Cianplan

Level 4: Reliability, security & cloud · L4.1 Availability math & SLOs

Counting nines

Compute serial and parallel availability and yearly downtime.

12 min · Pro

What you take away

  • Availability is a property of the composition, not of any one component: dependencies in series multiply, so the whole is always worse than its worst part.

And 3 more inside the class, in Pro.

How the class is laid out

  1. The problem
  2. The simple version
  3. In series, availability multiplies
  4. In parallel, failure multiplies
  5. The cheapest nine is usually MTTR
  6. Check your understanding