If you care only about 99.9% of your requests being on time, it means that one in a 1000 can time-out and die Your argument is mathematically correct, but from

If you care only about 99.9% of your requests being on time, it means that one in a 1000 can time-out and die Your argument is mathematically correct, but from an engineering point of view, doesn't make sense. Of course in theory a system which responds 99.9% of the time means that it's possible for 1 of 1000 requests to die horribly. But we are engineers. If I achieve 99.99% of my target, it means that it's highly likely that that one request will also behave with pretty good performance, but I can't be sure that it will be 100ms. It might be 150ms. Or 200ms. But as an engineer, I claim that I am not concerned and don't care about knowing precisely that number. I think this is what the author of the original article misses completely: he is applying advanced math and refuses to consider that engineering tradeoffs are practical. Of course in theory they aren't. But in practice they are.