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Add an Arbiter to Replica Set

On this page

  • Considerations
  • Primary-Secondary-Arbiter Replica Sets
  • Add an Arbiter

In some circumstances (such as you have a primary and a secondary but cost constraints prohibit adding another secondary), you may choose to add a mongod instance to a replica set as an arbiter to vote in elections.

Arbiters are mongod instances that are part of a replica set but do not hold data (i.e. do not provide data redundancy). They can, however, participate in elections.

Arbiters have minimal resource requirements and do not require dedicated hardware. You can deploy an arbiter on an application server or a monitoring host.

Important

Do not run an arbiter on systems that also host the primary or the secondary members of the replica set.

Warning

Avoid deploying more than one arbiter in a replica set. See Concerns with Multiple Arbiters.

To add an arbiter to an existing replica set:

  • Typically, if there are two or fewer data-bearing members in the replica set, you might need to first set the cluster wide write concern for the replica set.

  • See cluster wide write concern for more information on why you might need to set the cluster wide write concern.

You do not need to change the cluster wide write concern before starting a new replica set with an arbiter.

Tip

See also:

Default write concern formula

Considerations

Arbiters are not supported with quarterly rapid releases. If your deployment includes arbiters, only use LTS releases.

Primary-Secondary-Arbiter Replica Sets

If you are using a three-member primary-secondary-arbiter (PSA) architecture, consider the following:

  • The write concern "majority" can cause performance issues if a secondary is unavailable or lagging. For advice on how to mitigate these issues, see Mitigate Performance Issues with PSA Replica Set.

  • If you are using a global default "majority" and the write concern is less than the size of the majority, your queries may return stale (not fully replicated) data.

Replica Set Protocol Version

Note

For the following MongoDB versions, pv1 increases the likelihood of w:1 rollbacks compared to pv0 (no longer supported in MongoDB 4.0+) for replica sets with arbiters:

  • MongoDB 3.4.1

  • MongoDB 3.4.0

  • MongoDB 3.2.11 or earlier

See Replica Set Protocol Version.

Arbiter

An arbiter does not store data, but until the arbiter's mongod process is added to the replica set, the arbiter will act like any other mongod process and start up with a set of data files and with a full-sized journal.

IP Binding

Warning

Before you bind your instance to a publicly-accessible IP address, you must secure your cluster from unauthorized access. For a complete list of security recommendations, see Security Checklist. At minimum, consider enabling authentication and hardening network infrastructure.

MongoDB binaries, mongod and mongos, bind to localhost by default. If the net.ipv6 configuration file setting or the --ipv6 command line option is set for the binary, the binary additionally binds to the localhost IPv6 address.

By default mongod and mongos that are bound to localhost only accept connections from clients that are running on the same computer. This binding behavior includes mongosh and other members of your replica set or sharded cluster. Remote clients cannot connect to binaries that are bound only to localhost.

To override the default binding and bind to other IP addresses, use the net.bindIp configuration file setting or the --bind_ip command-line option to specify a list of hostnames or IP addresses.

Warning

Starting in MongDB 5.0, split horizon DNS nodes that are only configured with an IP address fail startup validation and report an error. See disableSplitHorizonIPCheck.

For example, the following mongod instance binds to both the localhost and the hostname My-Example-Associated-Hostname, which is associated with the IP address 198.51.100.1:

mongod --bind_ip localhost,My-Example-Associated-Hostname

In order to connect to this instance, remote clients must specify the hostname or its associated IP address 198.51.100.1:

mongosh --host My-Example-Associated-Hostname
mongosh --host 198.51.100.1

Important

To avoid configuration updates due to IP address changes, use DNS hostnames instead of IP addresses. It is particularly important to use a DNS hostname instead of an IP address when configuring replica set members or sharded cluster members.

Use hostnames instead of IP addresses to configure clusters across a split network horizon. Starting in MongoDB 5.0, nodes that are only configured with an IP address fail startup validation and do not start.

Add an Arbiter

Warning

Avoid deploying more than one arbiter in a replica set. See Concerns with Multiple Arbiters.

To add an arbiter to an existing replica set:

  • Typically, if there are two or fewer data-bearing members in the replica set, you might need to first set the cluster wide write concern for the replica set.

  • See cluster wide write concern for more information on why you might need to set the cluster wide write concern.

You do not need to change the cluster wide write concern before starting a new replica set with an arbiter.

Tip

See also:

Default write concern formula

Important

To avoid configuration updates due to IP address changes, use DNS hostnames instead of IP addresses. It is particularly important to use a DNS hostname instead of an IP address when configuring replica set members or sharded cluster members.

Use hostnames instead of IP addresses to configure clusters across a split network horizon. Starting in MongoDB 5.0, nodes that are only configured with an IP address fail startup validation and do not start.

  1. Create a data directory (e.g. storage.dbPath) for the arbiter. The mongod instance uses the directory for configuration data. The directory will not hold the data set. For example, create the /var/lib/mongodb/arb directory:

    mkdir /var/lib/mongodb/arb

  2. Start the arbiter, specifying the data directory and the name of the replica set to join. The following starts an arbiter using the /var/lib/mongodb/arb as the dbPath and rs for the replica set name:

    Warning

    Before you bind your instance to a publicly-accessible IP address, you must secure your cluster from unauthorized access. For a complete list of security recommendations, see Security Checklist. At minimum, consider enabling authentication and hardening network infrastructure.

    mongod --port 27017 --dbpath /var/lib/mongodb/arb --replSet rs --bind_ip localhost,<hostname(s)|ip address(es)>

  3. Connect to the primary and add the arbiter to the replica set. Use the rs.addArb() method, as in the following example which assumes that m1.example.net is the hostname associated with the specified ip address for the arbiter:

    rs.addArb("m1.example.net:27017")

    This operation adds the arbiter running on port 27017 on the m1.example.net host.

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