Getting Started

Using Outbox with RabbitMQ

Component: NServiceBus
NuGet Package: NServiceBus (9.x)

This sample demonstrates how the Outbox feature works to ensure the atomic processing of a message in RabbitMQ, ensuring that messages sent and received are kept consistent with any modifications made to business data in a database.

This sample uses Docker Compose to provide dependencies. It is not necessary to have installed instances of RabbitMQ or SQL Server.


  1. Install Docker.
  2. Install Docker Compose.
  3. If running Docker on Windows, set Docker to use Linux containers.
  4. In the sample directory, execute the following to set up the RabbitMQ and SQL Server instances:
> docker-compose up --detach

Once complete, the RabbitMQ administration can be reached via http://localhost:15672/ with username rabbitmq and password rabbitmq.

A connection to the SQL Server instance can be made using Server name localhost,11433, login sa, and password NServiceBus!. If connecting with SQL Server Management Studio, adjust the Connection Properties either by unchecking the Encrypt connection checkbox, or checking the Trust server certificate box.

Running the project

The code consists of a single NServiceBus endpoint project, which simulates receiving duplicated messages (normally received due to at-least-once delivery guarantees of the message broker) and processing them under three different circumstances.

  1. Without protection, resulting in duplicated processing of messages.
  2. Using the Outbox but with a maximum message concurrency of 1.
  3. Using the Outbox but with multiple messages being processed simultaneously, relying on the concurrency exception thrown by the database to ensure exactly-once successful processing of messages.

Selecting each step is accomplished via commenting and uncommenting code in the Program.cs file:

// STEP 1: Run code as is, duplicates can be observed in console and database

// STEP 2: Uncomment this line to enable the Outbox. Duplicates will be suppressed.

// STEP 3: Comment out this line to allow concurrent processing. Concurrency exceptions will
// occur in the console window, but only 5 entries will be made in the database.

Step 1: No protection

First, run the sample as-is. It's easy to see from the console output that each MessageId is processed twice. The endpoint has no way to know that it's handling duplicated messages.

Endpoint started. Press Enter to send 5 sets of duplicate messages...

Processing MessageId f4589be7-efaa-4d7b-8fc7-414ac6e5ddfa
Processing MessageId f4589be7-efaa-4d7b-8fc7-414ac6e5ddfa
Processing MessageId 78c4be6d-02f1-4dc6-9aa0-e1298dcb28f9
Processing MessageId 78c4be6d-02f1-4dc6-9aa0-e1298dcb28f9
Processing MessageId 96ed249b-ac80-4eea-be16-eede5f368b09
Processing MessageId 96ed249b-ac80-4eea-be16-eede5f368b09
Processing MessageId 95c6f325-22e2-4832-8a2a-0988ce318f40
Processing MessageId 95c6f325-22e2-4832-8a2a-0988ce318f40
Processing MessageId 1333e38d-b076-41e1-92d6-a2b7a699f62f
Processing MessageId 1333e38d-b076-41e1-92d6-a2b7a699f62f
Press any key to exit

The message handler also writes the received MessageId to the BusinessObject table in the database, as a simulation of writing business data. To see it, execute select * from BusinessObject when connected to the database:


Step 2: Outbox, 1 message at a time

Next, uncomment the line in Program.cs commented as Step 2, which enables the Outbox feature.

It's clear from the console output that each MessageId is only processed a single time, and the message handler does not execute for the duplicate messages:

Endpoint started. Press Enter to send 5 sets of duplicate messages...

Processing MessageId 4d9b207b-f7e3-48f3-a8a9-8bf00afe0bd5
Processing MessageId cfda075b-2ebd-4256-b127-20fbf719873c
Processing MessageId 48cb433c-2cda-4835-a40c-1f06e7c4ace5
Processing MessageId 93132c07-e39d-4eb1-9d34-db4dee493f17
Processing MessageId 247706a4-c231-441c-b5fd-9f2defe6bb6f
Press any key to exit

The same is true in the database:


Step 3: Concurrent Outbox

Lastly, comment out the line in Program.cs commented as Step 3, so that the endpoint is no longer limited to concurrently processing only one message at a time. The default concurrency level (2 * the number of logical processors) will now be used.

The resulting console output will likely be difficult to follow, as different threads competing to commit transactions involving the same MessageId in the Samples_RabbitMQ_Outbox_OutboxData table will result in SqlExceptions being thrown:

NServiceBus.RecoverabilityExecutor Immediate Retry is going to retry message '01dc0013-6954-4289-9351-073b2008a17c' because of an exception:
System.Exception: Failed to ExecuteNonQuery. CommandText:

insert into [dbo].[Samples_RabbitMQ_Outbox_OutboxData]
) ---> System.Data.SqlClient.SqlException: Violation of PRIMARY KEY constraint 'PK__Samples___C87C0C9D6CE4EEA0'. Cannot insert duplicate key in object 'dbo.Samples_RabbitMQ_Outbox_OutboxData'. The duplicate key value is (01dc0013-6954-4289-9351-073b2008a17c).
The statement has been terminated.

After hitting one of these errors, the message is retried, and as the Outbox lookup in the Samples_RabbitMQ_Outbox_OutboxData reveals that the message was already processed successfully, it will skip executing the message handler again.

The database output in the BusinessObject table will be the same.

Code walk-through

In Program.cs, an NServiceBus endpoint is created and configured to use the RabbitMQ transport, connecting to the broker instance hosted in Docker:

var endpointConfiguration = new EndpointConfiguration("Samples.RabbitMQ.Outbox");

var rabbitMqTransport = new RabbitMQTransport(RoutingTopology.Conventional(QueueType.Classic), "host=localhost;username=rabbitmq;password=rabbitmq")
    TransportTransactionMode = TransportTransactionMode.ReceiveOnly

Next, SQL Persistence is configured to connect to the Microsoft SQL Server instance hosted in Docker. SQL Server will store the storage for the Outbox feature in the Samples_RabbitMQ_Outbox_OutboxData table, as well as simulated business data in the BusinessObject table.

A non-standard port 11433 is used in case another SQL Server instance is already running on the machine:

// for SqlExpress use Data Source=.\SqlExpress;Initial Catalog=NsbRabbitMqOutbox;Integrated Security=True;Max Pool Size=100;Encrypt=false
// Password must match value in docker-compose.yml
var connectionString = @"Server=localhost,11433;Initial Catalog=NsbRabbitMqOutbox;User Id=SA;Password=NServiceBus!;Max Pool Size=100;Encrypt=false";

var persistence = endpointConfiguration.UsePersistence<SqlPersistence>();
persistence.ConnectionBuilder(() => new SqlConnection(connectionString));

MyHandler.cs contains the message handler, which shows a bare-bones example of accessing business data via SQL Persistence using ADO.NET, so that the business data is manipulated using the same SQL connection and transaction used by NServiceBus for the Outbox data.

public async Task Handle(MyMessage message, IMessageHandlerContext context)
    Console.WriteLine($"Processing MessageId {context.MessageId}");

    var sqlPersistenceSession = context.SynchronizedStorageSession.SqlPersistenceSession();

    using (var command = sqlPersistenceSession.Connection.CreateCommand())
        command.CommandText = $"insert into BusinessObject (MessageId) values ('{context.MessageId}')";
        command.Transaction = sqlPersistenceSession.Transaction;
        await command.ExecuteNonQueryAsync(context.CancellationToken);

The message handler:

  1. Logs the MessageId to the console.
  2. Retrieves the SynchronizedStorageSession in use by NServiceBus, containing the DbConnection and DbTransaction currently in use.
  3. Inserts the MessageId into the BusinessObject table.

Cleaning up

Once finished with the sample, the RabbitMQ and SQL Server instances can be cleaned up using:

> docker-compose down

Related Articles

  • RabbitMQ Transport
    An overview of the RabbitMQ transport.
  • SQL Persistence
    A persister that targets relational databases, including SQL Server, Oracle, MySQL, PostgreSQL, AWS Aurora MySQL and AWS Aurora PostgreSQL.