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Author: tomasjurasek

_posts/trace-baggage-context-propagation-open-telementry.md

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+---
+layout: single
+title:  "Trace & Baggage Context Propagation .NET with OpenTelemetry"
+---
+
+Context Propagation is an essential concept of distributed tracing. As the name suggests, it propagates the `trace` and `baggage` context over a network to other services.
+
+
+[Trace Context](https://www.w3.org/TR/trace-context/) and [Baggage Context](https://www.w3.org/TR/baggage/) are defined by the W3C standard. The propagation is by HTTP headers.
+
+## TraceContext
+
+The `traceparent` header represents incomming information about previous trace context.
+
+`traceparent: 00-0af7651916cd43dd8448eb211c80319c-b7ad6b7169203331-01`
+
+Format
+* version
+* trace-id
+* parent-id
+* trace-flags
+
+## BaggageContext
+
+The `baggage `header represents incomming baggage context. Its `key=value` representation. 
+
+`baggage: key1=value1`
+
+The information is transferred in plaintext. Be aware of personally sensitive information.
+
+# .NET Usage
+
+Tracing configuration registers `TraceContextPropagator` and `BaggageContextPropagator` out of the box. 
+
+
+ ```csharp 
+builder.Services.AddOpenTelemetry()
+    .UseOtlpExporter()
+    .WithTracing(tracing =>
+        {
+            tracing.AddSource(builder.Environment.ApplicationName);
+        });
+ ```
+
+You can use your own propagators (zipking,...)
+ ```csharp 
+Sdk.SetDefaultTextMapPropagator(new CompositeTextMapPropagator(
+    new List<TextMapPropagator>() {
+        new MyCustomPropagator()
+    }));
+ ```
+
+To enable propagation (trace & baggege) over HTTP register 
+`AddAspNetCoreInstrumentation` and 
+`AddHttpClientInstrumentation` to do it by default.
+ ```csharp 
+builder.Services.AddOpenTelemetry()
+    .UseOtlpExporter()
+    .WithTracing(tracing =>
+        {
+            tracing
+            .AddSource(builder.Environment.ApplicationName)
+            .AddAspNetCoreInstrumentation()
+            .AddHttpClientInstrumentation()
+        });
+
+
+ ```
+
+ If you are interested how the HTTP Propagation is working - [Incomming HTTP](https://github.com/open-telemetry/opentelemetry-dotnet-contrib/blob/main/src/OpenTelemetry.Instrumentation.AspNetCore/Implementation/HttpInListener.cs#L109), [OutComming HTTP](https://github.com/open-telemetry/opentelemetry-dotnet-contrib/blob/main/src/OpenTelemetry.Instrumentation.Http/Implementation/HttpHandlerDiagnosticListener.cs#L94).
+
+
+ 
+Propagate contexts over other protocols - messaging, database, etc. You need to do it manually with the `Inject` and then `Extract` methods through transfer attributes by `Propagators.DefaultTextMapPropagator` that provides registered propagators.