Art of Codeartofcode
Lesson 17 of 18

3. Client and Server

1
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?
13
Memory card and where the analogy breaks
14
Practice: five levels
15
Interview questions
16
Solutions
17
Sources
18
Code lab
3. Client and Server

Sources

3 min read+10 XP

Source notes

  • HTTP methods, safety and idempotency, status codes (including 401, 403, 502, 503, 504): RFC 9110, HTTP Semantics. PATCH: RFC 5789. 429 Too Many Requests: RFC 6585.
  • HTTP/1.1 persistent connections: RFC 9112. HTTP/2 multiplexing: RFC 9113. HTTP/3: RFC 9114. QUIC: RFC 9000.
  • TLS 1.3 full handshake in one round trip: RFC 8446.
  • TCP three-way handshake: RFC 9293. UDP: RFC 768.
  • Cookies and the Set-Cookie attributes HttpOnly and Secure: RFC 6265. SameSite is defined in the cookie specification updates (RFC 6265bis drafts) and implemented by major browsers.
  • JSON Web Token: RFC 7519.
  • Round-trip numbers in Level 2 are assumptions; arithmetic computed 2026-10-05.
  • Level 3 and Level 4 code: tested 2026-10-05 (code/ch03_client_server.py, all checks pass).