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Messaging
Core Principles
- 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.
- Outbox pattern for reliability — Always use the transactional outbox to ensure messages are published only when the database transaction succeeds.
- 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.
- 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 automaticallyDon'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) |
