What is Caching?
Caching is storing the result of expensive work so the next request can reuse it instead of recomputing. Done well it makes software dramatically faster; done carelessly it serves stale or wrong data, which is why cache invalidation is famously hard.
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Why it matters
Most performance wins come from not doing work twice. A cache turns a slow query or an expensive computation into an instant lookup, which is often the cheapest path to a fast product.
In practice
The hard part is not caching, it is invalidation: knowing when the cached value is stale and must be refreshed. Cache the things that are expensive and change rarely, and be deliberate about how they expire.