What is ASGI?

Definition

ASGI (Asynchronous Server Gateway Interface) is the spiritual successor to WSGI, designed to handle asynchronous Python web applications. It provides a standard interface between async web servers and Python frameworks.

ASGI supports HTTP, WebSockets, and other protocols through a single interface. It enables frameworks like FastAPI and Starlette to handle thousands of concurrent connections efficiently.

How ASGI Works

An ASGI application is a callable that takes three arguments:

• scope: A dictionary containing connection information (type, path, headers).
• receive: An async callable to receive incoming messages.
• send: An async callable to send outgoing messages.

Unlike WSGI's synchronous request-response model, ASGI's message-based design supports long-lived connections, streaming, and bidirectional communication.

ASGI Servers

Popular ASGI servers include:

• Uvicorn: Lightning-fast ASGI server built on uvloop and httptools. The most popular choice for FastAPI.
• Hypercorn: ASGI server supporting HTTP/1, HTTP/2, and HTTP/3.
• Daphne: Reference ASGI server, developed as part of the Django Channels project.
• Granian: A Rust-based ASGI server with excellent performance.

Code Example

# Raw ASGI application
async def app(scope, receive, send):
    if scope['type'] == 'http':
        await send({
            'type': 'http.response.start',
            'status': 200,
            'headers': [[b'content-type', b'text/plain']],
        })
        await send({
            'type': 'http.response.body',
            'body': b'Hello from ASGI!',
        })

# In practice, use a framework like FastAPI
# Run with: uvicorn main:app --host 0.0.0.0 --port 8000

Frequently Asked Questions

What is the difference between ASGI and WSGI?

WSGI is synchronous - it handles one request at a time per worker. ASGI is asynchronous - it can handle many concurrent connections with a single worker. ASGI also supports WebSockets and long-lived connections, which WSGI cannot. ASGI is the modern standard for new Python web applications.