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Replace a Config Server

Important

The following procedure applies to 4.0 config servers. For earlier versions of MongoDB, refer to the corresponding version of the MongoDB Manual.

Overview

If the config server replica set becomes read only, i.e. does not have a primary, the sharded cluster cannot support operations that change the cluster metadata, such as chunk splits and migrations. Although no chunks can be split or migrated, applications will be able to write data to the sharded cluster.

If one of the config servers is unavailable or inoperable, repair or replace it as soon as possible. The following procedure replaces a member of a config server replica set with a new member.

The tutorial is specific to MongoDB 4.0. For earlier versions of MongoDB, refer to the corresponding version of the MongoDB Manual.

Considerations

The following restrictions apply to a replica set configuration when used for config servers:

  • Must have zero arbiters.
  • Must have no delayed members.
  • Must build indexes (i.e. no member should have buildIndexes setting set to false).

Procedure

1

Start the replacement config server.

Start a mongod instance, specifying the --configsvr, --replSet, --bind_ip options, and other options as appropriate to your deployment.

Warning

Before you bind to other ip addresses, consider enabling access control and other security measures listed in Security Checklist to prevent unauthorized access.

mongod --configsvr --replSet <replicaSetName> --bind_ip localhost,<hostname(s)|ip address(es)>
2

Add the new config server to the replica set.

Connect a mongo shell to the primary of the config server replica set and use rs.add() to add the new member.

Tip

When a newly added secondary has its votes and priority settings greater than zero, during its initial sync, the secondary still counts as a voting member even though it cannot serve reads nor become primary because its data is not yet consistent.

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, once the member has transitioned into SECONDARY state, use rs.reconfig() to update its priority and votes.

rs.add( { host: "<hostnameNew>:<portNew>", priority: 0, votes: 0 } )

The initial sync process copies all the data from one member of the config server replica set to the new member without restarting.

mongos instances automatically recognize the change in the config server replica set members without restarting.

3

Update the newly added config server’s votes and priority settings.

  1. Ensure that the new member has reached SECONDARY state. To check the state of the replica set members, run rs.status():

    rs.status()
    
  2. Reconfigure the replica set to update the votes and priority of the new member:

    var cfg = rs.conf();
    
    cfg.members[n].priority = 1;  // Substitute the correct array index for the new member
    cfg.members[n].votes = 1;     // Substitute the correct array index for the new member
    
    rs.reconfig(cfg)
    

    where n is the array index of the new member in the members array.

Warning

  • The rs.reconfig() shell method can force the current primary to step down, which causes an election. When the primary steps down, the mongod closes all client connections. While this typically takes 10-20 seconds, try to make these changes during scheduled maintenance periods.
  • Avoid reconfiguring replica sets that contain members of different MongoDB versions as validation rules may differ across MongoDB versions.
4

Shut down the member to replace.

If replacing the primary member, step down the primary first before shutting down.

5

Remove the member to replace from the config server replica set.

Upon completion of initial sync of the replacement config server, from a mongo shell connected to the primary, use rs.remove() to remove the old member.

rs.remove("<hostnameOld>:<portOld>")

mongos instances automatically recognize the change in the config server replica set members without restarting.

6

If necessary, update mongos configuration or DNS entry.

With replica set config servers, the mongos instances specify in the --configdb or sharding.configDB setting the config server replica set name and at least one of the replica set members.

As such, if the mongos instance does not specify the removed replica set member in the --configdb or sharding.configDB setting, no further action is necessary.

If, however, a mongos instance specified the removed member in the --configdb or configDB setting, either:

  • Update the setting for the next time you restart the mongos, or
  • Modify the DNS entry that points to the system that provided the old config server, so that the same hostname points to the new config server.