Web Caching

Hit Ratios

How can we measure the effectiveness of our caches? One such measurement is called the cache hit ratio. This is the percentage of requests that are satisfied as cache hits. Usually, this includes both validated and unvalidated hits. Validated hits can be tricky because these requests are forwarded to origin servers, incurring slight bandwidth and latency penalties. Note that the cache hit ratio tells you only how many requests are hits—it doesn’t tell you how much bandwidth or latency has been saved.

The measurement that does tell you about bandwidth is called the byte hit ratio. Instead of counting only requests, this measure is based on the number of bytes transferred. Cache hits for large objects contribute more to the byte hit ratio than do small objects. The byte hit ratio measures how much bandwidth your cache has saved, but there are different ways to calculate it.

One way is to compare the sum of object sizes for cache hits and cache misses. However, this technique has a couple of shortcomings. For example, it doesn’t include request traffic. Counting the request traffic probably doesn’t matter much because it’s relatively small, and most of the data flows in the other direction (into your network, not out of it). This technique might not count the small 304 (Not Modified) responses either. However, the bigger problem is in accounting for requests aborted by the user. Consider a cache miss for a 100KB object. If the cache downloads the entire response, but the ...