Getting sampled spans when using const sampler and param 0

[andremarianiello]

Requirement - what kind of business use case are you trying to solve?

I am trying to disable all non-debug sampling, so that I specifically get traces only when I ask for them via the debug header.

Problem - what in Jaeger blocks you from solving the requirement?

I am using the following config:

{
  "service_name": "avm-nginx",
  "sampler": {
        "type": "const",
        "param": 0
  },
  "reporter": {
        "localAgentHostPort": "jaeger-agent.tracing:6831"
  }
}

via the nginx-opentracing module, but traces are still being sampled.

Proposal - what do you suggest to solve the problem or improve the existing situation?

Make const sampler work as documented when param is set to 0.

I believe the issue is that if the param option is not set, the yaml config parser sets it to 0. A value of 0 is later interpreted as the param being not set, so the param is set to 0.001 because that is default probabilistic sampling rate. Then even later, any non-zero param is interpreted as always sample for the ConstSampler, so there is no way to make the ConstSampler have a false decision. I don't expect this to be intended behavior.

If some other sentinel value was used, like -1, then specifying param = 0 would make ConstSampler and ProbablisticSampler work as documented, and not specifying the param would still work the same.