How to implement multi-process concurrent processing of HTTP requests?

Detailed explanation of Uvicorn multi-process model and HTTP request allocation mechanism

This article discusses in depth how Uvicorn can implement the concurrent processing of HTTP requests by multiple processes, focusing on the mechanism by which multiple processes listen to the same socket at the same time and the allocation strategy of HTTP requests among multiple worker processes. The key to understanding this is to master the principle of Uvicorn (and similar servers Gunicorn and uWSGI) to efficiently handle concurrent HTTP requests.

Uvicorn does not directly allow multiple processes to listen to the same socket at the same time, because the operating system does not allow this operation. It adopts the "listener-worker process" model: a main process (master process) is responsible for listening to the specified socket. When an HTTP request arrives, the main process forwards the request to one of the multiple worker processes (worker processes) it manages.

So, how does this forwarding mechanism work? The operating system will not send the request directly to the worker process. The main process receives all arriving requests and assigns the requests to an idle worker process through the Inter-process Communication (IPC) mechanism. This is not an operating system poll or a worker process actively preempts, but rather the main process maintains a worker process pool and allocates according to preset policies (such as polling or more complex algorithms). The specific allocation strategy depends on Uvicorn's configuration, but the goal is always to balance the load and maximize server throughput.

Gunicorn and uWSGI also adopt a similar architecture, managing multiple worker processes through the main process to process concurrent requests, and relying on the IPC mechanism to allocate requests, but the specific implementation details may vary. These servers usually provide multiple worker types (such as prefork, gevent), each type corresponds to different request processing and allocation strategies, and the ultimate goal is to optimize resource utilization and server performance.

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