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Clarify distribution summary docs #182

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Aug 21, 2024
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Clarify distribution summary docs #182

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Replace references to latency in dist operators to size and clarify d…
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9 changes: 7 additions & 2 deletions docs/spectator/core/meters/dist-summary.md
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Expand Up @@ -28,7 +28,7 @@ via a filter, or use one of the operators below to generate a useful response.

### Average Measurement (:dist-avg)

To compute the average latency across an arbitrary group, use the [:dist-avg] function:
To compute the average size across an arbitrary group, use the [:dist-avg] function:

@@@ atlas-stacklang
/api/v1/graph?q=nf.cluster,foo,:eq,name,http.req.payload.size,:eq,:and,:dist-avg,(,nf.asg,),:by
Expand All @@ -38,12 +38,17 @@ To compute the average latency across an arbitrary group, use the [:dist-avg] fu

### Maximum Measurement (:dist-max)

To compute the maximum latency across a group, use [:dist-max]:
To compute the maximum size across a group, use [:dist-max]:
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Good catch, thanks.


@@@ atlas-stacklang
/api/v1/graph?q=nf.cluster,foo,:eq,name,http.req.payload.size,:eq,:and,:dist-max,(,nf.asg,),:by
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!!! Note
Distribution summaries do not aggregate well over dimensions. Each measurement is recorded with a mapping of tags.
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This doesn't really have anything to do with :dist-max specifically, it should probably be moved to the initial section or possibly just as a warning in the IPC spec for the inflight stuff. Distribution summaries aggregate just fine is used as intended, i.e. summarizing recorded samples such as payload sizes for requests. The :dist-max will be the maximum recorded sample that matches the query criteria. The :dist-avg will be the average of the recorded samples that match the query criteria. Possible rewording:

Distribution summaries are intended for use-cases tracking the size of recorded samples. For example, request payload sizes to understand the average or maximum recorded payload size for a given scope. It should not be used for use-cases trying to perform hierarchical accumulation such as number of inflight requests across different levels.

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I moved this to the IPC spec for inflight metrics, and modified the wording to include some of your rewording. I don't want to make it sound like we shouldn't use distribution summary at all for inflight metrics, since we aren't deprecating it (for now). I still think it can be useful for customers to see the dist-max on certain filters, but we've added new dashboards for queueing approximation.

Therefore, when using [:dist-max] to query over a set of filters, the response will represent the maximum size for a
given set of tag values within the group, **not** the accumulate value of measurements across the entire group.

[:dist-max]: ../../../asl/ref/dist-max.md

### Standard Deviation of Measurement (:dist-stddev)
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