Programming language: C#
License: Apache License 2.0
Tags: Queue    

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SlimMessageBus <!-- omit in toc -->

SlimMessageBus is a client façade for message brokers for .NET. It comes with implementations for specific brokers (Apache Kafka, Azure EventHub, MQTT/Mosquitto, Redis Pub/Sub) and in-memory message passing (in-process communication). SlimMessageBus additionally provides request-response implementation over message queues.

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Key elements of SlimMessageBus

  • Consumers:
    • IConsumer<in TMessage> - subscriber in pub/sub (or queue consumer)
    • IRequestHandler<in TRequest, TResponse> - request handler in request-response
  • Producers:
    • IPublishBus - publisher in pub/sub (or queue producer)
    • IRequestResponseBus - sender in req/resp
    • IMessageBus - extends IPublishBus and IRequestResponseBus
  • Misc:
    • IRequestMessage<TResponse> - marker for request messages
    • MessageBus - static accessor for current context IMessageBus


  • [Introduction](docs/intro.md)
  • Providers:
    • [Apache Kafka](docs/provider_kafka.md)
    • [Azure ServiceBus](docs/provider_azure_servicebus.md)
    • [Azure EventHubs](docs/provider_azure_eventhubs.md)
    • [Redis](docs/provider_redis.md)
    • [Memory](docs/provider_memory.md)
    • [Hybrid](docs/provider_hybrid.md)
  • [Serialization Plugins](docs/serialization.md)
  • [Validation using FluentValidation](docs/plugin_fluent_validation.md)


Name Description NuGet
SlimMessageBus The core API for SlimMessageBus NuGet
Transport providers
.Host.Kafka Transport provider for Apache Kafka NuGet
.Host.AzureServiceBus Transport provider for Azure Service Bus NuGet
.Host.AzureEventHub Transport provider for Azure Event Hubs NuGet
.Host.Redis Transport provider for Redis NuGet
.Host.Memory Transport provider implementation for in-process (in memory) message passing (no messaging infrastructure required) NuGet
.Host.Hybrid Bus implementation that composes the bus out of other transport providers and performs message routing NuGet
.Host.Serialization.Json Serialization plugin for JSON (Newtonsoft.Json library) NuGet
.Host.Serialization.SystemTextJson Serialization plugin for JSON (System.Text.Json library) NuGet
.Host.Serialization.Avro Serialization plugin for Avro (Apache.Avro library) NuGet
.Host.Serialization.Hybrid Plugin that delegates serialization to other serializers based on message type NuGet
.Host.Serialization.GoogleProtobuf Serialization plugin for Google Protobuf NuGet
IoC Container
.Host.MsDependencyInjection DI adapter for Microsoft.Extensions.DependencyInjection NuGet
.Host.AspNetCore Integration for ASP.NET Core (DI adapter, config helpers) NuGet
.Host.Autofac DI adapter for Autofac container NuGet
.Host.Unity DI adapter for Unity container NuGet
.Host.Interceptor Core interface for interceptors NuGet
.Host.FluentValidation Validation for messages based on FluentValidation NuGet NuGet

Typically your application components (business logic, domain) only need to depend on SlimMessageBus which is the facade, and ultimately your application hosting layer (ASP.NET, Windows Service, Console App) will reference and configure the other packages (SlimMessageBus.Host.*) which are the providers and plugins.


Basic usage

Some service (or domain layer) sends a message:

IMessageBus bus; // injected

await bus.Publish(new SomeMessage());

Another service (or application layer) handles the message:

public class SomeMessageConsumer : IConsumer<SomeMessage>
   public async Task OnHandle(SomeMessage message)
       // handle the message

Note: It is also possible to avoid having to implement the interface IConsumer<T> (see [here](docs/intro.md#consumer)).

The bus also supports request-response implemented via queues (or topics - depending on what the chosen transport provider supports). The sender side sends a request message:

var messageResponse = await bus.Send(new MessageRequest());

Note: It is possible to configure the bus to timeout a request when the response does not arrive within the allotted time (see [here](docs/intro.md#produce-request-message)).

The receiving side handles the request and replies back:

public class MessageRequestHandler : IRequestHandler<MessageRequest, MessageResponse>
   public async Task<MessageResponse> OnHandle(MessageRequest request)
      // handle the request message and return response

The bus will ask the chosen DI to provide the consumer instances (SomeMessageConsumer, MessageRequestHandler).

Configuration with MsDependencyInjection

When Microsoft.Extensions.DependencyInjection is used, then SMB can be configured (requires either MsDependencyInjection or AspNetCore plugins):

IServiceCollection services;

services.AddSlimMessageBus(mbb =>
         .AddChildBus("Bus1", (builder) => 
               .Produce<SomeMessage>(x => x.DefaultTopic("some-topic"))
               .Consume<SomeMessage>(x => x
                  //.KafkaGroup("some-kafka-consumer-group") //  Kafka provider specific
                  //.SubscriptionName("some-azure-sb-topic-subscription") // Azure ServiceBus provider specific
               // ...
               .WithProviderKafka(new KafkaMessageBusSettings("localhost:9092")); // requires SlimMessageBus.Host.Kafka package
               // Use Azure Service Bus transport provider
               // Use Azure Azure Event Hub transport provider
               // Use Redis transport provider
               // Use in-memory transport provider
         // Add other bus transports, if needed
         //.AddChildBus("Bus2", (builder) => {})
         .WithSerializer(new JsonMessageSerializer()) // requires SlimMessageBus.Host.Serialization.Json package
         .WithProviderHybrid(); // requires SlimMessageBus.Host.Hybrid package
   // Option 1 (optional)
   addConsumersFromAssembly: new[] { Assembly.GetExecutingAssembly() }, // auto discover consumers and register into DI (see next section)
   addInterceptorsFromAssembly: new[] { Assembly.GetExecutingAssembly() }, // auto discover interceptors and register into DI (see next section)
   addConfiguratorsFromAssembly: new[] { Assembly.GetExecutingAssembly() } // auto discover modular configuration and register into DI (see next section)

// Option 2 (optional)

Configuration with Autofac or Unity

See the [Dependency Resolver](docs/intro.md#dependency-resolver) for more information.

Use Case: Domain Events (in-process pub/sub messaging)

This example shows how SlimMessageBus and SlimMessageBus.Host.Memory can be used to implement the Domain Events pattern. The provider passes messages in the same app domain process (no external message broker is required).

The domain event is a simple POCO:

// domain event
public record OrderSubmittedEvent(Order Order, DateTime Timestamp);

The event handler implements the IConsumer<T> interface:

// domain event handler
public class OrderSubmittedHandler : IConsumer<OrderSubmittedEvent>
   public Task OnHandle(OrderSubmittedEvent e)
      Console.WriteLine("Customer {0} {1} just placed an order for:", e.Order.Customer.Firstname, e.Order.Customer.Lastname);
      foreach (var orderLine in e.Order.Lines)
         Console.WriteLine("- {0}x {1}", orderLine.Quantity, orderLine.ProductId);

      Console.WriteLine("Generating a shipping order...");
      return Task.Delay(1000);

The domain event handler (consumer) is obtained from the dependency resolver at the time of event publication. It can be scoped (per web request, per unit of work) as configured in your favorite DI container.

Somewhere in your domain layer, the domain event gets raised:

// aggregate root
public class Order
   public Customer Customer { get; }
   public OrderState State { get; private set; }

   private IList<OrderLine> lines = new List<OrderLine>();
   public IEnumerable<OrderLine> Lines => lines.AsEnumerable();

   public Order(Customer customer)
      Customer = customer;
      State = OrderState.New;

   public OrderLine Add(string productId, int quantity) { }

   public Task Submit()
      State = OrderState.Submitted;

      return MessageBus.Current.Publish(new OrderSubmittedEvent(this)); // raise domain event

Some sample logic executed in your domain:

var john = new Customer("John", "Whick");

var order = new Order(john);
order.Add("id_machine_gun", 2);
order.Add("id_grenade", 4);

await order.Submit(); // events fired here

Notice the static MessageBus.Current property might actually be configured to resolve a scoped IMessageBus instance (web request-scoped or pick up message scope from an external bus).

The SlimMessageBus configuration for in-memory provider looks like this:

IServiceCollection services; // for MsDependencyInjection or AspNetCore

// Cofigure the message bus
services.AddSlimMessageBus(mbb => 
         .AutoDeclareFrom(Assembly.GetExecutingAssembly()); // Find types that implement IConsumer<T> and IRequestHandler<T, R> and declare producers and consumers for them
   addConsumersFromAssembly: new[] { Assembly.GetExecutingAssembly() } // Auto discover consumers and register inside DI container

In Startup.cs for the ASP.NET project, set up the MessageBus.Current helper (if you want to use it):

public void Configure(IApplicationBuilder app, IWebHostEnvironment env)
   // Set the MessageBus provider, so that IMessageBus are resolved from the current request scope

See the complete sample for ASP.NET Core where the handler and bus are web-request scoped.

Use Case: Request-response over Kafka topics

See sample.


  • Types of messaging patterns supported:
    • Publish-subscribe
    • Request-response
    • Queues
    • Hybrid of the above (e.g. Kafka with multiple topic consumers in one group)
  • Modern async/await syntax and TPL
  • Fluent configuration
  • Because SlimMessageBus is a facade, you have the ability to swap broker implementations
    • Using NuGet pull another broker provider
    • Reconfigure SlimMessageBus and retest your app
    • Try out the messaging middleware that works best for your app (Kafka vs. Redis) without having to rewrite your app.


  • The core of SlimMessageBus is "slim"
    • Simple, common and friendly API to work with messaging systems
    • No external dependencies.
    • The core interface can be used in domain model (e.g. Domain Events)
  • Plugin architecture:
    • DI integration (Microsoft.Extensions.DependencyInjection, Autofac, CommonServiceLocator, Unity)
    • Message serialization (JSON, XML)
    • Use your favorite messaging broker as a provider by simply pulling a NuGet package
  • No threads created (pure TPL)
  • Async/Await support
  • Fluent configuration
  • Logging is done via Microsoft.Extensions.Logging.Abstractions so that you can connect to your favorite logger provider.


Apache License 2.0


cd src
dotnet build
dotnet pack --output ../dist

NuGet packaged end up in dist folder


To run tests you need to update the respective appsettings.json to match your own cloud infrastructure or local infrastructure. SMB has some message brokers set up on Azure for integration tests (secrets not shared).

Run all tests:

dotnet test

Run all tests except integration tests that require local/cloud infrastructure:

dotnet test --filter Category!=Integration


Thanks to Gravity9 for providing an Azure subscription that allows to run our integration test infrastructure.

Thanks to the following service cloud providers for providing free instances for our integration tests:

If you want to help and sponsor, please write to me.

*Note that all licence references and agreements mentioned in the SlimMessageBus README section above are relevant to that project's source code only.