How does Uvicorn continuously listen for HTTP requests without serving_forever()?

Uvicorn: Detailed explanation of the continuous listening mechanism of lightweight ASGI server

As a lightweight ASGI-based web server, Uvicorn's ability to continuously listen for HTTP requests is its core feature. However, unlike traditional blocking servers, Uvicorn's code does not use explicit loop calls like server.serve_forever() directly. This article will explore in-depth how Uvicorn implements this clever continuous monitoring mechanism.

The key code snippet for Uvicorn startup contains server = await loop.create_server(...) . This line of code does not directly create a listening loop, but uses the asynchronous event loop mechanism provided by Python's asyncio library. loop.create_server function asynchronously creates a TCP server and registers it into the asyncio event loop.

asyncio event loop itself is an infinite loop that continuously listens to network events, including new connection requests. When a new connection arrives, asyncio automatically calls the protocol handler function specified in create_server (in Uvicorn, this is the function that handles ASGI requests) to handle the connection. Therefore, Uvicorn does not implement continuous listening through explicit serve_forever() , but cleverly utilizes the mechanism of asyncio event loop.

It should be noted that in the example code, if there is no async with server: and await server.serve_forever() , after asyncio.run(main()) function is executed, asyncio event loop will terminate, causing the server to stop listening. Adding these two lines of code is actually to manually control the operation of asyncio event loop. This is not the running mechanism of Uvicorn itself, but to control the server life cycle more granularly in specific scenarios. Uvicorn itself takes full advantage of the features of asyncio to integrate server listening and request processing into the event loop, and can achieve continuous listening without explicitly calling serve_forever() . loop = asyncio.get_running_loop() gets the current running event loop of asyncio . It is this event loop that ensures the continuous running of Uvicorn.

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