[hox]

Make the changes on a non-active replica database (assuming your database supports replication), then promote the replica to be primary. If you don’t have replicas, the concept of “zero downtime” is really at risk as your primary database becomes a single point of failure.

[michaeldejong]

That's not great either. Moving the "master" role from one server to a replica isn't instantaneous (meaning you have a period of read-only state), and while your replica is performing the schema operation you still need to keep the data in sync with changes made on the master server (which to the best of my knowledge is non-trivial).

There are tools that "replay" data changes though: - FB's OSC (https://www.facebook.com/notes/mysql-at-facebook/online-sche...) - GH's gh-ost (https://github.com/github/gh-ost)