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Hello ASGI

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Python programming language has WSGI (Web Server Gateway Interface) standard, where web servers can talk to apps using same standartized protocol. Simplest WSGI application looks like this:

import json

def wsgi(environ, start_response):
    start_response('200 OK', [
        ('Content-Type', 'application/json'),
    ])
    yield json.dumps(environ, default=repr).encode()

Response:

{
  "wsgi.errors": "<gunicorn.http.wsgi.WSGIErrorsWrapper object at 0x7fe381c1aef0>",
  "wsgi.version": [1, 0],
  "wsgi.multithread": false,
  "wsgi.multiprocess": false,
  "wsgi.run_once": false,
  "wsgi.file_wrapper": "<class 'gunicorn.http.wsgi.FileWrapper'>",
  "SERVER_SOFTWARE": "gunicorn/19.7.1",
  "wsgi.input": "<gunicorn.http.body.Body object at 0x7fe381c1af60>",
  "gunicorn.socket": "<socket.socket fd=9, family=AddressFamily.AF_INET, type=SocketKind.SOCK_STREAM, proto=0, laddr=('127.0.0.1', 8000), raddr=('127.0.0.1', 59066)>",
  "REQUEST_METHOD": "GET",
  "QUERY_STRING": "",
  "RAW_URI": "/",
  "SERVER_PROTOCOL": "HTTP/1.1",
  "HTTP_HOST": "localhost:8000",
  "HTTP_USER_AGENT": "curl/7.59.0",
  "HTTP_ACCEPT": "*/*",
  "HTTP_CONNECTION": "Upgrade, HTTP2-Settings",
  "HTTP_UPGRADE": "h2c",
  "HTTP_HTTP2_SETTINGS": "AAMAAABkAARAAAAAAAIAAAAA",
  "wsgi.url_scheme": "http",
  "REMOTE_ADDR": "127.0.0.1",
  "REMOTE_PORT": "59066",
  "SERVER_NAME": "127.0.0.1",
  "SERVER_PORT": "8000",
  "PATH_INFO": "/",
  "SCRIPT_NAME": ""
}

Now there is a new standard emerging, called ASGI (Asynchronous Server Gateway Interface). This new standard tries to fix WSGI limitations and provides more flexible interface, that can support multiple protocols like HTTP, HTTP/2, WebSockets and others. In addition ASGI provides possibility to use asyncio in web apps. Simples ASGI application looks like this:

import json


def asgi(scope):
    async def chat(receive, send):
        await receive()
        await send({
            'type': 'http.response.start',
            'status': 200,
            'headers': [
                [b'Content-Type', b'application/json'],
            ],
        })
        await send({
            'type': 'http.response.body',
            'body': json.dumps(scope, default=bytes.decode).encode(),
        })
        await send({
            'type': 'http.disconnect',
        })
    return chat

Response:

{
  "type": "http",
  "http_version": "2",
  "method": "GET",
  "path": "/",
  "root_path": "",
  "scheme": "https",
  "query_string": "",
  "headers": [
    ["host", "localhost:8000"],
    ["user-agent", "curl/7.59.0"],
    ["accept", "*/*"]
  ],
  "client": ["127.0.0.1", 59012],
  "server": ["127.0.0.1", 8000]
}

ASGI is a bit more complicated, but gives support for modern web protocols and asyncio.