Anatomy of an HTTP Request

lesson 1-3 · ~11 min · 3/29

HTTP, the language of requests

Clients and servers talk in a text protocol called HTTP (HyperText Transfer Protocol). A raw request is literally lines of text:

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

Line by line:

The request line: a method (GET), a path (/users/42), and the protocol version.
Headers: Name: value pairs carrying metadata. Host names the server the request is meant for; during development that is localhost:3000, where localhost means “this very machine” and 3000 is the port from lesson 1-2. Accept says what format the client wants back.
Optionally a blank line and then a body with data (used when sending things, like a signup form).

The method is the verb. The four you will use constantly: GET (read), POST (create), PUT/PATCH (update), DELETE (remove).

And the response

The server answers in the same style:

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

{"id": 42, "name": "Ada"}

The status line carries the status code: 200 OK, 404 not found, 500 server crashed. Unit 3 covers the full families.
Headers again, like Content-Type telling the client the body is JSON.
The body: the actual content.

Frameworks will parse this text for you, but a backend engineer should be able to read it raw. Let's prove you can, by writing the parser yourself.

Code exercise · javascript

Run this parser for the request line. split(" ") cuts the string at each space.

Quiz

A client sends: POST /orders HTTP/1.1 with a JSON body describing a pizza. What is the client most likely doing?

Code exercise · javascript

Your turn. Write parseHeader(line) that turns "Content-Type: application/json" into { name: "content-type", value: "application/json" }. Header names are case-insensitive, so lowercase the name. The value may contain colons (think "Host: localhost:3000"), so split only at the FIRST colon.

Code exercise · javascript

Your turn. Now build the other direction: formatResponse(status, reason, body) must produce the raw response text, i.e. the status line "HTTP/1.1 <status> <reason>", a Content-Type header, a blank line, then the body. Join the parts with \n (real HTTP uses \r\n line endings, but \n keeps the sandbox output readable).