Glossary term
What is Availability / "nines"? Meaning for developers
% of time (or requests) the system works. Example: 99.9% ≈ 8.8 hours of downtime per year.
Learn it properly · System DesignLesson 9: Availability and "The Nines" Explained“Five nines” is 26 seconds of downtime a month. What the nines really mean, why chained dependencies multiply away your availability while redundancy multiplies it back, and why fast recovery is often the cheapest nine you can buy.