Appendix_Socket_server_sample

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Create a server that expose server_socket listen on a specified port (5555 in my case)
The server_port is register in selector object which can be viewed as book keeping
Keep a loop to monitor all of events comming in server and check with the selector object. If the event matching with listening then process:
- Accept connection: In first time client connect. This actually register client socket
- Handle connection (after finishing acceptance): From client socket which already register in selector, we can:
  - read data from socket sock.recv
  - send data from socket sock.sendall

For testing:
- Run this code in server: python this-code.py
- From client (for example in the same machine):
  - In MacOS
```
nc 127.0.0.1 5555
```
  - In Windows
```
telnet 127.0.0.1 5555
```

import selectors
import socket

sel = selectors.DefaultSelector()

# --- Step 1: Create listening socket ---
server_sock = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
server_sock.bind(("0.0.0.0", 5555))
server_sock.listen()
server_sock.setblocking(False)

# Register the server socket for "read" events (incoming connections)
sel.register(server_sock, selectors.EVENT_READ)


def accept_connection(sock):
    client_sock, addr = sock.accept()  # non-blocking accept
    print(f"Accepted connection from {addr}")
    client_sock.setblocking(False)
    # Register client for read events
    sel.register(client_sock, selectors.EVENT_READ)


def handle_client(sock):
    data = sock.recv(1024)
    if data:
        print("Received:", data.decode())
        sock.sendall(b"Message received\n")
    else:
        print("Client disconnected")
        sel.unregister(sock)
        sock.close()


print("Server listening on port 5555...")

# --- Step 2: Main event loop ---
while True:
    events = sel.select()  # waits until some socket is ready
    for key, mask in events:
        sock = key.fileobj

        if sock is server_sock:
            accept_connection(sock)
        else:
            handle_client(sock)

Appendix_Socket_server_sample

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