cloud if your company policy allows.
For example, the following 5 member replica set distributes its members 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:
Connectivity
Verify that your network configuration allows each member to connect to every other member.