Three Member Replica Sets¶

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Primary with Two Secondary Members
Primary with a Secondary and an Arbiter

The minimum architecture of a replica set has three members. A three member replica set can have either three members that hold data, or two members that hold data and an arbiter.

Primary with Two Secondary Members¶

A replica set with three members that store data has:

One primary.
Two secondary members. Both secondaries can become the primary in an election.

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

These deployments provide two complete copies of the data set at all times in addition to the primary. These replica sets provide additional fault tolerance and high availability. If the primary is unavailable, the replica set elects a secondary to be primary and continues normal operation. The old primary rejoins the set when available.

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

Primary with a Secondary and an Arbiter¶

A three member replica set with a two members that store data has:

One primary.
One secondary member. The secondary can become primary in an election.
One arbiter. The arbiter only votes in elections.

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

Since the arbiter does not hold a copy of the data, these deployments provides only one complete copy of the data. Arbiters require fewer resources, at the expense of more limited redundancy and fault tolerance.

However, a deployment with a primary, secondary, and an arbiter ensures that a replica set remains available if the primary or the secondary is unavailable. If the primary is unavailable, the replica set will elect the secondary to be primary.

Diagram of an election of a new primary. In a three member replica set with a secondary and an arbiter, the primary becomes unreachable. The loss of a primary triggers an election where the secondary becomes new primary.