Core · NetworksLesson 4 of 6Free to read

HTTP requests and responses · Computer Networks

What does a browser actually send to a web server?

Why you need it

HTTP is plain text with a strict shape. Once you can read a request and a response line by line, debugging an API, a redirect loop or a cookie problem stops being guesswork, and writing a backend makes sense.

The intuition

A request:

GET /products?page=2 HTTP/1.1     ← method, path, version
Host: shop.example.com            ← headers: name: value
Accept: application/json
                                  ← an empty line ends the headers
(body, for POST and PUT)

A response starts with a status line, HTTP/1.1 200 OK, then headers, an empty line and the body.

Status codes by first digit: 2xx success, 3xx go elsewhere (redirect), 4xx the client's mistake (404 not found, 401 not logged in, 403 not allowed), 5xx the server's mistake.

Methods: GET reads, POST creates, PUT/PATCH change, DELETE removes. HTTPS is the same text sent inside an encrypted TLS connection.

Definition

Status code

A three-digit number at the start of a response that says how the request went: 200 OK, 301 moved, 404 not found, 500 server error.

Header

A `Name: value` line that carries extra information: Content-Type, Cookie, Authorization, Location, Cache-Control.

Examples

Parse a request

Try it yourself

Split the text into the request line, the headers and the body.

raw = "GET /products?page=2 HTTP/1.1\r\nHost: shop.example.com\r\nAccept: application/json\r\nCookie: session=abc123\r\n\r\n"
head, body = raw.split("\r\n\r\n", 1)
lines = head.split("\r\n")
method, target, version = lines[0].split(" ")
headers = {}
for line in lines[1:]:
    name, value = line.split(": ", 1)
    headers[name.lower()] = value
path, _, query = target.partition("?")
print(method, path, query)
print(headers["host"], headers["accept"])

Step by step

L2The empty line separates headers from body.
L4The first line has three parts.
L8Header names are case-insensitive, so store them in lower case.
L10partition splits the path from the query at ?.GET /products page=2

Build a response

Try it yourself

A response is the same shape: status line, headers, empty line, body.

import json

def respond(status, data):
    reasons = {200: "OK", 404: "Not Found"}
    body = json.dumps(data)
    return (f"HTTP/1.1 {status} {reasons[status]}\r\n"
            f"Content-Type: application/json\r\n"
            f"Content-Length: {len(body)}\r\n\r\n" + body)

print(respond(404, {"error": "no such product"}))

Step by step

L5Turn the dict into JSON text.
L8Tells the client how many bytes of body to read.
L10Status line, two headers, an empty line, then the JSON.HTTP/1.1 404 Not Found

Common mistakes

200 with an error inside

An API that returns 200 with {"error": …} breaks clients, caches and monitoring that trust the status code. Use 4xx or 5xx for failures.

401 versus 403

401: we don't know who you are (log in). 403: we know, and you're not allowed.

Quick quiz

1. A page moved permanently to a new URL. Which status and header tell the browser where to go?

A301 with Location
B404 with Host
C500 with Content-Type

Show the answer

Answer: A

3xx means "go elsewhere", and Location says where.

2. What does this print?

def kind(status):
    return {2: "success", 3: "redirect", 4: "client error", 5: "server error"}[status // 100]

print(kind(201), "/", kind(503), "/", kind(403))

A
```
success / server error / client error
```
B
```
success / client error / server error
```
C
```
redirect / server error / client error
```

Show the answer

Answer: A

Only the first digit matters: 2, 5 and 4.

Practice

3 exercises in three steps: predict what a program prints, change a program to meet a new requirement, then write one from scratch. Your code runs in the browser and is checked the moment you submit.

PredictRead a response
Type exactly what this program prints.
ModifyHeaders are case-insensitive
get_header(raw, name) should return a header's value or None. It fails when the request writes content-type but you ask for Content-Type. Compare names without caring about case.
WriteParse a query string
Write parse_query(target) that takes a request target like /search?q=lamp&page=2 and returns a dict of its query parameters: {"q": "lamp", "page": "2"}. No ? means an empty dict. (Don't use urllib.)

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Terms in this lesson

Term	Chinese	Definition
HTTP	超文本传输协议	The text protocol browsers and servers use: requests and responses.
Status code	状态码	The three-digit result of an HTTP request, such as 404.
HTTP header	HTTP 头部	A Name: value line with extra information about a request or response.