FastAPI AnyIO Structured TaskGroup Cooperative Cancellation Invariant

--- description: FastAPI AnyIO Structured TaskGroup Cooperative Cancellation Invariant - Enforces strict structured concurrency task tree scoping across all FastAPI routes and dependencies using AnyIO TaskGroups. Eliminates orphaned background coroutine leaks by ensuring task cancellation cascades cooperatively when an HTTP client disconnects or an upstream timeout triggers. globs: app/**/*.py, src/**/*.py, api/**/*.py alwaysApply: true verified_by: "agentrules.org" license: "CC-BY-4.0" source: "https://agentrules.org/rules/fastapi-anyio-taskgroup-cancellation-invariant" --- # FastAPI AnyIO Structured TaskGroup Cooperative Cancellation Invariant > **Severity**: CRITICAL | **Category**: backend > **Anti-Pattern Warning**: Spawning Unbound Coroutines via asyncio.create_task Leading to Zombie Process Leaks on Client Disconnect ## System Constraints You are a Principal Software Architect enforcing strict structured concurrency and cooperative cancellation invariants in FastAPI applications using AnyIO. You must reject any naked `asyncio.create_task`, fire-and-forget coroutines, or naked `asyncio.gather` calls within request lifecycles. Ensure every child operation is bound to an `anyio.create_task_group()` context or delegated to an out-of-process durable queue (e.g., Celery, ARQ). Cooperative cancellation exceptions (such as `anyio.get_cancelled_exc_class()`) must NEVER be caught and suppressed. ## Strict Guidelines - Engineers MUST ALWAYS manage concurrent sub-tasks within an `async with anyio.create_task_group() as tg:` context block. - Engineers MUST NOT spawn detached coroutines using `asyncio.create_task()`, `asyncio.ensure_future()`, or Starlette `BackgroundTasks` for unbounded or resource-heavy operations. - Code MUST NEVER suppress or catch AnyIO cancellation exceptions (`except Exception:` must re-raise or code must catch `Exception` only after AnyIO cancellation classes are allowed to propagate). - Services MUST explicitly use `anyio.CancelScope(shield=True)` ONLY around critical, non-idempotent rollback or cleanup operations that must execute during teardown. - Components NEVER leak unawaited tasks past the termination of an ASGI request scope. ## Bad vs Good Implementations ### Concurrent Fan-Out in FastAPI Endpoint #### Bad Antipattern (AI Common Hallucination) > **Why it fails**: Using `asyncio.create_task` and `asyncio.gather` creates unmanaged coroutines detached from FastAPI's ASGI cancellation signals. If a client terminates the connection mid-flight, Starlette cancels the endpoint, but `user_task` and `order_task` can continue executing indefinitely as zombies, consuming database connections and computing power. ```python import asyncio from fastapi import APIRouter, HTTPException router = APIRouter() async def fetch_user_data(user_id: str) -> dict: await asyncio.sleep(2) # Simulates I/O return {"id": user_id, "profile": "data"} async def fetch_order_history(user_id: str) -> list: await asyncio.sleep(3) # Simulates I/O return [{"order": 101}] @router.get("/users/{user_id}/dashboard") async def get_dashboard(user_id: str): # ANTI-PATTERN: asyncio.create_task leaves tasks orphaned if client disconnects user_task = asyncio.create_task(fetch_user_data(user_id)) order_task = asyncio.create_task(fetch_order_history(user_id)) try: user, orders = await asyncio.gather(user_task, order_task) return {"user": user, "orders": orders} except Exception as e: # Swallows cancellation or leaves background tasks running upon early abort raise HTTPException(status_code=500, detail=str(e)) ``` #### Production Standard (Correct) > **Standard**: By utilizing `anyio.create_task_group()`, all child tasks are bound to the parent HTTP request lifecycle. If the client disconnects, ASGI propagates cancellation to the task group, which immediately and cooperatively terminates all active sub-tasks, preventing resource and memory leaks. ```python import anyio from fastapi import APIRouter, HTTPException router = APIRouter() async def fetch_user_data(user_id: str, results: dict) -> None: await anyio.sleep(2) # Cooperative cancellation point results["user"] = {"id": user_id, "profile": "data"} async def fetch_order_history(user_id: str, results: dict) -> None: await anyio.sleep(3) # Cooperative cancellation point results["orders"] = [{"order": 101}] @router.get("/users/{user_id}/dashboard") async def get_dashboard(user_id: str): results: dict = {} try: # INVARIANT: Structured concurrency guarantees all children cancel if parent cancels async with anyio.create_task_group() as tg: tg.start_soon(fetch_user_data, user_id, results) tg.start_soon(fetch_order_history, user_id, results) except ExceptionGroup as eg: raise HTTPException(status_code=500, detail=f"Aggregation failed: {eg}") return results ```

Architectural Specification & Invariant Rationale

Architectural Invariant Design: FastAPI AnyIO Structured TaskGroup Cooperative Cancellation Invariant

FastAPI AnyIO Structured TaskGroup Cooperative Cancellation Invariant defines an industrial-grade behavioral standard for backend engineering. Enforces strict structured concurrency task tree scoping across all FastAPI routes and dependencies using AnyIO TaskGroups. Eliminates orphaned background coroutine leaks by ensuring task cancellation cascades cooperatively when an HTTP client disconnects or an upstream timeout triggers. Modern software architectures require strict adherence to deterministic patterns to guarantee high availability, maintainability, and zero security vulnerabilities.

Framework Paradigm Shift & Drift MechanicsLLMs are trained on millions of open-source repositories containing outdated code, legacy idioms, and unpatched antipatterns. When coding assistants generate implementations for this stack, they default to common statistical tokens: Spawning Unbound Coroutines via asyncio.create_task Leading to Zombie Process Leaks on Client Disconnect Without an explicit system instruction file, developers spend hours debugging silent failures.

Failure Modes & Production Blast Radius

Hazard Analysis: What Breaks Without This Rule

The following failure modes have been cataloged from hundreds of hours of production code reviews where AI assistants produced non-compliant implementations:

Failure Mode	AI Symptom (The Anti-Pattern)	Severity	Production Impact & Remedy
AI Hallucination & Deprecation Drift	Spawning Unbound Coroutines via asyncio.create_task Leading to Zombie Process Leaks on Client Disconnect	Critical	Violating this industrial standard introduces regression bugs, security risks, and unexpected runtime failures in production. Enforce explicit behavioral system directives across all AI coding assistants in your repository.
Silent Architecture Inconsistency	AI assistants produce mixed paradigms across files, creating fragmented codebase conventions.	Medium	Increases cognitive load during code reviews and leads to maintenance debt across engineering teams. Pin project conventions in .cursor/rules, CLAUDE.md, and AGENTS.md so all team agents follow identical guardrails.

Architectural Directives in Detail

Step-by-Step Production Directives

Each rule constraint below is injected into the AI agent's system prompt to establish deterministic boundaries:

Engineers MUST ALWAYS manage concurrent sub-tasks within an `async with anyio.create_task_group() as tg

Engineers MUST ALWAYS manage concurrent sub-tasks within an `async with anyio.create_task_group() as tg:` context block.

Rationale:AI models frequently generate unconstrained code here because statistical language distributions lack project-specific architectural context. Mandating this directive ensures that generated code adheres to verified industrial production requirements.