Caching

Caching is a technique that stores copies of data in a faster storage location to speed up data retrieval and reduce load on slower resources. A cache holds recently used or frequently accessed items so that subsequent requests can be served quickly. Caching leverages locality of reference: data that has been accessed recently is likely to be requested again soon.

Caches exist at multiple levels and domains. CPU caches (L1, L2, and sometimes L3) sit between the processor and main memory to speed up instruction and data fetches. Disk and operating system page caches keep file system data in memory to improve file access. Web caching includes browser caches, reverse proxies, and content delivery networks that store web objects to speed up delivery for many users. Databases, application servers, and memoization techniques store results of expensive computations or queries. Cache policies govern what data is kept and for how long.

Common cache policies include eviction rules such as least recently used, least frequently used, and first-in-first-out. Write strategies describe how updates propagate to the underlying data store, including write-through, write-back, and write-around. Cache invalidation and coherence are critical in multi-core or distributed environments to prevent stale data; coherence protocols and versioning help maintain consistency across caches.

Performance benefits come with trade-offs. Caching can dramatically reduce latency and increase throughput, but introduces complexity, potential data staleness, and additional memory overhead. Effective caching requires careful policy selection, cache sizing, invalidation strategies, and monitoring to balance speed, accuracy, and resource use.