Institute of Information Science - University of Miskolc

To illustrate the difference with a blocking socket approach, we'll create a simple blocking TCP server and a corresponding client. This server will handle one connection at a time in a blocking manner, meaning it will wait (or block) on I/O operations like accepting new connections or receiving data.

import socket

HOST = '127.0.0.1'  # Standard loopback interface address (localhost)
PORT = 65432        # Port to listen on (non-privileged ports are > 1023)

# Create a socket
with socket.socket(socket.AF_INET, socket.SOCK_STREAM) as server_socket:
    server_socket.bind((HOST, PORT))
    server_socket.listen()
    print(f"Server is listening on {HOST}:{PORT}")

    while True:
        # Accept a new connection
        conn, addr = server_socket.accept()
        with conn:
            print(f"Connected by {addr}")
            while True:
                data = conn.recv(1024)
                if not data:
                    break  # No more data from client, close connection
                print(f"Received {data.decode()} from {addr}")
                response = "This is a response from the server.".encode()
                conn.sendall(response)

import socket

HOST = '127.0.0.1'  # The server's hostname or IP address
PORT = 65432        # The port used by the server

# Create a socket
with socket.socket(socket.AF_INET, socket.SOCK_STREAM) as s:
    s.connect((HOST, PORT))
    print("Connected to the server")

    # Send data
    message = 'Hello, server'.encode()
    s.sendall(message)
    print("Message sent to the server")

    # Wait for a response
    data = s.recv(1024)
    print("Received response from the server")

print(f"Received: {data.decode()}")

Creating a non-blocking TCP socket server in Python involves setting up a socket to listen for connections without blocking the main execution thread of the program. Below is a simple example of a non-blocking TCP server that accepts multiple client connections and handles them asynchronously. This server uses the select method, which is a way to check for I/O readiness on sockets, making it possible to manage multiple connections without blocking on any single one.

import socket
import select

HOST = '127.0.0.1'  # Standard loopback interface address (localhost)
PORT = 65432        # Port to listen on (non-privileged ports are > 1023)

# Create a socket
server_socket = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
server_socket.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEADDR, 1)

# Bind the socket to the address and port
server_socket.bind((HOST, PORT))

# Listen for incoming connections
server_socket.listen()

print(f"Listening on {HOST}:{PORT}")

# Set the server socket to non-blocking mode
server_socket.setblocking(0)

# Keep track of input sockets
inputs = [server_socket]
outputs = []

while inputs:
    # Wait for at least one of the sockets to be ready for processing
    readable, writable, exceptional = select.select(inputs, outputs, inputs)

    for s in readable:
        if s is server_socket:
            # Accept new connection
            connection, client_address = s.accept()
            print(f"New connection from {client_address}")
            connection.setblocking(0)
            inputs.append(connection)
        else:
            data = s.recv(1024)
            if data:
                # A readable client socket has data
                print(f"Received {data} from {s.getpeername()}")
                # Add output channel for response
                if s not in outputs:
                    outputs.append(s)
            else:
                # Interpret empty result as closed connection
                print(f"Closing {client_address}")
                if s in outputs:
                    outputs.remove(s)
                inputs.remove(s)
                s.close()

    for s in writable:
        response = b'This is a response from the server.'
        s.send(response)
        # Once response has been sent, we don't need to write anymore
        outputs.remove(s)

    for s in exceptional:
        print(f"Handling exceptional condition for {s.getpeername()}")
        # Stop listening for input on the connection
        inputs.remove(s)
        if s in outputs:
            outputs.remove(s)
        s.close()

To test the non-blocking TCP server, you can create a simple client that connects to the server, sends a message, and then waits to receive a response. Below is an example of a basic TCP client in Python that interacts with our non-blocking server.

import socket

HOST = '127.0.0.1'  # The server's hostname or IP address
PORT = 65432        # The port used by the server

# Create a socket
with socket.socket(socket.AF_INET, socket.SOCK_STREAM) as s:
    # Connect to the server
    s.connect((HOST, PORT))
    print("Connected to server")

    # Send data
    message = 'Hello, server'.encode()
    s.sendall(message)
    print("Message sent to server")

    # Wait for a response
    data = s.recv(1024)
    print("Received response from server")

print(f"Received: {data.decode()}")