Fundamentals
Availability
The fraction of time a system serves requests successfully, expressed in nines. 99.9% ≈ 43 minutes downtime per month.
Why interviewers ask about it
Dependencies in series multiply. Six services at 99.9% on the critical path give 99.4% - the hidden cost of microservices.
Related terms
This term is part of the free System Design interview preparation module - browse the full glossary for every definition.