I have three questions to ask about DB migrations (which I can guess the answers to but would love to hear directly), if that's okay:
How do you handle a DB migration with a staged rollout (two 2 of N production servers)?
How do you organise timing between a migration deploy and code deploy if one is done before the other?
How do you handle rollbacks?
It's also really important that migrations don't affect the running app code. New columns shouldn't be used yet. Removed columns or tables need to have all references removed first before running the migration. We confirm this with a deprecation helper that sends metrics to Graphite.
That's about all I can answer from the app side :)
Was assuming that a column rename would duplicate the column during the first migration — so that the old and new codebase would work correctly. I guess the only complication is that you need to keep track of which branches have been successfully rebased/merged to master so you can run the second cleanup migrations.
Thanks again!
One question I have which is not addressed by the article is how to deal with database changes. Every database has difficulty with schema migrations to one degree or another, but MySQL (which IIRC is what GitHub uses) is particularly bad. In my organization, we are VERY careful with any deploy that contains a migration.
(I suppose this is where GitHub's staging environments with real world datasets come in.)
One workaround I've considered is automatically deploying code that doesn't contain a migration (which is the vast majority) and forcing a more manual approach to database migrations, to make sure people are on hand to roll it back if necessary.