Say you have a `user` table and a `post` table with `post.user_id` being a FOREIGN KEY on `user.user_id`. Without disabling FK integrity you would not be able to restore a post without restoring the user first. When restoring in parallel this might or might not work out.
They scale poorly in MySQL, and they lose a lot of purpose in a massively sharded environment anyway. For example, say you like a status post on Facebook, or friend another user. It's very unlikely that the liked status or friended user exists on the same shard as your account, and there's no way to enforce a foreign key relationship in an inherently non-distributed database like MySQL.
So instead integrity is handled at the application layer, with additional background processes to fix the occasional integrity problem and detect integrity anomalies.
We always diff against the same base and have 5 days in between subsequent full dumps. The number of days just comes from a trade off between space occupied by the backups and time it takes to generate them.
> Is there no better way to handling this than polling?
There's definitely different ways to approach this, we find polling works well for us. We also use the same database for crash recovery, so doing the assignments through it serves both purposes.
> Presumably you can only get this parallelism by disabling FK integrity. Is it re-enabled in the following VERIFY stage?
I'm not sure what you mean by parallelism through disabling FK integrity. Splitting the backup into its tables means we can restore a subset of tables instead of the entire backup. This allows us to load individual tables concurrently, but also not have to wait to load a massive database if all we need is a few small tables.