Messaging Skill: .NET, RabbitMQ, Azure Service Bus

--- name: messaging description: > Asynchronous messaging patterns for .NET applications. Covers Wolverine and MassTransit, outbox pattern, saga and choreography, and broker configuration for RabbitMQ and Azure Service Bus. Load this skill when implementing event-driven communication, background processing, module-to-module messaging, or when the user mentions "Wolverine", "MassTransit", "message bus", "RabbitMQ", "Azure Service Bus", "event", "publish", "consumer", "outbox", "saga", "integration event", "queue", or "pub/sub". --- # Messaging ## Core Principles 1. **Wolverine is the recommended default** — MIT licensed, combines mediator + messaging in one library with built-in outbox, saga support, and convention-based handlers. MassTransit is an alternative but requires a commercial license from v9. 2. **Outbox pattern for reliability** — Always use the transactional outbox to ensure messages are published only when the database transaction succeeds. 3. **Choreography for simple flows, saga for complex** — If a workflow has 2-3 steps, use event choreography. If it has compensating actions or complex state, use a saga. 4. **Messages are contracts** — Put message types in a shared contracts project. Keep them as simple records with primitive types. ## Patterns ### Wolverine Setup ```csharp // Program.cs builder.Host.UseWolverine(opts => { // Auto-discover handlers from this assembly opts.Discovery.IncludeAssembly(typeof(Program).Assembly); // RabbitMQ transport opts.UseRabbitMq(rabbit => { rabbit.HostName = "localhost"; // Or from configuration: // rabbit.HostName = builder.Configuration["RabbitMq:Host"]!; }) .AutoProvision() // Create queues/exchanges automatically .AutoPurgeOnStartup(); // Dev only — clear queues on startup // Enable transactional outbox with EF Core opts.Services.AddDbContextWithWolverineIntegration<AppDbContext>(x => x.UseNpgsql(builder.Configuration.GetConnectionString("Default"))); opts.Policies.AutoApplyTransactions(); // Wrap handlers in DB transactions }); ``` **Why**: `UseWolverine()` registers handler discovery, transport, and outbox in one place. `AutoProvision()` eliminates manual broker setup during development. ### Publishing Events Wolverine supports two publishing styles: cascading messages (return values) and explicit publishing. ```csharp // Message contract (in shared Contracts project) public record OrderCreated(Guid OrderId, string CustomerId, decimal Total, DateTimeOffset CreatedAt); // Style 1: Cascading messages — return the event from the handler // Wolverine automatically publishes returned messages after the handler completes. public static class CreateOrder { public record Command(string CustomerId, List<OrderItem> Items); public record Response(Guid OrderId, decimal Total); public static async Task<(Response, OrderCreated)> HandleAsync( Command command, AppDbContext db, TimeProvider clock, CancellationToken ct) { var order = Order.Create(command.CustomerId, command.Items, clock.GetUtcNow()); db.Orders.Add(order); await db.SaveChangesAsync(ct); var response = new Response(order.Id, order.Total); var @event = new OrderCreated(order.Id, order.CustomerId, order.Total, order.CreatedAt); return (response, @event); // Both are published automatically } } ``` ```csharp // Style 2: Explicit publishing via IMessageBus public static class CreateOrder { public record Command(string CustomerId, List<OrderItem> Items); public record Response(Guid OrderId, decimal Total); public static async Task<Response> HandleAsync( Command command, AppDbContext db, IMessageBus bus, TimeProvider clock, CancellationToken ct) { var order = Order.Create(command.CustomerId, command.Items, clock.GetUtcNow()); db.Orders.Add(order); await db.SaveChangesAsync(ct); await bus.PublishAsync(new OrderCreated( order.Id, order.CustomerId, order.Total, order.CreatedAt)); return new Response(order.Id, order.Total); } } ``` **Why**: Cascading messages (tuple return) are simpler and testable — the handler is a pure function. Use explicit `IMessageBus` when publishing is conditional or requires multiple events. ### Consuming Events Wolverine uses convention-based handlers — no interface, no base class. Just a `Handle` method with the message type as the first parameter. ```csharp // Notifications module — handles OrderCreated from Orders module public static class OrderCreatedHandler { public static async Task HandleAsync( OrderCreated message, NotificationsDbContext db, ILogger logger, CancellationToken ct) { logger.LogInformation("Processing OrderCreated: {OrderId}", message.OrderId); var notification = new OrderNotification(message.OrderId, message.CustomerId); db.Notifications.Add(notification); await db.SaveChangesAsync(ct); } } ``` **Why**: Convention-based handlers have zero ceremony. Wolverine discovers them by signature: any public method named `Handle`/`HandleAsync`/`Consume`/`ConsumeAsync` with the message type as the first parameter. ### Transactional Outbox Ensures messages are only published if the database transaction succeeds. ```csharp // 1. Register DbContext with Wolverine integration builder.Host.UseWolverine(opts => { opts.Services.AddDbContextWithWolverineIntegration<AppDbContext>(x => x.UseNpgsql(builder.Configuration.GetConnectionString("Default"))); opts.Policies.AutoApplyTransactions(); }); // 2. DbContext — add Wolverine outbox tables public class AppDbContext(DbContextOptions<AppDbContext> options) : DbContext(options) { public DbSet<Order> Orders => Set<Order>(); protected override void OnModelCreating(ModelBuilder modelBuilder) { // Wolverine inbox/outbox tables — required for transactional messaging modelBuilder.AddIncomingWolverineMessageTable(); modelBuilder.AddOutgoingWolverineMessageTable(); } } ``` **Why**: `AddDbContextWithWolverineIntegration` + `AutoApplyTransactions` wraps every handler in a transaction that includes outbox writes. Messages are only sent after the transaction commits — no dual-write problem. ### Saga (Stateful Orchestration) Wolverine sagas use a `Saga<T>` base class with `Start` and `Handle` methods. Cascading messages drive the saga forward. ```csharp public record OrderSagaState(Guid Id) { public string? CustomerId { get; set; } public bool PaymentReceived { get; set; } } public class OrderSaga : Saga<OrderSagaState> { public Guid Id { get; set; } // Start the saga when an OrderCreated event arrives public static (OrderSagaState, ProcessPayment) Start(OrderCreated message) { var state = new OrderSagaState(message.OrderId) { CustomerId = message.CustomerId }; var command = new ProcessPayment(message.OrderId, message.Total); return (state, command); // State is persisted, command is sent } // Handle payment result public CompleteOrder Handle(PaymentCompleted message) { PaymentReceived = true; MarkCompleted(); // Ends the saga return new CompleteOrder(Id); } // Compensating action on failure public CancelOrder Handle(PaymentFailed message) { MarkCompleted(); return new CancelOrder(Id); } } ``` **Why**: Wolverine sagas use simple C# methods instead of a state machine DSL. Each handler returns cascading messages to drive the workflow. `MarkCompleted()` cleans up the saga state. ### Alternative: MassTransit MassTransit is a mature alternative with a commercial license requirement from v9+. Key API surface: ```csharp // Setup builder.Services.AddMassTransit(x => { x.SetKebabCaseEndpointNameFormatter(); x.AddConsumers(typeof(Program).Assembly); x.UsingRabbitMq((context, cfg) => { cfg.Host(builder.Configuration.GetConnectionString("RabbitMq")); cfg.ConfigureEndpoints(context); }); }); // Publishing await publishEndpoint.Publish(new OrderCreated(...), ct); // Consuming — requires IConsumer<T> interface public class OrderCreatedConsumer(AppDbContext db) : IConsumer<OrderCreated> { public async Task Consume(ConsumeContext<OrderCreated> context) { var message = context.Message; // Handle event... } } // Outbox x.AddEntityFrameworkOutbox<AppDbContext>(o => { o.UsePostgres(); o.UseBusOutbox(); }); // Saga — uses MassTransitStateMachine<TState> public class OrderSaga : MassTransitStateMachine<OrderSagaState> { /* ... */ } ``` > **License note**: MassTransit v9+ requires a commercial license for production use. Wolverine (MIT) is the recommended default for new projects. ## Anti-patterns ### Don't Publish Events Without Outbox ```csharp // BAD — if SaveChanges succeeds but Publish fails, data is inconsistent await db.SaveChangesAsync(ct); await bus.PublishAsync(new OrderCreated(...)); // GOOD — use transactional outbox (messages are in the same transaction) // Configure AddDbContextWithWolverineIntegration() + AutoApplyTransactions() // Wolverine handles this automatically ``` ### Don't Put Complex Logic in Message Contracts ```csharp // BAD — behavior in a message public record OrderCreated(Guid OrderId) { public decimal CalculateShipping() => /* logic */; // DON'T } // GOOD — messages are pure data public record OrderCreated(Guid OrderId, string CustomerId, decimal Total, DateTimeOffset CreatedAt); ``` ### Don't Use Fire-and-Forget for Important Events ```csharp // BAD — no guarantee of delivery _ = Task.Run(() => bus.PublishAsync(new OrderCreated(...))); // GOOD — await the publish (with outbox, this is transactional) await bus.PublishAsync(new OrderCreated(...)); ``` ## Decision Guide | Scenario | Recommendation | |----------|---------------| | Module-to-module communication (new project) | Wolverine with events (MIT, free) | | Module-to-module communication (existing MassTransit) | MassTransit (commercial license required from v9) | | Reliable event publishing | Transactional outbox (both Wolverine and MassTransit support this) | | Simple 2-3 step workflow | Event choreography | | Complex workflow with compensation | Wolverine saga or MassTransit saga | | Local development broker | RabbitMQ (via Docker or Aspire) | | Production cloud broker | Azure Service Bus or RabbitMQ | | Want single lib for mediator + messaging | Wolverine (replaces both Mediator and MassTransit) |