Feature Flags

The feature flag behavior allows you to conditionally enable or disable message handlers based on feature flags, without deploying new code.

Installation

dotnet add package TurboMediator.FeatureFlags

# Optional: Microsoft.FeatureManagement integration
dotnet add package TurboMediator.FeatureFlags.FeatureManagement

Using the Attribute

[FeatureFlag("new-checkout")]
public record NewCheckoutCommand(Guid CartId) : ICommand<CheckoutResult>;

[FeatureFlag("beta-search", FallbackBehavior = FeatureFallback.ReturnDefault)]
public record BetaSearchQuery(string Term) : IQuery<SearchResult>;

[FeatureFlag("premium-export", FallbackBehavior = FeatureFallback.Throw)]
public record ExportReportCommand(Guid ReportId) : ICommand<byte[]>;

[FeatureFlag("per-user-feature", PerUser = true)]
public record PersonalizedFeedQuery : IQuery<Feed>;

Attribute Properties

Property	Default	Description
Feature name	Required	The feature flag name
`FallbackBehavior`	`Throw`	What to do when flag is disabled: Throw, ReturnDefault, Skip
`PerUser`	`false`	Evaluate per-user instead of globally

IFeatureFlagProvider

public interface IFeatureFlagProvider
{
    ValueTask<bool> IsEnabledAsync(string featureName, CancellationToken cancellationToken = default);
    ValueTask<bool> IsEnabledAsync(string featureName, string userId, CancellationToken cancellationToken = default);
}

In-Memory Provider

For development and testing:

builder.Services.AddTurboMediator(m =>
{
    m.WithFeatureFlags(ff =>
    {
        ff.UseInMemoryProvider()
          .WithFeature("new-checkout", true)
          .WithFeature("beta-search", false)
          .WithFeature("premium-export", enabled: true);
    });
});

Microsoft.FeatureManagement Integration

Integrate with Microsoft.FeatureManagement for configuration-driven flags:

dotnet add package TurboMediator.FeatureFlags.FeatureManagement

// appsettings.json
{
  "FeatureManagement": {
    "new-checkout": true,
    "beta-search": {
      "EnabledFor": [
        {
          "Name": "Percentage",
          "Parameters": { "Value": 50 }
        }
      ]
    }
  }
}

// Registration
builder.Services.AddFeatureManagement();
builder.Services.AddMicrosoftFeatureFlags();

builder.Services.AddTurboMediator(m =>
{
    m.WithFeatureFlags(ff =>
    {
        // FeatureManagement provider is auto-detected
    });
});

FeatureFlagOptions

builder.Services.AddTurboMediator(m =>
{
    m.WithFeatureFlags(ff =>
    {
        ff.UseInMemoryProvider()
          .UseStrictMode() // or .UseLenientMode()
          .WithUserIdProvider(() => currentUserService.GetUserId());
    });
});

Option	Description
`UseStrictMode()`	Sets default fallback to `FeatureFallback.Throw`
`UseLenientMode()`	Sets default fallback to `FeatureFallback.ReturnDefault`
`WithUserIdProvider(Func<string?>)`	Parameterless function to get user ID

Custom Provider

public class LaunchDarklyProvider : IFeatureFlagProvider
{
    private readonly ILdClient _client;

    public LaunchDarklyProvider(ILdClient client) => _client = client;

    public ValueTask<bool> IsEnabledAsync(string featureName)
    {
        return new ValueTask<bool>(
            _client.BoolVariation(featureName, Context.Default, false));
    }

    public ValueTask<bool> IsEnabledAsync(string featureName, string userId)
    {
        var context = Context.Builder(userId).Build();
        return new ValueTask<bool>(
            _client.BoolVariation(featureName, context, false));
    }
}

Feature flags are evaluated at the pipeline level, so disabled features never reach the handler. This provides a clean way to do gradual rollouts, A/B testing, and kill switches.

Feature Flags

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