What is Cache Miss?

A cache miss is an event in which a caching system, such as a Content Delivery Network (CDN) edge server or a reverse proxy, does not contain the requested resource at the time of the client request. This means the cache cannot fulfill the request from its stored content. The cache must then forward the request to the origin server, which processes it and returns the full response. The cache typically stores this response before delivering it to the client, so subsequent identical requests can be served from cache as a cache hit.

Cache misses are classified into three types: cold miss (first request for a resource), capacity miss (resource was evicted due to cache size limits), and conflict miss (resource was displaced by another cached object due to cache indexing constraints). The miss rate, defined as the number of cache misses divided by total requests, is a key performance metric. A high miss rate indicates inefficient caching, increasing latency and origin server load. CDN operators tune cache-control headers, TTL values, and eviction policies (e.g., LRU, LFU) to minimize misses.

In the wider internet infrastructure stack, cache misses directly impact user experience and operational cost. Each miss adds round-trip time to the origin, which can be hundreds of milliseconds for distant servers. CDNs and browsers both implement caching; a browser cache miss triggers a network request, while a CDN cache miss may further propagate to the origin. Protocols like HTTP/1.1 (RFC 7234) define caching semantics, and the Cache-Status HTTP response header (proposed in RFC 9213) can indicate whether a response was a hit or miss. Reducing cache misses is a primary goal of CDN configuration and cache optimization strategies.