Noncacheable

Noncacheable is an adjective used to describe data, resources, or operations that should not be stored in caches at any layer of a system, including client browsers, intermediate proxies, reverse proxies, content delivery networks, and server-side caches. When something is noncacheable, caches are instructed not to persist it for reuse, or to revalidate the data with the origin before serving a cached copy.

In web technology, noncacheable behavior is commonly implemented through HTTP caching directives. The Cache-Control header can express noncacheable intent using directives such as no-store, which asks caches not to store the response at all, and no-cache, which allows caching but requires validation with the origin before reuse. Older HTTP/1.0 compatibility can use Pragma: no-cache. Expires headers may also influence caching decisions, but modern semantics focus on Cache-Control. The exact interpretation can vary between user agents and intermediaries, so explicit directives are important for predictable behavior.

Rationale and implications for noncacheable content include data freshness, privacy, and security considerations, especially for personalized, sensitive, or rapidly changing information. The trade-off is higher server load and latency, since caches cannot serve the data without contacting the origin. Noncacheable content is common for login pages, user dashboards with personalized data, financial quotes, real-time results, and other dynamic content where stale information is unacceptable.

In practice, noncacheable is a broad, nonstandard term used across domains to indicate that caching should be avoided or limited, with precise behavior defined by the specific caching mechanism or directive in use.