You need a minimum of three instances in an InnoDB Cluster to make it tolerant to the failure of one instance. Adding further instances increases the tolerance to failure of an InnoDB Cluster.

Group Replication implements compatibility policies which consider the version of the instances, and the Cluster.addInstance() operation detects this and in the event of an incompatibility the operation terminates with an error. See Checking the MySQL Version on Instances and Combining Different Member Versions in a Group.

Use the Cluster.addInstance(instance) function to add an instance to the cluster, where instance is connection information to a configured instance, see Section 7.4.2, “Configuring Production Instances for InnoDB Cluster Usage”. For example:

mysql-js> cluster.addInstance('icadmin@ic-2:3306')
A new instance will be added to the InnoDB cluster. Depending on the amount of
data on the cluster this might take from a few seconds to several hours.
Please provide the password for 'icadmin@ic-2:3306': ********
Adding instance to the cluster ...
Validating instance at ic-2:3306...
This instance reports its own address as ic-2
Instance configuration is suitable.
The instance 'icadmin@ic-2:3306' was successfully added to the cluster.

The options dictionary of the addInstance(instance[, options]) function provides the following attributes:

When a new instance is added to the cluster, the local address for this instance is automatically added to the group_replication_group_seeds variable on all online cluster instances in order to allow them to use the new instance to rejoin the group, if needed.

If you are using MySQL 8.0.17 or later you can choose how the instance recovers the transactions it requires to synchronize with the cluster. Only when the joining instance has recovered all of the transactions previously processed by the cluster can it then join as an online instance and begin processing transactions. For more information, see Section 7.4.6, “Using MySQL Clone with InnoDB Cluster”.

You can configure how Cluster.addInstance() behaves, letting recovery operations proceed in the background or monitoring different levels of progress in MySQL Shell.

Depending on which option you choose to recover the instance from the cluster, you see different output in MySQL Shell. Suppose that you are adding the instance ic-2 to the cluster, and ic-1 is the seed or donor.

To cancel the monitoring of the recovery phase, issue CONTROL+C. This stops the monitoring but the recovery process continues in the background. The recoveryProgress integer option can be used with the Cluster.addInstance() operation to display the progress of the recovery phase.

To verify the instance has been added, use the cluster instance's status() function. For example this is the status output of a sandbox cluster after adding a second instance:

mysql-js> cluster.status()
{
    "clusterName": "testCluster",
    "defaultReplicaSet": {
        "name": "default",
        "primary": "ic-1:3306",
        "ssl": "REQUIRED",
        "status": "OK_NO_TOLERANCE",
        "statusText": "Cluster is NOT tolerant to any failures.",
        "topology": {
            "ic-1:3306": {
                "address": "ic-1:3306",
                "mode": "R/W",
                "readReplicas": {},
                "role": "HA",
                "status": "ONLINE"
            },
            "ic-2:3306": {
                "address": "ic-2:3306",
                "mode": "R/O",
                "readReplicas": {},
                "role": "HA",
                "status": "ONLINE"
            }
        }
    },
    "groupInformationSourceMember": "mysql://icadmin@ic-1:3306"
}

How you proceed depends on whether the instance is local or remote to the instance MySQL Shell is running on, and whether the instance supports persisting configuration changes automatically, see Section 6.2.3, “Persisting Settings”. If the instance supports persisting configuration changes automatically, you do not need to persist the settings manually and can either add more instances or continue to the next step. If the instance does not support persisting configuration changes automatically, you have to configure the instance locally. This is essential to ensure that instances rejoin the cluster in the event of leaving the cluster.

Once you have your cluster deployed you can configure MySQL Router to provide high availability, see Section 6.10, “Using MySQL Router with AdminAPI, InnoDB Cluster, and InnoDB ReplicaSet”.