Web Caching

Service Times

Table A-11 shows the mean and median service times for different types of requests. The type is taken directly from the fourth field of Squid’s access.log file. The three-digit number following the slash is the HTTP status code. Here I show only 200 (OK) and 304 (Not Modified) replies.

Table A-11. Mean and Median Service Times by Cache Result (IRCache Data)

Type	Mean (sec)	Median (sec)
TCP_REFRESH_HIT/200	5.165	0.281
TCP_MISS/200	4.590	0.466
TCP_REFRESH_MISS/200	4.091	0.468
TCP_CLIENT_REFRESH_MISS/200	1.889	0.215
TCP_HIT/200	1.819	0.050
TCP_MISS/304	1.151	0.294
TCP_REFRESH_HIT/304	1.107	0.219
TCP_IMS_HIT/304	0.102	0.042

The string before the slash indicates how the cache handled the request. TCP_MISS means the object wasn’t found in the cache at all. TCP_HIT indicates an unvalidated cache hit. TCP_IMS_HIT occurs when Squid receives an If-modified-since request and returns a 304 (Not Modified) reply immediately because the response is fresh according to the local configuration. TCP_CLIENT_REFRESH_MISS means that the user-agent sent the no-cache directive. TCP_REFRESH_HIT occurs when Squid validates its cached response and learns that the object hasn’t changed. If the client’s request includes an If-modified-since header, Squid returns the 304 message. Otherwise, it sends the entire object, and the status is 200. Finally, TCP_REFRESH_MISS means Squid sent a validation request to the origin server, and the cached object was out-of-date.

This data is derived from the second field of Squid’s access.log ...