How can I automatically trace MongoDB queries in my C# .NET Core app using OpenTelemetry?

Below is an overview of the steps needed to have MongoDB queries automatically traced via OpenTelemetry in your .NET Core app.

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What’s Required

1. MongoDB Diagnostic Integration:
   The key piece is installing the NuGet package
   MongoDB.Driver.Core.Extensions.DiagnosticSource
   This package hooks into the diagnostic events provided by the MongoDB driver so that when the auto instrumentation is running, the Mongo events are picked up and converted into trace spans.

2. Auto Instrumentation Setup:
   You already have the following in place:

   • The Kubernetes components (kube‑prometheus‑stack, opentelemetry‑operator, etc.)
   • The .NET OpenTelemetry auto instrumentation package (OpenTelemetry.AutoInstrumentation)
     With this combination, installing the MongoDB diagnostic package should make the driver “speak” the diagnostics language that OpenTelemetry understands.

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How It Works

When you add the MongoDB.Driver.Core.Extensions.DiagnosticSource package to your project, it subscribes to the diagnostic events emitted by the MongoDB .NET driver. Then, the .NET auto instrumentation agent automatically picks those events up as long as:

• The MongoDB driver version is compatible (generally version 2.9.0 or later).
• The instrumentation is initialized early (before any driver calls).

If everything is wired up correctly, each MongoDB query will produce one or more diagnostic events that the auto instrumentation converts into spans. Those spans will then be sent along with your other traces (such as incoming HTTP requests) to your OpenTelemetry collector, which is later viewed in Grafana Tempo.

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Troubleshooting Steps

If MongoDB activity doesn’t appear in your Tempo dashboard despite the setup:

• Ensure Package Versions and Order:
  Verify that you’re using a MongoDB driver version that supports diagnostic source events and that the MongoDB.Driver.Core.Extensions.DiagnosticSource package is loaded in your app before any MongoDB operations occur.

• Environment Variables & Startup Hooks:
  Make sure that the .NET auto instrumentation startup hooks (set via environment variables like DOTNET_STARTUP_HOOKS) are active. They are responsible for enabling automatic instrumentation.

• Check Initialization:
  Confirm that the auto instrumentation configuration isn’t overridden and that the diagnostic source from MongoDB is enabled by default.

• Logs and Diagnostics:
  Enable verbose logging for the auto instrumentation to see if it reports subscribing to diagnostic events for MongoDB. This can help determine if the instrumentation is active but not forwarding spans.

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Example

While no code change is required to “switch on” the diagnostic-based tracing, your project file should include the following package reference:

XMLCopied
<ItemGroup>
  <PackageReference Include="MongoDB.Driver.Core.Extensions.DiagnosticSource" Version="YOUR_VERSION_HERE" />
</ItemGroup>

And ensure that your application's startup logic does nothing that might delay the loading of these packages before any MongoDB operations are performed.

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Summary

To automatically trace MongoDB queries:
• Install the MongoDB.Driver.Core.Extensions.DiagnosticSource package.
• Ensure that your MongoDB driver version supports diagnostic source events and that it’s initialized early in the app’s lifecycle.
• With OpenTelemetry’s .NET auto instrumentation in place (and properly configured via startup hooks/environment settings), the MongoDB driver’s events are captured and converted into trace spans, which will then be visible in your Grafana Tempo dashboard.

If after verifying these points your MongoDB queries still aren’t showing, double-check your auto instrumentation logs and configuration settings to ensure that nothing is blocking or filtering out the MongoDB diagnostic events.