Living note

Choosing Cache Boundaries

How to decide what belongs in browser, edge, application, or data-layer caches.

A cache is a consistency decision with a performance benefit, not merely a faster key-value lookup.

Boundary checklist

Cache boundary trade-offs
Layer	Best for	Main risk
Browser	Private, reusable assets	Hard invalidation
Edge	Shared public responses	Regional drift
Application	Computed domain data	Memory pressure

Name the invalidation event

If the team cannot name what invalidates a value, the cache policy is unfinished.

cache-policy.tsTS

export const articleCache = {
  key: (slug: string) => `article:${slug}`,
  revalidateOn: ["article.published", "article.unpublished"],
  ttlSeconds: 300,
};