[processor/tailsamplingprocessor] documentation to drop trace

[mangkoran]

Component(s)

processor/tailsampling

Describe the issue you're reporting

In tailsampling readme there is no mention on how to drop a trace. What I understand from this part is that each policy will result in either "sample" or "not sample" decision. However, there is also no mention how to create a "not sample" decision.

How can I drop a certain trace? e.g. match the trace using string_attribute and drop the matched trace. Initially I thought this could be done using filter processor. However, as stated in https://github.com/open-telemetry/opentelemetry-collector-contrib/issues/31355 filter processor can only drop span and not the entire trace.

[github-actions[bot]]

Pinging code owners:

• processor/tailsampling: @jpkrohling

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[jpkrohling]

You can set an "invert" match: if a trace has a span with a specific attribute value for a given attribute, it would normally yield a "Sample" decision. An inverted match would yield a "Not Sample" decision.

[mangkoran]

Thank you for the explanation. However I still couldn't understand the explanation in the docs:

• When there's an "inverted not sample" decision, the trace is not sampled;
• When there's a "sample" decision, the trace is sampled;
• When there's a "inverted sample" decision and no "not sample" decisions, the trace is sampled;
• In all other cases, the trace is NOT sampled

There's "inverted sample" mentioned. This is contradictory to your explanation:

An inverted match would yield a "Not Sample" decision.

From how I understand the docs, there could be 4 policy outcome:

1. Sample, not inverted
2. Sample, inverted
3. Not sample, not inverted
4. Not sample, inverted

CMIIW

[jpkrohling]

I think both are right:

• the "string" policy, for instance, would do an invert match on a specific string
• the invert match would generate an inverted don't sample, which means "do not sample"

Here's an example from the docs:

         {
            name: test-policy-9,
            type: string_attribute,
            string_attribute: {key: http.url, values: [\/health, \/metrics], enabled_regex_matching: true, invert_match: true}
         },

In this case, if the URL contains /metrics, we are dropping the trace.

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