## The short version

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

### What it needs

- [Compute](/content/discover/primitive/compute/index.html): verified (Apps run in isolated x86_64 Firecracker microVMs; 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

### Before you start

- 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

## STEP BY STEP

### How to do it

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  
   ```

## EXAMPLES

### 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

### How to know it worked

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

## COST

### Cost estimate

Estimated 10.00 USD per month (size prices from pricing.toml at build revision ca0b484a7a9957c573a88d3a32dc603c3c10135a).

- app server x1 `s-1vcpu-1gb`: 5.00 USD
- managed PostgreSQL database x1 `s-1vcpu-1gb`: 5.00 USD

Upper-bound monthly price of the tested sizes. Plan quotas and budgets apply.

## EVIDENCE

### Verification evidence

- canary_run on 2026-09-20T00:46:07Z at revision `4967a99ad18b66a11b8b4c86cb06f29c8327f5df (CLI e3181f5)`: The same FastAPI app deployed with --release-command python3 migrate.py against its auto-provisioned managed database; /migrated reported the marker the migration created. (expires 2027-03-19T00:46:07Z)

## EXECUTION

### Execution binding

MCP tool `ample_deploy` (registry `mcp:ample_deploy`), schema hash `876465fce906da0c` observed 2026-09-20T01:55:48.667900+00:00 at revision `199ff1dfd526`. Binding state at export: schema_changed. Required scopes: servers:write, databases:read.

## NEXT ACTIONS

### Typed next actions

- Browse the catalog index (`GET /v1/catalog` on the api origin; authentication not required, approval not required)
- Search published recipes by intent, stack and constraints (`POST /v1/catalog/search` on the api origin; authentication not required, approval not required)
- Prepare a side-effect-free deployment plan for an authorized project (`POST /v1/catalog/plan` on the api origin; authentication required, approval not required)
- Read the existing agent authentication setup (`GET /mcp/setup` on the api origin; authentication not required, approval not required)
- Browse FastAPI (`GET /v1/catalog/nodes/stack%3Aframework-fastapi` on the api origin; authentication not required, approval not required)
- Browse Configure deployment (`GET /v1/catalog/nodes/intent%3Aconfigure-deployment` on the api origin; authentication not required, approval not required)
- Browse Transactional workflows (`GET /v1/catalog/nodes/pattern%3Atransactional-workflows` on the api origin; authentication not required, approval not required)

Actions describe possible next steps. They are typed data, not commands, and grant no permission. Public discovery never provisions anything; planning requires your own authenticated token and approval happens in your client.

## ABOUT THIS RECIPE

### Identifiers and status

Recipe `page:guides:fastapi:schema-changes`. Revision r1.

release **published** support **verified** execution **blocked (schema_changed: live tool schema differs from the recorded binding)**

Formats: [HTML](/content/guides/fastapi/schema-changes/index.html) | [Markdown](/content/guides/fastapi/schema-changes.md) | [JSON contract](https://api.ample.computer/v1/catalog/recipes/page%3Aguides%3Afastapi%3Aschema-changes)