HTTPcaches

HTTPcaches are systems that store responses to HTTP requests in order to serve subsequent requests more quickly and to save bandwidth. They exist at multiple levels, including client side (browser), corporate or Internet service provider proxies, and content delivery networks (CDNs). Cached responses are identified by the request URL and, in some cases, by the request headers that affect the response, such as Accept-Language or Accept-Encoding.

Caching decisions are guided by HTTP headers. The Cache-Control and Expires headers express how long a response may be reused without contacting the origin server. Public and private directives indicate whether a cached copy may be shared among users. Public caches often respect s-maxage, while others use max-age for private caches. Validation mechanisms like ETag/If-None-Match and Last-Modified/If-Modified-Since allow a cache to check with the origin whether a stored response is still fresh via conditional requests. If the origin indicates the resource is unchanged (304 Not Modified), the cache can reuse the stored response; if it has changed, the cache updates the copy.

The Vary header signals that different responses may be served for different request headers, which affects cacheability. Caches should also consider content encoding, cookies, and other headers when storing entries.

Caching improves latency and reduces load on origin servers but can lead to stale content if freshness is mismanaged. Invalidation and purging policies, versioning of URLs, and careful use of cache directives help mitigate these risks. Adoption is widespread in browsers, corporate proxies, and CDNs, forming a key component of modern web performance optimization.