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Domain Events – Salvation

Sunday, June 14th, 2009.

I’ve been hearing from people that have had a great deal of success using the Domain Event pattern and the infrastructure I previously provided for it in Domain Events – Take 2. I’m happy to say that I’ve got an improvement that I think you’ll like. The main change is that now we’ll be taking an approach that is reminiscent to how events are published in NServiceBus.


Before diving right into the code, I wanted to take a minute to recall how we got here.

It started by looking for how to create fully encapsulated domain models.

The main assertion being that you do *not* need to inject anything into your domain entities.

Not services. Not repositories. Nothing.

Just pure domain model goodness.

Make Roles Explicit

I’m going to take the advice I so often give. A domain event is a role, and thus should be represented explicitly:

   1:  public interface IDomainEvent {}

If this reminds you of the IMessage marker interface in nServiceBus, you’re beginning to see where this is going…

How to define domain events

A domain event is just a simple POCO that represents an interesting occurence in the domain. For example:

   1:  public class CustomerBecamePreferred : IDomainEvent 
   2:  {
   3:      public Customer Customer { get; set; }
   4:  }

For those of you concerned about the number of events you may have, and therefore are thinking about bunching up these events by namespaces or things like that, slow down. The number of domain events and their cohesion is directly related to that of the domain model.

If you feel the need to split your domain events up, there’s a good chance that you should be looking at splitting your domain model too. This is the bottom-up way of identifying bounded contexts.

How to raise domain events

In your domain entities, when a significant state change happens you’ll want to raise your domain events like this:

   1:  public class Customer
   2:  {
   3:      public void DoSomething()
   4:      {
   5:          DomainEvents.Raise(new CustomerBecamePreferred() { Customer = this });
   6:      }
   7:  }

We’ll look at the DomainEvents class in just a second, but I’m guessing that some of you are wondering “how did that entity get a reference to that?” The answer is that DomainEvents is a static class. “OMG, static?! But doesn’t that hurt testability?!” No, it doesn’t. Here, look:

Unit testing with domain events

One of the things we’d like to check when unit testing our domain entities is that the appropriate events are raised along with the corresponding state changes. Here’s an example:

   1:  public void DoSomethingShouldMakeCustomerPreferred()
   2:  {
   3:      var c = new Customer();
   4:      Customer preferred = null;
   6:      DomainEvents.Register<CustomerBecamePreferred>(
   7:          p => preferred = p.Customer
   8:              );
  10:      c.DoSomething();
  11:      Assert(preferred == c && c.IsPreferred);
  12:  }

As you can see, the static DomainEvents class is used in unit tests as well. Also notice that you don’t need to mock anything – pure testable bliss.

Who handles domain events

First of all, consider that when some service layer object calls the DoSomething method of the Customer class, it doesn’t necessarily know which, if any, domain events will be raised. All it wants to do is its regular schtick:

   1:  public void Handle(DoSomethingMessage msg)
   2:  {
   3:      using (ISession session = SessionFactory.OpenSession())
   4:      using (ITransaction tx = session.BeginTransaction())
   5:      {
   6:          var c = session.Get<Customer>(msg.CustomerId);
   7:          c.DoSomething();
   9:          tx.Commit();
  10:      }
  11:  }

The above code complies with the Single Responsibility Principle, so the business requirement which states that when a customer becomes preferred, they should be sent an email belongs somewhere else.

Notice that the key word in the requirement – “when”.

Any time you see that word in relation to your domain, consider modeling it as a domain event.

So, here’s the handling code:

   1:  public class CustomerBecamePreferredHandler : Handles<CustomerBecamePreferred>
   2:  { 
   3:     public void Handle(CustomerBecamePreferred args)
   4:     {
   5:        // send email to args.Customer
   6:     }
   7:  } 

This code will run no matter which service layer object we came in through.

Here’s the interface it implements:

   1:  public interface Handles<T> where T : IDomainEvent
   2:  {
   3:      void Handle(T args); 
   4:  } 

Fairly simple.

Please be aware that the above code will be run on the same thread within the same transaction as the regular domain work so you should avoid performing any blocking activities, like using SMTP or web services. Instead, prefer using one-way messaging to communicate to something else which does those blocking activities.

Also, you can have multiple classes handling the same domain event. If you need to send email *and* call the CRM system *and* do something else, etc, you don’t need to change any code – just write a new handler. This keeps your system quite a bit more stable than if you had to mess with the original handler or, heaven forbid, service layer code.

Where domain event handlers go

These handler classes do not belong in the domain model.

Nor do they belong in the service layer.

Well, that’s not entirely accurate – you see, there’s no *the* service layer. There is the part that accepts messages from clients and calls methods on the domain model. And there is another, independent part that handles events from the domain. Both of these will probably make use of a message bus, but that implementation detail shouldn’t deter you from keeping each in their own package.

The infrastructure

I know you’ve been patient, reading through all my architectural blah-blah, so here it is:

   1:  public static class DomainEvents
   2:  { 
   3:      [ThreadStatic] //so that each thread has its own callbacks
   4:      private static List<Delegate> actions;
   6:      public static IContainer Container { get; set; } //as before
   8:      //Registers a callback for the given domain event
   9:      public static void Register<T>(Action<T> callback) where T : IDomainEvent
  10:      {
  11:         if (actions == null)
  12:            actions = new List<Delegate>();
  14:         actions.Add(callback);
  15:     }
  17:     //Clears callbacks passed to Register on the current thread
  18:     public static void ClearCallbacks ()
  19:     {
  20:         actions = null;
  21:     }
  23:     //Raises the given domain event
  24:     public static void Raise<T>(T args) where T : IDomainEvent
  25:     {
  26:        if (Container != null)
  27:           foreach(var handler in Container.ResolveAll<Handles<T>>())
  28:              handler.Handle(args);
  30:        if (actions != null)
  31:            foreach (var action in actions)
  32:                if (action is Action<T>)
  33:                    ((Action<T>)action)(args);
  34:     }
  35:  } 

Notice that while this class *can* use a container, the container isn’t needed for unit tests which use the Register method.

When used server side, please make sure that you add a call to ClearCallbacks in your infrastructure’s end of message processing section. In nServiceBus this is done with a message module like the one below:

   1:  public class DomainEventsCleaner : IMessageModule
   2:  { 
   3:      public void HandleBeginMessage() { }
   5:      public void HandleEndMessage()
   6:      {
   7:          DomainEvents.ClearCallbacks();
   8:      }
   9:  }

The main reason for this cleanup is that someone just might want to use the Register API in their original service layer code rather than writing a separate domain event handler.


Like all good things in life, 3rd time’s the charm.

It took a couple of iterations, and the API did change quite a bit, but the overarching theme has remained the same – keep the domain model focused on domain concerns. While some might say that there’s only a slight technical difference between calling a service (IEmailService) and using an event to dispatch it elsewhere, I beg to differ.

These domain events are a part of the ubiquitous language and should be represented explicitly.

CustomerBecamePreferred is nothing at all like IEmailService.

In working with your domain experts or just going through a requirements document, pay less attention to the nouns and verbs that Object-Oriented Analysis & Design call attention to, and keep an eye out for the word “when”. It’s a critically important word that enables us to model important occurrences and state changes.

What do you think? Are you already using this approach? Have you already tried it and found it broken in some way? Do you have any suggestions on how to improve it?

Let me know – leave a comment below.

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  1. Bala Says:

    Domain event could alter multiple aggregates which is common, wouldn’t you be updating multiple aggregates in a single transaction?

  2. udidahan Says:


    The more common case is where those multiple aggregates are updated in separate transactions, usually as a result of some kind of “service bus” event being transmitted from the domain events. That service bus event gets routed to multiple subscribers, behind which you’d have each of the respective aggregates that would updated in their own transactions.

  3. Bala Says:

    If we put message to service bus using domain event and for some reason the transaction fails and rollback. How do we handle the message in the service bus?

  4. udidahan Says:


    If you’re using something like NServiceBus which can enlist into the transaction that caused the domain event, then the message will also be rolled back.

  5. Anestis Kivranoglou Says:

    In order to Avoid Registering and Unregistering Event Handlers on every Request, Since those Handlers are Static Functions and i want only to map Handlers on Domain Events.

    What are the Drawbacks if i use a Static Dictionary storing each registered event Handler per Event Type?
    private static Dictionary<Type, List> Handlers

  6. Rupesh Kumar Tiwari Says:

    HI Udi,
    You explained domain should use in memory domainevent publisher. Then how could we publish service bus events ? Are service bus events separate than domain events ? If I want to publish some service bus event on a specific domain event then how should it be ? Also some place you responded that in order to update different aggregate we should publish event via NServiceBus. Then how will we use domain event publishers ?
    According to your whole description what i found is in domain model aggregate root will publish domain event via nservice bus so that another aggregate root will act on that event eventually with durable manner. So that any time error happens it can roll back also via nservice enlisting. If this is the case then where we want to use in memory domainevent publishers?

  7. Rupesh Kumar Tiwari Says:

    Hi Udi,
    If we do not want to update multiple aggregates within one transaction you said we should use nservice bus and use real bus rather than in memory domaineventpublishers. Then is it fine to use other publishers in domain than static domaineventpublishers ?

  8. Rupesh Kumar Tiwari Says:

    HI Udi,
    I found this https://blogs.msdn.microsoft.com/cesardelatorre/2017/02/07/domain-events-vs-integration-events-in-domain-driven-design-and-microservices-architectures/ article where talks about Domain Events and Integration Events.
    I think that answers my question. What he says within a bounded context we can keep one logical transaction using in memory domain events. However once one transaction is committed within bounded context then we send one way messaging using Service Bus that message is called integration message. This is the message on which other bounded context are interested.

  9. udidahan Says:

    Hi Anestis,

    Be careful of using static – it is particularly problematic in async/await.

  10. udidahan Says:

    Hi Rupesh,

    These days, instead of recommending domain events I prefer to model things as sagas with NServiceBus. I find that it simplifies things greatly – effectively turning the saga into an instance of the domain model pattern, where the events are regular NServiceBus events.

  11. Rupesh Kumar Tiwari Says:

    Thank Udi,
    Initially, I was under assumption that DDD domain models and domain layer should not have any third party dependencies. Therefore, I was hesitant to introduce any third party messaging system inside domain layer. We are still using DomainEventPublisher which does in memory messaging across aggregates.
    However, if I understood correctly, you are saying we can use NService Bus messaging system for doing event publish/subscribe across aggregates within the domain layer also to get Retry and Durable Messaging benefit. It make sense.

    I watched https://www.youtube.com/watch?v=fWU8ZK0Dmxs&t=599s
    There I found that:
    1) When the problem domain is a collaborative domain then we should use CQRS style.
    2) In CQRS Saga is almost required and covers many hidden business requirements.

    Why we need SAGA in CQRS ?
    My understanding :-
    And the answer that I found is in CQRS if a domain model has if conditions then it is hiding lots of the business requirements.
    Therefore, we should create Saga as domain model. And get rid of all if conditions from domain model for that ask business and create explicit actions to update the saga as a long running process to get rid of if conditions in domain model.

    However, if we are not in a collaborative domain then our domain model may not be a saga it could be a regular aggregate with all of its assertions within its methods right ?

  12. udidahan Says:

    Hi Rupesh,

    One of the messages from the presentation you linked to is that if you have a domain model that contains if statements about the state of your entities, then it is, in fact, a collaborative domain – even if it didn’t look that way from the beginning.

  13. DamienD Says:

    Thanks for this enlighting serie !

    As a former military signal corps member in Swiss Army in late 80’s, i’ve learned that we can provide reliable communications services when based on unreliable channel transports only if we strictly implement message based protocols. Failover is in base design of such type of communications.

    A great lesson when thinking about today’s requirements in distributed applications on unreliable conditions.

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always with a smile. Udi is indeed a top-league professional!”

Ben Scheirman Ben Scheirman, Lead Developer at CenterPoint Energy
“Udi is one of those rare people who not only deeply understands SOA and domain driven design, but also eloquently conveys that in an easy to grasp way. He is patient, polite, and easy to talk to. I'm extremely glad I came to his workshop on SOA.”

Scott C. Reynolds Scott C. Reynolds, Director of Software Engineering at CBLPath
“Udi is consistently advancing the state of thought in software architecture, service orientation, and domain modeling.
His mastery of the technologies and techniques is second to none, but he pairs that with a singular ability to listen and communicate effectively with all parties, technical and non, to help people arrive at context-appropriate solutions. Every time I have worked with Udi, or attended a talk of his, or just had a conversation with him I have come away from it enriched with new understanding about the ideas discussed.”

Evgeny-Hen Osipow, Head of R&D at PCLine
“Udi has helped PCLine on projects by implementing architectural blueprints demonstrating the value of simple design and code.”

Rhys Campbell Rhys Campbell, Owner at Artemis West
“For many years I have been following the works of Udi. His explanation of often complex design and architectural concepts are so cleanly broken down that even the most junior of architects can begin to understand these concepts. These concepts however tend to typify the "real world" problems we face daily so even the most experienced software expert will find himself in an "Aha!" moment when following Udi teachings.
It was a pleasure to finally meet Udi in Seattle Alt.Net OpenSpaces 2008, where I was pleasantly surprised at how down-to-earth and approachable he was. His depth and breadth of software knowledge also became apparent when discussion with his peers quickly dove deep in to the problems we current face. If given the opportunity to work with or recommend Udi I would quickly take that chance. When I think .Net Architecture, I think Udi.”

Sverre Hundeide Sverre Hundeide, Senior Consultant at Objectware
“Udi had been hired to present the third LEAP master class in Oslo. He is an well known international expert on enterprise software architecture and design, and is the author of the open source messaging framework nServiceBus. The entire class was based on discussion and interaction with the audience, and the only Power Point slide used was the one showing the agenda.
He started out with sketching a naive traditional n-tier application (big ball of mud), and based on suggestions from the audience we explored different solutions which might improve the solution. Whatever suggestions we threw at him, he always had a thoroughly considered answer describing pros and cons with the suggested solution. He obviously has a lot of experience with real world enterprise SOA applications.”

Raphaël Wouters Raphaël Wouters, Owner/Managing Partner at Medinternals
“I attended Udi's excellent course 'Advanced Distributed System Design with SOA and DDD' at Skillsmatter. Few people can truly claim such a high skill and expertise level, present it using a pragmatic, concrete no-nonsense approach and still stay reachable.”

Nimrod Peleg Nimrod Peleg, Lab Engineer at Technion IIT
“One of the best programmers and software engineer I've ever met, creative, knows how to design and implemet, very collaborative and finally - the applications he designed implemeted work for many years without any problems!

Jose Manuel Beas
“When I attended Udi's SOA Workshop, then it suddenly changed my view of what Service Oriented Architectures were all about. Udi explained complex concepts very clearly and created a very productive discussion environment where all the attendees could learn a lot. I strongly recommend hiring Udi.”

Daniel Jin Daniel Jin, Senior Lead Developer at PJM Interconnection
“Udi is one of the top SOA guru in the .NET space. He is always eager to help others by sharing his knowledge and experiences. His blog articles often offer deep insights and is a invaluable resource. I highly recommend him.”

Pasi Taive Pasi Taive, Chief Architect at Tieto
“I attended both of Udi's "UI Composition Key to SOA Success" and "DDD in Enterprise Apps" sessions and they were exceptionally good. I will definitely participate in his sessions again. Udi is a great presenter and has the ability to explain complex issues in a manner that everyone understands.”

Eran Sagi, Software Architect at HP
“So far, I heard about Service Oriented architecture all over. Everyone mentions it – the big buzz word. But, when I actually asked someone for what does it really mean, no one managed to give me a complete satisfied answer. Finally in his excellent course “Advanced Distributed Systems”, I got the answers I was looking for. Udi went over the different motivations (principles) of Services Oriented, explained them well one by one, and showed how each one could be technically addressed using NService bus. In his course, Udi also explain the way of thinking when coming to design a Service Oriented system. What are the questions you need to ask yourself in order to shape your system, place the logic in the right places for best Service Oriented system.

I would recommend this course for any architect or developer who deals with distributed system, but not only. In my work we do not have a real distributed system, but one PC which host both the UI application and the different services inside, all communicating via WCF. I found that many of the architecture principles and motivations of SOA apply for our system as well. Enough that you have SW partitioned into components and most of the principles becomes relevant to you as well. Bottom line – an excellent course recommended to any SW Architect, or any developer dealing with distributed system.”

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