Sampling should recommend ParentBasedSampler with TraceIDRatioBasedSampler

[thomaschaaf]

What needs to be changed?
The sampling page https://github.com/open-telemetry/opentelemetry.io/blob/main/content/en/docs/languages/js/sampling.md currently does not mention ParentBasedSampler. In my opinion the default should be ParentBasedSampler with TraceIDRatioBasedSampler as that is what most newcomers are looking for.

What is the name + path of the page that needs changed? content/en/docs/languages/js/sampling.md

[tiffany76]

Thanks for raising this issue, @thomaschaaf.

@open-telemetry/javascript-approvers, PTAL. Thanks!

[cartermp]

I wrote this originally. At the time, I chose not to focus on the combination of the two, but I think now it's fine to do both.

Unfortunately, there is a downside to both behaviors:

• With ParentBased included, your own service will be (often surprisingly) impacted by any service upstream of you that happens to head sample differently. At my day job I've run into people getting really confused about why some traces weren't present because an upstream service they don't control dictated sampling.
• With ParentBased excluded, you can get missing spans unexpectedly.

The best way to have really coherent traces with sampling is to use tail-based sampling. But SDKs don't do that.

At any rate, I think including ParentBased is probably less...weird? Surprising? Less common to have weird behavior?

[cijothomas]

As per spec, the default sampler is ParentBased(root=AlwaysOn) . Is that what js uses by default?

[cartermp]

Yes, JS does that by default if you don't change sampling in any way. However, since this is for documenting adjusting the sampling rate at the head, the suggestion is to do ParentBased(TraceIdRatioBased=foo))

[cijothomas]

Thanks for clarifying. yes, that makes sense.