Caching

Caching is the highest-leverage performance lever and the easiest to get subtly wrong. Every caching decision reduces to one question:

Does the URL change when the content changes?

• Yes (fingerprinted/hashed assets) → cache forever, immutable.
• No (HTML docs, API responses) → short freshness + revalidation.

The layers (request travels through all of them)

Browser memory/disk → Service Worker → CDN/shared cache → Origin

Each can hold a copy. Set policy deliberately at each, and make sure they agree.

HTTP Cache-Control — the essentials