🔗 Relay - Adaptive Dependency Injection

Relay your dependency injection to the next level! A powerful, fluent library that extends Microsoft.Extensions.DependencyInjection with adaptive patterns for conditional routing, multi-relays, adapter chains, and dynamic service resolution.

Terms of use

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• You unequivocally condemn Russia and its military aggression against Ukraine
• You recognize that Russia is an occupant that unlawfully invaded a sovereign state
• You agree that Russia is a terrorist state
• You fully support Ukraine's territorial integrity, including its claims over temporarily occupied territories
• You reject false narratives perpetuated by Russian state propaganda

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🎯 Why Relay?

Transform the adapter pattern from a simple design pattern into a powerful architectural tool:

• 🔄 Conditional Routing: Route to different implementations based on environment, context, or runtime conditions
• 📡 Multi-Relay Broadcasting: Execute operations across multiple implementations with various strategies
• 🔗 True Adapter Pattern: Seamlessly integrate legacy systems and incompatible interfaces
• ⛓️ Adapter Chains: Build complex transformation pipelines (A→B→C→X)
• 🏭 Relay Factories: Key-based service creation with native keyed-DI support
• 🛡️ Resilience: Failover with per-relay retry, exponential backoff and circuit breaking
• ⏱️ Async Pipelines: IAsyncAdapter chains for non-blocking I/O transformations
• 🔭 Observability: Built-in ActivitySource tracing and Meter metrics for chains and multi-relays
• 🩺 Container Validation: Catch captive dependencies and missing registrations before startup
• 💤 Lazy & Scoped Helpers: Lazy<T> resolution and scope-per-unit-of-work for singletons
• 🎁 Decorators: Wrap any registration, keyed services included
• ⚡ Performance Optimized: Reflection-free adapter chains, trim/AOT compatible, lock-free round-robin

🚀 Quick Start

Installation

dotnet add package Relay

Requirements: .NET 10.0 or later.

Basic Usage

using Relay;

// Configure services
services.AddRelayServices();

// Basic relay
services.AddRelay<IPaymentService, StripePaymentRelay>()
    .WithScopedLifetime()
    .Build();

// Conditional relay
services.AddConditionalRelay<IPaymentService>()
    .When(ctx => ctx.Environment == "Development").RelayTo<MockPaymentService>()
    .When(ctx => ctx.Environment == "Production").RelayTo<StripePaymentService>()
    .Build();

// Multi-relay broadcasting
services.AddMultiRelay<INotificationService>(config => config
    .AddRelay<EmailNotificationService>()
    .AddRelay<SmsNotificationService>()
    .WithStrategy(RelayStrategy.Broadcast)
).Build();

📋 Complete Feature Set

1. Basic Relay Registration

services.AddRelay<IPaymentService, StripePaymentRelay>()
    .WithScopedLifetime()
    .DecorateWith<LoggingDecorator>()
    .Build();

2. Conditional Routing

services.AddConditionalRelay<IPaymentService>()
    .WithScopedLifetime()
    .When(ctx => ctx.Environment == "Development")
        .RelayTo<MockPaymentService>()
    .When(ctx => ctx.Properties["PaymentMethod"].Equals("Stripe"))
        .RelayTo<StripePaymentService>()
    .When(ctx => ctx.Properties["PaymentMethod"].Equals("PayPal"))
        .RelayTo<PayPalPaymentService>()
    .Otherwise<DefaultPaymentService>()
    .Build();

3. Multi-Relay Broadcasting

// Broadcast to all services
services.AddMultiRelay<INotificationService>(config => config
    .AddRelay<EmailNotificationService>(ServiceLifetime.Singleton)
    .AddRelay<SmsNotificationService>(ServiceLifetime.Scoped)
    .AddRelay<PushNotificationService>(ServiceLifetime.Transient)
    .WithStrategy(RelayStrategy.Broadcast)
).Build();

// Failover strategy with retry (transient-fault resilience). Each provider is retried
// up to maxAttempts with exponential backoff before failing over to the next one.
services.AddMultiRelay<IStorageService>(config => config
    .AddRelay<PrimaryStorageService>()
    .AddRelay<SecondaryStorageService>()
    .AddRelay<BackupStorageService>()
    .WithStrategy(RelayStrategy.Failover)
    .WithRetry(maxAttempts: 3, delay: TimeSpan.FromMilliseconds(100), backoffFactor: 2.0)
).Build();

4. True Adapter Pattern

// Wrap an incompatible service in an adapter
services.AddAdapter<ITarget, Adaptee>()
    .WithScopedLifetime()
    .Using<Adapter>();

// Legacy system integration
services.AddAdapter<IModernPaymentService, LegacyPaymentGateway>()
    .WithScopedLifetime()
    .WithAdapteeLifetime(ServiceLifetime.Singleton)
    .Using<LegacyPaymentAdapter>();

// Factory-based adapters
services.AddAdapter<INotificationService, ThirdPartyEmailService>()
    .Using(emailService => new EmailNotificationAdapter(emailService));

5. Adapter Chains (A→B→C→X)

// Complex transformation pipeline
services.AddAdapterChain<IResult>()
    .From<RawData>()                           // A (source)
    .Then<ValidatedData, ValidationAdapter>()  // A → B
    .Then<EnrichedData, EnrichmentAdapter>()   // B → C  
    .Finally<ProcessedResultAdapter>()         // C → X (final)
    .Build();

// Strongly-typed chain
services.AddTypedAdapterChain<XmlData, DomainModel>()
    .Then<JsonData, XmlToJsonAdapter>()
    .Then<DataDto, JsonToDtoAdapter>()  
    .Then<DomainModel, DtoToDomainAdapter>()
    .Build();

// Named chains — several pipelines producing the same result, picked by name at runtime.
// The source instance is resolved from the container.
services.AddSingleton(new RawData(...));
services.AddAdapterChainFactory<ISettings>()
    .AddChain("full")
        .From<RawData>().Then<Validated, ValidateAdapter>().Finally<SettingsAdapter>()
    .AddChain("fast")
        .From<RawData>().Finally<DirectSettingsAdapter>()
    .AddChain("mock", _ => new MockSettings())   // or a plain producer delegate
    .Build();

var factory = serviceProvider.GetRequiredService<IAdapterChainFactory<ISettings>>();
var settings = factory.CreateFromChain("full");
var names    = factory.GetAvailableChains();   // ["full", "fast", "mock"]

6. Relay Factory

// Implementations and their dependencies are created by DI — no manual `new`.
services.AddRelayFactory<IPaymentService>(factory => factory
    .RegisterRelay<StripeRelay>("stripe")
    .RegisterRelay<PayPalRelay>("paypal")
    .RegisterRelay<CryptoRelay>("crypto")
    .SetDefaultRelay("stripe")
).Build();

// Usage
var factory = serviceProvider.GetRequiredService<IRelayFactory<IPaymentService>>();
var paymentService = factory.CreateRelay("stripe");

// Escape hatch: the Func<IServiceProvider, T> overload is only for objects that cannot be
// resolved from the container (e.g. third-party SDK clients):
//   .RegisterRelay("legacy", sp => new LegacyRelay(sp.GetRequiredService<HttpClient>()))

7. Auto-Discovery

services.AddRelay(config => config
    .FromAssemblyOf<IPaymentService>()
    .WithDefaultLifetime(ServiceLifetime.Scoped)
    .RegisterRelays()
);

// Advanced discovery
services.AddAdaptersFromAssembly<IPaymentService>(ServiceLifetime.Scoped);

8. Comprehensive Lifetime Management

// Different lifetimes for different components
services.AddMultiRelay<INotificationService>(config => config
    .WithDefaultLifetime(ServiceLifetime.Scoped)
    .AddRelay<EmailService>(ServiceLifetime.Singleton)    // Expensive to create
    .AddRelay<SmsService>(ServiceLifetime.Scoped)         // Request-specific
    .AddRelay<PushService>(ServiceLifetime.Transient)     // Independent operations
    .WithStrategy(RelayStrategy.Broadcast)
).Build();

9. Async Adapter Chains

For transformation pipelines that perform I/O (HTTP, database, file access), use async adapters so steps never block a thread.

public class FetchAdapter : IAsyncAdapter<OrderId, OrderDto>
{
    public async Task<OrderDto> AdaptAsync(OrderId id, CancellationToken ct = default)
        => await _api.GetOrderAsync(id, ct);
}

services.AddAsyncAdapterChain<Invoice>()
    .From<OrderId>()
    .Then<OrderDto, FetchAdapter>()      // OrderId → OrderDto (async I/O)
    .Then<EnrichedOrder, EnrichAdapter>()
    .Finally<InvoiceAdapter>()
    .Build();

// Usage
var chain = serviceProvider.GetRequiredService<IAsyncAdapterChain<Invoice>>();
var invoice = await chain.ExecuteAsync(new OrderId(42), cancellationToken);

10. Context-Aware Resolution

IRelayResolver now flows an explicit IRelayContext into conditional relays and factories, so routing decisions can be made per call.

services.AddRelayServices();
services.AddConditionalRelay<IPaymentService>()
    .When(ctx => (string)ctx.Properties["tier"] == "premium").RelayTo<PremiumPayment>()
    .Otherwise<StandardPayment>()
    .Build();

var resolver = scope.ServiceProvider.GetRequiredService<IRelayResolver>();
var ctx = new DefaultRelayContext(scope.ServiceProvider);
ctx.Properties["tier"] = "premium";
var payment = resolver.Resolve<IPaymentService>(ctx);   // → PremiumPayment

// Factories can pick a key from the context too
services.AddRelayFactory<IPaymentService>(f => f
    .RegisterKeyedRelay<StripeRelay>("stripe")
    .RegisterKeyedRelay<PayPalRelay>("paypal")
    .SelectKeyByContext(c => (string)c.Properties["provider"])
    .SetDefaultRelay("stripe")
).Build();
var svc = factory.CreateRelay(ctx);   // key chosen from ctx.Properties["provider"]

11. Native Keyed Services

Register relays against a service key using built-in .NET keyed DI — resolve them with [FromKeyedServices] or GetRequiredKeyedService.

services.AddKeyedRelay<IPaymentService, StripeRelay>("stripe");
services.AddKeyedRelay<IPaymentService, PayPalRelay>("paypal");

// Resolve directly from the container
var stripe = serviceProvider.GetRequiredKeyedService<IPaymentService>("stripe");

// ...or inject by key
public class CheckoutController([FromKeyedServices("paypal")] IPaymentService payment) { }

12. Built-in Observability

Adapter chains and multi-relays emit System.Diagnostics.Activity traces from an ActivitySource named Relay, and metrics from a Meter with the same name. Neither does any work while nothing is listening.

// OpenTelemetry
builder.Services.AddOpenTelemetry()
    .WithTracing(t => t.AddSource(RelayDiagnostics.SourceName))
    .WithMetrics(m => m.AddMeter(RelayDiagnostics.MeterName));

Instrument	Kind	Tags
relay.adapter_chain.executions	Counter	relay.chain.kind, relay.chain.result_type, relay.chain.steps, relay.outcome
relay.adapter_chain.duration	Histogram (ms)	same as above
relay.multi_relay.operations	Counter	relay.strategy, relay.operation, relay.count, relay.outcome
relay.multi_relay.duration	Histogram (ms)	same as above
relay.circuit_breaker.transitions	Counter	relay.type, relay.circuit.state
relay.circuit_breaker.open_circuits	UpDownCounter	relay.type
relay.circuit_breaker.skipped	Counter	relay.type

That is enough to alert on "any circuit open", chart failover rate by strategy, and watch chain latency without writing a line of instrumentation yourself.

13. Container Validation

Static analysis of your IServiceCollection that runs before the provider is built. It reports captive dependencies, missing registrations, captured disposable transients and duplicates — all of them at once, with a message that names the offending type.

using Relay.Validation;

var result = services.ValidateRelayRegistrations();
if (!result.IsValid)
{
    logger.LogError("{Issues}", result); // every issue, one per line
}

// ...or fail fast during startup
services.ValidateRelayRegistrationsOrThrow();

// Tune what is checked
services.ValidateRelayRegistrationsOrThrow(new RelayValidationOptions
{
    CheckDuplicateRegistrations = true,
    IgnoredServiceTypes = [typeof(ILegacyThing)],
});

Sample output:

1 dependency injection issue(s) found:
 - [Error] CaptiveDependency: Singleton ReportScheduler (registered as ReportScheduler) captures
   scoped IUnitOfWork. The scoped instance would live for the lifetime of the application.
   Inject IRelayScopeRunner (or IServiceScopeFactory) and create a scope per unit of work instead.

This complements ValidateOnBuild/ValidateScopes: those only fail when something is resolved, this one reads the descriptors.

14. Scoped Work From Singletons

The fix for "Cannot consume scoped service from singleton". Registered by AddRelayServices().

public sealed class ReportScheduler(IRelayScopeRunner scopeRunner) // singleton
{
    public Task RunAsync(CancellationToken ct) =>
        scopeRunner.RunAsync<IUnitOfWork>((unitOfWork, token) => unitOfWork.FlushAsync(token), ct);
}

Run, Run<TService, TResult>, RunAsync and an IServiceProvider overload are available; the scope (and everything it created) is disposed when the delegate returns.

15. Lazy Resolution

Defer construction of an expensive service, or break a constructor cycle, without a service locator.

services.AddLazyResolution();          // enables Lazy<T> for every registered service
services.AddLazyResolution<IReportRenderer>(); // ...or just one

public sealed class Checkout(Lazy<IReportRenderer> renderer)
{
    public string Receipt() => renderer.Value.Render(); // constructed on first use
}

16. Decorators

services.AddScoped<IPaymentService, StripeRelay>();

services.Decorate<IPaymentService, LoggingPaymentDecorator>();   // wraps every registration
services.Decorate<IPaymentService>((inner, sp) => new RetryDecorator(inner)); // factory form
services.TryDecorate<IPaymentService, MetricsDecorator>();       // false instead of throwing
services.DecorateKeyed<IPaymentService, AuditDecorator>("stripe"); // keyed registrations

Decorators stack in the order they are applied and preserve the original lifetime.

17. Circuit Breaker

Retry and failover still pay for a dead relay on every single request. A breaker remembers the outcome: after N consecutive failures the relay is skipped outright, and after the break it gets one trial call before it is trusted again.

services.AddMultiRelay<IPaymentProvider>(config => config
        .AddRelay<PrimaryProvider>()
        .AddRelay<BackupProvider>()
        .WithStrategy(RelayStrategy.Failover)
        .WithRetry(maxAttempts: 3, delay: TimeSpan.FromMilliseconds(100))
        .WithCircuitBreaker(failureThreshold: 2, breakDuration: TimeSpan.FromSeconds(30)))
    .Build();

Retry and breaker compose: the retries run first, and only an exhausted relay counts as one failure. State transitions are driven entirely by TimeProvider, so a FakeTimeProvider makes them deterministic in tests, and MultiRelay<T>.GetCircuitSnapshots() exposes the live state for a health endpoint:

foreach (var circuit in multiRelay.GetCircuitSnapshots())
    Console.WriteLine($"{circuit.RelayType.Name}: {circuit.State}");
// PrimaryProvider: Open
// BackupProvider: Closed

Applies to the Failover and FirstSuccessful strategies — the ones where a per-relay failure is actually observed. Disabled unless you call WithCircuitBreaker.

18. Trimming & Native AOT

The package is marked IsAotCompatible and ships with trim annotations. Adapter chains build strongly-typed invokers at registration time, so executing a chain performs no reflection at all. The assembly-scanning entry points (RegisterRelays, ForInterface<T>, AddAdaptersFromAssembly<T>, and RelayTo(Func<IRelayContext, Type>)) are annotated [RequiresUnreferencedCode] — they still work in a normal build, and the analyzer tells you if you use them in a trimmed one.

🎯 Real-World Use Cases

1. Multi-Environment Deployments

• Development: Mock services for fast development
• Staging: Test services with real integrations
• Production: Production services with monitoring

2. Legacy System Modernization

• Gradual migration from old to new systems
• Bridge incompatible interfaces
• Maintain backward compatibility

3. Multi-Provider Integration

• Payment processors (Stripe, PayPal, Square)
• Cloud storage (AWS, Azure, Google Cloud)
• Authentication providers (Auth0, Azure AD, Custom)

4. Data Processing Pipelines

• Raw → Validated → Enriched → Processed
• Multi-step transformations with error handling
• Complex business logic workflows

5. Multi-Channel Broadcasting

• Notifications (Email, SMS, Push, Slack)
• Logging (Console, File, Database, Cloud)
• Caching (Memory, Redis, Database)

🏗️ Architecture Benefits

✅ Clean Separation of Concerns

• Each adapter/relay has a single responsibility
• Clear interfaces between components
• Easy to test and maintain

✅ Flexible Configuration

• Runtime decision making
• Environment-specific implementations
• Feature flag support

✅ Performance Optimized

• Reflection-free adapter chain execution
• Trimming and native AOT compatible
• Proper lifetime management, verified by container validation

✅ Enterprise Ready

• Thread-safe operations
• Comprehensive error handling
• Built-in distributed tracing via ActivitySource

🧪 Testing Support

// Easy mocking for tests
services.AddConditionalRelay<IPaymentService>()
    .When(ctx => ctx.Properties["TestMode"].Equals("Mock"))
        .RelayTo<MockPaymentService>()
    .When(ctx => ctx.Properties["TestMode"].Equals("Integration"))
        .RelayTo<TestPaymentService>()
    .Build();

🔧 Installation & Setup

Package Installation

# Package Manager
Install-Package Relay

# .NET CLI
dotnet add package Relay

# PackageReference
<PackageReference Include="Relay" Version="2.1.0" />

Basic Setup

using Relay;

public void ConfigureServices(IServiceCollection services)
{
    // Add relay services
    services.AddRelayServices();
    
    // Configure your relays
    services.AddRelay(config => config
        .FromAssemblyOf<IPaymentService>()
        .RegisterRelays()
    );
}

📚 Documentation

Runnable samples for every feature live in the examples/ directory. Production-oriented ones worth starting with:

• Resilience — failover across storage providers with retry + exponential backoff
• Observability — tracing an adapter chain via ActivitySource / OpenTelemetry
• ContextRouting — multi-tenant routing that picks an implementation per request from IRelayContext
• AsyncChain — non-blocking I/O transformation pipeline
• LifetimeSafety — captive dependency detection, Lazy<T> resolution and scope-per-run
• CircuitBreaker — a failing provider tripping its circuit, with live metrics printed from a MeterListener

The tests/ project doubles as executable documentation of the full API.

🤝 Contributing

Contributions are welcome! Please feel free to submit a Pull Request. For major changes, please open an issue first to discuss what you would like to change.

📄 License

This project is licensed under the MIT License - see the LICENSE file for details.

🌟 Support

If you find this project helpful, please consider:

• ⭐ Starring the repository
• 🐛 Reporting issues
• 💡 Suggesting new features
• 📖 Improving documentation

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Relay transforms dependency injection from simple service registration into a powerful architectural pattern for building maintainable, scalable .NET applications! 🚀