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HTTP and APIs

Beginner 10 min read

How a browser or another program talks to a server: requests, methods, status codes and JSON APIs.

Why it matters

Every web application, mobile app and integration is built on HTTP. Reading a request and its status code is the first step in debugging almost any backend problem.

Where it fits

  1. Plan & Design
  2. ● Develop › Backend
  3. Version control
  4. Verify
  5. Build & Package
  6. CI/CD
  7. Deploy
  8. Operate

What you will learn

  • Read an HTTP request and response: method, path, headers, body, status.
  • Choose the right method and status code for an API endpoint.
  • Explain why a server must validate every request it receives.

Before you start

A request and its response

Example 
GET /api/projects/42 HTTP/1.1
Host: example.com
Accept: application/json

HTTP/1.1 200 OK
Content-Type: application/json

{"id": 42, "name": "My project"}
The client asks for a resource with a method and a path; the server answers with a status code, headers and a body.

Methods and status codes you meet every day

  • GET reads, POST creates, PUT/PATCH change, DELETE removes. GET must never change data.
  • 2xx success (200 OK, 201 Created, 204 No Content).
  • 4xx the client's mistake (400 bad input, 401 not signed in, 403 not allowed, 404 not found, 422 validation failed).
  • 5xx the server's mistake (500 a bug, 502/504 the app behind the proxy did not answer).

Watch out: Treat every request as untrusted: validate input, check that the user may access the resource, and never return more data than the caller needs.

Tip: Open your browser's developer tools (Network tab) on any site to see real requests, headers and status codes.

Official documentation and references

Guides

Reference

Specifications

Practise in Zerethon IDE

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Reviewed 2026-10-23. Links go to the official documentation unless marked otherwise.