Why Does Removing Data Echoing from a Python Socket Server Cause `socket.recv()` to Return Nothing on Subsequent Calls?

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)

This line implied that the server would echo the received data back to the client until the client closed its end of the connection.

In the modified code, the behavior changed as follows:

break

When the conn.recv(1024) method was called for the second time, it would immediately terminate, leaving no data available for the client.

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:

1. Close the client's outgoing connection to indicate completion.
2. Implement multiple recv() calls to account for potential fragmentation in data transmission.

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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