How should caching and revalidation work?

Definition:

Caching saves a previously computed or fetched result so it can be reused, while revalidation refreshes or invalidates that stored result. A cache design needs a key, a freshness rule and a plan for changes. In Next.js there can be several caching layers, so one invalidation mechanism may not describe every layer's behavior.

Key points:

• Decide which data may be cached, for how long, and for which user or tenant.

• Invalidate affected data after writes; never share private results across users.

• Next.js defaults vary by version and router, so check the project's actual configuration.

• After a mutation, identify which pages or data keys depend on it instead of clearing unrelated cached data blindly.

Interview point:

• Explain exactly what is cached, who may reuse it and what event makes it stale.