This is a problem we wrestled with a lot at Netflix. What counts as down? It was rare for the platform to be completely unavailable. Most downtime was partial, where just some people had issues. How do you calculate that?

We ended up settling on predicting how much traffic there should be at any given second, and then going from there. For example, if there should have been 3000 stream starts in that second, and we got 2000 starts, that would count as 1/3 of a second of downtime.

Our goal was 4 nines, which allowed for a total of one minute of downtime a week. We managed to achieve that on many weeks of the year. One week in the three years I kept close track we managed 100%. It was near Christmas, when no one was working and deploying code. Shooting ourselves in the foot was the number one cause of downtime.