Udi Dahan   Udi Dahan – The Software Simplist
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MSDN Magazine Domain Model Article

Sunday, August 2nd, 2009.

MSDN magazine

My article on “employing the domain model pattern” has been published in the August edition of MSDN Magazine.

Here’s a short excerpt:

“In this article, we’ll go through the reasons to (and not to) employ the domain model pattern, the benefits it brings, as well as provide some practical tips on keeping the overall solution as simple as possible.”

Continue reading…

  
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26 Comments

  1. Victor Kornov Says:

    The MSDN article is an interesting read, as usual.
    I was put off a bit by the comment “…If you don’t need the behavior found on those domain model classes, why plough through them to get at their data?..” This is totally reasonable when you really do not need domain behavior/rules in your reports (queries). But what if you do need them? I would like the article to at least mention those cases 😉 Otherwise it looks like you intentionally didn’t mention that to prove main point of the article. I mean things like exceptions vs. returns codes did get a least a short comment.


  2. roxin Says:

    Hi,

    customer.Purchase(shoppingCart);

    Wouldn’t that line be better off on a domain service rather than the customer ? When purchasing you might check for some discounts that you can apply to the customer (with a list of discount levels coming from the db ) or some other stuff – would you still have the method on the customer ?

    I’m thinking that having “customer.Purchase(shoppingCart)” will make the entity quite a large class after a while .


  3. udidahan Says:

    Victor,

    Finer requirements around reports/queries usually deal with horizontal filtering – which rows is this specific user authorized to see.

    That behavior really doesn’t belong on the entities themselves. A better place to put it would be as a set of filter objects which acts upon the rows before they are shown to the user. This is a good candidate for a pluggable design allowing multiple bounded contexts to contribute their own filters.

    I hope you’ll forgive me not mentioning this important point.


  4. udidahan Says:

    Roxin,

    It really depends on a lot of other detail that I wasn’t able to fit in to the article. For example, the shopping cart itself could have already taken into account all the discounts before the customer actually performs the purchase. That logic could also be put in a different bounded context.

    That keeps any entity from getting too large.


  5. Joe Says:

    Hi Udi,

    I really enjoyed reading your article. I went back and read your post about domain events “Domain Events – Salvation Sunday, June 14th, 2009”. I am a bit confused on the relationship between domain events and service buses like NServiceBus. For example, the customer.DoSomething() method raises a domain event, could it not send a message with NServiceBus and do the same thing? How would you choose one implementation over the other?


  6. udidahan Says:

    Joe,

    Glad you liked it.

    While the domain object *could* have an instance of the bus injected into it, that would break its encapsulation – requiring it to know about the message schema as well. Better for it to just say what happened – logically, and let something else handle the rest.

    Does that answer your question?


  7. Bjorn Says:

    Hi Udi and thanks for another great article.

    In your section on concurrency you let the consuming code decide that the purchase method of the customer model should be a transaction. Is this just an example on how to deal with concurrency or do you mean that transactions should actually NOT be handled in domain objects?

    /Bjorn


  8. udidahan Says:

    Bjorn,

    I did mean “that transactions should actually NOT be handled in domain objects”.

    Hope that helps.


  9. Bjorn Says:

    What about when you have vanilla DAOs below your repos instead of some OR mapper. Then a repository operation on an aggregate would likely involve hand-coded SQL-operations in the DAOs on more than one table. Is it still the service layer that should start the transaction or should it be done by the repository that encapsulates the DAOs?


  10. AT Says:

    Hi Udi,
    I really liked your article. Big thanks for sharing knowledge.
    Couple of questions.
    1. Are domain events are for fire and forget operations only or they participate in business transactions i.e.
    2. What are resposiblities of service layer? If there is single api for domain process then there is little for service layer besides starting transaction. i.e. Adding item to shopping cart.
    Service layer coordinating steps.
    a. Check inventory
    b. Check product discounts
    c. Check special customer discounts (preferred custormer)
    d. call shoppingcart.additem(product)

    Service layer just calling single Api domain model.
    call shoppingcart.additem(product) which encapsulates above steps.

    First process is more customizable/flexible for different scenarios. Different discounts but in a way business rules are implemented there.
    Second hides everything away but loses flexiblity.


  11. udidahan Says:

    Bjorn,

    Since you’d want all those activities to succeed or fail together, you’d want them all running in the same transaction, meaning that the service layer would still need to manage things.


  12. udidahan Says:

    AT,

    My pleasure.
    To your questions:

    1. Domain events call synchronously back into registered code. If that code decides to fire and forget some messages elsewhere, that’s its choice. There could be two different handlers, one that does fire and forget, the other that calls back down into the same transaction.

    2. The service layer’s responsibility is to handle the message that arrives while delegating business logic to the domain model.

    The process that you describe is probably not what would necessarily happen – inventory can be handled by a separate transaction later. Discounts could also be handled by a separate bounded context, being consistent within a couple of seconds of the first.

    These processes are usually handled by events between bounded contexts – not the service layer. There’s plenty of flexibility in that approach.

    Hope that answers your questions.


  13. Erik Says:

    Udi,
    Good article. I have a couple of questions.

    1. Does a class own the events it Raises or Can multiple classes call the same events;

    2. With Bounded Contexts, do you always uses NService bus to communicate between bouded contexts, or do you ever do it in process in production or during a debug/testing phase. i.e would you ever see the case where a set of DomainEventHandlers became your context map between contexts (for a testing scenario) or say if the contexts were part of the same service?

    Thanks,

    Erik


  14. udidahan Says:

    Erik,

    Thanks.

    An entity/class doesn’t necessarily “own” the domain events it raises. Multiple entities can raise the same kind of event.

    Between bounded contexts, I do use a kind of message/service bus, yes. One of the things we’ve been targeting with nServiceBus is a lighter-weight debugging model, but there’s still some way to go.

    For a testing scenario, you could wire in your own handlers to do whatever you wanted, but I wouldn’t see that as a real context map.

    Hope that answers your questions.


  15. ryzam Says:

    Hi Udi

    What is the major different between DomainService and DomainEvent? After try your suggestion it seem that DomainEvent is similar to DomainService but instead of using normal call to object it will use event to call object and execute the method

    Thanks


  16. udidahan Says:

    Ryzam,

    A Domain Service actually does the work, a Domain Event calls back to those who do the work.

    Hope that helps.


  17. ryzam Says:

    Thanks Udi,
    I’m new to this approach, need times to really understand.I have another questions.

    1. Do you move all the business process responsiblity to handler? Example:

    Admin can change Customer LicenseStatus and notification email will send to that customer.

    public class Customer
    {
    public LicenseStatus CurrentLicenseStatus {private set; get;}

    public void ChangeLicenseStatus(SendEmailWhenStatusChanged msg)
    {
    //Do you change the status here and raise the event after that?
    //or
    //Handler will cater all the process?

    //Option 1
    this.LicenseStatus = LicenseStatus.Approved;
    //Do some other process the raise event

    DomainEvents.Raise(msg);

    //Option 2
    DomainEvents.Raise(msg);
    }
    }


  18. udidahan Says:

    Ryzam,

    In the example you give, I’d probably have Customer.ApproveLicense, which internally changes the LicenseStatus, raises an event – CustomerLicenseStatusChanged, which would be caught by a handler, which then sends the email.

    Does that answer your question?


  19. Jan Ove Olsen Says:

    Hi Udi

    Great article! This is an excellent way to keep both the service layer and the domain model pure. We’re using this clean up our service layer. We’re moving all the “do external stuff” to handlers on domain events instead of having it in the service layer. This leaves the service layer very thin, it basically just manages transactions and calls a domain object. Keep up the good work, it has really made a difference for the better in my current project.


  20. udidahan Says:

    Jan,

    Happy to hear that it’s working out well for you.


  21. Szymon Kulec Says:

    Greetings Udi,

    as always your article is a great source of knowledge. Although I’m stunned by the simplicity of your Query/Domain separation which seems to me to be the most readable CQS example I’ve ever seen, I have one doubt which is:

    presume that in UI, on a form, you have a dropdown, which should be filled with the types of the entity you are creating. Where these types should come from? Query or Domain? If there are from Query, how the mapping from a simple int-key to the Domain object should be done?


  22. udidahan Says:

    Szymon,

    It means a lot to me to hear that the time I invest in simplifying things pays off – so thanks very much for your kind words.

    The scenario you’re asking about sounds like a pure entity-manipulation story, one that doesn’t quite fit the purpose of CQS. Trying to shoe-horn it, it would be clear that the Query side would be responsible for filling in the dropdown, but creating an entity is rarely a command.

    See this blog post for more information:

    http://www.udidahan.com/2009/06/29/dont-create-aggregate-roots/

    Hope that helps.


  23. Mike Says:

    Hi Udi,

    Great article, I just have one question in the SubmitOrder example where the business logic is moved to the Purchase method and a separate handler would handle if the order exceeds the limit of unpaid orders how would this be communicated to the user? I was wondering if the UI would be subscribed to this event and could then display a message.

    Thanks,
    Mike


  24. udidahan Says:

    Mike,

    This would most likely be communicated via email.


  25. Ranjan Says:

    Hey Udi,

    Really it is interesting article. Could you please update the code sample, so that we can download and execute on my end?

    Thanks,
    Ranjan


  26. udidahan Says:

    Hey Ranjan,

    In all honesty, I’m probably not going to get around to it.

    Sorry,

    Udi


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“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.”

Consult with Udi

Guest Authored Books
Chapter: Introduction to SOA    Article: The Enterprise Service Bus and Your SOA

97 Things Every Software Architect Should Know



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