Should the traceparent header be a string or an array of strings?

[pwfarwell]

Our application handles trace headers from incoming event messages (via a NATS pipeline).
The json for the headers looks like this, where the b3 and traceparent headers are arrays of strings:

  "extensions": {
    "tenantId": "KWoKXpC5NQ2bUzvB0bmaRmsK9ONRLiwB",
    "header": {
      "b3": ["89404dabe42388d7a0940f9bb76492f8-2f0a4b7aae75ed8f-1"],
      "traceparent": ["00-89404dabe42388d7a0940f9bb76492f8-2f0a4b7aae75ed8f-01"]
    }
  }

To extract them, I have tried doing this

  private final TextMapGetter<Map<String, List<String>>> mapGetter = new TextMapGetter<Map<String, List<String>>>() {
    
    @Override
    public List<String> get(Map<String, List<String>> carrier, String key) {
      return carrier.get(key);
    }

    // etc.
  };

However, the W3C and B3 implementations in java otel reject a List of headers as invalid.
The only impl of a TextMapGetter that works for me is this:

  private final TextMapGetter<Map<String, String>> mapGetter = new TextMapGetter<Map<String, String>>() {
    
    @Override
    public String get(Map<String, String> carrier, String key) {
      return carrier.get(key);
    }

    // etc.
  };  

Should I be implementing the TextMapGetter differently?

[jkwatson]

I think you'll probably need to just grab the first element of the array of header values, or something like that.

[pwfarwell]

Thanks @jkwatson That makes sense to me, too. My co-workers tell me that the Go and NodeJs implementations of the otel propagators treat the traceparent and b3 headers as an array of strings. :)

[jkwatson]

What do they do with multiple trace parents?

[pwfarwell]

I will try to find out...

[jkwatson]

BTW, according to spec: https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/context/api-propagators.md#get

You're supposed to return the first value of the key.

[pwfarwell]

Thanks @jkwatson Thanks. So it appears that the spec allows trace headers to be provided as an array, but in practice the client should expect it to be a single string.
At any rate, the workaround of using the first value works for us.