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Rename ProbabilitySampler to TraceIdRatioBasedSampler and add requirements #611

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Aug 21, 2020
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Rename ProbabilitySampler to TraceIdRatioBasedSampler and add requirements #611

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c819f17
Add requirements for probability sampler
bogdandrutu May 18, 2020
3ce2497
Merge branch 'master' into probrequirements
bogdandrutu Aug 21, 2020
3a70c51
Merge branch 'master' into probrequirements
bogdandrutu Aug 21, 2020
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27 changes: 20 additions & 7 deletions specification/trace/sdk.md
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Expand Up @@ -90,7 +90,7 @@ It produces an output called `SamplingResult` which contains:

Returns the sampler name or short description with the configuration. This may
be displayed on debug pages or in the logs. Example:
`"ProbabilitySampler{0.000100}"`.
`"TraceIdRatioBased{0.000100}"`.

Description MUST NOT change over time and caller can cache the returned value.

Expand All @@ -109,16 +109,29 @@ The default sampler is `ParentBased(root=AlwaysOn)`.
* Returns `NOT_RECORD` always.
* Description MUST be `AlwaysOffSampler`.

#### Probability
#### TraceIdRatioBased

* The `ProbabilitySampler` MUST ignore the parent. To respect the parent
`SampledFlag`, the `ProbabilitySampler` should be used as a delegate of the
`ParentBased` sampler specified below.
* Description MUST be `ProbabilitySampler{0.000100}`.
* The `TraceIdRatioBased` MUST ignore the parent `SampledFlag`. To respect the
parent `SampledFlag`, the `TraceIdRatioBased` should be used as a delegate of
the `ParentBased` sampler specified below.
* Description MUST be `TraceIdRatioBased{0.000100}`.

TODO: Add details about how the `ProbabilitySampler` is implemented as a function
TODO: Add details about how the `TraceIdRatioBased` is implemented as a function
of the `TraceID`.

##### Requirements for `TraceIdRatioBased` sampler algorithm

* The sampling algorithm MUST be deterministic. A trace identified by a given
`TraceId` is sampled or not independent of language, time, etc. To achieve this,
implementations MUST use a deterministic hash of the `TraceId` when computing
the sampling decision. By ensuring this, running the sampler on any child `Span`
will produce the same decision.
* A `TraceIdRatioBased` sampler with a given sampling rate MUST also sample all
traces that any `TraceIdRatioBased` sampler with a lower sampling rate would
sample. This is important when a backend system may want to run with a higher
sampling rate than the frontend system, this way all frontend traces will
still be sampled and extra traces will be sampled on the backend only.
Comment on lines +127 to +131
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This is only relevant for requests that cross trust boundaries because otherwise the sampled flag in the incoming TraceState is honored by the parent-based sampler, right? This could mentioned to avoid confusion.

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Not really, we decided that the samplers like this will not honor any flag or TraceState, is the user responsibility to configure the ParentBased:

The TraceIdRatioBased MUST ignore the parent SampledFlag. To respect the parent SampledFlag, the TraceIdRatioBased should be used as a delegate of the ParentBased sampler specified below.

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Re-read, yes in general if parent-based is set it is not very important, but even if parent-based is not set this should be the same case.


#### ParentBased

* This is a composite sampler. `ParentBased` helps distinguished between the
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