Pre-deploy migrations

Run a command such as a database migration before each new version starts serving.

Set the command

In Settings > Deploy, set Pre-deploy command, for example npm run migrate or python manage.py migrate. Or deploy.preDeployCommand in railway.json.

How it runs

• It runs once per deployment, after the build and before any new replica starts, in a container from the new image with the service's variables and private network.
• If it exits non-zero, the deployment fails with "Pre-deploy command exited with code 1: <line>" and the old version keeps serving.
• It is stopped after 300 seconds by default (deploy.preDeployTimeoutSec, up to 3600).

Write safe migrations

The old version keeps serving while the migration runs and until the switch. Make migrations backwards compatible: add columns before using them, and drop them in a later deploy.