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Python Socket: Understanding the Impact of Data Return Behavior
Initially, a Python echo server example from the official documentation functioned flawlessly. However, upon modifying the code to eliminate the sending of data back to the client, an issue arose. The second invocation of the socket.recv() method yielded no return.
Different Implementation, Different Outcomes
The original code from the documentation employed a while loop that:
conn.sendall(data)
In the modified code, the behavior changed as follows:
break
The Nature of TCP Streams
To understand this behavior, it's essential to grasp the nature of TCP streams. They transmit data in a continuous flow, with no direct correlation between client and server operations. Moreover, the protocol determines the underlying communication rules.
In the original code, the protocol dictated that the server would echo each data packet it received until the client closed its outgoing connection. Upon closure, the server would close its socket.
Modified Protocol and Client Adjustments
The modified code introduced a new protocol where the server would discard incoming data until the client closed its outgoing connection. Subsequently, the server would send "ok" and close its socket.
To make the client work with this new protocol, it was necessary to:
Updated Server and Client
The following updated code samples demonstrate the revised protocol implementation:
Server:
import socket</p> <p>HOST = ''<br>PORT = 50007 <br>s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)<br>s.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEADDR, 1)<br>s.bind((HOST, PORT))<br>s.listen(1)</p> <p>conn, addr = s.accept()<br>print('Connected by', addr)</p> <p>while True:</p> <pre class="brush:php;toolbar:false">data = conn.recv(1024) if not data: break
conn.sendall(b'ok')
conn.shutdown(socket.SHUT_WR)
conn.close()
Client:
import socket</p> <p>HOST = 'localhost'<br>PORT = 50007<br>s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)<br>s.connect((HOST, PORT))<br>s.sendall(b'Hello, world')<br>s.shutdown(socket.SHUT_WR)<br>data = b''<br>while True:</p> <pre class="brush:php;toolbar:false">data = conn.recv(1024) if not data: break
s.close()
print('Received', repr(data))
With these revised implementations, the server efficiently discards incoming data, allowing the client to receive its response after closing.
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