Fitness Functions

When to Use This Skill

Use this skill when you need to:

• Enforce architectural boundaries between modules
• Test that dependencies follow prescribed rules
• Validate layer constraints (e.g., no UI → Domain)
• Create performance fitness functions
• Generate architecture test code
• Audit existing architecture for violations

Keywords: fitness functions, architecture tests, NetArchTest, ArchUnitNET, dependency rules, layer constraints, architectural boundaries, module isolation, architecture validation, performance tests

What Are Fitness Functions?

Fitness functions are automated tests that validate architectural characteristics. They provide objective, repeatable verification that the system maintains desired properties as it evolves.

Types of Fitness Functions

Type	Validates	Example
Dependency	Component relationships	"Domain cannot depend on Infrastructure"
Layer	Vertical slice rules	"Controllers only call Application layer"
Naming	Convention compliance	"Handlers must end with 'Handler'"
Performance	Runtime characteristics	"API response < 200ms at p95"
Cyclomatic	Code complexity	"No method > 10 cyclomatic complexity"

Quick Start

1. Install Required Package

# For NetArchTest (simpler, recommended for most cases)
dotnet add package NetArchTest.Rules

# For ArchUnitNET (more powerful, Java-like syntax)
dotnet add package ArchUnitNET
dotnet add package ArchUnitNET.xUnit  # or .NUnit

2. Create Test Project

dotnet new xunit -n YourSolution.ArchitectureTests
dotnet add YourSolution.ArchitectureTests reference src/YourSolution.Domain
dotnet add YourSolution.ArchitectureTests reference src/YourSolution.Application
dotnet add YourSolution.ArchitectureTests reference src/YourSolution.Infrastructure

3. Write First Test

public class DependencyTests
{
    [Fact]
    public void Domain_ShouldNotDependOn_Infrastructure()
    {
        var result = Types.InAssembly(typeof(Order).Assembly)
            .ShouldNot()
            .HaveDependencyOn("YourSolution.Infrastructure")
            .GetResult();

        Assert.True(result.IsSuccessful, result.FailingTypeNames?.FirstOrDefault());
    }
}

NetArchTest Patterns

NetArchTest provides a fluent API for testing architectural constraints.

Detailed patterns: See references/netarchtest-patterns.md

Common Rules

// Dependency constraints
Types.InAssembly(domainAssembly)
    .ShouldNot()
    .HaveDependencyOn("Microsoft.EntityFrameworkCore");

// Naming conventions
Types.InAssembly(applicationAssembly)
    .That()
    .ImplementInterface(typeof(IRequestHandler<,>))
    .Should()
    .HaveNameEndingWith("Handler");

// Layer isolation
Types.InNamespace("Domain")
    .ShouldNot()
    .HaveDependencyOnAny("Application", "Infrastructure", "Api");

ArchUnitNET Patterns

ArchUnitNET offers more expressive rules with a syntax similar to ArchUnit for Java.

Detailed patterns: See references/archunitnet-patterns.md

Common Rules

// Define architecture layers
private static readonly Architecture Architecture =
    new ArchLoader().LoadAssemblies(
        typeof(Order).Assembly,
        typeof(OrderHandler).Assembly,
        typeof(OrderRepository).Assembly
    ).Build();

private static readonly IObjectProvider<IType> DomainLayer =
    Types().That().ResideInNamespace("Domain").As("Domain Layer");

private static readonly IObjectProvider<IType> InfrastructureLayer =
    Types().That().ResideInNamespace("Infrastructure").As("Infrastructure Layer");

[Fact]
public void DomainLayer_ShouldNotDependOn_InfrastructureLayer()
{
    IArchRule rule = Types().That().Are(DomainLayer)
        .Should().NotDependOnAny(InfrastructureLayer);

    rule.Check(Architecture);
}

Dependency Rules Catalog

Common dependency rules for modular monoliths:

Full catalog: See references/dependency-rules.md

Module Isolation

[Fact]
public void Modules_ShouldNotCrossReference_CoreProjects()
{
    var orderingCore = Types.InAssembly(typeof(Order).Assembly);
    var inventoryCore = Types.InAssembly(typeof(Product).Assembly);

    // Ordering.Core cannot reference Inventory.Core
    var result = orderingCore
        .ShouldNot()
        .HaveDependencyOn("Inventory.Core")
        .GetResult();

    Assert.True(result.IsSuccessful);
}

Shared Kernel Constraints

[Fact]
public void SharedKernel_ShouldNotDependOn_AnyModule()
{
    var sharedKernel = Types.InAssembly(typeof(Entity).Assembly);

    var result = sharedKernel
        .ShouldNot()
        .HaveDependencyOnAny(
            "Ordering", "Inventory", "Shipping", "Customers")
        .GetResult();

    Assert.True(result.IsSuccessful);
}

Performance Fitness Functions

Test runtime characteristics to ensure performance standards.

Detailed guide: See references/performance-fitness.md

Response Time Test

[Fact]
public async Task Api_ShouldRespondWithin_200ms()
{
    var client = _factory.CreateClient();
    var stopwatch = Stopwatch.StartNew();

    var response = await client.GetAsync("/api/orders/123");

    stopwatch.Stop();
    Assert.True(stopwatch.ElapsedMilliseconds < 200,
        $"Response took {stopwatch.ElapsedMilliseconds}ms");
}

Memory Allocation Test

[Fact]
public void Handler_ShouldNotAllocateExcessiveMemory()
{
    var before = GC.GetTotalMemory(true);

    for (int i = 0; i < 1000; i++)
    {
        _handler.Handle(new GetOrderQuery(Guid.NewGuid()));
    }

    var after = GC.GetTotalMemory(true);
    var allocated = (after - before) / 1000;  // Per operation

    Assert.True(allocated < 10_000, $"Allocated {allocated} bytes per operation");
}

Code Generation Templates

Use these templates to quickly create architecture tests:

• references/templates/architecture-test-template.cs - Full test class scaffold
• references/templates/performance-test-template.cs - Performance test patterns

Quick Template Usage

# Copy template and customize
cp templates/architecture-test-template.cs tests/ArchitectureTests.cs

Integration with CI/CD

GitHub Actions Example

- name: Run Architecture Tests
  run: dotnet test --filter Category=Architecture
  continue-on-error: false  # Fail pipeline on violations

Test Categories

[Trait("Category", "Architecture")]
public class DependencyTests
{
    // Architecture tests run separately from unit tests
}

Integration with Event Storming

Fitness functions enforce the boundaries discovered through event storming:

Event Storming → Bounded Contexts
    ↓
Modular Architecture → Module Structure
    ↓
Fitness Functions → Enforce Boundaries

After event storming identifies bounded contexts:

1. Define modules based on contexts
2. Create dependency rules between modules
3. Add fitness functions to enforce isolation

Best Practices

1. Run in CI/CD - Catch violations before merge
2. Start with critical rules - Don't try to test everything at once
3. Clear failure messages - Make violations easy to understand
4. Categorize tests - Separate from unit/integration tests
5. Document intent - Explain why each rule exists
6. Review regularly - Update rules as architecture evolves

Troubleshooting

Common Issues

Test finds no types:

• Check assembly references in test project
• Verify namespace patterns match actual namespaces

False positives:

• Add exclusions for legitimate dependencies
• Check for indirect dependencies via shared packages

Performance tests flaky:

• Use warm-up runs before measuring
• Run in isolated environment
• Use statistical significance (multiple runs)

References

• references/netarchtest-patterns.md - NetArchTest usage patterns
• references/archunitnet-patterns.md - ArchUnitNET usage patterns
• references/performance-fitness.md - Performance testing patterns
• references/dependency-rules.md - Common dependency rules catalog
• references/templates/architecture-test-template.cs - Test class template
• references/templates/performance-test-template.cs - Performance test template

Version History

• v1.0.0 (2025-12-22): Initial release
  • NetArchTest and ArchUnitNET patterns
  • Dependency rules catalog
  • Performance fitness functions
  • Code generation templates
  • CI/CD integration guidance

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Last Updated

Date: 2025-12-22 Model: claude-opus-4-5-20251101