mika's open learning notebook

HTTP

HTTP is the request/response protocol used by the web.

A request includes things such as:

• method (GET, POST, PUT, PATCH, DELETE)
• URL
• headers
• optional body

A response includes:

• status code
• headers
• body

Status codes

Common groups:

• 2xx success
• 3xx redirect
• 4xx client/request problem
• 5xx server problem

A status code is evidence about where a failure occurred.

Headers

Headers carry metadata such as:

• content type
• authorization
• caching
• origin/CORS information

DNS

DNS translates names such as example.com into network addresses.

When a website fails, “the internet is broken” is too broad. The problem could be DNS, routing, TLS, HTTP, application code, or the server itself.

CORS

CORS is a browser security mechanism controlling whether one origin is allowed to access resources from another.

A CORS error is not automatically an API bug. It often means the browser is enforcing a server-defined cross-origin policy.

Browser DevTools

Useful places to inspect:

• Console
• Network tab
• request/response headers
• response payloads
• timing
• status codes

Postman / API clients

Testing an API outside the browser helps separate API behavior from browser-specific concerns such as CORS or front-end state.

SQL

SQL is used to query relational data.

Core concepts to practice:

• SELECT
• WHERE
• ORDER BY
• GROUP BY
• joins
• inserts/updates
• schema relationships

The important skill is translating a question about data into a precise query.

Debugging web requests

Useful sequence:

what request was sent?
→ did DNS/network connection work?
→ what status came back?
→ what headers came back?
→ what body came back?
→ how did the application handle it?

Main lesson

Web debugging becomes easier when I separate transport, HTTP, browser policy, server behavior, and application state instead of treating them as one system.