Std.Db.Migrate.apply

fn

This is a callable function.

What it does

Bring the schema to what the program expects, through any driver.

The plan is decided before any lock is taken, so a refusal holds nothing.

Each pending migration then runs in a transaction of its own, which on

PostgreSQL first takes a lock that ends with the transaction: it cannot be

left held by a failure, and a pooled connection goes back to the pool with

nothing attached to it. Inside the lock the record is read again, so a

process that waited for another finds its migration already there and

skips it rather than running it twice.

SQLite has one writer at a time already. A second process that reaches the

same file mid-migration is refused as busy rather than left waiting, and

running it again finds nothing to do.

Answers how many migrations this call applied.

Read the signature

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• Read arrows from left to right: inputs come first, and the final type is returned.
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• Names inside [ ] are type arguments, such as the item type held by a collection.
• Result makes success and recoverable failure part of the function's type.