Message Queues with RabbitMQ / Azure Service Bus in ASP.NET Core

Introduction

When ShopNest order is placed, inventory, email, and analytics react asynchronously via RabbitMQ and MassTransit — decoupling failure domains.

After this article you will

• Run RabbitMQ in Docker
• Publish OrderCreatedEvent with MassTransit
• Build consumers for email and inventory
• Understand dead letter queues and retries
• Compare Azure Service Bus for cloud

Prerequisites

• Article 51 — Microservices Introduction
• ShopNest API, EF Core, and DI from prior modules

Concept deep-dive

// Publish from Order API
await _publishEndpoint.Publish(new OrderCreatedEvent
{
    OrderId = order.Id,
    CustomerEmail = order.CustomerEmail,
    Lines = order.Lines.Select(l => new { l.ProductId, l.Qty }).ToList()
});

// Consumer
public class SendOrderEmailConsumer : IConsumer<OrderCreatedEvent>
{
    public async Task Consume(ConsumeContext<OrderCreatedEvent> context)
    {
        await _email.SendOrderConfirmationAsync(context.Message);
    }
}

builder.Services.AddMassTransit(x =>
{
    x.AddConsumer<SendOrderEmailConsumer>();
    x.AddConsumer<UpdateInventoryConsumer>();
    x.UsingRabbitMq((ctx, cfg) =>
    {
        cfg.Host("localhost", "/", h => { h.Username("guest"); h.Password("guest"); });
        cfg.ConfigureEndpoints(ctx);
    });
});

Saga: coordinate multi-step distributed workflow with compensating actions. DLQ: failed messages after retries for manual inspection.

Hands-on — ShopNest Order Processing with Async Events

1. docker run rabbitmq:3-management.
2. MassTransit OrderCreatedEvent + two consumers.
3. Simulate consumer failure — message retries then DLQ.
4. Note Azure Service Bus swap via MassTransit transport config.

Common errors & best practices

• No idempotent consumers — duplicate messages double-charge inventory.
• Sync HTTP chain pretending to be async — use real messaging.

Interview questions

Q: Why message queue?
A: Decouple producers/consumers; absorb spikes; retry on failure.

Q: MassTransit role?
A: Abstraction over RabbitMQ/ASB with consumers, sagas, retries built in.

Q: At-least-once delivery?
A: Consumers must be idempotent — handle duplicate events safely.

Summary

• OrderCreatedEvent triggers parallel downstream work
• RabbitMQ + MassTransit standard for .NET microservices
• DLQ and retries handle transient failures
• Azure Service Bus for managed cloud messaging

Previous: Microservices Introduction
Next: gRPC with ASP.NET Core

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