Add an Arbiter to Replica Set¶

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Arbiters are mongod instances that are part of a replica set but do not hold data. Arbiters participate in elections in order to break ties. If a replica set has an even number of members, add an arbiter.

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 the same system as a member of the replica set.

Considerations¶

Note

For the following MongoDB versions, pv1 increases the likelihood of w:1 rollbacks compared to pv0 for replica sets with arbiters:

MongoDB 3.4.1
MongoDB 3.4.0
MongoDB 3.2.11 or earlier

See Replica Set Protocol Versions.

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.

To minimize the default creation of data, set the following in the arbiter’s configuration file:

storage.journal.enabled to false

Warning

Never set storage.journal.enabled to false on a data-bearing node.
For MMAPv1 storage engine, storage.mmapv1.smallFiles to true

These settings are specific to arbiters. Do not set storage.journal.enabled to false on a data-bearing node. Similarly, do not set storage.mmapv1.smallFiles unless specifically indicated.

Add an Arbiter¶

Warning

In general, avoid deploying more than one arbiter per replica set.

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 /data/arb directory:
```
mkdir /data/arb
```
Start the arbiter, specifying the data directory and the replica set name. The following starts an arbiter using the /data/arb as the dbPath and rs for the replica set name:
```
mongod --port 30000 --dbpath /data/arb --replSet rs
```
Connect to the primary and add the arbiter to the replica set. Use the rs.addArb() method, as in the following example:
```
rs.addArb("m1.example.net:30000")
```
This operation adds the arbiter running on port 30000 on the m1.example.net host.

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