3. Client and Server
1
The picture to keep in your head2
3.1 What exactly are a client and a server?3
3.2 What's a request and a response?4
3.3 What happens when you type a URL and press Enter?5
3.4 What do GET, POST, PUT, PATCH and DELETE mean?6
3.5 What do status codes tell you?7
3.6 What goes in the headers and what goes in the body?8
3.7 What's the difference between TCP and UDP?9
3.8 How does HTTPS keep the conversation private?10
3.9 Why do keep-alive, HTTP/2 and HTTP/3 make things faster?11
3.10 What does "stateless" mean, and why do we want it?12
3.11 How do cookies, sessions and tokens remember who you are?13
Memory card and where the analogy breaks14
Practice: five levels15
Interview questions16
Solutions17
Sources18
Code lab3. Client and Server
3.1 What exactly are a client and a server?
3 min read+10 XP
The conversation
In the restaurant: The customer asks and the kitchen answers. It isn't about who's bigger or more important. It's about who starts the conversation and who does the work.
In the system:
- A client is any program that sends a request: your browser, a mobile app, or even another server calling an API.
- A server is a program that waits for requests and answers them. It usually runs on a machine in a data centre, listening on a port (443 for HTTPS, 80 for plain HTTP).
The words describe roles, not machines. The bookstore's app server is a server to your phone, but a client when it asks the database for book 42. One program can be both, at different moments.
Why split it like this? The kitchen holds the ingredients and the recipes (the data and the business rules), so they stay in one trusted place. Millions of customers can order without each needing their own kitchen.
Memory code: "Customers ask, kitchens cook." Client = whoever asks. Server = whoever answers. Roles, not machines.