Caching: speed up without serving stale data

Where to put it

In the browser, in the delivery network, in the app server, and in a shared key-value store. Each layer is faster and harder to invalidate. Most pain comes from layers piled up with no single deciding policy.

Three strategies

Time expiry. Simple and enough for most cases. Choose a short time for data that changes and a logical one for stable data.

Event invalidation. On write, delete the key. More precise, and requires remembering every place that writes.

Background refresh. Serve an existing copy and refresh in parallel — a smooth experience, in exchange for a slightly stale value.

Pitfalls

A cache key that doesn't include the user's identity or permissions — so one person's data is served to another. Mass expiry at the same moment that brings down the database. And a cache that papers over a slow query instead of fixing it.