Commercially available products, like Microsoft’s BizTalk Server, are a key part of many organizations’ SOA-style applications and application integration solutions. Freely available, open source solutions have been gaining acceptance in the broader marketplace. The acceptance of open source solutions is based on not only the maturity of the solutions, but also based on the accessibility and maturity of the underlying user groups that use, develop, and promote open source solutions. nServiceBus is a freely available, open source distributed, scalable, communications framework that’s based on the publish-subscribe model.

nServiceBus works by acting as a communications layer between publishers – entities that send information, and subscribers – entities that receive and process information. The information that passes between a publisher and subscriber is often referred to as a message – a message is, in Microsoft-based technologies, a type or class written in C# (or any other .NET-programming language).

nServiceBus provides the programming model, configuration model,  and binding to the underlying infrastructure to reliably handle messages and map message types to subscribers. The programming model in version 1.9 is considerably easier to use and understand as compared to previous versions.

One of the samples that comes out-of-the-box, called PubSub, is designed to demonstrate how easy it is to get nServiceBus working. The sample also demonstrates how to create message types and associate a message type with an endpoint.  The following walks you through some of the sample’s major features – you don’t have to have nServiceBus or the samples installed to follow-along.

nSerivceBus does not limit the definition of messages in any way – you describe your own messages in whatever way makes sense in your situation. nServiceBus only requires that you derive your message type(s) from the marker interface nServiceBus.IMessage (a marker interface - also referred to as an abscract type - is an interface that does not define any members). nServiceBus can operate on a concrete type (an instance of a class), and can operate on abstract types (an instance of class derived from an abstract type). This approach makes it possible for nServiceBus to operate on specific messages,  or messages that are based on a certain type.

The PubSub sample includes two messages, IEvent and EventMessage. For simplicity, the code that follows demonstrates how to publish the EventMessage type:

IEvent eventMessage;
eventMessage = new EventMessage();
// bus type defined elsewhere . . . 
bus.Publish(eventMessage);
 

The code appears to be simple becasue the underlying infrastructure that nServiceBus provides handles the details of ensuring that the message has a subscriber and handles the details of sending the message to the subscriber.

Subscribers handle messages as follows:

// Handle messages of type IEvent
public class EventMessageHandler : 
    IMessageHandler<IEvent>
{
        public void Handle(IEvent message)
        { . . . }
}
// Handle EventMessage
public class EventMessageHandler : 
    IMessageHandler<EventMessage>
{
        public void Handle(EventMessage message)
        { . . . }
}
 

 var bus = NServiceBus.Configure.With()
    .SpringBuilder()
    .MsmqSubscriptionStorage()
    .XmlSerializer()
    .MsmqTransport()
        .IsTransactional(true)
        .PurgeOnStartup(false)
    .UnicastBus()
        .ImpersonateSender(false)
    .CreateBus()
    .Start();

• nServiceBus home page: http://nServiceBus.com
• nServiceBus discussion group: http://tech.groups.yahoo.com/group/nservicebus/