Discuss Linux Foundation PCA Exam Topic 2 Question 17

How do you calculate the average request duration during the last 5 minutes from a histogram or summary called http_request_duration_seconds?

Arate(http_request_duration_seconds_sum[5m]) / rate(http_request_duration_seconds_count[5m])

Brate(http_request_duration_seconds_total[5m]) / rate(http_request_duration_second$_count[5m])

Crate(http_request_duration_seconds_total[5m]) / rate(http_request_duration_seconds_average[5m])

Drate(http_request_duration_seconds_sum[5m]) / rate(http_request_duration_seconds_average[5m])

Suggested Answer: A

In Prometheus, histograms and summaries expose metrics with _sum and _count suffixes to represent total accumulated values and sample counts, respectively. To compute the average request duration over a given time window (for example, 5 minutes), you divide the rate of increase of _sum by the rate of increase of _count:

text{Average duration} = frac{text{rate(http_request_duration_seconds_sum[5m])}}{text{rate(http_request_duration_seconds_count[5m])}}

Here,

http_request_duration_seconds_sum represents the total accumulated request time, and

http_request_duration_seconds_count represents the number of requests observed.

By dividing these rates, you obtain the average request duration per request over the specified time range.

Extracted and verified from Prometheus documentation -- Querying Histograms and Summaries, PromQL Rate Function, and Metric Naming Conventions sections.

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25 comments · newest first

Huey 5 days ago

C is confusing. Average isn’t needed here.

Helaine 10 days ago

B looks tempting, but I feel it’s not right.

Roselle 15 days ago

I agree, A makes sense. Sum over count.

Viva 20 days ago

I think it's A. Seems straightforward.

Junita 26 days ago

A feels right. Count gives a true average.

Louis 1 month ago

I’m not sure about C. Average in the rate? Doesn’t fit.

Phuong 1 month ago

A is definitely the best choice. It’s standard practice.

Andrew 1 month ago

D is confusing. Why use average in the denominator?

Lizette 2 months ago

I’m leaning towards A too. It’s clear and logical.

Michal 2 months ago

Option B looks tempting, but I feel the naming is off.

Christoper 2 months ago

I agree, A seems straightforward. Average needs both.

Micaela 2 months ago

I think option A is correct. It makes sense to use sum and count.

Leonardo 2 months ago

D) sounds off to me, not sure about that one.

Crissy 2 months ago

Wait, is C) even a valid option?

Whitney 3 months ago

I think B) makes more sense, though.

Alease 3 months ago

A) is the right choice!

Trina 3 months ago

How can you trust those rates over just raw counts?

Lauran 3 months ago

Totally agree with A), it makes the most sense.

Jaclyn 3 months ago

Wait, is there even an average metric in the options?

In 3 months ago

I think B) is a better choice.

Clare 4 months ago

A) is the correct formula!

Skye 4 months ago

I thought the average was derived from sum and count, so A makes sense, but I wonder if there's a catch with the time window.

Salina 4 months ago

I'm a bit confused about option B; it mentions total and count, but I don't recall if that's the standard way to calculate averages.

Shawna 4 months ago

I remember practicing a similar question, and I feel like the average request duration is calculated using the sum and count, so A seems right to me.

Trina 4 months ago

I think the answer might be A, but I'm not entirely sure about the difference between sum and total in this context.

Linux Foundation PCA Exam - Topic 2 Question 17 Discussion