akka-net-testing-patterns

What this skill does

# 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](examples.md): Complete code samples for all testing patterns (Patterns 1-8 plus Reminders)
- [anti-patterns-and-reference.md](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](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

```xml
<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:**

```csharp
// 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](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.

```csharp
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.

```csharp
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.

```csharp
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

| Pattern | Use Case |
|---------|----------|
| Basic Actor Test | Single actor with injected services |
| TestProbe | Verify actor sends messages to dependencies |
| Auto-Responder | Avoid `Ask` timeouts when testing |
| Persistent Actor | Test event sourcing and recovery |
| Cluster Sharding | Test sh