Art of Codeartofcode
Lesson 11 of 18

3. Client and Server

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The picture to keep in your head
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3.1 What exactly are a client and a server?
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3.2 What's a request and a response?
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3.3 What happens when you type a URL and press Enter?
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3.4 What do GET, POST, PUT, PATCH and DELETE mean?
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3.5 What do status codes tell you?
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3.6 What goes in the headers and what goes in the body?
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3.7 What's the difference between TCP and UDP?
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3.8 How does HTTPS keep the conversation private?
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3.9 Why do keep-alive, HTTP/2 and HTTP/3 make things faster?
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3.10 What does "stateless" mean, and why do we want it?
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3.11 How do cookies, sessions and tokens remember who you are?
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Memory card and where the analogy breaks
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Practice: five levels
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Interview questions
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Solutions
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Sources
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Code lab
3. Client and Server

3.10 What does "stateless" mean, and why do we want it?

3 min read+10 XP

Remembering the user

In the restaurant: One waiter remembers you: "same as yesterday?". Lovely, but only that waiter can serve you. If they go on break, nobody knows your order. A better system: every order slip says everything: table number, your loyalty number, the full order. Then any waiter can take it.

In the system:

  • A stateful server keeps information about you in its own memory between requests (for example, "this user is logged in").
  • A stateless server keeps nothing about you between requests. Each request carries what's needed (a token, an ID), and anything else lives in a shared store every server can reach (a database or Redis).

stateful: each server's own notebook; stateless: one shared notebook.

Figure: with in-memory sessions, a load balancer sending Asha to server B logs her out. With a shared store, any server can answer.

Why stateless wins:

  • Any server can handle any request, so a load balancer can spread traffic freely (Chapter 7).
  • Adding servers is easy: new ones need no copy of anyone's memory (Chapter 6).
  • A crashed server loses nothing important.

"Stateless" doesn't mean the system forgets you. It means the app servers don't hold the memory; it lives somewhere shared.

Memory code: "Every slip says it all." Keep servers forgetful; keep memory where everyone can reach it.