Replica Set Secondary Members

A secondary maintains a copy of the primary’s dataset. To replicate data, a secondary applies operations from theprimary’s oplog to its own data setin an asynchronous process. [1] A replica set can have one or moresecondaries.

The following three-member replica set has two secondarymembers. The secondaries replicate the primary’s oplog and applythe operations to their data sets.

Although clients cannot write data to secondaries, clients can readdata from secondary members. See Read Preference for moreinformation on how clients direct read operations to replica sets.

A secondary can become a primary.If the current primary becomes unavailable, the replica setholds an election to choose which of the secondariesbecomes the new primary.

In the following three-member replica set, the primary becomes unavailable.This triggers an election where one of the remainingsecondaries becomes the new primary.

SeeReplica Set Elections for more details.

You can configure a secondary member for a specific purpose. You canconfigure a secondary to:

• Prevent it from becoming a primary in an election, which allows it toreside in a secondary data center or to serve as a cold standby. SeePriority 0 Replica Set Members.
• Prevent applications from reading from it, which allows it to run applicationsthat require separation from normal traffic. SeeHidden Replica Set Members.
• Keep a running “historical” snapshot for use in recovery fromcertain errors, such as unintentionally deleted databases. SeeDelayed Replica Set Members.