listen() method parameters in python

Question

I'm new to Python and interested in socket programming these days. From the youtube videos, I'm building a simple server but I didn't get listen() method very well. I know "it listens" incoming connections but I didn't get the the idea of "maximum queued connections" in the document. Could you explain this concept in layman's terms so that I can understand better?

Answer 1

socket.listen in Python calls the underlying listen syscall:

listen() marks the socket referred to by sockfd as a passive socket, that is, as a socket that will be used to accept incoming connection requests using accept(2).

A passive socket is the one you'd informally call the server.

The backlog argument defines the maximum length to which the queue of pending connections for sockfd may grow. If a connection request arrives when the queue is full, the client may receive an error with an indication of ECONNREFUSED or, if the underlying protocol supports retransmission, the request may be ignored so that a later reattempt at connection succeeds.

In other words, when you call sock.listen(5) and 6 connection requests come in before you call accept, one of them is getting dropped. In practice, the value is only a hint to the OS.

Unless your application or its usage scenario is extraordinary, pass in any value - 5 is often quoted - and be done. Just make sure that you don't have too much overhead between accept calls, and the queue of pending connections will never be full and only rarely used in the first place.