What is Cache Hit?

A cache hit is the condition in which a content delivery network (CDN) edge node holds a valid copy of a requested object and delivers it to the requesting client without forwarding the request to the origin server. This is the ideal outcome in a caching system because it minimizes latency, reduces load on the origin, and lowers bandwidth costs. The object must be present in the cache and not expired or marked as stale according to the cache control headers (e.g., Cache-Control, Expires) set by the origin.

When a client request arrives at an edge server, the server first checks its local cache storage. If the requested URL matches a stored object and the object's freshness lifetime has not expired, the server returns a 200 OK response with the cached content, often including an Age header indicating how many seconds the object has been cached. The server does not contact the origin for validation unless the cache has a stale entry that requires revalidation via If-Modified-Since or If-None-Match headers. A cache hit reduces the round-trip time from potentially hundreds of milliseconds to single-digit milliseconds.

Cache hit ratio is a key performance metric for CDNs. It is calculated as the number of cache hits divided by total requests over a given period. High cache hit ratios (typically above 90% for static content) indicate efficient caching policies and good cache utilization. Factors that reduce cache hits include low TTL values, unique per-user URLs, query string variations, and cache busting mechanisms. CDN operators tune cache rules, use purge APIs, and implement tiered caching to maximize the hit rate and improve end-user experience.