page:recipes:rest api:fastapi:worker state

Host rest api with FastAPI: worker execution and job state

Summary

Deploy a rest api 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.

Representative Queries

• Host rest api with FastAPI: worker execution and job state
• Where can I host REST API built with FastAPI?
• I need a separately runnable worker, durable job records and idempotent retries; not a managed queue.

Resource Requirements

• Primitive: Compute
• Primitive: Postgres

Infrastructure 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.

Framework

FastAPI

Workload

REST API

Release Status

Published

Support Status

Verified

Execution Status

Ready

Prerequisites

• 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).
• 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.
• 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.
   Command: 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).
   Command: 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).
   Command: 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.
   Command: ample logs --kind runtime

Limits and Considerations

• 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 worker is a single long-running process supervised in its own microVM; there is no scheduler, cron or horizontal worker scaling in this recipe.

Cost Estimate

• Currency: USD
• Monthly Amount: 15.0
• Basis: Size prices from pricing.toml (loaded by the API) at build revision 1ac5595375130d45290090e82ed0f554ccd45405-dirty.

Components

• Web server: s-1vcpu-1gb, $5.0
• Worker server: s-1vcpu-1gb, $5.0
• Managed PostgreSQL database: s-1vcpu-1gb, $5.0

Evidence Summary

FastAPI web service plus a separate Python worker process deployed on Ample with ample plan --write then ample up. The web service enqueued a job that the private worker service consumed from the shared managed PostgreSQL database.

Next Actions

• Browse the catalog index: /v1/catalog
• Search published recipes by intent, stack and constraints: POST /v1/catalog/search

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 Reference: tests/deploy-canaries/fastapi-worker