MongoDB Documentation, Release 3.0.4 Add an Arbiter to Replica Set Considerations Add an Arbiter Convert a Standalone to a Replica Set To deploy a replica set without using a pre-existing mongod instance, see Deploy a Replica Set (page 603). This tutorial describes the process for converting a standalone mongod instance into a three-member replica set . Use standalone instances for testing and development, but always use replica sets in production. To install a standalone instance, see the installation tutorials (page 5). This operation adds the arbiter running on port 30000 on the m1.example.net host. rs.addArb("m1.example.net:30000") mongod --port 30000 --dbpath /data/arb --replSet rs mkdir /data/arb These settings are specific to arbiters. Do not set journal.enabled to false on a data-bearing node. Similarly, do not set smallFiles unless specifically indicated. Warning: Never set journal.enabled to false on a data-bearing node. To minimize the default creation of data, set the following in the arbiter’s configuration file : 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 . Important: Do not run an arbiter on the same system as a member of the replica set. Arbiters have minimal resource requirements and do not require dedicated hardware. You can deploy an arbiter on an application server or a monitoring host. Arbiters are mongod instances that are part of a replica set but do not hold data. Arbiters participate in elections (page 580) in order to break ties. If a replica set has an even number of members, add an arbiter. • journal.enabled to false • smallFiles to true 1. Create a data directory (e.g. 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: 2. Start the arbiter. Specify the data directory and the replica set name. The following, starts an arbiter using the /data/arb dbPath for the rs replica set: 3. Connect to the primary and add the arbiter to the replica set. Use the rs.addArb() method, as in the following example: Chapter 9. Replication 614