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

// 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.

// 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
    }
}

// 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.

// 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.

// 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.

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:

// 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

// 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

// 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

// 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)