This post is about a fairly technical detail of how Galera works. I'm writing it down in preparation for testing this feature so that I can agree with Alex whether to file a bug or not. I'm sharing it on my blog just in case someone else might benefit from learning this.
Galera 2.0 introduces rolling schema upgrades. This is a new way to do non-blocking schema changes in MySQL.
As the name suggests, it is done as a rolling upgrade. Having seen clusters doing rolling upgrades before, I assumed this is what happens:
- Execute alter table on Node 1.
- Node 1 is removed from the cluster and stops processing transactions.
- Node 1 completes alter table.
- Node 1 re-joins cluster and catches up so that it is in sync.