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Difficulty: MediumDefining and Managing SLOs, SLIs, and Error Budgets

An enterprise healthcare provider operates an electronic health records (EHR) API Gateway on Google Cloud with a defined Service Level Objective (SLO) of 99.9%99.9\% availability over a rolling 3030-day window (43,20043,200 minutes). Over the current 3030-day window, the service experienced two incidents while maintaining constant request volume: a database outage resulted in a complete service failure (100%100\% error rate) lasting 1515 minutes, and a downstream service throttling issue resulted in a partial degradation (40%40\% error rate) lasting 4545 minutes. What is the remaining allowable error budget in minutes for this 3030-day window?

Answer: 10.2 minutes

Answer

The remaining allowable error budget for the 30-day window is 10.2 minutes.
The total allowable error budget for a 99.9% SLO over 30 days (43,200 minutes) is 0.001×43,200=43.20.001 \times 43,200 = 43.2 minutes. Incident 1 (100% failure for 15 minutes) consumes 15.0 minutes. Incident 2 (40% failure for 45 minutes under uniform traffic) consumes 45×0.40=18.045 \times 0.40 = 18.0 minutes. Total consumed budget is 15.0+18.0=33.015.0 + 18.0 = 33.0 minutes. Subtracting the consumed budget from the total allowable budget leaves 43.2−33.0=10.243.2 - 33.0 = 10.2 minutes.

Step-by-Step Solution

1
Calculate the total allowable error budget for a 99.9% availability SLO over 30 days (43,200 minutes).
Total Error Budget = 43,200×(1−0.999)=43.243,200 \times (1 - 0.999) = 43.2 minutes.
An SLO of 99.9% permits a maximum error rate of 0.1% across the entire rolling window.
2
Compute the effective downtime minutes consumed during the total outage incident.
Incident 1 Budget Consumed = 15 minutes×100%=15.015 \text{ minutes} \times 100\% = 15.0 minutes.
A 100% error rate for 15 minutes consumes 15 full minutes of error budget.
3
Compute the effective downtime minutes consumed during the degraded state incident.
Incident 2 Budget Consumed = 45 minutes×40%=18.045 \text{ minutes} \times 40\% = 18.0 minutes.
Under uniform traffic, a partial outage with a 40% error rate for 45 minutes consumes equivalent error budget proportional to the failure fraction (45×0.4045 \times 0.40).
4
Sum the error budget consumed across both operational incidents.
Total Consumed = 15.0+18.0=33.015.0 + 18.0 = 33.0 minutes.
Combining the impact of full and partial outages gives the total budget spent.
5
Subtract total consumed budget from total allowable budget.
Remaining Error Budget = 43.2−33.0=10.243.2 - 33.0 = 10.2 minutes.
This yields the remaining margin of unreliability permitted before breaching the SLO target.

Key Concept

SLO Error Budget Calculation for Partial and Total Outages
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Topics

Defining and Managing SLOs, SLIs, and Error Budgets

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