This is #5 in a very long series of posts on Stack Overflow’s architecture. Previous post (#4): Stack Overflow: How We Do Monitoring - 2018 Edition
So…caching. What is it? It’s a way to get a quick payoff by not re-calculating or fetching things over and over, resulting in performance and cost wins. That’s even where the name comes from, it’s a short form of the “ca-ching!” cash register sound from the dark ages of 2014 when physical currency was still a thing, before Apple Pay. I’m a dad now, deal with it.
Let’s say we need to call an API or query a database server or just take a bajillion numbers (Google says that’s an actual word, I checked) and add them up. Those are all relatively crazy expensive. So we cache the result – we keep it handy for re-use.
I think it’s important here to discuss just how expensive some of the above things are. There are several layers of caching already in play in your modern computer. As a concrete example, we’re going to use one of our web servers which currently houses a pair of Intel Xeon E5-2960 v3 CPUs and 2133MHz DIMMs. Cache access is a “how many cycles” feature of a processor, so by knowing that we always run at 3.06GHz (performance power mode), we can derive the latencies (Intel architecture reference here – these processors are in the Haswell generation):