Per Buer is the CTO and founder of Varnish Software, the company behind the open source project Varnish Cache. Buer is a former programmer turned sysadmin, then manager turned entrepreneur. Runs, cross country skis and tries to keep his two boys from tearing down the house.
Phil Karlton once said, “There are only two hard things in Computer Science: cache invalidation and naming things.” This article is about the harder of these two: cache invalidation. It’s directed at readers who already work with Varnish Cache. To learn more about it, you’ll find background information in “Speed Up Your Mobile Website With Varnish.”
10 microseconds (or 250 milliseconds): That’s the difference between delivering a cache hit and delivering a cache miss. How often you get the latter will depend on the efficiency of the cache — this is known as the “hit rate.” A cache miss depends on two factors: the volume of traffic and the average time to live (TTL), which is a number indicating how long the cache is allowed to keep an object. As system administrators and developers, we can’t do much about the traffic, but we can influence the TTL.