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MySQL Shell 8.0  /  ...  /  Restoring a Cluster from Quorum Loss

7.8.2 Restoring a Cluster from Quorum Loss

If an instance (or instances) fail, then a cluster can lose its quorum, which is the ability to vote in a new primary. This can happen when there is a failure of enough instances that there is no longer a majority of the instances which make up the cluster to vote on Group Replication operations. See Fault-tolerance. When a cluster loses quorum you can no longer process write transactions with the cluster, or change the cluster's topology, for example by adding, rejoining, or removing instances. However if you have an instance online which contains the InnoDB Cluster metadata, it is possible to restore a cluster with quorum. This assumes you can connect to an instance that contains the InnoDB Cluster metadata, and that instance can contact the other instances you want to use to restore the cluster.

Important

This operation is potentially dangerous because it can create a split-brain scenario if incorrectly used and should be considered a last resort. Make absolutely sure that there are no partitions of this group that are still operating somewhere in the network, but not accessible from your location.

Connect to an instance which contains the cluster's metadata, then use the Cluster.forceQuorumUsingPartitionOf(instance) operation, which restores the cluster based on the metadata on instance, and then all the instances that are ONLINE from the point of view of the given instance definition are added to the restored cluster.

mysql-js> cluster.forceQuorumUsingPartitionOf("icadmin@ic-1:3306")

  Restoring replicaset 'default' from loss of quorum, by using the partition composed of [icadmin@ic-1:3306]

  Please provide the password for 'icadmin@ic-1:3306': ******
  Restoring the InnoDB cluster ...

  The InnoDB cluster was successfully restored using the partition from the instance 'icadmin@ic-1:3306'.

  WARNING: To avoid a split-brain scenario, ensure that all other members of the replicaset
  are removed or joined back to the group that was restored.

In the event that an instance is not automatically added to the cluster, for example if its settings were not persisted, use Cluster.rejoinInstance() to manually add the instance back to the cluster.

The restored cluster might not, and does not have to, consist of all of the original instances which made up the cluster. For example, if the original cluster consisted of the following five instances:

  • ic-1

  • ic-2

  • ic-3

  • ic-4

  • ic-5

and the cluster experiences a split-brain scenario, with ic-1, ic-2, and ic-3 forming one partition while ic-4 and ic-5 form another partition. If you connect to ic-1 and issue Cluster.forceQuorumUsingPartitionOf('icadmin@ic-1:3306') to restore the cluster the resulting cluster would consist of these three instances:

  • ic-1

  • ic-2

  • ic-3

because ic-1 sees ic-2 and ic-3 as ONLINE and does not see ic-4 and ic-5.