Is it expected that Activity.OperationName does not appear in the OTLP span?

[swythan]

I was rather suprised to find that if you create an Activity and set both the OperationName (via the constructor) and DisplayName properties, then it is the DisplayName that ends up as the name of the span exported by the OtlpExporter, and OperationName is not present in the output at all.

opentelemetry-dotnet/src/OpenTelemetry.Exporter.OpenTelemetryProtocol/Implementation/Serializer/ProtobufOtlpTraceSerializer.cs

Line 232 in ceebd45

writePosition = ProtobufSerializer.WriteStringWithTag(buffer, writePosition, ProtobufOtlpTraceFieldNumberConstants.Span_Name, activity.DisplayName);

Looking in to this further, I think this is intentional. The DisplayName is to all intents and purposes the actual name of the span, whilst the OperationName is presumably there for doing advanced filtering, enrichment, etc inside the process.

I actually came across this when looking at the network tracing spans created by .NET 9.

Example (source)

Availability	ActivitySource name	OperationName	DisplayName
.NET 9+	Experimental.System.Net.Http.Connections	Experimental.System.Net.Http.Connections.ConnectionSetup	HTTP connection_setup {address}:{port}

AFAICT the Experimental.System.Net.Http.Connections.ConnectionSetup identifier won't be available in the output at all, and the name field will be the much higher cardinality DisplayName.

Is that what everyone expects?

[cijothomas]

whilst the OperationName is presumably there for doing advanced filtering, enrichment, etc inside the process.

OperationName exists for legacy/back-compat reasons. (Pre-OpenTelemetry days). It should not be used at all in OpenTelemetry. Please share if there are docs showing its usage, so we can fix them.

[swythan]

I created this docs issue for the main Activity docs.

dotnet/dotnet-api-docs#11256

Also the .NET docs around the activities they publish (e.g. the table above from the link next to it) advise on there values of both properties. They're putting a high cardinality string in the DisplayName so I was surprised that this ends up being the name of the span.

I couldn't find any mention in the docs that the OperationName specified in StartActivity() is largely unused if DisplayName is specified.