How does file caching occur at the protocol level?

HTTP protocol-level caching is performed using headers and methods that allow the client and server to determine whether to re-download a resource or use it from the local cache. Key mechanisms:

1. Server caching headers:

   • Cache-Control: Defines caching directives, such as maximum age (max-age), caching permissions (public, private), cache prohibition (no-cache, no-store), and revalidation requirement (must-revalidate).
   • Expires: An outdated header indicating the date and time after which the resource is considered stale. It is overridden by Cache-Control with max-age.
   • ETag: Entity Tag - a unique identifier of the resource version generated by the server.
   • Last-Modified: The date and time of the last modification of the resource on the server.

2. Conditional request headers from the client:

   • If-None-Match: The client sends one or more ETags of the resource versions it has in cache. If none match the current ETag on the server, the server sends the entire resource with a 200 OK status. If they match, the server returns 304 Not Modified without a response body, and the client uses the cached version.
   • If-Modified-Since: The client sends the date of the last modification of the cached resource. If the resource on the server has been modified after this date, the server sends the entire resource with a 200 OK status. Otherwise, it returns 304 Not Modified.

Caching algorithm:

1. The client sends a request for the resource.
2. The server responds with the resource, adding Cache-Control, Expires, ETag, Last-Modified headers.
3. The client saves the resource (response body) along with these headers in the local cache.
4. On subsequent requests for the same resource, the client checks the headers in its cache:
   • If the resource is fresh according to Cache-Control (max-age) or Expires, the client uses the cached resource without requesting from the server (cache hit).
   • If the resource is stale, the client sends a conditional request using If-None-Match (with the saved ETag) and/or If-Modified-Since (with the saved Last-Modified).
5. The server receives the conditional request:
   • If the resource has not changed (checked via ETag/Last-Modified match), the server responds with 304 Not Modified.
   • If the resource has changed, the server responds with 200 OK with the new version of the resource and updated caching headers.
6. The client processes the response accordingly:
   • For 304 Not Modified, it uses the resource from the cache.
   • For 200 OK, it updates the cache with the new version and headers.

// Example server response headers:
// Cache-Control: public, max-age=3600
// Expires: Mon, 15 Aug 2022 12:00:00 GMT
// ETag: "abcdef123456"
// Last-Modified: Fri, 12 Aug 2022 10:00:00 GMT

// Example client conditional request:
// GET /path/to/resource HTTP/1.1
// Host: example.com
// If-None-Match: "abcdef123456"
// If-Modified-Since: Fri, 12 Aug 2022 10:00:00 GMT

// Server response example when ETag/Last-Modified match:
// HTTP/1.1 304 Not Modified
// Date: Mon, 15 Aug 2022 11:00:00 GMT
// Cache-Control: public, max-age=3600

Main caching headers table:

Header	Sender	Purpose	Example values
Cache-Control	Server	General caching rules	max-age=3600, no-cache, public
Expires	Server	Resource expiration date (stale)	Mon, 15 Aug 2022 12:00:00 GMT
ETag	Server	Resource version identifier	"abcdef123456"
Last-Modified	Server	Last modification date of the resource	Fri, 12 Aug 2022 10:00:00 GMT
If-None-Match	Client	Conditional request: use cache if ETag matches	"abcdef123456"
If-Modified-Since	Client	Conditional request: use cache if resource not changed after date	Fri, 12 Aug 2022 10:00:00 GMT

These mechanisms significantly reduce server load and speed up page loading for users, as browsers can avoid full re-downloads of resources.