When you have created a ReplicaSet, you can use the ReplicaSet.addInstance() operation to add an instance as a read-only secondary replica of the current primary of the ReplicaSet.

The primary of the ReplicaSet must be reachable and available during this operation. MySQL Replication is configured between the added instance and the primary, using an automatically created MySQL account with a random password. Before the instance can be an operational secondary, it must be in a synchronistic arrangement with the primary. This process is called recovery, and InnoDB ReplicaSet supports different methods which you configure with the recoveryMethod option.

For an instance to be able to join a ReplicaSet, various prerequisites must be satisfied. They are automatically checked by ReplicaSet.addInstance(), and the operation fails if any issues are found.

Use dba.configureReplicaSetInstance() to validate and configure binary log and replication related options before adding an instance. MySQL Shell connects to the target instance using the same user name and password used to obtain the ReplicaSet handle object. All instances of the ReplicaSet are expected to have the same administrator account with the same grants and passwords. You can create a custom administrator account with the required grants when you configure an instance with dba.configureReplicaSetInstance() option. See Section 9.2, “Configuring InnoDB ReplicaSet Instances”.