PCA Promql Practice Question
You need to calculate the 99th percentile HTTP request latency from a Prometheus histogram named http_request_duration_seconds. Which function is correct?
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]))
The histogram_quantile() function calculates the $\phi$-quantile from bucket time series of a histogram.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
rate(histogram_quantile(0.99, http_request_duration_seconds_bucket)[5m])
Why it's wrong here
histogram_quantile returns an instant vector, which cannot be wrapped directly in a range vector for rate().
- ✗
quantile_over_time(0.99, http_request_duration_seconds[5m])
Why it's wrong here
quantile_over_time operates on range vectors of gauges, not cumulative histograms.
- ✓
histogram_quantile(0.99, rate(http_request_duration_seconds_bucket[5m]))
Why this is correct
Correct. It takes the quantile, a rate of the bucket metric, and computes the value.
- ✗
histogram_quantile(http_request_duration_seconds_bucket[5m], 0.99)
Why it's wrong here
The function signature expects the quantile scalar first, followed by the rate of buckets.
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JA
Written and reviewed by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
Last reviewed August 2026 · checked against the official CNCF / Linux Foundation exam blueprint
This PCA practice question is part of Courseiva's free CNCF / Linux Foundation 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 PCA exam.