Exercises that are intended to check your knowledge of language or teach something about it are called Koans. For example you can find koans for JavaScript language here.

Often, I find that applying solution to some problem later turns out that it was a design pattern. So, I thought that it would be good to describe the context of a problem and leave it to figure out the solution as an exercise for you, dear reader. As the mentioned koans contain tests which initially fails, I will try to do the same and also create a suite of tests which should tell you if your solution is good.

Problem context

Let’s assume that we use domain event pattern for notifying about events in our system. This is also handy when we want to use Event Sourcing. We have a domain event definition that is simply marker interface and looks like this:

public interface IDomainEvent

We also have an interface for event handlers which are used to mark implementation that will handle these events:

public interface IEventHandler<T>
    void Handle(T domainEvent);

But, new requirement shows up and we need to introduce a MassTransit library because in the near future we want to use queue messaging system to reliably deliver domain events. So we change our IEventHandler<T> definition to implement IConsumer<T> interface from MassTransit.

public interface IEventHandler<T> : IConsumer<T> where T : class, IDomainEvent
    void Handle(T domainEvent);

Now, we can use our event handlers along with MassTransit.

You can find entire solution here. There is a simple console application that uses MassTransit to send messages over bus and shows that everything works. There are also unit tests which initially fails (tests are written using NUnit 3). The first failing test states that IEventHandler for UserCreated event should not be assignable from IConsumer<T> MassTransit interface. The second test states that DomainModel should not have any references to MassTransit library.

Why bother?

In spirit of the Ports and Adapters architecture we should try to make our domain context independent. Using interface that we do not own from external library make the domain polluted with infrastructural concerns. Also that domain is not reusable if we want to switch to another message library. New domain module, that deals with events must also take dependency to MassTransit. What we learned in blue book, is that we should isolate domain model from other code.


Every craftsmen who would like to become better must practice, as we can read in Manifesto for Software Craftsmanship. Therefore, I strongly encourage you to fork, and try to change the code to make all tests pass. As you know I designed it to apply design pattern to solve the problem. However, this is not only solution, that you may think of. But I designed it having in mind a particular design pattern, so that’s why I called it ‘design pattern koans’.

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05 December 2015