Pandatech MassTransit Outbox

Outbox and inbox pattern implementation for MassTransit with multiple DbContext support.

MassTransit's built-in outbox only supports a single DbContext. These packages let you reliably publish and consume messages across many modules, each with its own DbContext — designed for modular monolith and microservice architectures.

Package	Provider	Concurrency strategy
Pandatech.MassTransit.PostgresOutbox	PostgreSQL	FOR UPDATE SKIP LOCKED
Pandatech.MassTransit.SQLiteOutbox	SQLite	Lease-based (LeasedUntil)

Both packages are wire-compatible — a service using PostgreSQL for its outbox can publish to a service using SQLite for its inbox, and vice versa.

Features

• Multiple DbContext support — each module gets its own DbContext, outbox, and inbox
• Outbox pattern — messages are persisted atomically with your domain changes, then published by a background service
• Inbox pattern — idempotent message consumption prevents duplicate processing
• Background cleanup — processed messages are automatically removed after a configurable retention period
• Zero-allocation logging — uses [LoggerMessage] source generators throughout
• Multi-TFM — targets net9.0, and net10.0

Installation

# PostgreSQL dotnet add package Pandatech.MassTransit.PostgresOutbox # SQLite dotnet add package Pandatech.MassTransit.SqliteOutbox

Quick start

The API surface is identical for both providers. Examples below use the PostgreSQL package — replace the namespace with MassTransit.SQLiteOutbox for SQLite.

1. Configure your DbContext

Implement IOutboxDbContext, IInboxDbContext, or both, and call ConfigureInboxOutboxEntities in OnModelCreating:

using MassTransit.PostgresOutbox.Abstractions; using MassTransit.PostgresOutbox.Extensions; public class OrdersDbContext : DbContext, IOutboxDbContext, IInboxDbContext { public DbSet<OutboxMessage> OutboxMessages { get; set; } public DbSet<InboxMessage> InboxMessages { get; set; } protected override void OnModelCreating(ModelBuilder modelBuilder) { modelBuilder.ConfigureInboxOutboxEntities(); } }

PostgreSQL only — enable UseQueryLocks() for the FOR UPDATE SKIP LOCKED feature:

builder.Services.AddDbContextPool<OrdersDbContext>(options => options.UseNpgsql(connectionString) .UseQueryLocks());

2. Register services

using MassTransit.PostgresOutbox.Extensions; // Registers outbox publisher + outbox cleanup + inbox cleanup background services services.AddOutboxInboxServices<OrdersDbContext>();

To customize behavior, pass a Settings object:

services.AddOutboxInboxServices<OrdersDbContext>(new Settings { PublisherTimerPeriod = TimeSpan.FromSeconds(2), PublisherBatchCount = 50, OutboxRemovalBeforeInDays = 7, InboxRemovalBeforeInDays = 7 });

You can also register services individually:

services.AddOutboxPublisherJob<OrdersDbContext>(); services.AddOutboxRemovalJob<OrdersDbContext>(); services.AddInboxRemovalJob<OrdersDbContext>();

SQLite only — Settings has an additional LeaseDuration property (default: 5 minutes) that controls how long a message is leased before becoming available for reprocessing after a crash.

3. Publish messages (outbox)

Add your message to the outbox within the same SaveChangesAsync call as your domain changes:

dbContext.Orders.Add(new Order { Amount = 555, CreatedAt = DateTime.UtcNow }); dbContext.AddToOutbox(new OrderCreatedEvent { OrderId = orderId }); await dbContext.SaveChangesAsync();

To add multiple messages at once:

dbContext.AddToOutboxRange(event1, event2, event3); await dbContext.SaveChangesAsync();

Both methods return the generated outbox message ID(s) for correlation if needed.

The background publisher picks up new messages, publishes them via MassTransit, and marks them as done.

4. Consume messages (inbox)

Create a consumer that inherits from InboxConsumer<TMessage, TDbContext>:

using MassTransit.PostgresOutbox.Abstractions; using Microsoft.EntityFrameworkCore.Storage; public class OrderCreatedConsumer(IServiceProvider sp) : InboxConsumer<OrderCreatedEvent, OrdersDbContext>(sp) { protected override async Task ConsumeAsync( OrderCreatedEvent message, IDbContextTransaction transaction, CancellationToken ct) { // Your idempotent processing logic here. // The transaction is managed by InboxConsumer — just do your work. } }

The base class handles deduplication (by MessageId + ConsumerId) and concurrency. In PostgreSQL this uses FOR UPDATE SKIP LOCKED; in SQLite it uses atomic lease acquisition.

How it works

Outbox flow

Your code calls AddToOutbox() + SaveChangesAsync() → the message is persisted in the OutboxMessages table atomically with your domain changes → a background HostedService polls for new messages, publishes them via MassTransit, and marks them as done → a cleanup service deletes old processed messages.

Inbox flow

MassTransit delivers a message to your InboxConsumer → the base class inserts or finds the InboxMessage row → acquires an exclusive lock (PostgreSQL) or lease (SQLite) → calls your ConsumeAsync method → marks the message as done and commits → if your code throws, the transaction rolls back and the message is retried.

Cross-provider compatibility

Both packages serialize messages identically (System.Text.Json, same MassTransit header convention), so they are fully wire-compatible. A modular monolith can have some modules using PostgreSQL and others using SQLite — messages flow seamlessly between them via the shared message broker.

Settings reference

Property	Default	Description
PublisherTimerPeriod	1 second	How often the publisher polls for new outbox messages
PublisherBatchCount	100	Max messages published per tick
OutboxRemovalBeforeInDays	5	Days to retain processed outbox messages
OutboxRemovalTimerPeriod	1 day	How often outbox cleanup runs
InboxRemovalBeforeInDays	5	Days to retain processed inbox messages
InboxRemovalTimerPeriod	1 day	How often inbox cleanup runs
LeaseDuration (SQLite only)	5 minutes	How long a message lease is held

License

MIT