Distributions

A server that is slow on one request in a hundred sounds healthy. A user request that must wait for 100 such servers is slow 63% of the time, because it is fast only when all 100 are, and 0.99¹⁰⁰ = 0.366. No server got worse. The request inherited the distribution of the slowest of a hundred draws, a calculation that Jeffrey Dean and Luiz André Barroso published from Google's serving systems in 2013 and that most engineers who run distributed systems rediscover the hard way.