ASGI vs WSGI: Async vs Sync Python Web Server Interfaces
WSGI served Python web development for 20 years. ASGI is its async successor, enabling WebSockets, long-lived connections, and concurrent I/O handling.
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Quick Overview
ASGI
ASGI (Asynchronous Server Gateway Interface) is the modern standard for async Python web applications. It supports HTTP, WebSockets, and other protocols. Used by FastAPI, Starlette, and Django Channels.
WSGI
WSGI (Web Server Gateway Interface) is the traditional standard for synchronous Python web applications. Defined in PEP 3333, it's used by Django, Flask, and most Python web frameworks. Each request blocks a worker.
Architectural Differences
WSGI follows a synchronous request-response model. A WSGI application is a callable that receives the environment and a start_response callback, then returns the response body. Each request occupies one worker thread/process for its entire duration.
ASGI uses an asynchronous message-based model. An ASGI application is a coroutine that receives scope, receive, and send callables. This enables handling multiple concurrent connections per worker and supports long-lived connections like WebSockets.
Performance Impact
For I/O-bound applications (database queries, API calls), ASGI can handle 5-10x more concurrent connections per worker. While one request waits for a database response, the event loop handles other requests.
WSGI applications need one worker per concurrent request. Handling 1000 concurrent connections requires ~1000 worker processes/threads, consuming significant memory.
For CPU-bound work, the difference is minimal since Python's GIL limits true parallelism regardless of the interface.
Feature Comparison Table
| Feature | ASGI | WSGI |
|---|---|---|
| Concurrency Model | Async (event loop) | Sync (thread/process per request) |
| WebSocket Support | Native | Not supported |
| HTTP/2 Support | Yes (via Hypercorn) | Limited |
| Server Examples | Uvicorn, Hypercorn, Daphne | Gunicorn, uWSGI, mod_wsgi |
| Framework Support | FastAPI, Starlette, Django (partial) | Django, Flask, Bottle |
| Memory per Connection | Low (shared event loop) | High (dedicated worker) |
| Maturity | 2016+ | 2003+ (PEP 3333) |
| Ecosystem Size | Growing | Massive |
| Debugging | More complex (async) | Simpler (synchronous) |
Verdict: Which Should You Choose?
Choose ASGI if:
- You're building with FastAPI or another async framework
- WebSocket support is needed
- High concurrency with many simultaneous connections is expected
- I/O-bound operations dominate your workload
- You want to future-proof your Python web stack
Choose WSGI if:
- You're using Flask, Django (without Channels), or another sync framework
- Your application is primarily CPU-bound
- Your team isn't familiar with async Python
- You need maximum ecosystem compatibility
- Simpler debugging and reasoning about code flow is important
Frequently Asked Questions
Can Django use ASGI?
Yes. Django 3.0+ supports ASGI deployment, and Django 4.1+ supports async views. However, the ORM, middleware, and most of the ecosystem remain synchronous. For full async benefits, FastAPI or Starlette are better choices.
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