1.2.1 MySQL Router Read Replica Support

MySQL Router reads the values defined in the metadata field, v2_router_options.router_options.read_only_targets, to retrieve routing information for read-only traffic.

v2_router_options.router_options.read_only_targets is populated by the AdminAPI method cluster.setRoutingOption() which sets the routing policy to one of the following values using the read_only_targets option:

• all: all Read Replicas and Secondary cluster members are used for read-only traffic.

• read_replicas: only Read Replicas are used for read-only traffic.

• secondaries: only Secondary cluster members are used for read-only traffic.

Note

If read_only_targets is not present, or set to a value other than all, read_replicas, or secondaries, MySQL Router defaults to secondaries and logs a warning message.

MySQL Router does not use Read Replicas as a source for Cluster metadata. Also, it is not possible to use a Read Replica in a MySQL Router bootstrap command. An error is returned for any attempt to bootstrap with a Read Replica.

Failure Handling

MySQL Router does not route connections to Read Replicas in the following situations:

• If there is no quorum in the Cluster.

• If all Cluster members are in OFFLINE state.

• If no Cluster members can be reached when checking their Group Replication state.

MySQL Router routing policy is affected by configuration in the following ways:

• If the Cluster state is INVALID and the invalidated_cluster_policy is set to drop_all, Read Replicas are not used for new read-only connections and all existing connections to the Read Replicas are dropped.

• If the Cluster state is INVALID and the invalidated_cluster_policy is set to allow_ro, Read Replicas are used for new read-only connections and existing connections to the Read Replicas are unaffected.

MySQL Router uses the standard quarantine mechanism for Read Replicas, as defined by the desination_status configuration parameters. See Destination Status Options.