census-instrumentation
Change sampling decision after starting a span.
Currently, a sampling decision is made at the start of a span and can't be changed afterwards.
Envoy sometimes un-samples a span, e.g. if the span belongs to a health-check, and this wasn't known at start time.
in brave we call this "abandon" not sampling. because sampling is trace tier. literally we have a method span.abandon() with a hazard sign make sure you use this with extreme care. Often you don't have to create a span for reasons such as preventing health checks from becoming traced.. that's quite odd actually. span.abandon() was used for something harder to tell, for example if the message was a dupe.
I think this is similar with "abandon" from brave. So the feature that you want is not to turn on sampling, but to turn off sampling. Is that correct?
Envoy only uses abandon (true->false) for now, but in #217 we talk about a false->true transition, which might also be useful for a form of retroactive tracing.
This issue is generally about: should we allow this transition and, if so, in which directions?
provided instrumentation is written to not inspect noop state, you could possibly transition from false to true, but it is a bit dodgy. In any case, one child could inherit sampled false and another true.
I wouldn't recommend a false to true transition. abandon's opposite would be resurrect, and it wouldn't be possible to resurrect data never recorded.
Chase pointer: Envoy discussion about this: https://github.com/envoyproxy/envoy/issues/7422
How important is the "abandon" use-case?
Instead of abandon, could we satisfy this use-case by filtering out spans by name during export?
Instrumentation can control if abandon is present. otherwise they would need a buddy to filter it out later.. since export is different than the tracer api maybe from config pov it could be a little messy
On Thu, Aug 8, 2019, 9:53 AM easy [email protected] wrote:
How important is the "abandon" use-case?
Instead of abandon, could we satisfy this use-case by filtering out spans by name during export?
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