How is caching managed in HTTP?

HTTP cache management is performed using HTTP headers exchanged between the client (browser) and the server.

Main headers for cache control:

• Cache-Control: The most important header that defines caching directives. For example:
  • max-age=<seconds>: Indicates how long (in seconds) the resource is considered "fresh".
  • no-cache: Requires the cache to revalidate with the server before using the cached copy.
  • no-store: Prohibits caches from storing any part of the request or response.
  • public: The response can be cached by any cache (both private and shared).
  • private: The response can only be cached by a private cache (e.g., the user's browser).
• Expires: An outdated header that specifies the absolute date and time when the resource is considered "fresh" in RFC 1123 format. It has lower priority than Cache-Control.
• ETag: An entity tag, a weak or strong opaque value that uniquely identifies the version of a resource. The server sends it in the response. The client can include it in the If-None-Match header in subsequent requests for the same resource for conditional requests. If the ETag matches, the server returns 304 Not Modified.
• Last-Modified: The date and time of the last modification of the resource on the server. The client can include it in the If-Modified-Since header in subsequent requests for conditional requests. If the resource has not changed since the specified date, the server returns 304 Not Modified.

Example of header usage:

HTTP/1.1 200 OK
Content-Type: text/html
Cache-Control: max-age=3600, public // Cache for 1 hour, can be cached by both shared and private caches
ETag: "abcdef123"
Last-Modified: Wed, 21 Oct 2015 07:28:00 GMT

Caching mechanisms:

1. Heuristic caching: If neither Cache-Control nor Expires are specified, the cache can use a heuristic algorithm (for example, based on the Last-Modified header) to determine "freshness".
2. Conditional requests: The client uses headers If-None-Match (with ETag) or If-Modified-Since (with Last-Modified) to check if the resource has changed on the server. This helps avoid re-downloading the entire resource if it hasn't changed.

Cache management is an important aspect of optimizing web application performance.