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https://issues.apache.org/jira/browse/SOLR-15283?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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David Smiley resolved SOLR-15283.
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Resolution: Fixed
> Remove Solr trace sampling; let Tracer configuration/impl decide
> ----------------------------------------------------------------
>
> Key: SOLR-15283
> URL: https://issues.apache.org/jira/browse/SOLR-15283
> Project: Solr
> Issue Type: Task
> Reporter: David Smiley
> Assignee: David Smiley
> Priority: Blocker
> Fix For: main (9.0)
>
> Time Spent: 0.5h
> Remaining Estimate: 0h
>
> GlobalTracer should always have the Tracer produced by the
> TracerConfiguratorPlugin. Solr should not intervene by substituting a no-op
> version sometimes, and thus needn't have its ThreadLocal tracking either
> (which doesn't work well). The special {{samplePercentage}} cluster property
> should be removed.
> Background: When someone configures tracing (supplying TracerConfigurator
> plugin), Solr will "sample" tracing if an incoming request has no tracing
> information. By default this is 10% and is only configurable via a
> {{samplePercentage}} cluster property. If you're in the 90%, this results in
> a no-op Tracer -- no trace IDs. This is really confusing & annoying because
> Tracers themselves have notions of sampling, which means "reporting"
> (sending) the trace to a tracing server where it can be
> stored/analyzed/visualized. The point of a non-sampled trace is propagating
> IDs for logging (trace ID in MDC) -- very light-weight. Zipkin and Jaeger
> (and others?) have their own samplers. When Solr receives a request with a
> trace ID, in Zipkin it also includes the binary sampling decision (it's
> another header). The expectation is that if the trace says to sample, then
> this sampling decision is propagated downstream and thus the whole call tree
> is fully sampled (reported to a server).
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