Akka.NET Testing Patterns

When to Use This Skill

Use this skill when:

• Writing unit tests for Akka.NET actors
• Testing persistent actors with event sourcing
• Verifying actor interactions and message flows
• Testing actor supervision and lifecycle
• Mocking external dependencies in actor tests
• Testing cluster sharding behavior locally
• Verifying actor state recovery and persistence

Reference Files

• examples.md: Complete code samples for all testing patterns (Patterns 1-8 plus Reminders)
• anti-patterns-and-reference.md: Anti-patterns, traditional TestKit, CI/CD integration

Choosing Your Testing Approach

Use Akka.Hosting.TestKit (Recommended for 95% of Use Cases)

When:

• Building modern .NET applications with Microsoft.Extensions.DependencyInjection
• Using Akka.Hosting for actor configuration in production
• Need to inject services into actors (IOptions, DbContext, ILogger, HTTP clients, etc.)
• Testing applications that use ASP.NET Core, Worker Services, or .NET Aspire
• Working with modern Akka.NET projects (Akka.NET v1.5+)

Advantages:

• Native dependency injection support - override services with fakes in tests
• Configuration parity with production (same extension methods work in tests)
• Clean separation between actor logic and infrastructure
• Type-safe actor registry for retrieving actors

Use Traditional Akka.TestKit

When:

• Contributing to Akka.NET core library development
• Working in environments without Microsoft.Extensions (console apps, legacy systems)
• Legacy codebases using manual Props creation without DI

See anti-patterns-and-reference.md for traditional TestKit patterns.

Core Principles (Akka.Hosting.TestKit)

1. Inherit from Akka.Hosting.TestKit.TestKit - This is a framework base class, not a user-defined one
2. Override ConfigureServices() - Replace real services with fakes/mocks
3. Override ConfigureAkka() - Configure actors using the same extension methods as production
4. Use ActorRegistry - Type-safe retrieval of actor references
5. Composition over Inheritance - Fake services as fields, not base classes
6. No Custom Base Classes - Use method overrides, not inheritance hierarchies
7. Test One Actor at a Time - Use TestProbes for dependencies
8. Match Production Patterns - Same extension methods, different AkkaExecutionMode

Required NuGet Packages

<ItemGroup>
  <!-- Core testing framework -->
  <PackageReference Include="Akka.Hosting.TestKit" Version="*" />

  <!-- xUnit (or your preferred test framework) -->
  <PackageReference Include="xunit" Version="*" />
  <PackageReference Include="xunit.runner.visualstudio" Version="*" />
  <PackageReference Include="Microsoft.NET.Test.Sdk" Version="*" />

  <!-- Assertions (recommended) -->
  <PackageReference Include="FluentAssertions" Version="*" />

  <!-- In-memory persistence for testing -->
  <PackageReference Include="Akka.Persistence.Hosting" Version="*" />

  <!-- If testing cluster sharding -->
  <PackageReference Include="Akka.Cluster.Hosting" Version="*" />
</ItemGroup>

CRITICAL: File Watcher Fix for Test Projects

Akka.Hosting.TestKit spins up real IHost instances, which by default enable file watchers for configuration reload. When running many tests, this exhausts file descriptor limits on Linux (inotify watch limit).

Add this to your test project - it runs before any tests execute:

// TestEnvironmentInitializer.cs
using System.Runtime.CompilerServices;

namespace YourApp.Tests;

internal static class TestEnvironmentInitializer
{
    [ModuleInitializer]
    internal static void Initialize()
    {
        // Disable config file watching in test hosts
        // Prevents file descriptor exhaustion (inotify watch limit) on Linux
        Environment.SetEnvironmentVariable("DOTNET_HOSTBUILDER__RELOADCONFIGONCHANGE", "false");
    }
}

Why this matters:

• [ModuleInitializer] runs automatically before any test code
• Sets the environment variable globally for all IHost instances
• Prevents cryptic inotify errors when running 100+ tests
• Also applies to Aspire integration tests that use IHost

Testing Patterns Overview

Each pattern below has a condensed description. See examples.md for complete code samples.

Pattern 1: Basic Actor Test

The foundation pattern. Override ConfigureServices() to inject fakes, override ConfigureAkka() to register actors with the same extension methods as production.

public class OrderActorTests : TestKit
{
    private readonly FakeOrderRepository _fakeRepository = new();

    protected override void ConfigureServices(HostBuilderContext context, IServiceCollection services)
    {
        services.AddSingleton<IOrderRepository>(_fakeRepository);
    }

    protected override void ConfigureAkka(AkkaConfigurationBuilder builder, IServiceProvider provider)
    {
        builder.WithInMemoryJournal().WithInMemorySnapshotStore();
        builder.WithActors((system, registry, resolver) =>
        {
            registry.Register<OrderActor>(system.ActorOf(resolver.Props<OrderActor>(), "order-actor"));
        });
    }

    [Fact]
    public async Task CreateOrder_Success_SavesToRepository()
    {
        var orderActor = ActorRegistry.Get<OrderActor>();
        var response = await orderActor.Ask<OrderCommandResult>(
            new CreateOrder("ORDER-123", "CUST-456", 99.99m), RemainingOrDefault);
        response.Status.Should().Be(CommandStatus.Success);
        _fakeRepository.SaveCallCount.Should().Be(1);
    }
}

Pattern 2: TestProbe for Actor Interactions

Register a TestProbe in the ActorRegistry as a stand-in for a dependency actor. Use ExpectMsgAsync<T>() to verify messages were sent.

Pattern 3: Auto-Responding TestProbe

When the actor under test uses Ask to communicate with dependencies, create an auto-responder actor that forwards messages to a probe AND replies to avoid timeouts.

Pattern 4: Testing Persistent Actors

Use WithInMemoryJournal() and WithInMemorySnapshotStore(). Test recovery by killing the actor with PoisonPill and querying to force recovery from journal.

Pattern 5: Reuse Production Configuration

Always reuse production extension methods in tests instead of duplicating HOCON config. This ensures tests use the exact same configuration as production.

protected override void ConfigureAkka(AkkaConfigurationBuilder builder, IServiceProvider provider)
{
    builder
        .AddDraftSerializer()                                    // Same as production
        .AddOrderDomainActors(AkkaExecutionMode.LocalTest)      // Same, but local mode
        .WithInMemoryJournal().WithInMemorySnapshotStore();      // Test-specific overrides
}

Pattern 6: Cluster Sharding Locally

Use AkkaExecutionMode.LocalTest with GenericChildPerEntityParent to test sharding behavior without an actual cluster. Same extension methods, different mode.

Pattern 7: AwaitAssertAsync for Async Operations

Use AwaitAssertAsync when actors perform async operations. It retries assertions until they pass or timeout, preventing flaky tests.

await AwaitAssertAsync(() =>
{
    _fakeReadModelService.SyncCallCount.Should().BeGreaterOrEqualTo(1);
}, TimeSpan.FromSeconds(3));

Pattern 8: Scenario-Based Integration Tests

Test complete business workflows end-to-end with multiple actors and state transitions. Register all domain actors, verify state at each step.

Common Patterns Summary

Best Practices

1. One test class per actor - Keep tests focused
2. Override ConfigureServices/ConfigureAkka - Don't create base classes
3. Use fakes, not mocks - Simpler, more maintainable
4. Test one actor at a time - Use TestProbes for dependencies
5. Match production patterns - Same extension methods, different AkkaExecutionMode
6. Use AwaitAssertAsync for async - Prevents flaky tests
7. Test recovery - Kill and restart actors to verify persistence
8. Scenario tests for workflows - Test complete business flows end-to-end
9. Keep tests fast - In-memory persistence, no real databases
10. Use meaningful names - Scenario_FirstTimePurchase_SuccessfulPayment

Debugging Tips

1. Enable debug logging - Pass LogLevel.Debug to TestKit constructor
2. Use ITestOutputHelper - See actor system logs in test output
3. Inspect TestProbe - Check probe.Messages to see what was sent
4. Query actor state - Add state query messages for debugging
5. Use AwaitAssertAsync with logging - See why assertions fail
6. Check ActorRegistry - Verify actors are registered correctly

// Constructor with debug logging
public OrderActorTests(ITestOutputHelper output)
    : base(output: output, logLevel: LogLevel.Debug)
{
}