# Apply database schema changes for FastAPI

## Summary

Apply database schema changes for a FastAPI app on Ample with a release command. `python3 migrate.py` runs against the injected DATABASE_URL before the new release goes live, and the release fails instead of activating if the migration fails.

## Infrastructure requirements

- [Compute](/content/discover/primitive/compute/index.html): verified (Apps run in isolated x86_64 Firecracker microVMs that auto-pause when idle and wake on request; sizes are the priced VM sizes.)
- [Postgres](/content/discover/primitive/postgres/index.html): verified (Managed PostgreSQL 16 runs in its own microVM and is auto-provisioned when an app needs a database and no DATABASE_URL is supplied.)

## Prerequisites

- A migration command that reads DATABASE_URL and exits non-zero on failure
- A PostgreSQL database (supplied or managed)
- An Ample account token with servers:write and databases:read

## Exact tested configuration

- template: `python-3.12`
- runtime: `python`
- size: `s-1vcpu-1gb`
- install: `python3 -m pip install --target .ample/python -r requirements.txt`
- start: `PYTHONPATH=.ample/python:${PYTHONPATH:-} python3 run.py`

## Steps

1. **Write an idempotent migration.** Create tables and markers with IF NOT EXISTS or a migration framework so a re-run is safe.
2. **Deploy with the release command.** Ample runs the command before activation; a non-zero exit fails the release.
   
   ```
   ample deploy . --name <app-name> --public --start "python3 run.py" --release-command "python3 migrate.py"
   ```
3. **Verify the marker through the app.** Serve a route (/migrated) that reads something only the migration creates.
4. **Verify.** Fetch the live URL and run the success checks below. On failure read the build log, then the runtime log, fix the cause and deploy again; do not blind-retry.
   
   ```
   ample logs <deployment_id> --kind build
   ```

## Tested examples

- **FastAPI release-command canary** (tests/deploy-canaries/fastapi-postgres-api): python3 migrate.py runs before activation against the auto-provisioned managed database.

## Success checks

- migration marker is readable (`/migrated` on the live URL, expect migrated=release-ok)

## Limitations

- Verified on the node-22 template at s-1vcpu-1gb; other templates and sizes are not verified by this recipe.
- Region, compliance attestations and request-duration limits are unknown and not claimed.
- Apps auto-pause when idle and wake on the next request; always-on is an operator setting, not a plan feature.
- Managed PostgreSQL 16 only; extensions, connection limits and backup or restore procedures are not verified.
- Apps and their managed databases are placed together; cross-node shared databases are unsupported.
- The release command runs once per release on the app VM; there is no separate migration runner or rollback of a partially applied migration.

## Cost estimate

Estimated 10.00 USD per month.
- app server x1 `s-1vcpu-1gb`: 5.00 USD
- managed PostgreSQL database x1 `s-1vcpu-1gb`: 5.00 USD

## Verification evidence

- canary_run on 2026-09-20T00:46:07Z: The same FastAPI app deployed with --release-command python3 migrate.py against its auto-provisioned managed database; /migrated reported the marker the migration created.

### Execution binding

MCP tool `ample_deploy`, schema hash `876465fce906da0c` observed 2026-09-20T01:55:48.667900+00:00, binding state `current`, required scopes: servers:write, databases:read.