To prevent bloking the cluster while doing ALTERS (SCHEMA UPGRADES), do this per node:

SET wsrep_OSU_method='RSU';

Then Run the ALTER statement once done reset the Schema Upgrade method back to Total Order Isolation.

SET wsrep_OSU_method='TOI';

http://galeracluster.com/documentation-webpages/clusterstallonalter.html http://galeracluster.com/documentation-webpages/schemaupgrades.html