WSGI vs ASGI — Python Web Server Comparison

WSGI (Web Server Gateway Interface, PEP 3333) is the Python standard for synchronous web applications. A WSGI application is a callable that takes (environ, start_response) and returns an iterable of response bytes. The WSGI server (Gunicorn, uWSGI) calls this function per request, blocking until the response is complete. All of Django and Flask's core routing is built on WSGI.

ASGI (Asynchronous Server Gateway Interface) extends WSGI to support async Python. It handles three protocol types: HTTP, WebSocket, and lifespan events. An ASGI app is an async callable: async def app(scope, receive, send). This enables handling WebSocket connections, long-polling, server-sent events, and high-concurrency I/O-bound workloads — things that block an entire WSGI worker thread.

Yes. Django 3.1+ added ASGI support. You can run Django with Uvicorn or Daphne, and use async def views for I/O-bound handlers. However, Django's ORM is synchronous — you must use sync_to_async() wrappers or Django Channels for full async database access. FastAPI is natively async throughout, making it simpler for fully async workloads.

Gunicorn is a mature WSGI server that spawns multiple worker processes. Uvicorn is a lightning-fast ASGI server built on uvloop and httptools. A common production pattern: Gunicorn as the process manager with Uvicorn workers (--worker-class uvicorn.workers.UvicornWorker). This gives you Gunicorn's battle-tested process management with Uvicorn's async performance.

Not necessarily. ASGI makes I/O-bound concurrency faster — handling many simultaneous connections with long waits (DB queries, external APIs, WebSockets). CPU-bound workloads see no benefit from async. A poorly written async app (sync blocking code in an async function) can be slower than WSGI. The key benefit is handling many concurrent connections with fewer OS threads.

ASGI frameworks: FastAPI (full native support), Starlette (ASGI core), Django 3.1+ (partial, with sync ORM), Quart (async Flask clone), Litestar (formerly Ormar). WSGI-only frameworks: Flask, Bottle, Pyramid, Falcon (WSGI mode). Uvicorn, Daphne, and Hypercorn are the main ASGI servers. Gunicorn with UvicornWorker bridges WSGI management with ASGI performance.