Using Replica Set Arbiter nodes and non-voting nodes

Percona Server for MongoDB replication model is based on elections, when nodes of the Replica Set choose which node becomes the primary node.

The need for elections influences the choice of the number of nodes in the cluster. Elections are the reason to avoid even number of nodes, and to have at least three and not more than seven participating nodes.

Still, sometimes there is a contradiction between the number of nodes suitable for elections and the number of nodes needed to store data. You can solve this contradiction in two ways:

• Add Arbiter nodes, which participate in elections, but do not store data,
• Add non-voting nodes, which store data but do not participate in elections.

Adding Arbiter nodes

Normally, each node stores a complete copy of the data, but there is also a possibility, to reduce disk IO and space used by the database, to add an arbiter node . An arbiter cannot become a primary and does not have a complete copy of the data. The arbiter does have one election vote and can be the odd number for elections. The arbiter does not demand a persistent volume.

Percona Operator for MongoDB has the ability to create Replica Set Arbiter nodes if needed. This feature can be configured in the Replica Set section of the deploy/cr.yaml file:

• set arbiter.enabled option to true to allow Arbiter instances,
• use arbiter.size option to set the desired amount of Arbiter instances.

For example, the following keys in deploy/cr.yaml will create a cluster with 4 data instances and 1 Arbiter:

....
replsets:
  ....
  size: 4
  ....
  arbiter:
    enabled: true
    size: 1
    ....

Note

You can find description of other possible options in the replsets.arbiter section of the Custom Resource options reference.

Preventing Arbiter instances to share Kubernetes Nodes with Replica Set

By default Arbiter instances are allowed to run on the same host as regular Replica Set instances. This may be reasonable in terms of the number of Kubernetes Nodes required for the cluster. But as a result it increases possibility to have 50/50 votes division in case of network partitioning. You can use anti-affinity constraints to avoid such Pod alocation as follows:

....
arbiter:
  enabled: true
  size: 1
  affinity:
    antiAffinityTopologyKey: "kubernetes.io/hostname"
    advanced:
      podAntiAffinity:
        requiredDuringSchedulingIgnoredDuringExecution:
        - labelSelector:
            matchLabels:
              app.kubernetes.io/component: mongod
              app.kubernetes.io/instance: cluster1
              app.kubernetes.io/managed-by: percona-server-mongodb-operator
              app.kubernetes.io/name: percona-server-mongodb
              app.kubernetes.io/part-of: percona-server-mongodb
              app.kubernetes.io/replset: rs0
          topologyKey: kubernetes.io/hostname

Adding non-voting nodes

Non-voting member is a Replica Set node which does not participate in the primary election process. This feature is required to have more than 7 nodes, or if there is a node in the edge location , which obviously should not participate in the voting process.

Note

Non-voting nodes support has technical preview status and is not recommended for production environments.

Note

It is possible to add a non-voting node in the edge location through the externalNodes option. Please see cross-site replication documentation for details.

Percona Operator for MongoDB has the ability to configure non-voting nodes in the Replica Set section of the deploy/cr.yaml file:

• set nonvoting.enabled option to true to allow non-voting instances,
• use nonvoting.size option to set the desired amount of non-voting instances.

For example, the following keys in deploy/cr.yaml will create a cluster with 3 data instances and 1 non-voting instance:

....
replsets:
  ....
  size: 3
  ....
  nonvoting:
    enabled: true
    size: 1
    ....

Note

You can find description of other possible options in the replsets.nonvoting section of the Custom Resource options reference.

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Last update: 2024-09-15