Microsoft Certified: DevOps Engineer ExpertImplement a Site Reliability Engineering (SRE) strategyMedium
A team is designing an SRE strategy for a new critical service. They have defined a Service Level Objective (SLO) for the service's availability as 99.9%. They want to calculate the maximum allowable downtime per month to stay within this SLO. Assume a standard 30-day month.
- A7.2 hours
- B0.72 hours
- C4.32 minutes
- D43.2 minutes
Show answer & explanationAnswer & explanation
Correct answer: D. 43.2 minutes
A 99.9% availability SLO means the service can be unavailable for 0.1% of the time. First, calculate total minutes in a 30-day month: 30 days * 24 hours/day * 60 minutes/hour = 43,200 minutes. Then calculate 0.1% of this total downtime: 43,200 minutes * 0.001 = 43.2 minutes.
Why the other options are wrong
- A. This calculates 1% downtime (99% availability), which is too lenient.
- B. This calculates 0.01% downtime in hours, but it's still incorrect for 99.9% availability and needs to be in minutes for typical SRE discussions.
- C. This calculates 0.01% downtime (99.99% availability), which is too strict.
Availability Calculation
The process of determining the maximum permissible downtime or the expected uptime based on a Service Level Objective (SLO) for availability.
- Availability % = (Total Time - Downtime) / Total Time * 100.
- Error Budget % = 100% - Availability %.
- Downtime = Total Time * Error Budget %.
Memory trick: Turn the percentage into a fraction, then multiply by total time.