How many ninesThe 9s in an uptime figure: 99% is two nines, 99.9% three, 99.99% four. Each extra nine cuts allowed downtime roughly tenfold., really?

A system is only as available as the parts it leans on. Chain dependencies in seriesA chain where every part must be up for the whole to work — so a failure anywhere takes the system down, and availabilities multiply. and their availabilities multiply down — three 99.9% services in a row are just 99.7% together. Add redundancy in parallelRedundant copies where any one staying up keeps the system up — so you're only down when all of them fail at once. and the gaps multiply away instead. List your parts and read off the system number.