[bug] Adding a baggage entry for the current trace and replace key and value is not possible via Baggage API

[HHobeck]

Package

OpenTelemetry.Api

Package Version

Package Name	Version
OpenTelemetry.Api	1.10.0
OpenTelemetry	1.10.0
OpenTelemetry.Exporter.OpenTelemetryProtocol	1.10.0
OpenTelemetry.Extensions.Hosting	1.10.0
OpenTelemetry.Instrumentation.AspNetCore	1.10.1
OpenTelemetry.Instrumentation.Http	1.10.0

Runtime Version

net8.0

Description

Baggage allows you to add a key with a value as an attribute to every subsequent child span of the current application context. The documentation of the OpenTelemetry .NET API -> Beggage API states:

Baggage API allows users to add context to metric, traces, and logs. Baggage can be propagated out of proc using Propagators. OpenTelemetry SDK ships a BaggagePropagator and enables it by default.

and

The recommended way to add Baggage is to use the Baggage.SetBaggage() API. OpenTelemetry users should not use the Activity.AddBaggage method.

If I try this feature using the Baggage API this is not possible for a simple ASP.NET Core Web API.

Steps to Reproduce

Program.cs:

using OpenTelemetry;
using OpenTelemetry.Trace;
using System.Net;
using WebApplication1.Controllers;

namespace WebApplication1
{
    public class Program
    {
        public static void Main(string[] arguments)
        {
            WebApplicationBuilder applicationBuilder = WebApplication.CreateBuilder(arguments);

            // Add services to the container.
            OpenTelemetryBuilder openTelemetryBuilder = applicationBuilder.Services.AddOpenTelemetry();
            openTelemetryBuilder.WithTracing(builder => builder
                .AddAspNetCoreInstrumentation()
                .AddHttpClientInstrumentation()
                .AddOtlpExporter()
            );

            HttpClient.DefaultProxy = new WebProxy();
            applicationBuilder.Services.AddHttpClient(nameof(WeatherForecastController), configure =>
            {
                configure.BaseAddress = new Uri("http://localhost:5091/");
            });
            applicationBuilder.Services.AddControllers();
            applicationBuilder.Services.AddEndpointsApiExplorer();
            applicationBuilder.Services.AddSwaggerGen();

            WebApplication application = applicationBuilder.Build();
            if (application.Environment.IsDevelopment())
            {
                application.UseSwagger();
                application.UseSwaggerUI();
            }
            application.UseHttpsRedirection();
            application.UseAuthorization();
            application.MapControllers();
            application.Run();
        }
    }
}

WeatherForecastController.cs:

using Microsoft.AspNetCore.Mvc;
using OpenTelemetry;

namespace WebApplication1.Controllers
{
    [ApiController]
    [Route("[controller]")]
    public class WeatherForecastController(IHttpClientFactory httpClientFactory) : ControllerBase
    {
        [HttpGet("A", Name = "GetWeatherForecastA")]
        public void GetA()
        {
            Baggage.Current.SetBaggage("MyValue", "HelloWorld-1");
            string? myValue = Baggage.Current.GetBaggage("MyValue"); // this call returns null

            using HttpClient httpClient = httpClientFactory.CreateClient(nameof(WeatherForecastController));

            using HttpResponseMessage httpResponseMessage = httpClient
                .GetAsync("/WeatherForecast/B")
                .GetAwaiter().GetResult();
            httpResponseMessage.EnsureSuccessStatusCode();
        }

        [HttpGet("B", Name = "GetWeatherForecastB")]
        public void GetB()
        {
            string? myValue = Baggage.Current.GetBaggage("MyValue"); // this call returns null
        }
    }
}

Expected Result

The baggage value should be:

• returned when calling the GetBaggage function
• part of the trace record in otel exporter
• propageted as Header (Correlation-Context or PropagationObject??) of all sub calls via HttpClient
• part in all child spans

Actual Result

1. Baggage.Current.GetBaggage("MyValue") returns null
2. The value MyValue is not part of the trace record
3. The value will be not propagated via Http Header to sub request calls
4. Is not part of the child activity

Additional Context

No response

[rajkumar-rangaraj]

Why are you using Baggage.Current.SetBaggage instead of Baggage.SetBaggage?

Baggage.SetBaggage should be called once before accessing Baggage.Current. In your case, since Baggage.Current is not initialized, it will return null.

Refer to the Baggage API documentation for more details:
OpenTelemetry .NET Baggage API

[HHobeck]

Thank you very much for your answer. You are completely right. I have spent so much hours and didn't see this. Maybe I was inspired from Activity.Current.

Anyway, I would like to point out that the baggage information will be not part of any log, metric or trace records (which I was not aware off):

It is important to note that Baggage is not automatically attached to any telemetry. User can explicitly read Baggage and use it to enrich metrics, logs and traces.

That means the concepts around Baggage API has nothing to do with the OpenTelemetryProtocol and lives side by side. You can also use the .net Baggage API with Activity.Current.AddBaggage(..) it makes no different. That's why I'm also very surprised about the recommendation because there is no reason using the one over the other API:

The recommended way to add Baggage is to use the Baggage.SetBaggage() API. OpenTelemetry users should not use the Activity.AddBaggage method.