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Fix Single Span Sampling for parent spans - #4674
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…ate from a trace sampling priority, so child spans will be dropped even if the parent span is kept by SSS.
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bantonsson
approved these changes
Feb 7, 2023
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What Does This Do
Fix Single Span Sampling for root spans to keep an SSS decision separate from a trace sampling priority, so child spans will be dropped when the entire trace has to be dropped even if the parent span is kept by SSS.
Motivation
Fix for when a root span is sampled by the single span sampling but the entire span is dropped.
The problem is that when a span doesn’t have an explicitly set sampling priority then it gets it from the root span context. It’s done to preserve a sampling priority in case of partial flush (#3412). This logic was implemented before single span sampling was introduced.
Now, when a root span is sampled by the single span sampling mechanism, it overrides the already set dropping priority causing a child span (without priority -128) to use a single span sampling priority instead of a trace dropping priority that has been set for the entire trace.
Additional Notes
Relevant System Tests PR
incident-18687