What should the default DistributedContextPropagator be

[MihaZupan]

#50658 and #55556 added APIs for controlling distributed context propagation.
Part of those APIs is a CreateDefaultPropagator method that is used to seed the global static Current property.

HttpClient and ASP.NET Core both consume these APIs and as such inherit the default behavior.

CreateDefaultPropagator currently returns a LegacyPropagator, which is preserving pre-.NET 6 behavior of injecting context by default.

Having injection ON by default means some user scenarios work out of the box:

• ASP.NET Core services will preserve the trace across process boundaries even if they haven't been explicitly instrumented for distributed tracing
• For services that collect logs, actions performed across different services can still be attributed to a shared incoming request
• Even when some of the services are not instrumented, others still benefit from the trace as it continues to flow

On the other hand it brings some downsides:

• Some services are very strict about the headers they are expecting and may refuse requests that contain trace-related headers. This may be relatively difficult for users to diagnose as the behavior of HttpClient depends on whether it was used from within the ASP.NET Core pipeline or not
• We are spending a few cycles every request to decide on whether to do anything, or potentially sending extra headers that may just get ignored
• There are theoretical privacy concerns of correlating requests based on the trace id, outlined in the W3C Trace Context specification
• More discussion around the behavior: HttpClient automatically adds Request-Id HTTP header #35337

For 7.0, we should decide whether injecting context is the right default for the platform.

cc: @tarekgh @noahfalk @shirhatti @davidfowl @dotnet/ncl

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Issue Details

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#50658 and #55556 added APIs for controlling distributed context propagation.
Part of those APIs is a CreateDefaultPropagator method that is used to seed the global static Current property.

HttpClient and ASP.NET Core both consume these APIs and as such inherit the default behavior.

CreateDefaultPropagator currently returns a LegacyPropagator, which is preserving pre-.NET 6 behavior of injecting context by default.

Having injection ON by default means some user scenarios work out of the box:

• ASP.NET Core services will preserve the trace across process boundaries even if they haven't been explicitly instrumented for distributed tracing
• For services that collect logs, actions performed across different services can still be attributed to a shared incoming request
• Even when some of the services are not instrumented, others still benefit from the trace as it continues to flow

On the other hand it brings some downsides:

• Some services are very strict about the headers they are expecting and may refuse requests that contain trace-related headers. This may be relatively difficult for users to diagnose as the behavior of HttpClient depends on whether it was used from within the ASP.NET Core pipeline or not
• We are spending a few cycles every request to decide on whether to do anything, or potentially sending extra headers that may just get ignored
• There are theoretical privacy concerns of correlating requests based on the trace id, outlined in the W3C Trace Context specification
• More discussion around the behavior: HttpClient automatically adds Request-Id HTTP header #35337

For 7.0, we should decide whether injecting context is the right default for the platform.

cc: @tarekgh @noahfalk @shirhatti @davidfowl @dotnet/ncl

Author:	MihaZupan
Assignees:	-
Labels:	area-System.Diagnostics
Milestone:	7.0.0

[tommcdon]

Moving older issues to the Future milestone as it is unlikely we will have time to address this in the .NET 7 timeframe. Please feel free to move this issue back to .NET 7 if there is active work to address the feature request.

[MihaZupan]

Given we're not changing anything here for 7 and there appears to be no interest in doing so in the future, I'm fine with just closing this.
Feel free to reopen if you disagree.