Why HTTP Matters for CDNs

Before we dive into CDNs, we need to establish a shared understanding of HTTP—the protocol that powers the web. This isn't a comprehensive HTTP course, but these fundamentals are essential for understanding how CDNs work.

Here's the key insight: CDNs are HTTP-aware caches. They make decisions based on HTTP methods, headers, and status codes. If you understand these, you understand how to control CDN behavior.

The Request-Response Cycle

HTTP is a request-response protocol. A client (usually a browser) sends a request, and a server sends back a response. Every web interaction follows this pattern:

Client sends request — "Give me this resource"
Server processes — Finds or generates the resource
Server sends response — The resource (or an error)
Connection closes — (or stays open for reuse)

This is stateless—each request is independent. The server doesn't remember previous requests unless you explicitly pass state (via cookies, tokens, etc.).

HTTP Methods

HTTP methods (also called "verbs") indicate what action the client wants to perform:

Method	Purpose	Cacheable?	Safe?
GET	Retrieve a resource	Yes	Yes
POST	Submit data / create resource	Rarely	No
PUT	Replace a resource entirely	No	No
PATCH	Partially update a resource	No	No
DELETE	Remove a resource	No	No
HEAD	GET without response body	Yes	Yes
OPTIONS	Get allowed methods	No	Yes

Common HTTP Methods

Safe means the method doesn't modify server state—it only retrieves. Cacheable means responses can typically be stored and reused.

Anatomy of a Request

TEXT

GET /api/products/123 HTTP/1.1
Host: example.com
User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10_15_7)
Accept: application/json
Accept-Encoding: gzip, br
Cache-Control: no-cache

Example HTTP Request

A request consists of:

Request line — Method, path, and HTTP version
Headers — Metadata about the request (Host, Accept, etc.)
Body — (Optional) Data sent with POST/PUT requests

Anatomy of a Response

TEXT

HTTP/1.1 200 OK
Content-Type: application/json
Content-Length: 256
Cache-Control: public, max-age=3600
ETag: "abc123"
Age: 120

{"id": 123, "name": "Widget", "price": 29.99}

Example HTTP Response

A response consists of:

Status line — HTTP version, status code, and reason phrase
Headers — Metadata about the response (Content-Type, Cache-Control, etc.)
Body — The actual content (HTML, JSON, image bytes, etc.)

Status Codes

Status codes tell you what happened with your request. They're grouped by first digit:

Range	Category	Common Examples
1xx	Informational	100 Continue, 101 Switching Protocols
2xx	Success	200 OK, 201 Created, 204 No Content
3xx	Redirection	301 Moved Permanently, 304 Not Modified
4xx	Client Error	400 Bad Request, 401 Unauthorized, 404 Not Found
5xx	Server Error	500 Internal Server Error, 502 Bad Gateway, 503 Service Unavailable

HTTP Status Code Families

Headers: The Control Plane

Headers are key-value pairs that control HTTP behavior. For CDNs, certain headers are critical:

Header	Direction	Purpose
Cache-Control	Response	Primary caching directives (we'll cover this extensively)
ETag	Response	Version identifier for conditional requests
Vary	Response	Which request headers affect caching
Age	Response	How long content has been in cache
If-None-Match	Request	Conditional GET with ETag
Host	Request	Which origin to contact

CDN-Critical Headers (Preview)

We'll explore each of these in depth in Module 3 (Headers & Configuration). For now, just know that headers are how you control CDN caching behavior.

HTTP Versions

You'll encounter different HTTP versions:

HTTP/1.1 — Still widely used, text-based, one request per connection (or pipelining)
HTTP/2 — Binary, multiplexed (many requests over one connection), header compression
HTTP/3 — Uses QUIC instead of TCP, better performance on unreliable networks

The caching semantics are the same across versions—what changes is the transport efficiency. CDNs typically support all versions and often upgrade connections automatically.

Key Takeaway

Quick Check

Which HTTP methods are typically cached by CDNs?

POST and PUT GET and HEAD DELETE and PATCH All methods equally

Correct! Not quite.

Explanation

CDNs typically only cache GET and HEAD requests. These are 'safe' methods that don't modify server state—they only retrieve information. POST, PUT, DELETE, and PATCH modify data and are passed directly to the origin server.

Quick Check

What does the HTTP status code 304 indicate?

The resource was not found The request was redirected The cached version is still valid (Not Modified) The server encountered an error

Correct! Not quite.

Explanation

304 Not Modified means the cached content is still valid. This happens during cache revalidation—the client or CDN asks 'has this changed?' and the origin responds 'no, use what you have.' This saves bandwidth by not re-transferring unchanged content.

HTTP Essentials