Why does flyway produce a warning message when setting outOfOrder to true?

Question

I am getting the following warning message in my logs when outOfOrder is set to true:

WARNING: outOfOrder mode is active. Migration run may not be reproducible.

What is the exact meaning of this message? It will be good if someone can tell me what would happen when migration happens. Any example would be helpful.

Answer 1

Assume 3 migrations:

1. Create two names 'Tom' and 'Jerry'
2. Add 'Mickey' as a 3rd one
3. Turn names into uppercase

Running with outOfOrder may result in say your migrations applied like this:

1, 3, 2 -> In the DB: TOM, JERRY, Mickey

Re-running it later will produce

1, 2, 3 -> In the DB: TOM, JERRY, MICKEY

This is why outOfOrder is potentially dangerous and the first migration run might not be reproducible.

Answer 2

To add to Axel's answer, not only may the resulting data vary depending on the order, but the migration may not even be possible. Consider:

Migrations:

1. Create table foo
2. Add column foo.bar
3. Rename column foo.bar to foo.baz

Execution order:

• 1, 2, 3 → foo has column baz
• 1, 3, … → error applying 3: column foo.bar not found
• 2, … → error applying 2: table foo not found
• 3, … → error applying 3: table foo not found