What is a cache hit and a cache miss?

A cache hit is a request that the cache answers with a stored response, without contacting the origin. A cache miss is a request the cache cannot answer from storage, because it has no copy or its copy is stale, so it forwards the request to the origin. Hits are fast and cheap. Misses cost an origin round trip.

cf-cache-status: HIT
x-cache: MISS
x-vercel-cache: HIT
Cache-Status: ExampleCache; fwd=uri-miss

Between the two sit states each CDN names differently. Cloudflare's CF-Cache-Status, for example, has:

• HIT and MISS, and EXPIRED: found in cache but expired, and served from the origin.
• STALE: served from cache although expired, because the origin could not be reached. UPDATING: served stale while a refresh runs.
• REVALIDATED: a stale copy was checked with the origin and served from cache.
• BYPASS and DYNAMIC: not eligible for caching, so not a miss in the usual sense.

A `Cache-Status` header normalizes this across caches. The share of hits is the cache hit ratio, and a cold cache is one where every request is a miss.