Add Members to a Replica Set

Maximum Voting Members

A replica set can have a maximum of seven voting members. To add a member to a replica set that already has seven voting members, you must either add the member as a non-voting member or remove a vote from an existing member.

Init Scripts

In production deployments you can configure a init script to manage member processes.

Existing Members

You can use these procedures to add new members to an existing replica set.

Restore Former Members

You can use these procedures to re-add a node that was removed.

If the data on the removed node is relatively recent, the node recovers and catches up to the rest of the replica set.

Data Files

If you have a backup or snapshot of an existing member, you can move the data files (for example, the dbPath directory) to a new system and use them to quickly initiate a new member. The files must be:

• A valid copy of the data files from a member of the same replica set. See Back Up and Restore with Filesystem Snapshots document for more information.

  Important

  Always use filesystem snapshots to create a copy of a member of the existing replica set. Do not use mongodump and mongorestore to seed a new replica set member.

• More recent than the oldest operation in the primary's oplog. The new member must be able to become current by applying operations from the primary's oplog.

IP Binding

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.

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

Prepare the Data Directory

Before adding a new member to an existing replica set, prepare the new member's data directory using one of the following strategies:

• Make sure the new member's data directory does not contain data. The new member will copy the data from an existing member.

  If the new member is in a recovering state, it must exit and become a secondary before MongoDB can copy all data as part of the replication process. This process takes time but does not require administrator intervention.

• Manually copy the data directory from an existing member. The new member becomes a secondary member and will catch up to the current state of the replica set. Copying the data over may shorten the amount of time for the new member to become current.

  Ensure that you can copy the data directory to the new member and begin replication within the window allowed by the oplog. Otherwise, the new instance will have to perform an initial sync, which completely resynchronizes the data, as described in Resync a Member of a Replica Set.

  Use rs.printReplicationInfo() to check the current state of replica set members with regards to the oplog.

For background on replication deployment patterns, see the Replica Set Deployment Architectures document.

Add a Member to an Existing Replica Set

1. Start the new mongod instance. Specify the data directory and the replica set name. The following example specifies the /srv/mongodb/db0 data directory and the rs0 replica set:

   mongod --dbpath /srv/mongodb/db0 --replSet rs0  --bind_ip localhost,<hostname(s)|ip address(es)>

   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.

   For more information on configuration options, see the mongod manual page.

   Note

   Optional

   You can specify the data directory, replica set name, and the IP binding in the mongod.conf configuration file, and start the mongod with the following command:

   mongod --config /etc/mongod.conf

2. Connect to the replica set's primary.

   You can only add members while connected to the primary. If you do not know which member is the primary, log into any member of the replica set and issue the db.hello() command.

3. Use rs.add() to add the new member to the replica set. Pass the member configuration document to the method. For example, to add a member at host mongodb3.example.net, issue the following command:

   rs.add( { host: "mongodb3.example.net:27017" } )

   Warning

   Before MongoDB 5.0, a newly added secondary still counts as a voting member even though it can neither serve reads nor become primary until its data is consistent. If you are running a MongoDB version earlier than 5.0 and add a secondary with its votes and priority settings greater than zero, this can lead to a case where a majority of the voting members are online but no primary can be elected. To avoid such situations, consider adding the new secondary initially with priority :0 and votes :0. Then, run rs.status() to ensure the member has transitioned into SECONDARY state. Finally, use rs.reconfig() to update its priority and votes.