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Percona XtraDB Cluster (PXC) is a 100% synchronized cluster in regards to DML operations. It is ensured by the optimistic locking model and ability to rollback transaction which cannot be applied on all nodes. However, DDL operations are not transactional in MySQL. This adds complexity when you need to change the schema of the database.

Changes made by DDL may affect the results of the queries. Therefore all modifications must replicate on all nodes prior to the next data access. For operations that run momentarily, it can be easily achieved, but schema changes may take hours to apply. Therefore in addition to the safest synchronous blocking schema upgrade method: TOI, - PXC supports more relaxed, though not safe, method RSU.

RSU: Rolling Schema Upgrade is advertised to be non-blocking. But you still need to take care of updates, running while you are performing such an upgrade. Surprisingly, even updates on not related tables and schema can cause RSU operation to fail.

In this talk, I will uncover nuances of PXC schema upgrades and point to details you need to take special care about.

Further Information
Schema change is a frequent task, and many do not expect any surprises with it. However, the necessity to replay the changes to all synchronized nodes adds complexity. I made a webinar on a similar topic which was recorded and available for replay. Now I have found that I share a link to the webinar to my Support customers approximately once per week. Not having a good understanding of how schema change works in the cluster leads to lockups and operation failures. This talk will provide a checklist that will help to choose the best schema change method.