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Replication Methods

rs.addArb()

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  • Description
  • IP Binding

Description

rs.addArb(host)

Adds a new arbiter to an existing replica set.

rs.add(<host>, true) is functionally the same as rs.addArb(<host>). You can use these commands interchangeably.

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

The rs.addArb() method takes the following parameter:

Parameter
Type
Description
host
string
Specifies the hostname and optionally the port number of the arbiter member to add to replica set.

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.

IP Binding

Warning

Before binding to a non-localhost (e.g. publicly accessible) IP address, ensure you have secured 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 will fail startup validation and will not start.

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rs.add()