Python Async & FastAPI Scaling

Guidance for writing correct, non-blocking async Python and for adapting that code to distributed environments where multiple instances run concurrently.

Core principle

Single-instance async and distributed async solve different problems:

• Single instance: don't block the event loop; maximize concurrency within one process.
• Distributed (K8s/Lambda/Container Apps + queues): don't overload shared resources (databases, third-party APIs) across N instances; survive instance death mid-task; make work resumable/idempotent.

A semaphore, lock, or rate limiter that only exists in local process memory does nothing for a fleet of pods. That's the #1 mistake this skill exists to prevent.

Used alongside another skill

This skill is frequently pulled in mid-task by code-reviewer, feature-coder, or python-tutor when the code in front of them turns out to be async, FastAPI-based, or queue/worker-based. When that happens, don't replace the calling skill's workflow (review steps, TDD phases, hint format) — feed this skill's checklist items and reference guidance into whatever step is currently active (e.g. code-reviewer's "Performance" step, or feature-coder's Phase 1 planning/Phase 5 implementation). Only run this skill standalone, with its own pass over all reference files, when no other skill is already driving the task.

How to use this skill

1. Identify which situation applies and read the matching reference file before writing code:

2. Default to checking all relevant files for a task that touches FastAPI + scaling — most real tasks span 2-3 of these files (e.g., a new endpoint that calls a rate-limited third-party API needs fastapi-patterns.md + resilience-patterns.md).
3. When generating code, always state explicitly which concurrency boundary is being enforced and at what layer (in-process vs. cluster-wide), since that's the detail most often glossed over.
4. If the task maps cleanly onto one of the everyday scenarios below, go straight to its checklist in references/checklists.md instead of re-deriving it from the pattern files — it's the same guidance, pre-assembled for that scenario:
   • Writing a new async FastAPI endpoint
   • Adding a queue consumer / background worker
   • Calling a third-party / external API
   • Reviewing a PR that touches async code
   • Preparing to scale out (1 instance → N, or raising N)
   • Debugging "it hangs" / "it's slow" / "works on one pod but not at scale"
   • Writing tests for async code

Quick mistake checklist (apply before finalizing any async/FastAPI code)

• Any blocking call (requests, time.sleep, sync DB driver, open()) inside an async def? → fix or move to a thread/def route.
• Any asyncio.Semaphore/Lock/in-memory rate limiter meant to control cross-instance behavior? → must move to Redis or the queue layer instead.
• Any asyncio.create_task(...) without keeping a reference or joining it? → risk of silent drop, especially on Lambda freeze or pod SIGTERM.
• Any distributed lock without a TTL/expiry? → risk of permanent deadlock if the holder crashes.
• Any queue consumer without a bounded concurrency (prefetch_count / semaphore)? → reintroduces the overload problem the queue was meant to solve.
• Any job handler that isn't idempotent (including a provider-side idempotency key for side-effecting external calls)? → at-least-once delivery + retries will double-process it eventually.
• Any request handler doing work that could exceed the load balancer/gateway timeout? → should enqueue + return a job id instead of awaiting inline.
• Graceful shutdown handled (SIGTERM, Lambda freeze) so in-flight async work isn't silently dropped?
• Any background task created before the request-scoped contextvars it depends on are set? → it captured a snapshot at creation time, not a live view.
• Any async generator wrapping a resource (DB cursor, file, stream) relying on GC to run its finally block on early exit? → close it explicitly (aclose() / contextlib.aclosing).

Full explanations and code examples for each item are in the reference files above. This list is the fast, universal pass — for a deeper, scenario-specific version (e.g. "I'm specifically reviewing a PR" or "I'm specifically adding a queue consumer"), use references/checklists.md.