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Replication Introduction

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Replication is the process of synchronizing data across multiple servers.

Purposes of Replication

Replication provides redundancy and increases data availability. With multiple copies of data on different database servers, replication protects a database from the loss of a single server. Replication also allows you to recover from hardware failure and service interruptions. With additional copies of the data, you can dedicate a server to disaster recovery, reporting, or backup.

In some cases, you can use replication to increase read capacity. Clients have the ability to send read operations to different servers. You can also maintain copies in different data centers to increase the locality and availability of data for distributed applications.

Replication in MongoDB

A replica set is a group of mongod instances that host the same data set. One mongod, the primary, receives all write operations. All other instances, secondaries, apply operations from the primary so that they have the same data set.

The primary accepts all write operations from clients. A replica set can have only one primary. [1] To support replication, the primary records all changes to its data sets in its oplog. For more information on primary node operation, see Replica Set Primary.

Diagram of default routing of reads and writes to the primary.

The secondaries replicate the primary’s oplog and apply the operations to their data sets such that the secondaries’ data sets reflect the primary’s data set. If the primary is unavailable, the replica set will elect a secondary to be primary. For more information on secondary members, see Replica Set Secondary Members.

Diagram of a 3 member replica set that consists of a primary and two secondaries.

You may add an extra mongod instance to a replica set as an arbiter. Arbiters do not maintain a data set. The purpose of an arbiter is to maintain a quorum in a replica set by responding to heartbeat and election requests by other replica set members. Because they do not store a data set, arbiters can be a good way to provide replica set quorum functionality with a cheaper resource cost than a fully functional replica set member with a data set. If your replica set has an even number of members, add an arbiter to obtain a majority of votes in an election for primary. Arbiters do not require dedicated hardware. For more information on arbiters, see Replica Set Arbiter.

Diagram of a replica set that consists of a primary, a secondary, and an arbiter.

An arbiter will always be an arbiter whereas a primary may step down and become a secondary and a secondary may become the primary during an election.

Asynchronous Replication

Secondaries apply operations from the primary asynchronously. By applying operations after the primary, replica sets can continue to function despite the failure of one or more members.

For more information on replication mechanics, see Replica Set Oplog and Replica Set Data Synchronization.

Automatic Failover

When a primary does not communicate with the other members of the replica set for more than 10 seconds, the replica set will attempt to select another member to become the new primary. The first secondary that receives a majority of the votes becomes primary.

Diagram of an election of a new primary. In a three member replica set with two secondaries, the primary becomes unreachable. The loss of a primary triggers an election where one of the secondaries becomes the new primary

See Replica Set Elections and Rollbacks During Replica Set Failover for more information.

Read Operations

When a replica set has one and only one primary, reads from that primary provide strict consistency. [1]

By default, clients read from the primary; however, clients can specify a read preference to send read operations to secondaries. Asynchronous replication to secondaries means that reads from secondaries may return data that does not reflect the state of the data on the primary. For information on reading from replica sets, see Read Preference.

In MongoDB, clients can see the results of writes before they are made durable:

  • Regardless of write concern, other clients can see the result of the write operations before the write operation is acknowledged to the issuing client.
  • Clients can read data which may be subsequently rolled back.

Additional Features

Replica sets provide a number of options to support application needs. For example, you may deploy a replica set with members in multiple data centers, or control the outcome of elections by adjusting the priority of some members. Replica sets also support configuring dedicated members for reporting, disaster recovery, or backup functions.

See Priority 0 Replica Set Members, Hidden Replica Set Members and Delayed Replica Set Members for more information.

[1](1, 2) In some circumstances, two nodes in a replica set may transiently believe that they are the primary, but at most, one of them will be able to complete writes with {w: majority} write concern. The node that can complete {w: majority} writes is the current primary, and the other node is a former primary that has not yet recognized its demotion, typically due to a network partition. When this occurs, clients that connect to the former primary may observe stale data despite having requested read preference primary.

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