How ariga atlas works under the hood?

[lancejpollard]

I just discovered atlas from this HN post, I've been wondering why this hasn't been done before! Or maybe it has but I'm just not aware :). Rails requires you to write the steps to change the schema, but it looks like Atlas is like Terraform and you define the desired state and the system figures out the steps to change the schema, amazing!

My question is, how does this work? Is it / was it hard to implement? Is it just a simple matter of comparing the existing column (let's say) and to find the new column definition has nullable allowed, but the old one didn't, and then adding a migration step? Or is it extremely complicated sort of like dependency resolution?

If you could spare a moment and jot down some notes on how you implemented this feature at a high level, that would be really wonderful. I am working on a custom programming language for fun and have been wondering how to do this sort of thing (terraform for database migrations)! Thank you!