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From CRUD to Domain-Driven Fluency

Friday, February 15th, 2008.

I got a question about how to stay away from CRUD based service interfaces when the logic itself is like that, and I’ve found that this shift in thinking really needs more examples, so I’ve decided to put this out there:

For instance, in an HR system, the process of interviewing candidates – wouldn’t you just insert, update, and delete these Appointment objects?

If I were to put on my domain-driven hat, I would describe those requirements differently – interview appointments have a lifecycle: proposed, accepted, cancelled, etc. It seems that only a user of the role HR Interviewer should be able to make appointments for themselves, so the service layer code would probably look something like this:

using (ISession session = SessionFactory.OpenSession())
using (ITransaction tx = session.BeginTransaction())
    ICandidateInterviewer interviewer = session.Get<ICandidateInterviewer>(message.InterviewerId);
    ICandidate candidate = session.Get<ICandidate>(message.CandidateId);


The “ScheduleInterviewWith” method accepts an ICandidate and returns an IAppointment. IAppointment has a method “At” which accepts a DateTime parameter and returns void – just changes the data of the appointment. The state of the appointment at creation time would probably be proposed. The appointment object would probably be added to the list of appointments for that interviewer – that’s what will cause it to be persisted automatically.

Later, when the candidate accepts the meeting, we could have the following method on ICandidate – void Accept(IAppointment); that would obviously check that the candidate is the right person for that interview, the appointment’s current state (not cancelled), etc – finally updating its state. What part of this looks like create, update, delete? If that’s what your service layer to domain interaction looks like, do you now know what your messages will be looking like?CRUD seems to be what most of us are familiar with. Moving to domain-driven thinking takes time and practice, but is well worth it. Contrast this with a more traditional O/R mapping solution:

using (ISession session = SessionFactory.OpenSession())
using (ITransaction tx = session.BeginTransaction())
    ICandidateInterviewer interviewer = session.Get<ICandidateInterviewer>(message.InterviewerId);
    ICandidate candidate = session.Get<ICandidate>(message.CandidateId); 

    Appointment a = new Appointment(); 

    a.Interviewer = interviewer; 

    a.Candidate = candidate;

    a.Time = message.RequestedTime; 



As you can see, we’ve got simpler, more expressive, and more testable code when employing the domain model pattern, than using “just” O/R mapping. I’m not saying that the domain model pattern doesn’t need O/R mapping in the background for it to work. But that’s just it – the persistence gunk needs to be in the background and the business logic needs to be encapsulated.

So, while I’ll agree with Dave that the Domain Model is more lifestyle than pattern, I would argue against these conclusions:

If this post had a point, it’s only to share the idea that Domain Model is a big, big thing. It’s probably overkill in a lot of cases where you have simple applications that have very simple purposes.

As you just saw in the example above, there is no “overkill” to be seen. The domain model in the example wasn’t “a big, big thing”.

The domain model. Use it.

Why not have a better lifestyle?   ;-)

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  1. Amit Says:

    ICandidateInterviewer interviewer = session.Get(message.InterviewerId);
    ICandidate candidate = session.Get(message.CandidateId);


    Takes a candidate assigns him to interviewer with a time slot.

    Now a level deep, interviewer.ScheduleInterviewWith is certainly going to perform CRUD (CREATE READ UPDATE And Delete over some memory space).
    Isn’t it like CRUD with wrappers.

  2. udidahan Says:


    Of course the interviewer object will be creating an appointment object and adding it to its list. The thing is that that’s encapsulated. If it were to add other objects as well, you wouldn’t see that either.

    You can see that by working entirely in memory that way in the domain model we can more easily unit test those business rules.

    The service layer doesn’t have to explicitly INSERT those objects in the DB. CRUD usually refers to the act of actually working against the DB interfaces – creating an object in memory and connecting it to (and with) other objects is rarely viewed as CRUD.

    Does that make sense?

  3. Nick Says:

    Excellently well-put. It is hard to explain the difference between creating data-oriented domain objects and giving them real responsibilities.

    How do you handle the limitations that you hit when your domain objects need services in order to fulfil their responsibilities? Do you inject dependencies, pass dependencies through with method calls or keep your domain model self-contained and access external services from the calling code?

  4. Mark Says:

    Quick question:

    In the “traditional O/R mapping solution”, we have:

    using (ISession session = SessionFactory.OpenSession())
    using (ITransaction tx = session.BeginTransaction())



    In the “domain-driven” solution, I assume session.Save is called inside:


    So, how do you have access to the session object here, as its not explicitly passed? Is session effectively an alias to a singleton then (does OpenSession return an existing open session if there is one)? Is this typical when using ORM?

  5. udidahan Says:


    I’ve been seeing that question pop up multiple times on the DDD threads and have just stopped answering it there :)

    The Domain Model is a self-contained component – it doesn’t take dependencies on anything else. What it does is raise events that external code can use to make decisions about which external “service” to call.

    Does that make sense?

  6. udidahan Says:


    Session.Save is not called in “interviewer.ScheduleInterviewWith(candidate).At(message.RequestedTime);”

    The domain objects have no persistence code in them whatsoever.

    The O/R Mapper needs to be configured in such a way so that persistence occurs by virtue of the fact that there is an unsaved appointment object in the interviewer’s collection of appointments.

    Does that make sense?

  7. Nick Says:

    Hi Udi,

    Thanks for the answer – I might go and track down some of the earlier posts – always an interesting discussion :)


  8. Alberto Brandolini Says:

    Definitely well explained.

    I think CRUDs spoil the application, putting business logic in the user behaviour, instead that in the application. But the process of bringing the business logic back into the app is not always straightforward: you need to have smart people to interact with, different skills and motivation and probably some extra time.
    Sometimes this is worth the effort, but some other times a quick collection of CRUDs, maybe implemented fast with Ruby on Rails or Grails is all you need, especially for small applications.


  9. udidahan Says:


    It looks like there will be a lot more systems to rewrite in the future with the growing popularity of RoR, doesn’t it?


  10. Colin Jack Says:

    Good blog entry but I think you’ve been a little unfair with the “before” style code:

    Appointment appointment = new Appointment(message.RequestedTime, a.Candidate);

    Also whilst I like the example you provide I’d also be happy with the service layer calling methods on the Appointment directly such as appointment.Accept() rather than calling ICandidate.Accept(appointment).

    I also think theres really two points you are making here. One is that you think its good to take advantage of the features of the ORM so that when you save an interviewer its appointments are saved, the other is the fluent interface example.

    I’m also interested, do you use fluent interfaces a lot in the domain? So far I’ve only found them useful for simple value objects and entities, particularly creation.

  11. udidahan Says:


    Glad you liked it.

    Yes, I could have done that kind of constructor for Appointment. However, in terms of DDD, I don’t see there being an appointment that could exist without an ICandidate. I also worry about “generic” Accept methods as they may guide people to thinking about the visitor pattern when that’s not what is occuring.

    I’m not sure I’m so much “taking advantage of the features of the ORM” so much as designing a persistent ignorant domain model.

    On the issue of fluent interfaces – it just looks like I did something special. Actually,


    returns an IAppointmentThatHasNotBeenScheduled which has a method:

    At(DateTime time);

    So, while I don’t explicitly go looking for how to use fluent interfaces in the domain model, the fact that I use intention revealing interfaces often leads to something that looks fluent. But usually, there is none of the chaining behavior found so often when doing the fluent thing.

    Hope that makes sense.

  12. DDD & Many to Many Object Relational Mapping Says:

    […] From CRUD to Domain-Driven Fluency […]

  13. ryzam Says:

    Do you expose ISession directly in services?..

  14. udidahan Says:


    I’m not sure what you mean by exposing ISession, but service layer code does make use of ISession directly, yes.

  15. ryzam Says:

    Sorry, my previous question was wrong, actually i would like to ask do you use ISession directly from your services?.. You already answer this thanks.

    But if you are using directly, it seem that your services is taking repository function and how do you test it? What is the advantage to have services direct query/get data from dataaccess framework (for example NHibernate)


  16. udidahan Says:


    The service layer is inherently integration code and, as such, most suitable for integration testing (rather than unit testing).

    The advantage of services talking directly to ISession is simplicity, and the ability to use the power of the framework chosen (NHibernate).

  17. Andrew Says:

    Hi Udi,

    good post, always like reading your work but have a question, The original example (makes use of the session) looks more like a Unit of work service method than a traditional DDD method, is there anything wrong with the code you posted being placed in the service layer which makes use of the domain model? the reason I ask is I am struggling when thinking in domain methods as most of the domain methods i think of really just model crud behaviour. For example, I am working on some software right now where you basically have an estate which can have a number of sites and all I seem to be modelling is Estate.AddSite, Estate.RemoveSite etc. And the only thing I seem to be putting in my domain object is validation e.g. the Site description cannot be blank…

    Would you class these as domain methods or just service layer methods?
    I think you’re right in terms of this takes a whole different mind set !

  18. udidahan Says:


    I think that the issue is that the UI you’re supporting is exposing that kind of functionality to the user. DDD applies to UI design as well.

    The code in the post is indeed the implementation of service layer methods.

  19. Andrew Says:

    Hi udidahan,

    Sorry to bother you again, i have read Eric Evan’s book (maybe i should read it again) but I seem to struggle with what would appear in the domain side of things, everything seems to be a service/crud based method of the system. The examples given always seem to fit nicely with business logic (I think I may be interpreting my own system incorrectly) Would you mind suggesting a couple of typical domain methods for the following :

    If i have an estate and this contains regions in the country and these contain houses, the user can add new regions to the estate and houses to regions and view the houses details.

    Does this sound like something that warrents DDD or am I trying to fit something in that just doesnt fit, I cant believe this, as nearly all web based systems are just reflections over data at their core and merely provide facilities for the users to perform basic crud methods on a system. Would you model the crud methods e.g. AddRegion as a domain method or just at the service level?

    Sorry to be a pain, but this is one of the biggest hurdles I always have with DDD, if you can suggest any reading or articles, I would be extremely grateful (where did I put Erics book…)

  20. udidahan Says:


    That kind of behavior can be built using the Active Record pattern instead of the Domain Model pattern. As you said, it is simple data manipulation and, as such, doesn’t require it. That said, DDD is broader than the Domain Model pattern and there are many other useful elements to it that can still be applicable.

  21. Riana Says:

    Hi Udi,

    This is an old but still very interesting article ! :-)

    I have just a little question though.

    How do you implement real CRUD based screens as we used to have in many applications ?

    For example, I have an application with which it should be possible to create then register a new Client. Registering a client means here, that I have to enter its infos, address etc .. then save it.

    How do you handle those kind of scenario where your application needs to have a Domain Model (cause it has some complex business logic) but however, certain part of it are just simple CRUD like entering client infos for example. How do to marry both ?

    Thanks Udi, keep the good job on !


  22. udidahan Says:


    The trick is to separate the basic CRUD stuff (which can be implemented simply using, for example, datasets) and the stuff requiring the more complex logic. You can look at composition of DDD bounded contexts for more information.

  23. Setya Says:

    Hi Udi,

    Thank you for your nice blog. I’d like to ask what would your code look like if there’s no ORM involved (NHibernate in your case) ?


  24. udidahan Says:


    It would probably end up having a bunch of persistence code intermingled with the domain logic – not the most desirable result.

  25. Setya Says:

    Hi Udi,

    In your above example, is it just your intention for clarity sake to expose ISession and ITransaction, or can we just wrap them in a repository class having load(entity) & save(entity) to be invoked from service layer instead ?


  26. Setya Says:

    Hi Udi,

    I also want to ask regarding your response to Nick:

    ‘The Domain Model is a self-contained component – it doesn’t take dependencies on anything else. What it does is raise events that external code can use to make decisions about which external “service” to call’

    What if the domain model needs to enforce business validations which depends on external services from legacy system or outside bounded context, I can’t use events for this since event is for something already happened, while validation is preventing it from happening.

    My naive design would be to wrap those external services into a domain service than injecting it as method parameter as I can not find any other way.


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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.”

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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