htmlCntConstRate_filterOff: hit ratios

The hit ratios table shows measured hit ratios. Hits are calculated based on client- and server-side traffic comparison. Offered hits are counted for 'basic' transactions only (simple HTTP GET requests with '200 OK' responses). Measured hit stats are based on all transactions. Thus, 'offered' hit ratio are not the same as 'ideal' hit ratio in this context.

Measured hit count or volume is the difference between client- and server-side traffic counts or volumes. DHR, Document Hit Ratio, is the ratio of the total number of hits to the number of all transactions. BHR, Byte Hit Ratio, is the ratio of the total volume (a sum of response sizes) of hits to the total volume of all transactions. Negative measured hit ratios are possible if server-side traffic of a cache exceeds client-side traffic (e.g., due to optimistic prefetching or extra freshness checks) and if side measurements are out-of-sync. Negative measured BHR can also be due to aborted-by-robots transactions.

A less accurate way to measure hit ratio is to detect hits on the client-side using custom HTTP headers. A hit ratio table based on client-side tricks is available elsewhere.