Transactional Outbox

The transactional outbox pattern guarantees that events are published to a message broker if and only if the database transaction that triggered them commits. RCommon provides outbox integration for both MassTransit and Wolverine.

The problem the outbox solves

Without an outbox, there is a window between committing a database change and publishing an event where a crash can cause the event to be lost. Conversely, publishing an event before committing risks publishing for a transaction that is later rolled back. The outbox eliminates both failure modes by writing the event to the same database transaction as the business data, then relaying it to the broker after the transaction commits.

MassTransit outbox

Installation

NuGet Package

dotnet add package RCommon.MassTransit.Outbox

MassTransit's Entity Framework Core outbox stores pending messages in the same DbContext as your application data. The outbox tables are added to any DbContext that you supply.

Configuration

Call AddOutbox<TDbContext> on the IMassTransitEventHandlingBuilder returned by WithEventHandling:

using RCommon;
using RCommon.MassTransit;

builder.Services.AddRCommon()
    .WithEventHandling<MassTransitEventHandlingBuilder>(mt =>
    {
        mt.AddProducer<PublishWithMassTransitEventProducer>();
        mt.AddSubscriber<OrderShipped, OrderShippedHandler>();

        // Configure the transactional outbox backed by AppDbContext
        mt.AddOutbox<AppDbContext>(outbox =>
        {
            outbox.UseSqlServer();   // or outbox.UsePostgres()
            outbox.UseBusOutbox();
        });

        mt.UsingRabbitMq((ctx, cfg) =>
        {
            cfg.Host("rabbitmq://localhost");
            cfg.ConfigureEndpoints(ctx);
        });
    });

IMassTransitOutboxBuilder members

Method	Description
UseSqlServer()	Configures the outbox to use SQL Server dialect
UsePostgres()	Configures the outbox to use PostgreSQL dialect
UseBusOutbox(configure?)	Enables the bus outbox relay; accepts an optional IBusOutboxConfigurator action

Applying outbox migrations

The MassTransit outbox adds its own tables to the DbContext. Add a migration after configuring the outbox:

dotnet ef migrations add AddMassTransitOutbox
dotnet ef database update

Wolverine outbox

Installation

NuGet Package

dotnet add package RCommon.Wolverine.Outbox

Wolverine's outbox integrates with Entity Framework Core transactions, persisting messages alongside application data in the same transaction scope.

Configuration

Call AddOutbox on the IWolverineEventHandlingBuilder:

using RCommon;
using RCommon.Wolverine;

builder.Host.UseWolverine();

builder.Services.AddRCommon()
    .WithEventHandling<WolverineEventHandlingBuilder>(wlv =>
    {
        wlv.AddProducer<PublishWithWolverineEventProducer>();
        wlv.AddSubscriber<OrderShipped, OrderShippedHandler>();

        wlv.AddOutbox(outbox =>
        {
            outbox.UseEntityFrameworkCoreTransactions();
        });
    });

IWolverineOutboxBuilder members

Method	Description
UseEntityFrameworkCoreTransactions()	Integrates Wolverine's outbox with EF Core transaction management

How delivery works

Once the outbox is configured:

1. When IEventRouter.RouteEventsAsync is called, the producer writes the message to the outbox table inside the current database transaction rather than sending it directly to the broker.
2. After the transaction commits, a background relay process (managed by MassTransit or Wolverine) reads pending messages from the outbox table and forwards them to the broker.
3. Each message is deleted from the outbox only after the broker acknowledges receipt.

This gives at-least-once delivery semantics. Consumers must be idempotent to handle the rare case of duplicate delivery after a relay failure.

API summary

MassTransit outbox

Type	Package	Description
MassTransitOutboxBuilderExtensions.AddOutbox<TDbContext>	RCommon.MassTransit.Outbox	Registers the EF Core outbox for the given DbContext
IMassTransitOutboxBuilder	RCommon.MassTransit.Outbox	Builder for outbox dialect and bus relay options

Wolverine outbox

Type	Package	Description
WolverineOutboxBuilderExtensions.AddOutbox	RCommon.Wolverine.Outbox	Registers Wolverine's outbox via ConfigureWolverine
IWolverineOutboxBuilder	RCommon.Wolverine.Outbox	Builder for EF Core transaction integration