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A Sampling Configuration proposal #240

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A Sampling Configuration proposal #240

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Initial content
PeterF778 Nov 8, 2023
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Renaming file
PeterF778 Nov 8, 2023
0e87ea3
Updates based on Sampling SIG discussion
PeterF778 Nov 10, 2023
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Adding example
PeterF778 Nov 11, 2023
2970583
simplifying - flattening the RULE structure
PeterF778 Nov 13, 2023
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correcting spelling errors
PeterF778 Nov 13, 2023
e2d1a84
Cleanup and small changes
PeterF778 Nov 17, 2023
877a056
Removing fallback sampler.
PeterF778 Nov 21, 2023
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Revising the yaml syntax to allow sampler argument identification by …
PeterF778 Nov 22, 2023
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Updates after discussion in Configuration SIG.
PeterF778 Nov 28, 2023
76dbe46
Changing the description of the composite samplers.
PeterF778 Dec 1, 2023
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Added a note on limitations
PeterF778 Dec 20, 2023
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Adding some clarifications.
PeterF778 Jan 5, 2024
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52 changes: 51 additions & 1 deletion text/0240-Sampling_Configuration.md
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Expand Up @@ -114,8 +114,8 @@ For rule-based sampling (e.g. decision depends on Span attributes), we need a Ru
Note: The `opentelemetry-java-contrib` repository contains [RuleBasedRoutingSampler](https://github.com/open-telemetry/opentelemetry-java-contrib/blob/main/samplers/src/main/java/io/opentelemetry/contrib/sampler/RuleBasedRoutingSampler.java), with similar functionality.
```yaml
samplerType: RuleBased
spanKind: SERVER | CONSUMER | ... # optional
parameters:
- spanKind: SERVER | CONSUMER | ... # optional
- - RULE1
- RULE2
...
Expand All @@ -130,6 +130,56 @@ where each RULE is
The Predicates represent logical expressions which can access Span Attributes (or anything else available when the sampling decision is to be taken), and perform tests on the accessible values.
For example, one can test if the target URL for a SERVER span matches a given pattern.

## Example

Let's assume that a user wants to configure head sampling as follows:
- for root spans:
- drop all `/healthcheck` requests
- capture all `/checkout` requests
- capture 25% of all other requests
- for non-root spans
- follow the parent sampling decision
- however, capture all calls to service `/foo` (even if the trace will be incomplete)
- in any case, do not exceed 1000 spans/second

Such sampling requirements can be expressed as:
```yaml
samplerType: AllOf
parameters:
- - samplerType: AnyOf
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parameters:
- - samplerType: ParentBased
parameters:
- samplerType: RuleBased # for root spans
parameters:
- spanKind: SERVER
- - predicate: http.target == /healtcheck
sampler:
samplerType: AlwaysOff
- predicate: http.target == /checkout
sampler:
samplerType: AlwaysOn
- samplerType: TraceIdRatioBased # fallback sampler
parameters:
- 0.25
- samplerType: AlwaysOn # remote parent sampled
- samplerType: AlwaysOff # remote parent not sampled
- samplerType: AlwaysOn # local parent sampled
- samplerType: AlwaysOff # local parent not sampled
- samplerType: RuleBased
parameters:
- spanKind: CLIENT
- - predicate: http.url == /foo
sampler:
samplerType: AlwaysOn
- samplerType: AlwaysOff # fallback sampler
- samplerType: RateLimiting
parameters:
- 1000 # spans/second
```

The above example uses plain text representation of predicates. Actual representation for predicates is TBD. The example also assumes that a hypothetical `RateLimitingSampler` is available.
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Actual representation for predicates is TBD

Here are some possible directions, for reference:


## Strong and Weak Typing

Constructing a sampler instance may require using some data types which are not strings, numbers, samplers, or lists of those types. Obviously, the beforementioned rule-based sampler needs to get `Rules`, which, in turn, will reference `Predicates`. The knowledge of these types can be built-in to the YAML parser, to ensure proper support.
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