HTTP: How the Web Talks

What Is HTTP?

HTTP (HyperText Transfer Protocol): The common language of the web.

Purpose: Defines how to format requests and responses.

Runs on TCP: HTTP uses TCP for reliable delivery.

Most APIs you build: RESTful APIs using HTTP.

HTTP Request Format

Structure:

GET /api/users/123 HTTP/1.1
Host: api.example.com
User-Agent: Mozilla/5.0
Accept: application/json

(optional body)

Parts:

Request line: Method (GET), path (/api/users/123), version (HTTP/1.1)

Headers: Key-value metadata (Host, User-Agent, Accept)

Body (optional): Data sent to server (JSON, form data)

HTTP Methods

GET: Retrieve data. No body.

POST: Create resource. Has body.

PUT: Update entire resource. Has body.

DELETE: Remove resource. Usually no body.

PATCH: Partial update. Has body.

Example GET:

GET /api/products/456 HTTP/1.1
Host: store.com

Server returns product with ID 456.

Example POST:

POST /api/products HTTP/1.1
Host: store.com
Content-Type: application/json

{"name": "Laptop", "price": 999}

Server creates product, returns it with new ID.

HTTP Response Format

Structure:

HTTP/1.1 200 OK
Content-Type: application/json
Content-Length: 87

{"id": 123, "name": "John", "email": "john@example.com"}

Parts:

Status line: Version, status code (200), message (OK)

Headers: Content-Type, Content-Length, etc.

Body: Actual data (JSON, HTML, image bytes)

HTTP Status Codes

2xx Success:

• 200 OK: Request succeeded
• 201 Created: Resource created
• 204 No Content: Success, no body

4xx Client Errors:

• 400 Bad Request: Invalid syntax
• 401 Unauthorized: Auth required
• 404 Not Found: Resource missing
• 429 Too Many Requests: Rate limited

5xx Server Errors:

• 500 Internal Server Error: Server crashed
• 502 Bad Gateway: Upstream failed
• 503 Service Unavailable: Overloaded

The Connection Problem

HTTP/1.1 default behavior: Connection closes after response.

Flow:

1. TCP 3-way handshake
2. Send HTTP request
3. Receive response
4. TCP 2-way teardown

For next request: Repeat entire flow!

Why? HTTP designed for simple document retrieval. One request per page.

Modern reality: Page load needs 50-100 requests (HTML, CSS, JS, images, API calls).

Problem: New TCP connection per request is extremely expensive.

The Solution: Keep-Alive Header

Tell server to keep connection open.

Request:

GET /api/data HTTP/1.1
Host: api.example.com
Connection: keep-alive

Response:

HTTP/1.1 200 OK
Connection: keep-alive

(data)

Connection stays open. Next request reuses it. No new handshake.

Modern browsers: Automatically include keep-alive.

Modern servers (Nginx, Apache): Support it by default.

Connection Pooling

Backend services maintain pool of persistent connections.

Example: Node.js app → Database.

Without pooling: New TCP connection per query. Slow.

With pooling: Maintain 10 persistent connections. Reuse them. Fast.

Why HTTP Dominates

Universal: Every language has HTTP libraries.

Human readable: Debug with curl, Postman, browser tools.

Stateless: Each request independent. Easy to scale.

Standardized: Everyone understands GET, POST, status codes.

HTTP is the shared language of the internet.