Dotnet Messaging Patterns

dotnet-messaging-patterns

Durable messaging patterns for .NET event-driven architectures. Covers publish/subscribe, competing consumers, dead-letter queues, saga/process manager orchestration, and delivery guarantee strategies using Azure Service Bus, RabbitMQ, and MassTransit.

Scope

• Publish/subscribe and competing consumer patterns
• Dead-letter queues and poison message handling
• Saga/process manager orchestration
• Delivery guarantee strategies (at-least-once, exactly-once)
• Azure Service Bus, RabbitMQ, and MassTransit integration

Out of scope

• Background service lifecycle and IHostedService registration -- see [skill:dotnet-background-services]
• Resilience pipelines and retry policies -- see [skill:dotnet-resilience]
• JSON/binary serialization configuration -- see [skill:dotnet-serialization]
• In-process producer/consumer queues with Channel -- see [skill:dotnet-channels]

Cross-references: [skill:dotnet-background-services] for hosting message consumers, [skill:dotnet-resilience] for fault tolerance around message handlers, [skill:dotnet-serialization] for message envelope serialization, [skill:dotnet-channels] for in-process queuing patterns.

Messaging Fundamentals

Message Types

Commands are sent to a specific queue; events are published to a topic/exchange and delivered to all subscribers. This distinction drives the choice between point-to-point and pub/sub topologies.

Delivery Guarantees

At-least-once with idempotent consumers is the standard approach for durable messaging. True exactly-once requires distributed transactions (which most brokers do not support) or consumer-side deduplication.

Publish/Subscribe

Azure Service Bus Topics

// Publisher -- send event to a topic
await using var client = new ServiceBusClient(connectionString);
await using var sender = client.CreateSender("order-events");

var message = new ServiceBusMessage(
    JsonSerializer.SerializeToUtf8Bytes(new OrderPlaced(orderId, total)))
{
    Subject = nameof(OrderPlaced),
    ContentType = "application/json",
    MessageId = Guid.NewGuid().ToString()
};

await sender.SendMessageAsync(message, cancellationToken);

```text

```csharp

// Subscriber -- process events from a subscription
await using var processor = client.CreateProcessor(
    topicName: "order-events",
    subscriptionName: "billing-service",
    new ServiceBusProcessorOptions
    {
        MaxConcurrentCalls = 10,
        AutoCompleteMessages = false
    });

processor.ProcessMessageAsync += async args =>
{
    var body = args.Message.Body.ToObjectFromJson<OrderPlaced>();
    await HandleOrderPlacedAsync(body);
    await args.CompleteMessageAsync(args.Message);
};

processor.ProcessErrorAsync += args =>
{
    logger.LogError(args.Exception, "Error processing message");
    return Task.CompletedTask;
};

await processor.StartProcessingAsync(cancellationToken);

```text

**Key packages:**

```xml

<PackageReference Include="Azure.Messaging.ServiceBus" Version="7.*" />

```xml

### RabbitMQ Fanout Exchange

```csharp

// Publisher -- declare exchange and publish
var factory = new ConnectionFactory { HostName = "localhost" };
await using var connection = await factory.CreateConnectionAsync();
await using var channel = await connection.CreateChannelAsync();

await channel.ExchangeDeclareAsync(
    exchange: "order-events",
    type: ExchangeType.Fanout,
    durable: true);

var body = JsonSerializer.SerializeToUtf8Bytes(
    new OrderPlaced(orderId, total));

await channel.BasicPublishAsync(
    exchange: "order-events",
    routingKey: string.Empty,
    body: body);

```text

**Key packages:**

```xml

<PackageReference Include="RabbitMQ.Client" Version="7.*" />

```xml

### MassTransit Publish

MassTransit abstracts the broker, providing a unified API for Azure Service Bus, RabbitMQ, Amazon SQS, and in-memory
transport.

```csharp

// Registration
builder.Services.AddMassTransit(x =>
{
    x.AddConsumer<OrderPlacedConsumer>();

    x.UsingRabbitMq((context, cfg) =>
    {
        cfg.Host("localhost", "/", h =>
        {
            h.Username("guest");
            h.Password("guest");
        });
        cfg.ConfigureEndpoints(context);
    });
});

// Publisher
public sealed class OrderService(IPublishEndpoint publishEndpoint)
{
    public async Task PlaceOrderAsync(
        Guid orderId, decimal total, CancellationToken ct)
    {
        // Process order...
        await publishEndpoint.Publish(
            new OrderPlaced(orderId, total), ct);
    }
}

// Consumer
public sealed class OrderPlacedConsumer(
    ILogger<OrderPlacedConsumer> logger)
    : IConsumer<OrderPlaced>
{
    public async Task Consume(ConsumeContext<OrderPlaced> context)
    {
        logger.LogInformation(
            "Processing order {OrderId}", context.Message.OrderId);
        await ProcessAsync(context.Message);
    }
}

// Message contract (use records in a shared contracts assembly)
public record OrderPlaced(Guid OrderId, decimal Total);

```text

**Key packages:**

```xml

<PackageReference Include="MassTransit" Version="8.*" />
<!-- Pick ONE transport: -->
<PackageReference Include="MassTransit.RabbitMQ" Version="8.*" />
<!-- OR -->
<PackageReference Include="MassTransit.Azure.ServiceBus.Core" Version="8.*" />

```text

---

## Competing Consumers

Multiple consumer instances process messages from the same queue in parallel. The broker delivers each message to
exactly one consumer, distributing load across instances.

### Pattern

```text

Queue: order-processing
  ├── Consumer Instance A  (picks message 1)
  ├── Consumer Instance B  (picks message 2)
  └── Consumer Instance C  (picks message 3)

```text

### Azure Service Bus -- Scaling Consumers

```csharp

// Multiple instances reading from the same queue automatically compete.
// MaxConcurrentCalls controls per-instance parallelism.
var processor = client.CreateProcessor("order-processing",
    new ServiceBusProcessorOptions
    {
        MaxConcurrentCalls = 20,
        PrefetchCount = 50,
        AutoCompleteMessages = false
    });

```text

### MassTransit -- Concurrency Limits

```csharp

x.AddConsumer<OrderProcessor>(cfg =>
{
    cfg.UseConcurrentMessageLimit(10);
});

```text

### Ordering Considerations

Competing consumers sacrifice strict ordering for throughput. When order matters:

- **Azure Service Bus**: Use sessions (`RequiresSession = true`) to guarantee FIFO within a session ID (e.g., per
  customer)
- **RabbitMQ**: Use a single consumer per queue, or consistent-hash exchange to partition by key
- **MassTransit**: Configure `UseMessagePartitioner` for key-based ordering

---

## Dead-Letter Queues

Dead-letter queues (DLQs) capture messages that cannot be processed after exhausting retries. They prevent poison
messages from blocking the main queue.

### Why Messages Are Dead-Lettered

| Reason                         | Trigger                                      |
| ------------------------------ | -------------------------------------------- |
| Max delivery attempts exceeded | Message failed processing N times            |
| TTL expired                    | Message sat in queue past its time-to-live   |
| Consumer rejection             | Consumer explicitly dead-letters the message |
| Queue length exceeded          | Queue overflow policy routes to DLQ          |

### Azure Service Bus DLQ

```csharp

// Dead-letter a message with reason
await args.DeadLetterMessageAsync(
    args.Message,
    deadLetterReason: "ValidationFailed",
    deadLetterErrorDescription: "Missing required field: CustomerId");

// Read from the dead-letter sub-queue
await using var dlqReceiver = client.CreateReceiver(
    "order-processing",
    new ServiceBusReceiverOptions
    {
        SubQueue = SubQueue.DeadLetter
    });

while (true)
{
    var message = await dlqReceiver.ReceiveMessageAsync(
        TimeSpan.FromSeconds(5), cancellationToken);
    if (message is null) break;

    logger.LogWarning(
        "DLQ message: {Reason} - {Description}",
        message.DeadLetterReason,
        message.DeadLetterErrorDescription);

    // Inspect, fix, and re-submit or discard
    await dlqReceiver.CompleteMessageAsync(message);
}

```text

### MassTransit Error/Fault Queues

MassTransit automatically creates `_error` and `_skipped` queues. Failed messages after retry exhaustion move to the
error queue with fault metadata.

```csharp

// Configure retry before dead-lettering
x.AddConsumer<OrderProcessor>(cfg =>
{
    cfg.UseMessageRetry(r => r.Intervals(
        TimeSpan.FromSeconds(1),
        TimeSpan.FromSeconds(5),
        TimeSpan.FromSeconds(15)));
});

```text

### DLQ Monitoring

Always monitor DLQ depth with alerts. Unmonitored DLQs accumulate silently until data is lost or stale.

---

## Saga / Process Manager

Sagas coordinate multi-step business processes across services. Each step publishes events that trigger the next step,
with compensation logic for failures.

### Choreography vs Orchestration

| Style             | How it works                                                   | Use when                                                |
| ----------------- | -------------------------------------------------------------- | ------------------------------------------------------- |
| **Choreography**  | Services react to events independently; no central coordinator | Simple flows, few steps, loosely coupled                |
| **Orchestration** | A saga/process manager directs each step                       | Complex flows, compensation needed, visibility required |

### MassTransit State Machine Saga

```csharp

// Saga state
public class OrderState : SagaStateMachineInstance
{
    public Guid CorrelationId { get; set; }
    public string CurrentState { get; set; } = default!;
    public Guid OrderId { get; set; }
    public decimal Total { get; set; }
    public DateTime? PaymentReceivedAt { get; set; }
}

// State machine definition
public sealed class OrderStateMachine : MassTransitStateMachine<OrderState>
{
    public State Submitted { get; private set; } = default!;
    public State PaymentPending { get; private set; } = default!;
    public State Completed { get; private set; } = default!;
    public State Faulted { get; private set; } = default!;

    public Event<OrderSubmitted> OrderSubmitted { get; private set; } = default!;
    public Event<PaymentReceived> PaymentReceived { get; private set; } = default!;
    public Event<PaymentFailed> PaymentFailed { get; private set; } = default!;

    public OrderStateMachine()
    {

## Detailed Examples

See [references/detailed-examples.md](references/detailed-examples.md) for complete code samples and advanced patterns.