ASGI Spec & High-Concurrency Servers

Learning outcomes

• Understand ASGI scope and channel events
• Deploy high-concurrency async servers

Mental model

ASGI (Asynchronous Server Gateway Interface) is the async successor to WSGI, defining a standardized interface between async web servers (Uvicorn, Granian) and Python frameworks (FastAPI, Starlette).

Theory

An ASGI 3.0 application is a single async callable with signature app(scope, receive, send):

• scope: Dictionary containing request metadata (HTTP method, headers, path).
• receive: Async function to receive incoming request body chunk events (http.request).
• send: Async function to transmit outgoing response events (http.response.start, http.response.body).

async def raw_asgi_app(scope: dict, receive: callable, send: callable):
    assert scope["type"] == "http"
    event = await receive()

    await send({
        "type": "http.response.start",
        "status": 200,
        "headers": [[b"content-type", b"application/json"]],
    })
    await send({
        "type": "http.response.body",
        "body": b'{"status": "ok", "framework": "raw-asgi"}',
    })

Alternatives and trade-offs

• WSGI (Gunicorn): Synchronous single-request-per-worker model. Fails at WebSockets or streaming.
• Uvicorn (uvloop + httptools): Async server using Cython bindings for libuv and C HTTP parser.
• Granian (Rust core): Rust-based ASGI/WSGI server using hyper for high throughput.

Failure modes and misconceptions

1. WSGI Middleware on ASGI: Wrapping WSGI blocking middleware around FastAPI breaks asynchronous execution.
2. Missing http.response.body: Forgetting to send the body event leaves HTTP client connections hanging.

Decision scenario

Deploy production FastAPI applications with multi-worker ASGI process managers (Gunicorn + Uvicorn workers or Granian) to maximize async multi-core CPU throughput.

Learning outcomes

• Deconstruct the raw app(scope, receive, send) ASGI specification callable.
• Compare WSGI synchronous request lifecycles with ASGI asynchronous event channels.
• Deploy Uvicorn and Granian servers with optimal process worker topologies.

Trade-offs

ASGI servers support high-concurrency HTTP/WebSocket connections, but require non-blocking async architecture throughout all route handlers.

Primary sources

• FastAPI Framework Architecture & Dependency Injection Specification — Tiangolo, verified 2026-07-22