HTTPcaching

HTTP caching is the process of storing copies of HTTP responses so that future requests for the same resources can be served more quickly and with less network traffic. Caches can reside in web browsers, intermediate proxies, or content delivery networks. A cache uses response headers to decide if a response is reusable, how long it may be kept (its freshness), and when it must be revalidated with the origin server.

Determining cacheability involves both the request method and response headers. GET and HEAD responses are typically cacheable, while POST responses are usually not cached unless explicitly indicated. HTTP/1.1 introduces Cache-Control directives such as max-age and s-maxage for freshness, public and private scope, no-store and no-cache to control visibility and revalidation, and, in some cases, stale-while-revalidate for continued serving of stale copies while revalidation occurs.

Freshness and validation govern how cached responses are used. A cache uses freshness information (for example, max-age or Expires) to decide if a cached copy is still fresh. If it becomes stale, the cache can revalidate with the origin using conditional requests (If-Modified-Since or If-None-Match). If the resource has not changed, the server replies with 304 Not Modified and the cached body remains valid.

Cache hierarchy includes private caches (such as a user’s browser) and shared caches (proxies and CDNs). The Vary header indicates when different request headers produce different responses, guiding caches to store separate variants. Security considerations include avoiding caching of sensitive data in shared caches and applying appropriate directives to protect privacy and integrity.