7.5.4 Configuring Failover Consistency

Group Replication provides the ability to specify the failover guarantees (eventual or “read your writes”) if a primary failover happens in single-primary mode (see Configuring Transaction Consistency Guarantees). You can configure the failover guarantees of an InnoDB Cluster at creation by passing the consistency option (prior to version 8.0.16 this option was the failoverConsistency option, which is now deprecated) to the dba.createCluster() operation, which configures the group_replication_consistency system variable on the seed instance. This option defines the behavior of a new fencing mechanism used when a new primary is elected in a single-primary group. The fencing restricts connections from writing and reading from the new primary until it has applied any pending backlog of changes that came from the old primary (sometimes referred to as “read your writes”). While the fencing mechanism is in place, applications effectively do not see time going backward for a short period of time while any backlog is applied. This ensures that applications do not read stale information from the newly elected primary.

The consistency option is only supported if the target MySQL server version is 8.0.14 or later, and instances added to a cluster which has been configured with the consistency option are automatically configured to have group_replication_consistency the same on all cluster members that have support for the option. The variable default value is controlled by Group Replication and is EVENTUAL, change the consistency option to BEFORE_ON_PRIMARY_FAILOVER to enable the fencing mechanism. Alternatively use consistency=0 for EVENTUAL and consistency=1 for BEFORE_ON_PRIMARY_FAILOVER.

Note

Using the consistency option on a multi-primary InnoDB Cluster has no effect but is allowed because the cluster can later be changed into single-primary mode with the Cluster.switchToSinglePrimaryMode() operation.