# Host mobile backend with FastAPI: worker execution and job state

## Summary
Deploy a mobile backend built with FastAPI as a public web service plus a dedicated worker process on Ample. `ample plan --write` discovers both services and their shared database and writes ample.toml; `ample up` reconciles the topology: a managed PostgreSQL 16 database, a public web microVM and a private worker microVM (kind worker, no public URL), both reading DATABASE_URL. Verified on FastAPI: a separately deployed worker service (kind worker, no public URL) sharing a declared managed PostgreSQL database with the web service, consuming a jobs table with row locks and marking jobs done once (worker=done attempts=1). Not separately tested: your job payloads, retry policy and scheduling; treat the mobile backend-specific behavior as your application code.

## Representative Queries
- Host mobile backend with FastAPI: worker execution and job state
- Where can I host Mobile backend built with FastAPI?
- I need a separately runnable worker, durable job records and idempotent retries; not a managed queue.

## Resource Requirements
- **Compute**: Apps run in isolated x86_64 Firecracker microVMs that auto-pause when idle and wake on request; sizes are the priced VM sizes.
- **Postgres**: 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
1. A repository with the web app and the worker as separate service directories (the fixture uses apps/web and apps/worker), each building and starting with the documented commands (pip install into .ample/python from requirements.txt, then uvicorn from run.py reading PORT on the python-3.12 template; the worker starts with python3 worker.py and binds no port).
2. Both services read DATABASE_URL at runtime and share one jobs table; the worker claims rows with SELECT ... FOR UPDATE SKIP LOCKED and marks them done once.
3. An Ample account token with servers:write and databases:write (ample up creates the database).

## Workflow Steps
1. **Plan the topology**: Run the planner once. It discovers the web and worker services, infers kind = "worker" for the process without a port, declares the database each service needs, and writes ample.toml. Exit 2 means it left questions in the manifest; answer them with --answer or by editing the file. 
   ```bash
   ample plan --write .
   ```
2. **Share one database**: Keep a single [databases.main] with engine = "postgres", drop any per-service database the planner added, and set DATABASE_URL = { database = "main" } under both [services.web.env] and [services.worker.env]. Python services also need an explicit start command in the manifest (the fixture sets start for both services).
   ```bash
   ample plan --offline .
   ```
3. **Apply**: Reconcile the whole manifest in dependency order: the database first, then both services. It is idempotent and never destructive; exit 0 means everything applied, 2 needs input, 1 a failed resource (independent siblings still proceed and re-running resumes).
   ```bash
   ample up .
   ```
4. **Verify**: Fetch the web service URL and its worker status route (/p/worker-queue/status in the example) and confirm the worker processed the enqueued job; on failure read the worker service's runtime logs.
   ```bash
   ample logs --kind runtime
   ```

## Success Checks
- **Web service responds on its public URL**: 
  - **Kind**: http_get
  - **Path**: /  
  - **Expect**: ample canary fastapi patterns
- **Worker consumed the enqueued job**: 
  - **Kind**: http_get
  - **Path**: /p/worker-queue/status  
  - **Expect**: worker=done attempts=1 result=processed-by-worker

## Limitations
- Verified on the python-3.12 template at s-1vcpu-1gb for both services with the example fixture; other sizes, templates and FastAPI major versions are not verified.
- The mobile backend itself (your job payloads, retry policy and scheduling) is application code and was not separately tested; the worker-queue check is what was verified.
- The worker is a single long-running process supervised in its own microVM; there is no scheduler, cron or horizontal worker scaling in this recipe, and the check ran immediately after deploy so idle behavior of the worker VM is not verified.
- The worker service is private: it has no public URL and is reached only through the shared database; both services are placed with their database.
- Managed PostgreSQL 16 only; extensions, connection limits and backup or restore procedures are not verified; apps and their databases are placed together.
- Region, compliance attestations and request-duration limits are unknown and not claimed.

## Cost Estimate
- **Currency**: USD
- **Monthly Amount**: 15.0
- **Components**:
  - Web server: s-1vcpu-1gb, Quantity: 1.0, Monthly Amount: 5.0
  - Worker server: s-1vcpu-1gb, Quantity: 1.0, Monthly Amount: 5.0
  - Managed PostgreSQL database: s-1vcpu-1gb, Quantity: 1.0, Monthly Amount: 5.0
- **Note**: Always-on monthly price of the tested sizes for the web service, the worker service and the database; apps auto-pause when idle.

## Example
- **Title**: FastAPI web + worker fixture
- **Description**: A FastAPI web app (apps/web) and a worker process (apps/worker) sharing one managed PostgreSQL database; the queue module is under tests/deploy-canaries/_pattern-modules.
- **Source Ref**: tests/deploy-canaries/fastapi-worker

## Evidence Summary
- **Kind**: canary_run
- **Summary**: FastAPI web service plus a separate Python worker process deployed on Ample with `ample plan --write` then `ample up` (pip install into .ample/python from requirements.txt, then uvicorn from run.py reading PORT on the python-3.12 template); the web service enqueued a job that the private worker service consumed from the shared managed PostgreSQL database. Pattern proof: a separately deployed worker service (kind worker, no public URL) sharing a declared managed PostgreSQL database with the web service, consuming a jobs table with row locks and marking jobs done once (worker=done attempts=1).
- **Observed At**: 2026-09-20T03:56:46Z
- **Implementation Revision**: 03b6402b1b3e19178985f08ea4033804521d85d4-dirty (CLI b75e104)
- **Expires At**: 2027-03-19T03:56:46Z
- **Scope**: 
  - Checks: ["/","/p/worker-queue/status"] 
  - Pattern: pattern:dedicated-worker-process 
  - Services: ["web","worker"] 
  - Template: python-3.12 
  - Tool: ample_up