Galera Cluster for MySQL is a 100% synchronized cluster in regards to data modification operations (DML). It is ensured by the optimistic locking model and ability to rollback a transaction, which cannot be applied on all nodes. However, schema changes (DDL operations) are not transactional in MySQL, which adds complexity when you need to perform an upgrade or change schema of the database.
Changes made by DDL may affect results of the queries. Therefore all modifications must replicate on all nodes prior next data access. For operations which run momentarily it can be easily achieved, but schema changes may take hours to apply. Therefore in addition to safest synchronous blocking schema upgrade method TOI Galera also supports more relaxed, thought not safe, method RSU.
In her talk Sveta will describe which pitfalls you can hit while performing the change using one or another method, why and how to avoid them.