# 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.

## Representative Queries
- Apply database schema changes for FastAPI
- Where can I host Apply database schema changes built with FastAPI?
- I need a framework-specific schema migration and rollback procedure; no managed migration service is assumed.

## 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

## Resource Requirements
- primitive:compute
- primitive:postgres

## Infrastructure Requirements
- **Compute**  
  Status: verified  
  Summary: Apps run in isolated x86_64 Firecracker microVMs that auto-pause when idle and wake on request; sizes are the priced VM sizes.

- **Postgres**  
  Status: verified  
  Summary: Managed PostgreSQL 16 runs in its own microVM and is auto-provisioned when an app needs a database and no DATABASE_URL is supplied.

## Workflow 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.  
   Command: `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.  
   Command: `ample logs <app-name> --kind build`

## Success Checks
- Migration marker is readable on `/migrated` path which expects `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
- **Currency:** USD  
- **Monthly Amount:** $10.00  
- **Basis:** Size prices from pricing.toml (loaded by the API) at build revision 1ac5595375130d45290090e82ed0f554ccd45405-dirty.  
- **Components:**
  - App server: s-1vcpu-1gb, Quantity: 1.0, Monthly Amount: $5.00
  - Managed PostgreSQL database: s-1vcpu-1gb, Quantity: 1.0, Monthly Amount: $5.00  
- **Note:** Apps and managed databases auto-pause when idle; the estimate is the always-on monthly price of the tested sizes. Plan quotas and budgets apply.

## Examples
- **FastAPI release-command canary**  
  Description: `python3 migrate.py` runs before activation against the auto-provisioned managed database.  
  Source Reference: tests/deploy-canaries/fastapi-postgres-api.

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