Cache

A cache stores the result of expensive work so the next request can be answered from the stored copy instead of doing the work again. It might hold a rendered page, a database query result, or a response from a third-party API.

It is usually the highest-return performance change available, because most systems answer the same handful of questions over and over. It also protects what sits behind it: a cached response is a request that never reaches the database or the paid API.

The cost is staleness, and the difficult part is deciding when a cached copy stops being true. A short expiry is the simple answer; explicitly clearing entries when the underlying data changes is the correct one and the easier of the two to get wrong.