Clarify TraceIdRatioBased Sampler description

[MrAlias]

From the specification:

Description MUST be TraceIdRatioBased{0.000100}

Does this mean this sampler should always return the static string "TraceIdRatioBased{0.000100}" even if the ratio is not 0.000100?

Or should it return "TraceIdRatioBased{$RATIO}" with $RATIO replaced with a decimal representation of the sampling ratio? If so, what precision should this ratio be?

[MrAlias]

Survey of Existing Solutions

Go

"TraceIdRatioBased{$RATIO}" with $RATIO replaced with a decimal representation of the sampling ratio with the "smallest number of digits necessary to identify the value uniquely" level of precision.

https://github.com/open-telemetry/opentelemetry-go/blob/90bd4ab50c6e85743f2b8f3a1768a5afceeb1fc0/sdk/trace/sampling.go#L105

Java

"TraceIdRatioBased{$RATIO}" with $RATIO replaced with a decimal representation of the sampling ratio with 6 digits of precision.

https://github.com/open-telemetry/opentelemetry-java/blob/5c0ea253a53719774b16933191e8f8b1c46ff73d/sdk/trace/src/main/java/io/opentelemetry/sdk/trace/samplers/TraceIdRatioBasedSampler.java#L84

Python

"TraceIdRatioBased{$RATIO}" with $RATIO replaced with a decimal representation of the sampling ratio with up to 17 digits of precision (I think?).

https://github.com/open-telemetry/opentelemetry-python/blob/489a81e5d33690ec10f2d7606c7137168f85c17a/opentelemetry-sdk/src/opentelemetry/sdk/trace/sampling.py#L261

JS

"TraceIdRatioBased{$RATIO}" with $RATIO replaced with a decimal representation of the sampling ratio. I'm not sure on the precision.

https://github.com/open-telemetry/opentelemetry-js/blob/bf99144ad4e4fa38120e896d70e9e5bcfaf27054/packages/opentelemetry-core/src/trace/sampler/TraceIdRatioBasedSampler.ts#L40

.NET

"TraceIdRatioBased{$RATIO}" with $RATIO replaced with a decimal representation of the sampling ratio with 6 digits of precision (I think?).

https://github.com/open-telemetry/opentelemetry-dotnet/blob/21c440a284e2efe533a4fdf7c97c8978fbb6ecae/src/OpenTelemetry/Trace/TraceIdRatioBasedSampler.cs#L47

[MrAlias]

Based on what SIGs are doing (especially the ones that have a released a 1.0.0 version) it would follow the specification needs to be updated to reflect the description should be dynamic. E.g.

Description MUST return a string of the form `"TraceIdRatioBased{RATIO}"` with `RATIO` replaced with the Sampler instances' trace sampling ratio represented as a decimal number.
The precision of the number SHOULD follow language standards and SHOULD be high enough to identify when Samplers have different ratios.
For example, if a TraceIdRatioBased Sampler had a sampling ratio of 1 to every 10,000 spans it could return `"TraceIdRatioBased{0.000100}"` as its description.

[arminru]

I'm glad no one interpreted it as "it has to be this constant string regardless of the actual ratio" 😄

Thanks for digging up the status-quo and presenting it here, Tyler!
Your proposal sounds good to me. Could you please create a PR for that?

[MrAlias]

Your proposal sounds good to me. Could you please create a PR for that?

Sure thing 👍