KCNA Cloud Native Observability Practice Question
An organization uses Prometheus and Grafana for monitoring. They want to alert when the 99th percentile of request latency exceeds 500ms for more than 5 minutes. Which PromQL query should they use in the alert rule?
⚠ Common exam trap
The trap here is that candidates often pick a 1-minute rate window (Option A) thinking it provides faster detection, but the question explicitly requires a 5-minute sustained condition, and Prometheus alert rules evaluate the query over the rule evaluation interval, not the rate window.
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
histogram_quantile(0.99, rate(http_request_duration_seconds_bucket[5m])) > 0.5
It uses `histogram_quantile(0.99, rate(...[5m]))` to calculate the 99th percentile request latency over a 5-minute window, matching the requirement to alert when this value exceeds 500ms (0.5 seconds) for more than 5 minutes. The `rate()` function with a 5m range computes the per-second increase of bucket counters over that duration, which is necessary for accurate quantile estimation in Prometheus.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
histogram_quantile(0.99, rate(http_request_duration_seconds_bucket[1m])) > 0.5
Why it's wrong here
Range of 1m is too short; need 5m to match requirement.
- ✓
histogram_quantile(0.99, rate(http_request_duration_seconds_bucket[5m])) > 0.5
Why this is correct
Correctly calculates 99th percentile over 5 minutes, then compares to 0.5 seconds.
- ✗
avg(rate(http_request_duration_seconds_bucket[5m])) > 0.5
Why it's wrong here
This averages bucket rates, not percentile.
- ✗
max(rate(http_request_duration_seconds_bucket[5m])) > 0.5
Why it's wrong here
Max of bucket rates is not percentile.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This KCNA practice question is part of Courseiva's free CNCF certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the KCNA exam.