Socket

A socket is an endpoint of a two-way communication link, exposed by the operating system as an API. A connection is uniquely identified by the 5-tuple of protocol, source IP, source port, destination IP and destination port.

Why it matters

Every network program, from a browser to a database client, uses sockets. A server creates a listening socket on a port and accepts incoming connections, each of which gets its own socket. Limits on open sockets or file descriptors are a classic cause of "too many open files" errors and exhausted ports under heavy load.

Example

import socket
s = socket.create_connection(("example.com", 443), timeout=5)
print(s.getpeername())
s.close()

This opens a TCP socket to port 443 and prints the remote address. Failure raises a timeout or connection refused error.

Common mistakes

• Forgetting to close sockets, which leaks descriptors until the process fails.
• Not setting timeouts so a hung peer blocks your program forever.
• Confusing a Unix domain socket, which is local and file based, with a network socket.

Related

• Port
• TCP
• UDP
• Keep-alive

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