Sample: OrderService — Split Outbox + MassTransit RabbitMQ

Location: samples/outbox-relay/
Framework: ASP.NET Core 9 Minimal API · .NET 9 Worker Service
Transport: MassTransit over RabbitMQ — Hermodr.Publisher.MassTransit
Pattern: Transactional Outbox (external relay process)

Overview

This sample demonstrates the split Transactional Outbox pattern using two separate processes that share a SQLite database:

Process	Project	Role
Minimal API	`OrderService.Api`	Accepts HTTP requests; writes CloudEvents to the outbox only
Console worker	`OrderService.RelayWorker`	Polls the shared outbox; forwards events to RabbitMQ via MassTransit

The API has no knowledge of RabbitMQ or MassTransit — it only writes to the outbox. This clean separation means the transport layer can be swapped (e.g. Azure Service Bus, Amazon SQS) by changing the worker, without touching the API.

Architecture

HTTP request
    │
    ▼
OrderService.Api
    │  IEventPublisher.PublishAsync(event)
    ▼
OutboxPublishChannel  ──── INSERT into shared SQLite (outbox.db)

                    ┌── separate process ──────────────────────────────┐
                    │  OrderService.RelayWorker                        │
                    │                                                  │
                    │  OutboxRelayService (BackgroundService)          │
                    │      polls outbox.db every 5 s                   │
                    │      │                                           │
                    │      ▼                                           │
                    │  MassTransitPublishChannel                       │
                    │      │                                           │
                    └──────│───────────────────────────────────────────┘
                           │
                           ▼
                    RabbitMQ broker

What this sample demonstrates

1. API — outbox write path only

Program.cs in OrderService.Api calls .AddEntityFrameworkOutbox() with .WithFactory<>() but without .WithRelay():

var events = builder.Services.AddEventPublisher(options =>
{
    options.Source = new Uri("https://example.com/services/order-service");
});

events
    .AddEntityFrameworkOutbox(opts =>
        opts.UseSqlite(builder.Configuration.GetConnectionString("Outbox")))
    .WithFactory<OrderOutboxMessageFactory>();
    // ↑ No .WithRelay() — forwarding is handled by the external relay worker

The API project references only Hermodr.Publisher.Outbox.EntityFramework — there is no dependency on any transport package.

2. Worker — relay + MassTransit

Program.cs in OrderService.RelayWorker first configures MassTransit with the RabbitMQ transport, then registers the outbox channel with .WithRelay() and adds the MassTransit publish channels:

// ── MassTransit ──────────────────────────────────────────────────────────
builder.Services.AddMassTransit(mt =>
{
    mt.UsingRabbitMq((ctx, cfg) =>
    {
        cfg.Host(host, h =>
        {
            h.Username(username);
            h.Password(password);
        });

        cfg.ConfigureEndpoints(ctx);
    });
});

// ── Hermodr outbox relay → MassTransit ─────────────────────────────
var events = builder.Services.AddEventPublisher(options =>
{
    options.Source = new Uri("https://example.com/services/order-service");
});

// Outbox repository — same database as the API
events
    .AddEntityFrameworkOutbox(opts =>
        opts.UseSqlite(builder.Configuration.GetConnectionString("Outbox")))
    .WithFactory<OrderOutboxMessageFactory>()
    .WithRelay(opts =>
    {
        opts.Interval     = TimeSpan.FromSeconds(5);
        opts.MaxBatchSize = 50;
    });

// MassTransit channels — used only by the relay
events
    .AddMassTransit()                  // generic catch-all channel
    .AddMassTransit<OrderCreated>()    // typed per-event channels
    .AddMassTransit<OrderConfirmed>()
    .AddMassTransit<OrderShipped>()
    .AddMassTransit<OrderDelivered>()
    .AddMassTransit<OrderCancelled>();

MassTransit must be registered before the Hermodr channels so that IPublishEndpoint and ISendEndpointProvider are available in DI when the MassTransitPublishChannel resolves them.

3. Event types — no AMQP coupling in the API

Event classes in OrderService.Api carry only the [Event] annotation:

[Event("order.created", "1.0", Description = "A new order was placed by a customer")]
public sealed class OrderCreated
{
    public Guid OrderId { get; set; }
    public string CustomerId { get; set; } = default!;
    public IReadOnlyList<OrderCreatedItem> Items { get; set; } = [];
    public decimal TotalAmount { get; set; }
    public DateTimeOffset CreatedAt { get; set; }
}

The worker defines only stub event marker types — topic routing is handled by MassTransit's topology, not by AMQP annotations:

[Event("order.created", "1.0")]
public sealed class OrderCreated { }

4. Shared outbox database

Both processes point at the same SQLite file:

Process	`ConnectionStrings:Outbox`
API	`Data Source=outbox.db` (written in the API working directory)
Worker	`Data Source=../OrderService.Api/outbox.db` (relative path)

Production note: replace SQLite with SQL Server or PostgreSQL when running the two processes on different hosts or containers. SQLite file-locking is not safe for concurrent cross-process writes under high load.

Prerequisites

Tool	Version
.NET SDK	9.0 +
Docker	any recent version

Running the sample

1. Start RabbitMQ

cd samples/outbox-relay
docker compose up -d

The RabbitMQ management UI is available at http://localhost:15672 (guest / guest).

2. Start the API (terminal 1)

cd samples/outbox-relay/OrderService.Api
dotnet run

The API listens on http://localhost:5000. The SQLite database (outbox.db) is created automatically.

3. Start the relay worker (terminal 2)

cd samples/outbox-relay/OrderService.RelayWorker
dotnet run

The worker connects to the same outbox.db via the relative path in appsettings.json and starts polling every 5 seconds.

Tip: override the path with an environment variable if the working directories differ:
ConnectionStrings__Outbox="Data Source=/absolute/path/outbox.db" dotnet run

Exercise the full lifecycle

# 1. Create an order
ORDER_ID=$(curl -s -X POST http://localhost:5000/orders \
  -H "Content-Type: application/json" \
  -d '{
    "customerId": "cust-42",
    "items": [
      { "productId": "sku-001", "productName": "Widget", "quantity": 2, "unitPrice": 9.99 }
    ]
  }' | jq -r '.id')

echo "Created order: $ORDER_ID"

# 2. Confirm
curl -s -X PUT "http://localhost:5000/orders/$ORDER_ID/confirm" | jq

# 3. Ship
curl -s -X PUT "http://localhost:5000/orders/$ORDER_ID/ship" \
  -H "Content-Type: application/json" \
  -d '{ "trackingNumber": "1Z999AA10123456784", "carrier": "UPS" }' | jq

# 4. Deliver
curl -s -X PUT "http://localhost:5000/orders/$ORDER_ID/deliver" | jq

Within ~5 seconds after each request, the relay worker logs a Sending update followed by a Sent confirmation. The messages appear in the RabbitMQ management UI as MassTransit-formatted messages on the Hermodr exchange topology.

Configuration reference

API (`OrderService.Api/appsettings.json`)

Key	Default	Description
`ConnectionStrings:Outbox`	`Data Source=outbox.db`	SQLite outbox database (written by the API)

Worker (`OrderService.RelayWorker/appsettings.json`)

Key	Default	Description
`ConnectionStrings:Outbox`	`Data Source=../OrderService.Api/outbox.db`	Path to the shared SQLite database
`Events:MassTransit:RabbitMq:Host`	`localhost`	RabbitMQ broker hostname
`Events:MassTransit:RabbitMq:Username`	`guest`	RabbitMQ username
`Events:MassTransit:RabbitMq:Password`	`guest`	RabbitMQ password

Comparison with the in-process topology

	In-process relay (outbox-inapp)	External relay (this sample)
Process count	1 — API + relay in the same host	2 — API and relay run independently
Transport	RabbitMQ (direct AMQP channel)	MassTransit (broker-agnostic abstraction)
API dependency on transport	Yes — `Hermodr.Publisher.RabbitMq`	No — API only refs the outbox package
Relay restart	Restarts the entire API process	Relay restarts independently
Independent scaling	❌ Relay scales with the API	✅ Scale relay separately
Swap broker later	Requires API changes	Change worker only

Choose the external-relay topology when fault isolation, independent scaling, or transport-agnosticism are priorities.

Overview​

Architecture​

What this sample demonstrates​

1. API — outbox write path only​

2. Worker — relay + MassTransit​

3. Event types — no AMQP coupling in the API​

4. Shared outbox database​

Prerequisites​

Running the sample​

Exercise the full lifecycle​

Configuration reference​

API (OrderService.Api/appsettings.json)​

Worker (OrderService.RelayWorker/appsettings.json)​

Comparison with the in-process topology​

Related documentation​