# Migrate Heroku app: FastAPI

Move FastAPI compute from Heroku app to Ample, one component at a time. Destination verified on Ample: the app built, started and answered on its public HTTPS URL with encrypted configuration delivered.

## Summary
Source procedure: Export config vars with `heroku config -s -a` into an env file kept out of the repository. Not migrated automatically: Heroku add-ons other than Postgres, release-phase scheduling, dyno autoscaling and Heroku Scheduler are not migrated; use `--release-command` for release tasks.

Cutover: Add the custom domain with `ample domain add`, update DNS from the Heroku DNS target, keep the Heroku app until verified, then scale it down.

Rollback: keep the source untouched until you confirm; nothing at the source is changed or deleted by this guide.

## Representative Queries
- Migrate Heroku app: FastAPI
- Where can I host FastAPI built with FastAPI?
- I need a component-scoped export/import or reconfiguration procedure for FastAPI, with compatibility checks, verification and rollback.

## Resource Requirements
- **Compute**
- **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.

## Prerequisites
- Authorized access to the Heroku app source and its export tooling
- An inventory of every component in scope and out of scope
- A validated backup or copy before any cutover
- An Ample account token with servers:write, databases:read

## Migration Steps
1. **Inventory the source**  
   List what Heroku app provides beyond the component you are moving. Out of scope here: Heroku add-ons other than Postgres, release-phase scheduling, dyno autoscaling and Heroku Scheduler are not migrated; use `--release-command` for release tasks.
2. **Source step 1**  
   Export config vars with `heroku config -s -a` into an env file kept out of the repository.
3. **Source step 2**  
   Translate the `Procfile` web process into the start command (`--start` or the package manifest's start script); worker processes need a separate `--kind worker` service.
4. **Source step 3**  
   Note buildpacks and add-ons in use; managed databases replace Heroku Postgres, while other add-ons need their own plan.
5. **Deploy on Ample in parallel**  
   `pip install` into .ample/python from requirements.txt, then uvicorn from run.py reading PORT on the python-3.12 template. Pass the exported variables with --env; keep the source running.
   ```bash
   ample deploy . --name <name> --public --start "python3 run.py" --env-file .env.production
   ```
6. **Validate before cutover**  
   Run the app's own checks and, for data, compare counts and checksums; example checks include self-tests (/p/configuration-secrets, /p/relational-records).
7. **Cut over**  
   Add the custom domain with `ample domain add`, update DNS from the Heroku DNS target, keep the Heroku app until verified, then scale it down.
8. **Rollback**  
   Point DNS or configuration back to the source. The source was never modified; deletion is a separate, user-executed step after validation.

## Success Checks
- **Destination app responds on its public URL**  
  - Kind: `http_get`  
  - Path: `/`  
  - Expect: `ample canary fastapi patterns`
- **Configuration-secrets check**  
  - Kind: `http_get`  
  - Path: `/p/configuration-secrets`  
  - Expect: `see the pattern fixture checks`
- **Relational-records check**  
  - Kind: `http_get`  
  - Path: `/p/relational-records`  
  - Expect: `see the pattern fixture checks`
- **Data or object counts and checksums match the source**  
  - Kind: `manual`  
  - Expect: `operator comparison before cutover`

## Limitations
- Documentation only: nothing is executed automatically and no execution binding is offered.
- The source-side procedure is documented from Heroku app's standard tooling and was not executed in this catalog's evidence; the destination side was verified with the pattern fixture.
- No full source-product parity is claimed: Heroku add-ons other than Postgres, release-phase scheduling, dyno autoscaling and Heroku Scheduler are not migrated; use `--release-command` for release tasks.
- Verified on the python-3.12 template at s-1vcpu-1gb; region, compliance and request-duration limits are unknown.
- Managed PostgreSQL 16 only; extensions, connection limits and backup or restore procedures on the managed side are not verified beyond the pattern checks.

## Cost Estimate
- **Currency:** USD  
- **Monthly Amount:** $10.00  
- **Note:** Destination always-on monthly price of the tested sizes; apps auto-pause when idle. Source costs are unknown to Ample.
- **Components:**  
  - App server  
    - Size: s-1vcpu-1gb  
    - Monthly Amount: $5.00  
  - Managed PostgreSQL database  
    - Size: s-1vcpu-1gb  
    - Monthly Amount: $5.00

## Evidence Summary
- **Kind:** canary_run  
  - Summary: Destination side verified: the FastAPI pattern fixture deployed on Ample and its checks passed. The source-side export from Heroku app is documented and was not executed by this catalog's evidence.
  - Observed At: 2026-09-21T01:14:18Z  
  - Implementation Revision: 50dbac567d2c5cc8e55e6c748a646be0025d9cfb-dirty (CLI 0.1.21)  
  - Expires At: 2027-03-20T01:14:18Z  
  - Scope:  
    - Checks: `/p/configuration-secrets`, `/p/relational-records`  
    - Destination Only: true  
    - Template: python-3.12

## Last Verified At
- 2026-09-21T01:14:18Z.