Infrastructure & Operations › Cloud Computing

Server

A machine, or a program on one, that provides a service to clients.

Also known as: web server, server machine, backend server

“Server” is used for two related things:

1. A machine (physical or virtual) that is always on and waiting to serve requests.
2. A program running on it that listens on a port and answers requests from clients: a web server, a database server, a mail server.

A client is anything that sends the request: a browser, a mobile app, another service. The same machine can run many server programs at once, each on its own port.

# A tiny server program: listens on port 8000 and answers every request
from http.server import BaseHTTPRequestHandler, HTTPServer

class Hello(BaseHTTPRequestHandler):
    def do_GET(self):
        self.send_response(200)
        self.end_headers()
        self.wfile.write(b"hello")

HTTPServer(("0.0.0.0", 8000), Hello).serve_forever()

Visit http://localhost:8000 and you are the client. See localhost.

Why it matters

Your code usually runs as a server: it starts, waits for requests, and keeps going. That changes how you think about it. It must handle many clients at once, survive bad input, and restart cleanly.

Common confusions

• “Server” is relative. Your API is a server to the browser and a client to the database.
• Listening address. 127.0.0.1 accepts only local connections; 0.0.0.0 accepts connections on all of the machine’s addresses, which may expose it to the network.
• A server isn’t special hardware. Your laptop can be a server. What differs in production is reliability: monitoring, backups, and redundancy.
• One server is a single point of failure. Real services run several and put a load balancer in front.