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Difficulty: Very hardDefining and Managing SLOs, SLIs, and Error Budgets

A mission-critical payment gateway hosted on Google Kubernetes Engine (GKE) processes 20,000,00020,000,000 valid HTTP requests over a rolling 30-day measurement window. The Site Reliability Engineering (SRE) team defines an Availability Service Level Objective (SLO) of 99.9%99.9\%, measured as the ratio of successful (non-HTTP 5xx) requests to total valid requests. During a bad canary release, the service logged 12,00012,000 HTTP 500 responses before being rolled back. No other failure events occurred during the 30-day window. What percentage of the total 30-day error budget remains after this incident?

Answer: 40 %

Answer

The remaining error budget is 40.0%.
The total error budget for a 30-day period with 20,000,00020,000,000 requests at a 99.9%99.9\% SLO target is 20,000,000×0.001=20,00020,000,000 \times 0.001 = 20,000 allowable failed requests. The incident consumed 12,00012,000 requests (60%60\% of the budget). Therefore, 20,000−12,000=8,00020,000 - 12,000 = 8,000 allowed failed requests remain, which represents (8,000/20,000)×100=40.0%(8,000 / 20,000) \times 100 = 40.0\% of the original error budget.

Step-by-Step Solution

1
Calculate the total allowable error budget (in number of failed requests) from the SLO target.
Total Error Budget = 20,000,000×(1−0.999)=20,00020,000,000 \times (1 - 0.999) = 20,000 requests.
An Availability SLO of 99.9%99.9\% allows an error rate of 0.1%0.1\% (0.0010.001) across all incoming valid requests.
2
Subtract the consumed bad requests from the total allowable error budget.
Remaining Error Budget (in requests) = 20,000−12,000=8,00020,000 - 12,000 = 8,000 requests.
The canary incident consumed 12,00012,000 failed requests out of the 20,00020,000 total allowed budget.
3
Calculate the percentage of the error budget remaining relative to the original error budget.
Remaining Percentage = (8,000/20,000)×100=40.0%(8,000 / 20,000) \times 100 = 40.0\%.
Error budget remaining is expressed as a fraction of the total allowable error budget, not the total request volume.

Key Concept

Error Budget Calculation based on Request-Based SLIs and SLO targets
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Topics

Defining and Managing SLOs, SLIs, and Error Budgets

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