Pausing and resuming a socket server in Python

I needed to write a socket server in Python that would allow me to intermittently pause the server loop for a while, run something else, then get back to the previous request-handling phase; repeating this iteration until the heat death of the universe. Initially, I opted for the low-level socket module to write something quick and dirty. However, the implementation got hairy pretty quickly. While the socket module gives you plenty of control over how you can tune the server’s behavior, writing a server with robust signal and error handling can be quite a bit of boilerplate work.

Thankfully, I found out that Python is already shipped with a higher level library named socketserver¹ that uses the socket module underneath but gives you more tractable hooks to latch onto and build fairly robust servers where the low-level details are handled for you. Not only that, socketserver makes it easy to write a sever that can concurrently handle multiple clients either by spinning child threads or forking child processes.

While all this sounds good and dandy, my primary objective was to be able to write a server that can pause serving the clients every now and then, do some work and then come back to the previous work. Here’s how I did it with a multi-threaded socket server:

from __future__ import annotations

import logging
import socket
import socketserver
import time

logging.basicConfig(level=logging.INFO)


class RequestHandler(socketserver.BaseRequestHandler):
    def setup(self) -> None:
        logging.info("Start request.")

    def handle(self) -> None:
        conn = self.request
        while True:
            data = conn.recv(1024)

            if not data:
                break

            logging.info(f"recv: {data!r}")
            conn.sendall(data)

    def finish(self) -> None:
        logging.info("Finish request.")


class ThreadingTCPServer(socketserver.ThreadingTCPServer):
    _timeout = 5  # seconds
    _start_time = time.monotonic()

    def server_activate(self) -> None:
        logging.info("Server started on %s:%s", *self.server_address)
        super().server_activate()

    def get_request(self) -> tuple[socket.socket, str]:
        conn, addr = super().get_request()
        logging.info("Connection from %s:%s", *addr)
        return conn, addr

    def service_actions(self) -> None:
        if time.monotonic() - self._start_time > self._timeout:
            logging.info("Server paused, something else is running...")
            self._start_time = time.monotonic()


if __name__ == "__main__":
    with ThreadingTCPServer(("localhost", 9999), RequestHandler) as server:
        server.serve_forever()

This is a simple echo server that receives client connections and reflects back the data sent by the clients. The server can handle multiple client connections simultaneously using the ThreadingTCPServer class. This class is derived from the socketserver.ThreadingTCPServer class and is responsible for implementing the server’s main loop, which listens for incoming client connections and creates a separate thread for each one to handle the incoming request.

The RequestHandler class is used to handle each incoming request. This class is derived from the socketserver.BaseRequestHandler class and is responsible for handling the connection between a client and the server. It implements the setup, handle, and finish methods to perform any necessary initialization work, handle the incoming data, and clean up after the request has been processed. In the setup and finish methods, we’re only printing some message to indicate that these methods are called before and after the handle method respectively. In the handle method, we’re collecting the data sent by the clients and echoing them back. Here, inside the while loop, conn.recv is a blocking method and will keep reading from the clients indefinitely. We need the server to break out from this, do something else, and then get back to it gracefully.

In the __main__ section of the code snippet, a ThreadingTCPServer object is created and the server is started using the serve_forever method. This method will continuously run the server loop, listen for incoming connections and create a separate thread for each one to handle the request.

The ThreadingTCPServer class implements server_activate and get_request methods. These two methods are already implemented in the base and we’re just calling the methods from there with some additonal logging. Here, server_activate prints out the server’s IP address and port. Similarly, the get_request method calls the eponymous method from the superclass and logs the IP and the port of the incoming clients.

The server also implements a service_actions method that is called by the server loop. This is where we’re periodically pausing the server and performing some blocking actions. In this case, the service_actions method checks the current time and compares it to the start time of the server. If the difference is greater than the specified timeout, the server is paused and a message is printed to the console indicating that something else is running. Then after one iteration, the start time is updated so that the server gets paused again after the timeout period.

To test the server out, here’s a simple client that sends some data to the server:

# client.py

import socket
import time
import logging

logging.basicConfig(level=logging.INFO)

HOST = "localhost"  # The server's hostname or IP address.
PORT = 9999  # The port used by the server.

with socket.socket(socket.AF_INET, socket.SOCK_STREAM) as s:
    s.connect((HOST, PORT))
    while True:
        time.sleep(1)
        s.sendall(b"hello world")
        data = s.recv(1024)
        logging.info(f"Received {data!r}")

This client connects to the server via port 9999 and sends the b'hello world' byte string. The server will capture and echo it back to the client which the client will print as Received .... You can run the server in one console with python server.py and the client in another one with the python client.py command.

You’ll see that the server will pause every 5 seconds, do something else in a blocking manner and then come back to handle the client requests. If you attach a second client from another console, you’ll see that the server can also handle that while retaining the expected behavior. The server will pause even if there’s no client sending requests to the server. You can test that behavior by detaching all the clients from the server.

Now, we could also make the work in the serving_actions non-blocking by spinning a new thread or process and doing the work there. However, for the task that I was tackling, simply running the function in a blocking manner was enough.

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