feat: Add trace when evaluating feature flags by kylejuliandev · Pull Request #755 · open-feature/dotnet-sdk

kylejuliandev · 2026-05-07T18:49:22Z

This PR

Add trace when evaluating feature flags. This is different from the TraceEnricherHook which enriches the current span or trace with an event, instead this more closely follows the semantic conventions: https://opentelemetry.io/docs/specs/semconv/feature-flags/feature-flags-events/. Also add OpenTelemetry logging to the sample application and ensure traces are always sampled.

Success tracwe

Error trace

Related Issues

Fixes #595
Fixes #541

Notes

Follow-up Tasks

How to test

Signed-off-by: Kyle Julian <[email protected]>

codecov · 2026-05-07T18:52:15Z

Codecov Report

❌ Patch coverage is 98.55072% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 93.63%. Comparing base (ec94acf) to head (6d64f4f).
⚠️ Report is 10 commits behind head on main.

Files with missing lines	Patch %	Lines
src/OpenFeature/OpenFeatureActivitySource.cs	96.87%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #755      +/-   ##
==========================================
+ Coverage   93.47%   93.63%   +0.16%     
==========================================
  Files          68       70       +2     
  Lines        2956     3050      +94     
  Branches      351      368      +17     
==========================================
+ Hits         2763     2856      +93     
  Misses        135      135              
- Partials       58       59       +1

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gemini-code-assist

Code Review

This pull request implements OpenTelemetry tracing for feature flag evaluations in the OpenFeature client, including the addition of an activity source and corresponding unit tests. The review feedback highlights several discrepancies with OpenTelemetry Semantic Conventions, suggesting the use of standard attribute names (e.g., 'feature_flag.provider_name', 'feature_flag.evaluation_reason', 'feature_flag.evaluation_value', 'feature_flag.variant') and recommending that span statuses be explicitly set to 'Error' when failures occur to improve telemetry accuracy.

Signed-off-by: Kyle Julian <[email protected]>

askpt

LGTM! I expect this change to be mentioned in a README since the consumer might need to add extra configuration to remove or add these traces.

cc: @beeme1mr, @open-feature/technical-steering-committee, @WeihanLi, @arttonoyan

askpt · 2026-05-13T10:12:58Z

+    static string GetLibraryVersion()
+        => typeof(OpenFeatureActivitySource).Assembly
+            .GetCustomAttribute<AssemblyInformationalVersionAttribute>()?
+            .InformationalVersion ?? "UNKNOWN";


Could you please remove the +<sha> in the version. You can see it in the screenshots you posted.

Also I wonder if we should "cache" the version string somewhere to prevent doing GetCustomAttribute all the time.

The ActivitySource is static, so it's initialized once during the first flag evaluation and then subsequent evaluations don't fetch a new version

I've opted to use AssemblyVersion instead of the InformationalVersion in 3b8eac5

In Common.prod.props we set the Assembly version

dotnet-sdk/build/Common.prod.props

Lines 22 to 24 in b3fa7f4

<VersionPrefix>$(VersionNumber)</VersionPrefix>

<AssemblyVersion>$(VersionNumber)</AssemblyVersion>

<FileVersion>$(VersionNumber)</FileVersion>

I do a ToString on the Version, so we get major.minor.patch in the trace:

askpt · 2026-05-13T10:15:27Z

        var resolveValueDelegate = providerInfo.Item1;
        var provider = providerInfo.Item2;

+        var activity = OpenFeatureActivitySource.StartActivity("feature_flag.evaluation");


Wonder if we should set it as Client. See: https://learn.microsoft.com/en-us/dotnet/api/system.diagnostics.activitykind?view=net-10.0

Wonder if we should set it as Client

personally think it's fine to keep it using default Internal

I think if the Provider (InMemory, Flagd, etc..) traces, they may opt to use Client in this example. I figured here at this point we don't necessarily know if the flag evaluation is remote. But we are confident we are delegating the actual evaluation to another component that is running in the same process

WeihanLi · 2026-05-13T11:40:03Z

        var resolveValueDelegate = providerInfo.Item1;
        var provider = providerInfo.Item2;

+        var activity = OpenFeatureActivitySource.StartActivity("feature_flag.evaluation");


Suggested change

var activity = OpenFeatureActivitySource.StartActivity("feature_flag.evaluation");

using var activity = OpenFeatureActivitySource.StartActivity("feature_flag.evaluation");

maybe we could add using for this

Great spot, this was probably leaving activity sources undisposed of (potentially leaving resources hanging). Fixed in 3b8eac5

WeihanLi · 2026-05-13T11:56:53Z

        var provider = providerInfo.Item2;

+        var activity = OpenFeatureActivitySource.StartActivity("feature_flag.evaluation");
+        activity?.SetTag("feature_flag.key", flagKey);


for the SetTag/SetStatus, think we could add tags in one place and add tags only when IsAllDataRequested

https://learn.microsoft.com/en-us/dotnet/api/system.diagnostics.activity.isalldatarequested?view=net-10.0

Fixed in 6e943fa - this was a great spot! It helps avoid allocations in high performance scenarios

I wonder if we should add feature_flag.provider.name in all instances or only when IsAllDataRequested is true

WeihanLi · 2026-05-13T12:02:36Z

maybe we want to deprecate the TraceEnricherHook related code also?

WeihanLi · 2026-05-13T12:07:31Z

+        var providerMetadata = provider.GetMetadata();
+        if (providerMetadata != null)
+        {
+            activity?.SetTag("feature_flag.provider.name", providerMetadata.Name);


Would we like to keep these tagName/activityName to be constants in a separated place?

Fixed in 6e943fa - moved the tag names (apart from error.type) to the OpenFeatureActivitySource class

WeihanLi · 2026-05-13T12:09:29Z

    .ConfigureResource(resource => resource.AddService("openfeature-aspnetcore-sample"))
    .WithTracing(tracing => tracing
        .AddAspNetCoreInstrumentation()
+        .SetSampler(new AlwaysOnSampler())


Maybe we should add AddSource("OpenFeature") to collect OpenFeature activity?

Added trace source in 3b8eac5

* Ensure OpenFeature traces are recorded in sample app * Ensure ActivitySource is disposed of Signed-off-by: Kyle Julian <[email protected]>

* Add check on IsAllDataRequested to save on allocations on the activity span Signed-off-by: Kyle Julian <[email protected]>

kylejuliandev · 2026-05-14T19:22:47Z

/gemini review

gemini-code-assist

Code Review

This pull request introduces OpenTelemetry instrumentation to the OpenFeature .NET SDK by adding a new OpenFeatureActivitySource and integrating activity tracking into the OpenFeatureClient evaluation logic. The changes include mapping evaluation results and errors to standard semantic conventions and adding comprehensive tests. Review feedback suggests aligning specific attribute names with OpenTelemetry standards (specifically provider_name and variant), normalizing reason strings to lowercase, and removing a redundant Dispose() call on an activity already managed by a using statement.

Signed-off-by: Kyle Julian <[email protected]>

kylejuliandev added 3 commits May 6, 2026 23:15

Add OpenTelemetry log exporting and ensure traces reliably sample

3b609d9

Signed-off-by: Kyle Julian <[email protected]>

Add code to start activity and initial unit tests

703798f

Signed-off-by: Kyle Julian <[email protected]>

Add additional unit tests and ensure tags are added

73ee185

Signed-off-by: Kyle Julian <[email protected]>

kylejuliandev requested a review from a team as a code owner May 7, 2026 18:49

gemini-code-assist Bot reviewed May 7, 2026

View reviewed changes

kylejuliandev added 4 commits May 7, 2026 20:12

Fix build issue

24ea6b6

Signed-off-by: Kyle Julian <[email protected]>

Address gemini code review comments

3dce87e

Signed-off-by: Kyle Julian <[email protected]>

Add tests to improve code coverage of OpenFeatureActivitySource

a6b8ec3

Signed-off-by: Kyle Julian <[email protected]>

Set activity kind to internal

7e99375

Signed-off-by: Kyle Julian <[email protected]>

askpt reviewed May 13, 2026

View reviewed changes

WeihanLi reviewed May 13, 2026

View reviewed changes

kylejuliandev added 2 commits May 14, 2026 20:04

Ensure major.minor.patch version is included in trace

3b8eac5

* Ensure OpenFeature traces are recorded in sample app * Ensure ActivitySource is disposed of Signed-off-by: Kyle Julian <[email protected]>

Move tag keys to string constants in separate class

6e943fa

* Add check on IsAllDataRequested to save on allocations on the activity span Signed-off-by: Kyle Julian <[email protected]>

gemini-code-assist Bot reviewed May 14, 2026

View reviewed changes

Comment thread src/OpenFeature/OpenFeatureActivitySource.cs

Comment thread src/OpenFeature/OpenFeatureActivitySource.cs

Comment thread src/OpenFeature/OpenFeatureActivitySource.cs

Comment thread src/OpenFeature/OpenFeatureClient.cs Outdated

Remove duplicate activity disposal

6d64f4f

Signed-off-by: Kyle Julian <[email protected]>

	<VersionPrefix>$(VersionNumber)</VersionPrefix>
	<AssemblyVersion>$(VersionNumber)</AssemblyVersion>
	<FileVersion>$(VersionNumber)</FileVersion>

	var activity = OpenFeatureActivitySource.StartActivity("feature_flag.evaluation");
	using var activity = OpenFeatureActivitySource.StartActivity("feature_flag.evaluation");

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WeihanLi commented May 13, 2026

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