HTTP Essentials
Why HTTP Matters for CDNs
Before CDNs, we need a shared view of HTTP. HTTP is the protocol that powers the web. HTTP sits at the heart of every CDN. This is not a full HTTP course. But these basics matter for CDNs.
Here's the key insight: CDNs are HTTP-aware caches. They act on HTTP methods, headers, and status codes. If you understand these, you understand how to control CDN behavior.
The Request-Response Cycle
HTTP runs on requests and responses. A client sends a request. Usually that is a browser. The server sends back a response. Every web call follows this pattern:
- Client sends request — "Give me this resource"
- Server processes. It finds or makes the resource.
- Server sends response. The resource, or an error.
- Connection closes. Or it stays open for reuse.
HTTP is stateless. Each request stands alone. The server forgets past requests. Unless you pass state along, like cookies or tokens. The server keeps no memory of past talks.
HTTP Methods
HTTP methods, or "verbs," say what the client wants to do:
| Method | Purpose | Cacheable? | Safe? |
|---|---|---|---|
| GET | Gets a resource from the server | Yes | Yes |
| POST | Sends data to create a resource | Rarely | No |
| PUT | Replaces the whole resource | No | No |
| PATCH | Updates part of a resource | No | No |
| DELETE | Removes a resource | No | No |
| HEAD | GET with no response body | Yes | Yes |
| OPTIONS | Lists the allowed methods | No | Yes |
Safe means the method does not change server state. It only gets data. Cacheable: the reply can be stored and reused. Both rules shape what CDNs store.
Anatomy of a Request
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-cacheA request has three main parts:
- Request line — Method, path, and HTTP version
- Headers. Info about the request, like Host and Accept.
- Body. Optional data sent with POST or PUT.
Anatomy of a Response
HTTP/1.1 200 OK
Content-Type: application/json
Content-Length: 45
Cache-Control: public, max-age=3600
ETag: "abc123"
Age: 120
{"id": 123, "name": "Widget", "price": 29.99}A response has three main parts:
- Status line — HTTP version, status code, and reason phrase
- Headers. They describe the response, like Content-Type and Cache-Control.
- Body. The real content, like HTML, JSON, or images.
Status Codes
Status codes tell you what happened with your request. The first digit sets the group. 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. |
Headers: The Control Plane
Headers are key-value pairs. They steer HTTP behavior. For CDNs, certain headers are critical:
| Header | Direction | Purpose |
|---|---|---|
| Cache-Control | Response | Primary caching directives. We'll cover these later. |
| ETag | Response | Version identifier for conditional requests. |
| Vary | Response | Which request headers affect caching. |
| Age | Response | Seconds since the origin made the reply. |
| If-None-Match | Request | Conditional GET with ETag. |
| Host | Request | Which origin to contact. |
Module 3 (Headers & Configuration) covers each one in depth. For now, know that headers control CDN caching. Headers are the main control you have.
HTTP Versions
You'll encounter different HTTP versions:
- HTTP/1.1. Still widely used. Text-based. One request at a time over a reused connection.
- HTTP/2. Binary. Many requests share one connection. Header compression built in.
- HTTP/3. Uses QUIC instead of TCP. Better on unreliable networks.
Caching rules are the same in all versions. Only the transport changes. CDNs typically support these versions and let you choose which to expose. All three work the same for caching.
Key Takeaway
Which HTTP methods do CDNs typically cache?
GET and HEAD only fetch data.
They do not change server state.
So CDNs cache them safely.
Methods that write, like POST, go to the origin.
What does the status code 304 mean?
304 Not Modified means the stored copy is valid.
The origin told the cache to keep using it.
No body is sent, so bandwidth is saved.
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