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Add optional Zero Threshold for Exponential Histograms to the metrics data model #2665

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merged 3 commits into from
Aug 4, 2022
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Add optional Zero Threshold for Exponential Histograms to the metrics data model #2665

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e5565b7
Add optional Zero Threshold for Exponential Histograms
gouthamve Jul 15, 2022
ab73334
Fix lint issues
gouthamve Aug 2, 2022
c4125f3
Merge branch 'main' into add-zero-threshold
jmacd Aug 4, 2022
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27 changes: 19 additions & 8 deletions specification/metrics/data-model.md
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Original file line number Diff line number Diff line change
Expand Up @@ -30,7 +30,7 @@ linkTitle: Data Model
* [ExponentialHistogram](#exponentialhistogram)
+ [Exponential Scale](#exponential-scale)
+ [Exponential Buckets](#exponential-buckets)
+ [Zero Count](#zero-count)
+ [Zero Count and Zero Threshold](#zero-count-and-zero-threshold)
+ [Producer Expectations](#producer-expectations)
- [Scale Zero: Extract the Exponent](#scale-zero-extract-the-exponent)
- [Negative Scale: Extract and Shift the Exponent](#negative-scale-extract-and-shift-the-exponent)
Expand Down Expand Up @@ -643,14 +643,25 @@ perfect subsetting.
| 6 | 1.68179 | 2**(6/8) |
| 7 | 1.83401 | 2**(7/8) |

#### Zero Count
#### Zero Count and Zero Threshold

The ExponentialHistogram contains a special `zero_count` field
containing the count of values that are either exactly zero or within
the region considered zero by the instrumentation at the tolerated
level of precision. This bucket stores values that cannot be
expressed using the standard exponential formula as well as values
that have been rounded to zero.
The ExponentialHistogram contains a special `zero_count` bucket and an optional
`zero_threshold` field where `zero_count` contains the count of values whose
absolute value is less than or equal to `zero_threshold`. The precise value
for the `zero_threshold` is arbitrary and not related to the scale.
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When `zero_threshold` is unset or `0`, this bucket stores values that cannot
be expressed using the standard exponential formula as well as values that
have been rounded to zero.

Histograms with different `zero_threshold` can still be merged easily by
taking the largest `zero_threshold` of all involved Histograms and merge the
lower buckets of Histograms with a smaller `zero_threshold` into the common
wider zero bucket. If a merged `zero_threshold` is in the middle of a populated
bucket, it needs to be increased to match the upper boundary of the bucket.

In special cases, a wider zero bucket could be used to limit the total number
of populated buckets.

#### Producer Expectations

Expand Down