Schema as code.
WHAT A MIGRATION IS
A file describing a change to the database structure.
WHY USE THEM
The structure is versioned alongside the code Changes can be applied consistently across environments Anyone can recreate the structure from nothing
WHAT EACH CONTAINS
How to apply the change, and how to reverse it.
WHAT TO ALWAYS PROVIDE
The reversal, even when you expect never to use it.
WHAT TO NEVER DO
Modify a migration that has already run in production.
WHY
Environments diverge, and the record of what was applied becomes wrong.
WHAT TO DO INSTEAD
Write a new migration.
WHAT TO BE CAREFUL WITH
Migrations that alter large tables, which can lock them Migrations that also move data
WHAT TO DO ABOUT THE SECOND
Separate structural change from data change.
WHAT TO RUN BEFORE MIGRATING IN PRODUCTION
A backup.
WHAT TO TEST
Applying and reversing, on a copy.
WHAT TO CHECK AFTER
That the structure matches expectations.
