ASGI (Asynchronous Server Gateway Interface) is a spiritual successor to WSGI, intended to provide a standard interface between async-capable Python web servers, frameworks, and applications.

Where WSGI provided a standard for synchronous Python apps, ASGI provides one for both asynchronous and synchronous apps, with a WSGI backwards-compatibility implementation and multiple servers and application frameworks.

WSGI

WSGI applications are a single, synchronous callable that takes a request and returns a response; this doesn’t allow for long-lived connections, like you get with long-poll HTTP or WebSocket connections

ASGI

ASGI is structured as a single, asynchronous callable. It takes a scope, which is a dict containing details about the specific connection, send, an asynchronous callable, that lets the application send event messages to the client, and receive, an asynchronous callable which lets the application receive event messages from the client.

FastAPI

• Uses an ASGI server, for production such as Uvicorn or Hypercorn
• NB : You can combine asgi it with Gunicorn, to have an asynchronous multi-process server.