Simple Python Non-blocking Socket Server
I have been looking for a way to make a simple non-blocking socket server. I found a few examples on the net, that use
select but they were complex and didn't show how to handle messages.
How did I get to reaching this question?
The simplest way to make a socket server for listening to raw socket connections (such as sensors in my case) is something like this (blocking):
server_socket = socket.socket(socket.AF_INET, socket.SOCK_STREAM) server_socket.bind(("0.0.0.0", port_number)) server_socket.listen(max_clients) while True: client_socket, client_address = server_socket.accept() handle_client(client_socket, client_address)
The problem with this is that is stalls at
server_socket.accept() and doesn't continue until there is a connection.
Bad, becuase it keeps the socket open indefinitely. You could put it in a thread, but then it gets ugly when you need to close the server, sometimes the thread persists in the background.
So I web-searched for "non-blocking python socket" and eventually found some examples that used
select but as mentioned, they are quite complex, handling every scenario when many are unnecessary (eg when you just need to receive messages).
"select" is a function available in most operating systems, and from my basic understanding, is a handler for inputs and outputs from processes, or functions in programming in this case.
For a simple server that just listens and does not respond, I find this code works well:
def handle_client(client, address): request_bytes = b"" + client.recv(1024) if not request_bytes: print("Connection closed") client.close() request_str = request_bytes.decode() print(request_str) server_socket = socket.socket(socket.AF_INET, socket.SOCK_STREAM) server_socket.bind(("0.0.0.0", port_number)) server_socket.listen(max_clients) inputs = [server_socket] outputs =  while True: readable, writable, exceptional = select.select( inputs, outputs, inputs, 1) for s in readable: if s is server_socket: connection, client_address = s.accept() connection.setblocking(0) inputs.append(connection) threading.Thread(target=handle_client, args=(connection, client_address)).start()
Note the "1" argument in
select.select() which has a timeout of 1 second, meaning it will keep looping through the
while loop regardless if there is a connection.
I tested it with sending messages every 150ms, using this code:
import socket import time data = "Hello Server!"; i = 0 while i < 15: clientSocket = socket.socket(socket.AF_INET, socket.SOCK_STREAM); clientSocket.connect(("127.0.0.1", 9001)); data_o = data + str(i) clientSocket.send(data_o.encode()); time.sleep(0.15) i += 1 clientSocket.close()
And it works fine, receving all messages.
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