An internal billing service commits to 99.9% availability measured over a rolling 30-day window. How much downtime does its error budget allow in that window?
Show answer and explanation
Answer: B
The error budget is 100 percent minus the availability target. For 99.9 percent, allowed unavailability is 0.1 percent. A 30-day window contains 43,200 minutes, and 0.1 percent of that is 43.2 minutes. Teams spend this budget on risky changes and maintenance, and they slow releases when it runs out.
Why the other options are wrong
- A. That figure corresponds to 0.01 percent unavailability, a ten-times smaller allowance than a 99.9 percent target permits. The decimal place is off by a factor of ten.
- C. 7.2 hours corresponds to 1 percent unavailability, which would match a 99 percent target rather than 99.9 percent. The budget here is far smaller.
- D. One day is about 3.3 percent of 30 days. That corresponds to a roughly 96.7 percent objective, which is far looser than the 99.9 percent commitment described.