What is ASGI?

ASGI stands for Asynchronous Server Gateway Interface.

It is a standard interface that defines how Python web applications communicate with web servers — especially for asynchronous (async) apps.

1️⃣ Why ASGI Exists

Before ASGI, Python mostly used WSGI.

🔸 WSGI (older)

• Handles only synchronous requests
• One request blocks one worker
• Good for simple apps (Django old-style, Flask)

🔸 ASGI (newer)

• Supports asynchronous code
• Handles many connections at once
• Required for:
  • WebSockets
  • Real-time apps
  • Streaming
  • FastAPI

2️⃣ Simple Definition (Easy)

ASGI is a bridge between your Python app and the web server that lets async code work properly.

3️⃣ Where ASGI Is Used

ASGI is used in:

• FastAPI
• Starlette
• Django (async mode)
• WebSockets
• Background tasks

4️⃣ ASGI Architecture (Simple)

Client (Browser)
      ↓
ASGI Server (Uvicorn, Daphne, Hypercorn)
      ↓
ASGI Application (FastAPI)
      ↓
Python Code (async / await)

5️⃣ ASGI vs WSGI (Quick Table)

Feature	WSGI	ASGI
Async support	❌ No	✅ Yes
WebSockets	❌ No	✅ Yes
Concurrency	Limited	High
Real-time apps	❌ No	✅ Yes
Used by FastAPI	❌	✅

6️⃣ Simple ASGI Example

async def app(scope, receive, send):
    await send({
        "type": "http.response.start",
        "status": 200,
        "headers": []
    })
    await send({
        "type": "http.response.body",
        "body": b"Hello ASGI"
    })

You normally don’t write this directly — FastAPI does it for you.

7️⃣ What is an ASGI Server?

An ASGI server runs your app:

Common ASGI servers:

• Uvicorn (most popular)
• Daphne
• Hypercorn

Example:

uvicorn main:app

8️⃣ Why FastAPI Needs ASGI

FastAPI uses:

• async def endpoints
• Non-blocking I/O
• High performance

ASGI makes this possible.

9️⃣ Real-Life Analogy

Imagine a restaurant:

• WSGI = One waiter per table (slow)
• ASGI = One waiter handling many tables efficiently

🔟 Final One-Line Summary

ASGI is the modern Python standard that allows asynchronous, high-performance web applications like FastAPI to work.

Keep Learning 🙂