TraceIdRatioBased : Clarify what "lower sampling rate" means

[tigrannajaryan]

TraceIdRatioBased spec says:

A TraceIdRatioBased sampler with a given sampling rate MUST also sample all traces that any TraceIdRatioBased sampler with a lower sampling rate would sample.

What does "lower sampling rate" mean? The concept of sampling rate is not defined anywhere. Does sampling rate mean "fraction of spans that are exported"?

I find the following sentence also confusing:

This is important when a backend system may want to run with a higher sampling rate than the frontend system, this way all frontend traces will still be sampled and extra traces will be sampled on the backend only.

What does this describe? That backend system will drop some additional traces that frontend did not drop? Or the opposite of this, that backend will never drop traces that were not dropped by the frontend? Ideally we will have some pictures to explain.

[jsuereth]

My own interpretation: Rate = {spans}/{time}.

Higher rate means MORE spans are sampled, which means (for TraceIdRatioBased) that the ratio is a larger number.

For the second sentence:

This is important when a backend system may want to run with a higher sampling rate than the frontend system, this way all frontend traces will still be sampled and extra traces will be sampled on the backend only.

Would it be better to write this as:

This is important, because we want to make sure that for some traces, we get a sample of spans from all distributed components.  With TraceIdRatioBased, we're guaranteed that there will be overlap as long as every system has non-zero ratio configured.

???