ASGI

ASGI (Asynchronous Server Gateway Interface) is a specification for Python web servers and applications that supports asynchronous and synchronous execution. It was designed as a replacement for WSGI (Web Server Gateway Interface) to enable high-performance, event-driven applications such as WebSockets, long polling, and background tasks.

Key Features of ASGI

• Supports Asynchronous and Synchronous Code: Works with both async and traditional blocking Python code.
• Handles Multiple Protocols: Supports HTTP, WebSockets, and other real-time communication.
• Non-Blocking Execution: Ideal for handling high-concurrency applications.
• Event-Driven: Can process requests in parallel, unlike WSGI.

ASGI Application Structure

An ASGI application is an async function that accepts three parameters:

• scope: Contains request metadata (e.g., type, headers, method).
• receive: An async function to receive incoming messages.
• send: An async function to send responses.

Example: Minimal ASGI App

async def app(scope, receive, send):
    assert scope["type"] == "http"
 
    response_body = b"Hello, ASGI!"
    await send({
        "type": "http.response.start",
        "status": 200,
        "headers": [(b"content-type", b"text/plain")],
    })
    await send({
        "type": "http.response.body",
        "body": response_body,
    })

This simple ASGI app handles an HTTP request and returns "Hello, ASGI!".

Lifespan Events

ASGI applications support startup and shutdown events for resource management.

async def app(scope, receive, send):
    if scope["type"] == "lifespan":
        while True:
            message = await receive()
            if message["type"] == "lifespan.startup":
                print("Application starting...")
            elif message["type"] == "lifespan.shutdown":
                print("Application shutting down...")
                break

These events can be used for database connections, background tasks, and resource cleanup.

In order to understand ASGI & WSGI standards better I suggest also watching this playlist