Understanding Nancy's Routing Mechanism

Routing at the Core

Nancy employs a URL-to-handler mapping system through modules (derived from NancyModule). Each module defines route handlers within its constructor using fluent syntax. Under the hood, Nancy builds a route cache that maps HTTP verbs and paths to executable delegates.

public class ProductModule : NancyModule {
    public ProductModule() {
        Get["/products"] = parameters => {...};
    }
}

Routing Cache Pitfalls

In large applications with dynamic assembly loading or auto-registered modules, the routing cache may not populate correctly. If the DI container (like TinyIoC or Autofac) fails to resolve dependencies or loads modules too late in the lifecycle, routes silently disappear.

Root Causes of Routing Failures

1. Improper Module Registration

When modules are scattered across assemblies or dynamically loaded, Nancy's internal bootstrapper might skip modules not explicitly registered. This is exacerbated when custom INancyBootstrapper implementations override default behavior without preserving base logic.

2. Faulty Conventions or Assembly Scanning

Enterprise systems often customize Nancy via INancyModuleCatalog to dynamically discover modules. However, subtle misconfigurations can lead to module scanning failures, especially in AOT or trimmed environments (e.g., .NET Core single-file deployment).

public class CustomCatalog : INancyModuleCatalog {
    public IEnumerable<ModuleRegistration> GetAllModules(NancyContext ctx) {
        return AppDomain.CurrentDomain.GetAssemblies()
            .SelectMany(a => a.GetTypes())
            .Where(t => typeof(NancyModule).IsAssignableFrom(t))
            .Select(t => new ModuleRegistration(t, false));
    }
}

3. Silent Resolution Failures

When dependencies of modules cannot be resolved (e.g., due to mismatched scopes or registrations), Nancy may suppress these exceptions. This results in the affected module being omitted entirely without raising startup errors—especially with TinyIoC.

Diagnosing Routing Anomalies

Step-by-Step Diagnostics

• Enable Diagnostics module and verify loaded routes.
• Check log output at startup—enable full stack traces.
• Manually inspect the internal IRouteCache and INancyModuleCatalog.
• Validate container registrations and lifecycle mismatches.
• Inspect assembly loading policies for trimmed dependencies or missing references.

// Enable diagnostics in Bootstrapper
protected override DiagnosticsConfiguration DiagnosticsConfiguration =>
    new DiagnosticsConfiguration { Password = "admin" };

Logging Internal Module Load Failures

To expose silent errors, wrap module instantiation in try-catch and log inner exceptions. Extend the bootstrapper to override module resolution behavior and insert debug logging.

Architectural Considerations

Explicit Registration over Convention

In enterprise systems, prefer deterministic module registration over dynamic scanning. Define a static registry of modules to ensure consistent behavior across environments and avoid runtime surprises.

public class Bootstrapper : DefaultNancyBootstrapper {
    protected override IEnumerable<ModuleRegistration> Modules => new[] {
        new ModuleRegistration(typeof(ProductModule), false),
        new ModuleRegistration(typeof(UserModule), false)
    };
}

Use Autofac or Lamar for Better Control

Advanced containers provide clearer error handling and lifecycle management compared to TinyIoC. They allow scoped injection, async resolution, and better diagnostics, which are critical in production-grade systems.

Fixing the Issue

Step-by-Step Resolution Strategy

1. Switch to explicit module registration if dynamic discovery is failing.
2. Replace TinyIoC with Autofac or Lamar to surface DI failures.
3. Add structured logging during container setup and module instantiation.
4. Use integration tests that validate the presence of all expected routes.
5. Enforce CI checks to verify routing maps before deployment.

Testing and Validation

Write end-to-end integration tests using an in-memory Nancy host. Validate response status codes for known routes and ensure modules load correctly.

[Fact]
public async Task ShouldRespondToProductRoute() {
    var browser = new Browser(with => with.Module<ProductModule>());
    var result = await browser.Get("/products", with => with.HttpRequest());
    Assert.Equal(HttpStatusCode.OK, result.StatusCode);
}

Best Practices

• Use explicit module registration in large codebases.
• Favor robust containers like Autofac over TinyIoC.
• Log route registrations and bootstrap errors verbosely.
• Test all modules and routes using automated E2E tests.
• Do not suppress exceptions during module instantiation.

Conclusion

While Nancy is excellent for lightweight APIs and services, scaling it to enterprise environments uncovers subtle issues with routing, module resolution, and DI integration. Understanding how modules are loaded and how the container behaves is critical to maintaining a healthy production system. By favoring explicit configuration, rich logging, and container replacement, you can avoid these silent routing failures and ensure your applications behave deterministically under load.

FAQs

1. Why do some Nancy routes disappear after deployment?

This typically happens when module discovery fails due to missing assemblies, suppressed DI exceptions, or environmental differences between dev and prod.

2. Can I dynamically load modules from plugins?

Yes, but you must implement a custom INancyModuleCatalog and ensure plugin assemblies are fully loaded before the bootstrapper builds the route cache.

3. Is TinyIoC production-ready?

While functional, TinyIoC lacks advanced diagnostics and scoped lifetime control. For large-scale systems, Autofac or Lamar is preferred.

4. How do I validate which modules Nancy loaded?

Use the Diagnostics module or inject and inspect IRouteCache to dump active route mappings and module registrations.

5. Can Nancy run on .NET 6 or later?

Yes, but with caveats. Nancy has no official .NET 6+ support, so you may need to fork or adapt dependencies manually for long-term use.