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Outbox Demo — Critter Stack Conversion

Original Project

Repository: MassTransit/Sample-Outbox License: Apache 2.0 Stack: MassTransit 8, EF Core (PostgreSQL), RabbitMQ, ASP.NET Core, OpenTelemetry/Jaeger

Demonstrates MassTransit's transactional outbox pattern: business data and messages are written in the same database transaction, guaranteeing at-least-once delivery without distributed transactions. Includes a state machine saga that orchestrates a multi-step registration workflow.

Domain: Event registration — a member registers for an event, triggering email notification, attendee tracking, and validation.

What Changed

Removed (3 projects collapsed to 1)

  • Sample.Api — ASP.NET Core Web API with MVC controller
  • Sample.Components — shared library with consumers, saga, services, contracts
  • Sample.Worker — background service for consumer processing
  • MassTransit — consumers, state machine saga, outbox configuration, consumer definitions
  • EF Core — DbContext, entity configurations, saga state mapping, outbox/inbox tables
  • Separate worker process — MassTransit required a separate worker for consumer hosting

Added

  • WolverineFx.Marten — integrated outbox (messages + documents in same transaction)
  • Wolverine handlers — static methods replacing MassTransit consumers
  • Cascade messages — handler return values automatically published via outbox

Before vs After

Aspect Original (MassTransit) Converted (Wolverine + Marten)
Projects 3 (Api, Components, Worker) 1
C# files ~25 5
Outbox config EF Core interceptor + BusOutbox + polling interval IntegrateWithWolverine() (one line)
Saga State machine class + state entity + definition + map Handler returning cascade tuple
Consumer IConsumer<T> implementation class + definition class Static Handle(T) method
Worker process Separate hosted service Built into web host
Inbox/Outbox tables 4 MassTransit-managed tables Wolverine envelope tables (auto-created)

The Outbox Comparison

MassTransit outbox setup (spread across multiple files):

// In each host:
x.AddEntityFrameworkOutbox<RegistrationDbContext>(o => {
    o.QueryDelay = TimeSpan.FromSeconds(1);
    o.UsePostgres();
    o.UseBusOutbox();
});
// Plus: DbContext extends SagaDbContext, outbox tables in migrations

Wolverine+Marten outbox setup (one line):

builder.Services.AddMarten(opts => { ... })
    .IntegrateWithWolverine();  // That's it. Outbox is built in.

Saga Simplification

The original MassTransit saga required 4 files:

  • RegistrationStateMachine.cs — state machine definition with transitions
  • RegistrationState.cs — saga state entity
  • RegistrationStateDefinition.cs — consumer retry and outbox config
  • RegistrationStateMap.cs — EF Core mapping for saga state

The converted version is a single handler method that returns a tuple of cascade messages:

public static (SendRegistrationEmail, AddEventAttendee) Handle(
    RegistrationSubmitted message, IDocumentSession session)

Wolverine sends both cascade messages via the Marten outbox in the same transaction.

Running

Requires PostgreSQL. Update the connection string in appsettings.json, then:

dotnet run

POST to /registration with:

{ "eventId": "conf-2026", "memberId": "member-1", "payment": 100.00 }

Swagger UI available at /swagger.