How Configuring a Dispatcher for an External Bus Works

In order to receive messages from Message Oriented Middleware (MoM) such as RabbitMQ or Kafka you have to configure a Dispatcher. The Dispatcher works with a Command Processor to deliver messages read from a queue or stream to your Request Handler. You write a Request Handler as you would for a request sent over an Internal Bus, and hook it up to Message Oriented Middleware via a Dispatcher.

For each message source (queue or stream) that you listen to, the Dispatcher lets you run one or more Performers. A Performer is a single-threaded message pump. As such, ordering is guaranteed on a Peformer. You can run multiple Peformers to utilize the Competing Consumers pattern, at the cost of ordering.

If you are using .NET Core Dependency Injection, we provide extension methods to HostBuilder to help you configure a Dispatcher. This information is then for background only, but may be useful when debugging. Just follow the steps outlined in BasicConfiguration.

If you are not using HostBuilder you will need to perform the following steps explicitly in your code.

Configuring the Dispatcher

We provide a Dispatch Builder that has a progressive interface to assist you in configuring a Dispatcher

You need to consider the following when configuring the Dispatcher

• Command Processor

• Message Mappers

• Channel Factory

• Connection List

Configuring the Command Processor is covered in How Configuring the Command Processor Works.

Message Mappers

You need to register your Message Mapper so that we can find it. The registry must implement IAmAMessageMapperRegistry. We recommend using Brighter's MessageMapperRegistry unless you have more specific requirements.

var messageMapperRegistry = new MessageMapperRegistry(messageMapperFactory)
{
    { typeof(GreetingCommand), typeof(GreetingCommandMessageMapper) }
};

Channel Factory

The Channel Factory is where we take a dependency on a specific Broker. We pass the Dispatcher an instances of InputChannelFactory which in turn has a dependency on implementation of IAmAChannelFactory. The channel factory is used to create channels that wrap the underlying Message-Oriented Middleware that you are using.

Creating a Builder

This code fragment shows putting the whole thing together

// create message mappers
var messageMapperRegistry = new MessageMapperRegistry(messageMapperFactory)
{
    { typeof(GreetingCommand), typeof(GreetingCommandMessageMapper) }
};

// create the gateway
var rmqMessageConsumerFactory = new RmqMessageConsumerFactory(logger);
_dispatcher = DispatchBuilder.With()
    .CommandProcessor(CommandProcessorBuilder.With()
        .Handlers(new HandlerConfiguration(subscriberRegistry, handlerFactory))
        .Policies(policyRegistry)
        .NoExternalBus()
        .RequestContextFactory(new InMemoryRequestContextFactory())
        .Build())
    .MessageMappers(messageMapperRegistry)
    .ChannelFactory(new InputChannelFactory(rmqMessageConsumerFactory))
    .Subscribers(subscriptions)
    .Build();

Running The Dispatcher

To ensure that messages reach the handlers from the queue you have to run a Dispatcher.

The Dispatcher reads messages of input channels. Internally it creates a message pump for each channel, and allocates a thread to run that message pump. The pump consumes messages from the channel, using the Message Mapper to translate them into a Message and from there a Command or Event. It then dispatches those to handlers (using the Brighter Command Processor).

To use the Dispatcher you need to host it in a consumer application. Usually a console application or Windows Service is appropriate.

We recommend using HostBuilder, but if not you will need to use something like Topshelf to host your consumers.

The following code shows an example of using the Dispatcher from Topshelf. The key methods are Dispatcher.Receive() to start the message pumps and Dispatcher.End() to shut them.

We do allow you to start and stop individual channels, but this is an advanced feature for operating the services.

internal class GreetingService : ServiceControl
{
    private Dispatcher _dispatcher;

    public GreetingService()
    {
       /* Configfuration Code Goes here*/
    }

    public bool Start(HostControl hostControl)
    {
        _dispatcher.Receive();
        return true;
    }

    public bool Stop(HostControl hostControl)
    {
        _dispatcher.End().Wait();
        _dispatcher = null;
        return false;
    }

    public void Shutdown(HostControl hostcontrol)
    {
        if (_dispatcher != null)
            _dispatcher.End();
        return;
    }
}