ASGI 非同步伺服器閘道介面

ASGI，或是非同步伺服器閘道介面是 Channel 和 Daphne 構建的規範，旨在解開來自特定應用程序服務器的通道應用程序，並提供編寫應用程序和中間件代碼的通用方法。

It’s a spiritual successor to WSGI, designed not only run in an asychronous fashion via asyncio, but also supporting multiple protocols.

The full ASGI spec can be found at https://github.com/django/asgiref/blob/master/specs/asgi.rst

Summary

An ASGI application is a callable that takes a scope and returns a coroutine callable, that takes receive and send methods. It’s usually written as a class:

class Application:

    def __init__(self, scope):
        ...

    async def __call__(self, receive, send):
        ...

The scope dict defines the properties of a connection, like its remote IP (for HTTP) or username (for a chat protocol), and the lifetime of a connection. Applications are instantiated once per scope - so, for example, once per HTTP request, or once per open WebSocket connection.

Scopes always have a type key, which tells you what kind of connection it is and what other keys to expect in the scope (and what sort of messages to expect).

The receive awaitable provides events as dicts as they occur, and the send awaitable sends events back to the client in a similar dict format.

A protocol server sits between the client and your application code, decoding the raw protocol into the scope and event dicts and encoding anything you send back down onto the protocol.

Composability

ASGI applications, like WSGI ones, are designed to be composable, and this includes Channels’ routing and middleware components like ProtocolTypeRouter and SessionMiddeware. These are just ASGI applications that take other ASGI applications as arguments, so you can pass around just one top-level application for a whole Django project and dispatch down to the right consumer based on what sort of connection you’re handling.

Protocol Specifications

The basic ASGI spec only outlines the interface for an ASGI app - it does not specify how network protocols are encoded to and from scopes and event dicts. That’s the job of protocol specifications: