What is caching?
Caching stores reusable data in a location that can be accessed more quickly. It reduces the work needed to reproduce a result or download the same resource again. Processor, browser and server caches operate at different layers.
Hits, validity and validation
A cache hit occurs when suitable data is available; otherwise there is a miss. Validity rules determine how long a stored copy can be used. Headers such as Cache-Control matter for web caches. Validators such as ETags help reuse an existing copy when the file has not changed, rather than transferring the entire content again. A cache is not a permanent backup: it should be disposable and reconstructible from source data. With limited memory, choosing which entries to evict is part of the design. A larger cache cannot correct invalid freshness rules.
Freshness and safe design
An outdated weather icon and an outdated account balance do not have equivalent consequences. Each data type needs appropriate lifetimes and invalidation after changes. Access rules and cache keys must prevent one user's private response from reaching another user. Many entries expiring simultaneously can overload the origin; coalescing concurrent requests can reduce that load. Disabling caching does not solve every problem, and clearing a cache is not necessarily a permanent fix. Measurement should consider freshness, error rates and origin load alongside response time. New designs should be introduced on a small scale and monitored for incorrect content.