W3C trace context not propagated for session-enabled Service Bus triggers (OpenTelemetry)

[tpiironen]

W3C trace context not propagated for session-enabled Service Bus triggers (OpenTelemetry)

Description

When using OpenTelemetry ("telemetryMode": "OpenTelemetry" in host.json) with session-enabled Service Bus triggers (IsSessionsEnabled = true), the W3C trace context (traceparent/tracestate) embedded in the Service Bus message is not used as the parent trace. Instead, the Functions host creates a new root activity, breaking distributed tracing across process boundaries.

Non-session Service Bus triggers propagate trace context correctly. The issue is specific to session-enabled triggers.

Root Cause

The Functions host subscribes to a hardcoded list of ActivitySource names in OpenTelemetryConfigurationExtensions.ConfigureTracing():

ReadOnlySpan<string> sources =
[
    OpenTelemetryConstants.ActivitySourceNames.ServiceBusProcessor,  // "Azure.Messaging.ServiceBus.ServiceBusProcessor"
    OpenTelemetryConstants.ActivitySourceNames.EventHubsProcessor,
    // ...
];

The list includes "Azure.Messaging.ServiceBus.ServiceBusProcessor" but is missing "Azure.Messaging.ServiceBus.ServiceBusSessionProcessor".

The Azure.Messaging.ServiceBus SDK's DiagnosticScopeFactory.GetActivitySource() (source) constructs ActivitySource names dynamically from the operation name:

string clientName = ns + "." + ((indexOfDot < 0) ? name : name.Substring(0, indexOfDot));
return ActivitySources.GetOrAdd(clientName, static n => new ActivitySource(n));

Combined with DiagnosticProperty.cs (source):

Trigger type	Activity name	Resulting ActivitySource name	Host subscribed?
Non-session	ServiceBusProcessor.ProcessMessage	Azure.Messaging.ServiceBus.ServiceBusProcessor	Yes
Session	ServiceBusSessionProcessor.ProcessSessionMessage	Azure.Messaging.ServiceBus.ServiceBusSessionProcessor	No

Because the host has no listener for the session ActivitySource:

1. The SDK's DiagnosticScope.Start() calls ActivitySource.StartActivity() which returns null (no listeners)
2. Activity.Current remains null when the function is invoked
3. ActivitySourceWrapper (source) sees Activity.Current == null and creates a root activity → new OperationId/trace

Reproduction

1. Create two Azure Functions (isolated worker, .NET):
   • Function A (HTTP trigger): Sends a message to a session-enabled Service Bus queue
   • Function B (Service Bus trigger with IsSessionsEnabled = true): Receives and processes the message
2. Configure host.json with "telemetryMode": "OpenTelemetry"
3. Configure OpenTelemetry with Azure Monitor exporter in both functions
4. Send a request to Function A
5. Observe in Application Insights that Function A and Function B have different Operation IDs

If you repeat the same test with sessions disabled (IsSessionsEnabled = false), Function A and Function B share the same Operation Id.

Expected Behavior

Session-enabled Service Bus triggers should propagate the W3C trace context from the message, maintaining the same Operation Id across the distributed trace — identical to non-session triggers.

Proposed Fix

Add the missing ActivitySource name to OpenTelemetryConstants.cs:

internal static class ActivitySourceNames
{
    internal const string WebJobs = "Microsoft.Azure.WebJobs";
    internal const string Host = "Microsoft.Azure.Functions.Host";
    internal const string ServiceBusProcessor = "Azure.Messaging.ServiceBus.ServiceBusProcessor";
+   internal const string ServiceBusSessionProcessor = "Azure.Messaging.ServiceBus.ServiceBusSessionProcessor";
    internal const string EventHubsProcessor = "Azure.Messaging.EventHubs.EventProcessor";
    // ...
}

And subscribe to it in OpenTelemetryConfigurationExtensions.ConfigureTracing():

ReadOnlySpan<string> sources =
[
    OpenTelemetryConstants.ActivitySourceNames.ServiceBusProcessor,
+   OpenTelemetryConstants.ActivitySourceNames.ServiceBusSessionProcessor,
    OpenTelemetryConstants.ActivitySourceNames.EventHubsProcessor,
    // ...
];

Environment

• Azure Functions Host: 4.x (confirmed against dev branch as of 2025-07-15)
• Azure.Messaging.ServiceBus: 7.20.1
• Microsoft.Azure.Functions.Worker.Extensions.ServiceBus: 5.24.0
• Microsoft.Azure.Functions.Worker.OpenTelemetry: 1.1.0
• .NET: 10
• Worker model: Isolated

[shivamkrch]

I'm also facing this. Do we have any updates on this? When is it expected to be released?