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Don’t Create Aggregate Roots

Monday, June 29th, 2009.


My previous post on Domain Events left some questions about how aggregate roots should be created unanswered. It would actually be more accurate to say how aggregate roots should *not* be created. It turns out that this is one of the less intuitive parts of domain-driven design and has been the source of many arguments on the matter. Let’s start with the wrong way:

   1:  using (ISession s = sf.OpenSession())
   2:  using (ITransaction tx = s.BeginTransaction())
   3:  {
   4:      Customer c = new Customer();
   5:      c.Name = "udi dahan";
   7:      s.Save(c);
   8:      tx.Commit();
   9:  }

I understand that the code above is representative of how much code is written when using an object-relational mapper. Many would consider this code to follow DDD principles – that Customer is an aggregate root. Unfortunately – that is not the case. The code above is missing the real aggregate root.

There’s also the inevitable question of validation – if the customer object isn’t willing to accept a name with a space in it, should we throw an exception? That would prevent an invalid entity from being saved, which is good. On the other hand, exceptions should be reserved for truly exceptional occurrences. But if we don’t use exceptions, using Domain Events instead, how do we prevent the invalid entity from being saved?

All of these issues are handled auto-magically once we have a true aggregate root.

Always Get An Entity

Let’s start with the technical guidance – always get an entity. At least one. Also, don’t add any objects to the session or unit of work explicitly – rather, have some other already persistent domain entity create the new entity and add it to a collection property.

Looking at the code above, we see that we’re not following the technical guidance.

But the question is, which entity could we possibly get from the database in this case? All we’re doing is adding a customer.

And that’s exactly where the technical guidance leads us to the business analysis that was missing in this scenario…

Business Analysis

Customers don’t just appear out of thin air.

Blindingly obvious – isn’t it.

So why would we technically model our system as if they did? My guess is that we never really thought about it – it wasn’t our job. So here’s the breaking news – if we want to successfully apply DDD we do need to think about it, it is our job.

Going back to the critical business question:

Where do customers come from?

In the real world, they stroll into the store. In our overused e-commerce example, they navigate to our website. New customers that haven’t used our site before don’t have any cookies or anything we can identify them with. They navigate around, browsing, maybe buying something in the end, maybe not.

Yet, the browsing process is interesting in its own right:

  • Which products did they look at?
  • Did they use the search feature?
  • How long did they spend on each page?
  • Did they scroll down to see the reviews?

If and when they do finally buy something, all that history is important and we’d like to maintain a connection to it.

Actually, even before they buy something, what they put in their cart is the interesting piece. The transition from cart to checkout is another interesting piece. Do they actually complete the checkout process, or do they abandon it midway through?

Add to that when we ask/force them to create a user/login in our system.

Are they actually a customer if they haven’t bought anything?

We’re beginning to get an inkling that almost every activity that results in the creation of an entity or storing of additional information can be traced to a transition from a previous business state.

In any transition, the previous state is the aggregate root.

In the beginning…

Let’s start at the very beginning then – someone came to our site. Either they navigated here from some other web page, they clicked on an email link someone sent them, or they typed in our URL. This can be designed as follows:

   1:  using (ISession s = sf.OpenSession())
   2:  using (ITransaction tx = s.BeginTransaction())
   3:  {
   4:     var referrer = s.Get<Referrer>(msg.URL);
   5:     referrer.BroughtVisitorWithIp(msg.IpAddress);
   7:     tx.Commit();
   8:  }

And our referrer code could look something like this:

   1:  public void BroughtVisitorWithIp(string ipAddress)
   2:  {
   3:     var visitor = new Visitor(ipAddress);
   4:     this.NewVisitors.Add(visitor);
   5:  }

This follows the technical guidance we saw at the beginning.

It also allows us to track which referrer is bringing us which visitors, through tracking those visitors as they become shoppers (by putting stuff in their cart), finally seeing which become customers.

We can solve the situation of not having a referrer by implementing the null object pattern which is well supported by all the standard object-relational mappers these days.

How it works internally

When we call a method on a persistent entity retrieved by the object-relational mapper, and the entity modifies its state like when it adds a new entity to one of its collection properties, when the transaction commits, here’s what happens:

The mapper sees that the persistent entity is dirty, specifically, that its collection property was modified, and notices that there is an object in there that isn’t persistent. At that point, the mapper knows to persist the new entity without us ever having to explicitly tell it to do so. This is sometimes known as “persistence by reachability”.

Where validation happens

Let’s consider the relatively trivial rule that says that a user name can’t contain a space.

Also, keep in mind that a registered user is the result of a transition from a visitor.

Here’s *one* way of doing that:

   1:  public class Visitor
   2:  {
   3:     public void Register(string username, string password)
   4:     {
   5:        if (username.Contains(" "))
   6:        {
   7:           DomainEvents.Raise<UsernameCantContainSpace>();
   8:           return;
   9:        }
  11:        var user = new User(username, password);
  12:        this.RegisteredUser = u;
  13:     }
  14:  }

This actually isn’t representative of most of the rules that will be found in the domain model, but it illustrates a way of preventing an entity from being created without our service layer needing to know anything. All the service layer does is get the visitor object and call the Register method.

Validation of string lengths, data ranges, etc is not domain logic and is best handled elsewhere (and a topic for a different post). The same goes for uniqueness.


The most important thing to keep in mind is that if your service layer is newing up some entity and saving it – that entity isn’t an aggregate root *in that use case*. As we saw above, in the original creation of the Visitor entity by the Referrer, the visitor class wasn’t the aggregate root. Yet, in the user registration use case, the Visitor entity was the aggregate root.

Aggregate roots aren’t a structural property of the domain model.

And in any case, don’t go saving entities in your service layer – let the domain model manage its own state. The domain model doesn’t need any references to repositories, services, units of work, or anything else to manage its state.

If you do all this, you’ll also be able to harness the technique of fetching strategies to get the best performance out of your domain model by representing your use cases as interfaces on the domain model like IRegisterUsers (implemented by Visitor) and IBringVisitors (implemented by Referrer).

And spending some time on business analysis doesn’t hurt either – unless customers really do fall out of the sky in your world 🙂

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  1. udidahan Says:


    The user object would have a reference to its cart object – check if its null, if so, create a new one, set the reference. The ORM would do the dirty tracking on the user object and know to persist the new cart / the modified old cart.

    Does that answer your question?

  2. Mike Scott Says:

    Hi Udi

    So if I’m following you correctly, the user would be the aggregate root and the user repository would built the aggregate, assigning the cart if one existed already or leaving it null if not.

    Then, in the user class’ domain logic, there would be code to create a new cart if it was null and this would be persisted by reachability?

    If so, I can see that the code to deal with the cart is in the repository if one exists, and in the domain object if not. The usual way of handling this would be to have the load/check and creation together in the same piece of code, though your suggestion seems reasonable 🙂

  3. udidahan Says:


    The repository in this case would need to be nothing more than a regular ORM that eagerly fetched the cart object (if there was one) along with the user object. You don’t need a custom repository for this.

    The same thing goes for “persistence by reachability”, since the user object gets dirty, the ORM knows to persist it and the objects connected to it.

    Hope that makes sense.

  4. Mike Scott Says:

    Sure, understand perfectly. But that leads to the next question 😉

    Unit testing! How do you unit test the code that depends on the ORM? Do you abstract it?

  5. udidahan Says:


    The only code that depends on the ORM is the service layer. The domain model doesn’t depend on anything other than itself – and it’s the thing you’ll be unit testing.

    Does that make sense?

  6. Mike Scott Says:

    Hi Udi

    It makes sense to test the domain model, of course, but it also makes sense to test the service layer to make sure it is orchestrating things correctly, no?

  7. udidahan Says:


    The service layer shouldn’t orchestrate – after getting a domain object or two, it calls a single method on one of them, that’s it.

    See my High Performance Domain Models presentation.

  8. Jørn Wildt Says:

    Udi, is it this your are talking about: http://www.udidahan.com/2007/10/26/teched-speaking-about-high-performance-persistent-domain-models/ when you say High Performance Domain Models presentation?

    The links on that page are not alive anymore 🙁

    Can it be found elsewhere?

  9. Mike Scott Says:


    I understand that the service layer only makes a single call into the domain layer, but I’m still not convinced that it’s not doing enough orchestrating that it doesn’t need unit testing. It has to work with the persistence layer as well as calling into the domain.

    However, I do see your pont that if you have NHibernate code creating a session and a transaction in the session layer, that you could just forego abstracting your persistence, ignore unit testing and rely on integration tests.

    Is this what you’re advocating?

  10. udidahan Says:


    Check out this link as well:


  11. udidahan Says:


    You bring up the point exactly:

    “It has to work with the persistence layer as well as calling into the domain”

    Would a unit test that mocks out the persistence layer give us much more confidence in the correctness of the code? So much so that we didn’t need the integration test? And if we do go and put an integration test in place, what incremental value does the unit test provide? Does it, in essence, assert that the code is implemented the way it’s implemented?

    Hope that makes sense.

  12. Mike Scott Says:

    Hi Udi,

    “Would a unit test that mocks out the persistence layer give us much more confidence in the correctness of the code?”

    Yes, I believe it would. I think the service code should be unit-tested as well, even though it’s simple. Any breaking changes will be detected in seconds, rather than waiting for them to fail in the integration tests and be more difficult to track down later.

    If you take your argument to its logical conclusion, we could do away with unit tests altogether and rely solely on automated integration testing, could we not?

    There’s also the issue of using unit testing – or better to say specifying – first in order to create more expressive interfaces.

  13. udidahan Says:

    Well Mike,

    Then we can agree to disagree 🙂

  14. Richard Dingwall » Life inside an Aggregate Root, part 2 Says:

    […] The new keyword is a bit of a smell here — as Udi Dahan recently stated in a blog post, customers don’t just appear out of thin air. Let’s flip it […]

  15. Jon Says:

    Hi Udi,

    I really like the idea of persistence by reachability. However, I’m trying to implement this using NHibernate and I’ve run into a problem with many to many relationships. How do you avoid loading up the entire collection when adding to it?


  16. Strengthening your domain: Aggregate Construction - Jimmy Bogard - Los Techies : Blogs about software and anything tech! Says:

    […] Through existing aggregate roots […]

  17. ryzam Says:

    Hi Udi, your case is for newly customer, but how about existing customer. For me an existing customer can be right candidate to act as an AggregateRoot.

    From general perspective, I see role archetype (peter coad) by default can be an AggregateRoot

  18. udidahan Says:


    Well, an existing customer may be an Aggregate Root for certain use cases in a given bounded context.

  19. Jørn Wildt Says:

    Hi Udi

    There’s a thing that bothers me here. Others have mentioned it already: the size of the collection. Not only the initial collection – but the complete object graph that results from this line of thinking.

    Let’s assume we have a Website with blogs with posts with comments. Following the above principle, we add blogs through websites, posts through blogs, and comments through posts.

    This means the root object, the Website, ends up containing a massive object graph where all possible content items on the website are reachable from the Website root. Is this really the intention?

    It also makes it difficult to implement a content plugin structure since the Website has to know all possible content types at compile time in order to create the AddBlog, AddArticle, AddPhoto, AddUser etc. methods. This is probably not your intention, so what am I missing?


  20. udidahan Says:


    First of all, aggregate roots (AR) are used on the command side of CQRS and not the query side, in which case theoretical reachability on the command side is less relevant.

    Second, an AR is only relevant for scenarios where we’re using a domain model – if the action is a simple insert operation, we can do that without using the domain model. Not all commands need to involve the same domain model.

    I’m pretty sure that that leaves you with more questions – but the answers to those questions ultimately are project-specific. These patterns are to help you know which questions to ask – not to provide you with answers 🙂

  21. Lionel Orellana Says:

    Hi Udi,

    I’ve gone through previous comments and couldn’t see the point of bi-directional associations being raised directly. Your approach will create a lot more bi-directional associations than are otherwise needed. I don’t think I’m taking Evans too literally when I say bi-directional associations are best avoided. From the DDD bible:

    “In real life, there are lots of many-to-many associations, and a great number are naturally bidirectional. The same tends to be true of early forms of a model as we brainstorm and explore the domain. But these general associations complicate implementation and maintenance. Furthermore, they communicate very little about the nature of the



  22. udidahan Says:


    I agree that we don’t want many (any) bi-directional associations when using the domain model pattern. Don’t take the guidance given in this article as being complete – it is here to illustrate a very specific part of a more comprehensive set of techniques.

    Also, see my more recent posts talking about how the domain model pattern shouldn’t necessarily be used for all “business logic” and definitely not for the purposes of all persistence.

    Kind regards.

  23. Benjamin Says:

    Hi Udi,

    I hardly try to change my way of thinking with all these DDD/CQRS approches and every time I read something about ARs and Consistency Boundaries, I need to reconsider everything…hard process 😉

    And this post is not an exception. Take the dummy Blog/Post/Category/Tags example, where, well, domain model isn’t rich but is enough for creating mind troubles :p

    The old fashion way would be to create a Post object and set a Category reference instance on it (the user would select the category from a pre-loaded drop down list on the UI).

    But reading your post, I know tend to think that in creation context (meaning the first time a Post is created), maybe the Category would the AR, and the Post would be added to this Category. Or am I overlooking at it ?

    The thing is, even in this dummy example, I can’t make a design choice, everything seems right and wrong at the same time.

    And there are also other questions like “When should the Tags be created ? Persisted ? Do we need the Post::compose() method acting as a factory method and thus creating Tag instances, the whole thing being persisted by cascading through the Post object ?”

    Or “The user being able to associate tags to his new post either by selecting existing ones or by entering new ones. In the command handler, do we need to pre-load every existing tags and pass them to the Post::compose() method ?”

    Or even “If in the Post AR, Tags are referenced using VOs (tag identities) instead of hard association, who is responsible of creating the Tag instances during the compose command handling ?”

    So many questions…. :/

    Imagining all this in a very complex business domain seems decouraging !

  24. Hank Says:

    Missing from all this is the why. Why do it this way? What is gained vs. the “wrong way”?

  25. rh Says:


    in response to Andrew Davey’s question, it seems weird to me that admin should be considered as the aggregate root (so when you do admin.addItemToShop(..)) Does this mean that basically all the operations should be put under admin now? whatever you do in the shop, because the admin “executes” them? That doesnt seem right, or am I wrong? That would bloat admin quite a bit. (So admin has changeShopItemName, and so on? – or for that operation we can simply say now it’s the item what’s the aggregate root, but then is that case, since it is still the admin who can change it regardless, is that being checked in an application serivce and literally not being used after?)

  26. udidahan Says:


    The first question is whether the domain model patterns is appropriate for the complexity of that bounded context. If all that is needed is some simple persistence, you don’t need to worry about all this.

  27. Behrouz Says:

    I have a question perhaps not directly related to the topic. One of my biggest struggle in defining my domain objects is to know exactly what behavior belongs to what domain object an aggregate root in particular. For instance, with regard to your Visitor class example, in a real world, is it the visitor who register herself or someone else? Why did you define Register behavior in Visitor class?

  28. udidahan Says:


    I’m afraid the only answer I can give you is that each case is special and depends on the specifics of the domain. I would add to that, as a general rule of thumb, to not decide what your domain objects should be before you’ve explored in significant detail the behavior needed.

  29. rh Says:

    Hi Udi,

    Sorry to be a pain again, I seem to have found another gap between this article and your 2011 DDD exchange presentation, maybe it is just me and I am missing something, but if you get a minute could you answer this for me please?

    So in this article you say “always get an entity. At least one. Also, don’t add any objects to the session or unit of work explicitly – rather, have some other already persistent domain entity create the new entity and add it to a collection property”

    I really like this approach, however in this presentation:
    from minute 34 to 36, you say that “if all we need is some dirt simple persistance, shove data into DB and back, we don’t need to gold-plate that by involving a lot of other things”. So how does that work with the creation? I assume when you say get an entity, that entity is in your domain model, so then you do need the domain model in there even when the only thing you have is some “dirt simple persistance” or did I miss something?


  30. udidahan Says:


    You’ve got it – if all you need is some simple persistence, you don’t need a Domain Model. Sure, you could use an ORM and push some Data Model objects around, but don’t confuse that with a Domain Model.


  31. rh Says:

    Hi Udi,

    ok that part is clear, where I am a bit conflicted with this however is that I was trying to follow this recommendation:
    combined with what this article has about always taking an entity when creating something (and use a factory method like create on it). So you are saying that this entity is not neccessarily in the domain model (which model is it then)? Does that also mean that I am not encapsulating creation in my domain then?
    I still seem to be missing something.


  32. udidahan Says:


    Please understand that it isn’t always possible to follow generic guidance in all cases. One factor that influences things is whether it is possible to break things up effectively into SOA Services / DDD Bounded Contexts (aligned to Sub-Domains). There are others.

    The specifics of your project will dictate what makes sense.

    Sorry for not being able to be more help.

  33. rh Says:

    Hi Udi,

    No worries, you have been a lot of help anyway, just to close down this thread. Could you point out in your experience which of the following rules to prioritize over thoe others if not all of them can be followed?
    So basically what would be the order of these for you, because they seem to conflict a bit in some cases?:
    1./ Aggregates should only model true invariants
    2./ creation shoule be encapsulated in the domain
    3./ Each BC should represent a linguistic domain on its own (or phrased differently, I shouldn’t split a BC into 2 if those 2 would have the same language for convenience, or should I?)

    these 3 sometimes contradict a bit to me, because I want to do “1” but in order to be able to do that I would need to violate “3”, but if “3” would be allowed, then I could do “1” and “2” for sure.


  34. udidahan Says:


    With regards to #3, splitting BCs into 2 is rarely more convenient than having just one BC. In any case, for me I focus first on #3 as that strongly influences the responsibility of any aggregates (#1). I’d say #2 is probably the most mechanical so I guess I’d rate it lowest in terms of importance *when I’m doing my work*.

    The utility and priority of these guidelines are probably different for different people.

    Hope that helps.

  35. plalx Says:


    Your article has some quite interesting ideas, even though it seems to go against what some of the best DDD practitionners such as Vaughn Vernon advocates regarding aggregate roots, especially if we think of them as transactionnal boundaries, which seems to be the most important rule.

    Factory methods on aggregate roots that creates other aggregate roots is great, but isin’t there any risk of concurrency errors when you also implement persitence by reachability? I might be wrong as I only read about DDD and ORMs and never actually implemented anything yet 😉

    What I’ve seen so far instead is application services that could look like the following:

    Visitor = referrer.BroughtVisitorWithIp(msg.IpAddress);

    Then we may ask if referrer really has to be persitent?

    Visitor = new Referrer(msg.URL).BroughtVisitorWithIp(msg.IpAddress);

    Also, using domain events rather than exceptions to communicate violated domain rules seems interesting, but how would you communicate back the error to the client code?

    An approach I really like in DDD is to extensively use VO to model the domain to a fine-grained level. For instance, have a Username VO which would be responsible to handle rules such as “Username cannot contain spaces” rather than passing strings around in the domain. How would you reconciliate both approaches?


  36. udidahan Says:


    I agree wholeheartedly with the recommendation around value objects.

    I find that programming styles where application services are constantly new-ing up entities and saving them via repositories tend to limit the kind of questions that developers ask about the domain resulting in a lower quality ubiquitous language.

    The fact that concurrency issues come up as a result of persistence by reachability further forces developers to understand and model the collaborative nature of the domain. I tend to talk a lot about the impact of collaboration on a domain – see my posts about CQRS:


    With regards to having aggregate roots represent transactional boundaries, in my experience, the best way to drive to that is by starting with Bounded Contexts and having those strongly aligned with Sub Domains.


  37. raminxtar Says:

    I could be totally wrong but isn’t this an overloading of the term aggregate root?
    Maybe what you refer to would better be called root entity of a scenario/use-case. As far as I have understood aggregate roots are only about consistency boundaries not use case scenarios. And I guess aggregate roots ARE a structural property of the domain model

  38. udidahan Says:


    In order for an aggregate root to be a consistency boundary – it needs to be what you call a root entity, otherwise the transactions could start elsewhere and potentially end somewhere else as well.

  39. George Mauer Says:

    So how does this advice work for *deleting* aggregate roots?

    I have a model with Slideset, Slide, and UsedSlide entities that following the analysis methodologies outlined in Vaughn Vernon’s Effective Aggregate Design I’ve determined are each their own aggregates.

    Now a Slideset acts as a factory for UsedSlide

    public UsedSlide UseSlide(int slideId) {
    DomainEvents.Enqueue(new SlideUsed(this.Id, slideId));
    return new UsedSlide(this.Id, slideId);

    This works fine when adding – I only modify the UsedSlide AR in the transaction. But deleting gets a bit murky – do I delete directly against my ORM in a controller? Where does the domain event get raised from? What about validation?

  40. udidahan Says:


    Take a look at my post on deleting stuff:


  41. Jeroen Benckhuijsen Says:

    Hi Udi,

    Great post, a bit one from the past, but still great value in it. Don’t know if you’ve been up to some new posts with additional insight into this? However, we’ve been trying to figure out how to apply this pattern in our system. We ran into the following situation, perhaps you could shed some light on your take on this?

    – Some of our aggregates roots have an generated identity, either partially or completely.
    – We’ve made our Repository of that root responsible for generating a new identity (i.e. we’re using the “Persistence Mechanism Generates Identity” approach from Vaughn Vernons book). Our repository thus has a method like “Id nextIdentity()”. This is quite common in our system.
    – We’re trying to apply your pattern, on an root A, which creates another root B. B has the trait that it needs a generated identity as mentioned above.

    In this case, how can A get a hold of the new Identity to create the new B? This would require A to use the repository for B. However, this would require either exposing B’s repository as a Singleton, or by passing B’s repository as a parameter to the business method (i.e. using parameter injection).

    Both are ways I’m not really happy about:
    – All singleton services and accessing them from domain entities seems like a anti-pattern to me. E.g. it hardens testing them in unit-tests.
    – Passing in the repository clutters the business method, with internal logic dependencies.

    Have you come across this pattern, and any insight into how to solve it?


  42. Samuel Says:

    Hi Udi,

    Is it always necessary to add the new aggregate to a collection on the aggregate used as a factory, even if there is no business rule relating the collection items???
    Would this create a unnecessary contention in the system???
    Is It not possible to use the aggregate to create the new one and then to persist only the new one???

  43. udidahan Says:


    > Is it always necessary to add the new aggregate to a collection on the aggregate used as a factory, even if there is no business rule relating the collection items?

    Then the question would be why choose that other aggregate as the one to serve as a factory?

  44. SetNug Says:

    Hi Udi,

    I really enjoy your article.

    What’s the general rule to pickup Referrer in your example as the AR creator ?

    In an order system a logged user picks up a product then creates order on it, should we model it as Product#createOrder or User#createOrder ?

    If user is the creator, then it’s the creator of everything hence it must have createProduct, createProductCategory, createProductCatalogue, etc ?


  45. udidahan Says:

    Hi SetNug,

    Consider that any entity, whether it’s an Order or a Product often doesn’t get “created” out of whole cloth – just like the Customer example I gave, data often is collected bit by bit. Model those “interim states” more explicitly and it will often become clearer how this can work.

  46. Harrison Brown Says:

    Udi, whilst I understand what you’re advocating here from a modelling point of view, I still can’t quite see *why* you’re advocating for this approach. I may have completely missed it while reading the post and comments, but I’m mainly seeing discussion around the persistence mechanism (as you cleverly put it, “persistence by reachability”). What do we actually *gain* by handling creating aggregate roots in this way?

    On a much more practical level, I have two other questions:

    1. Another commenter gave a good example of a landlord (Mike Scott, comment #42). You and he were saying that the aggregate root that creates the landlord could be the employee physically entering their data, or the advert the landlord saw, or maybe even the partner API that the landlord’s data was provided by. Given that there could many ‘parent’ aggregate roots for the landlord, does that mean that all the landlords in the system could ‘belong to’ any of an advert, user, API, etc? Or, should I think of this more as there being some parent aggregate root which does the creation of the landlord during the landlord-creating command but then the next time that landlord is retrieved from a repository it’s just pulled out by itself (i.e. it acts like a child entity during creation but acts like a standalone aggregate root for later interactions, such as adding a property to a landlord, for example)?

    2. At a code level, would I still have a single class for my Landlord aggregate root (as I do in my DDD applications now) or would there be a ‘proper’ Landlord aggregate root class which is used for most interactions but some different class which represents the landlord only during creation (when it’s acting like a child entity of it’s creating parent aggregate)

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Bryan Wheeler, Director Platform Development at msnbc.com
Udi Dahan is the real deal.

We brought him on site to give our development staff the 5-day “Advanced Distributed System Design” training. The course profoundly changed our understanding and approach to SOA and distributed systems.

Consider some of the evidence: 1. Months later, developers still make allusions to concepts learned in the course nearly every day 2. One of our developers went home and made her husband (a developer at another company) sign up for the course at a subsequent date/venue 3. Based on what we learned, we’ve made constant improvements to our architecture that have helped us to adapt to our ever changing business domain at scale and speed If you have the opportunity to receive the training, you will make a substantial paradigm shift.

If I were to do the whole thing over again, I’d start the week by playing the clip from the Matrix where Morpheus offers Neo the choice between the red and blue pills. Once you make the intellectual leap, you’ll never look at distributed systems the same way.

Beyond the training, we were able to spend some time with Udi discussing issues unique to our business domain. Because Udi is a rare combination of a big picture thinker and a low level doer, he can quickly hone in on various issues and quickly make good (if not startling) recommendations to help solve tough technical issues.” November 11, 2010

Sam Gentile Sam Gentile, Independent WCF & SOA Expert
“Udi, one of the great minds in this area.
A man I respect immensely.”

Ian Robinson Ian Robinson, Principal Consultant at ThoughtWorks
"Your blog and articles have been enormously useful in shaping, testing and refining my own approach to delivering on SOA initiatives over the last few years. Over and against a certain 3-layer-application-architecture-blown-out-to- distributed-proportions school of SOA, your writing, steers a far more valuable course."

Shy Cohen Shy Cohen, Senior Program Manager at Microsoft
“Udi is a world renowned software architect and speaker. I met Udi at a conference that we were both speaking at, and immediately recognized his keen insight and razor-sharp intellect. Our shared passion for SOA and the advancement of its practice launched a discussion that lasted into the small hours of the night.
It was evident through that discussion that Udi is one of the most knowledgeable people in the SOA space. It was also clear why – Udi does not settle for mediocrity, and seeks to fully understand (or define) the logic and principles behind things.
Humble yet uncompromising, Udi is a pleasure to interact with.”

Glenn Block Glenn Block, Senior Program Manager - WCF at Microsoft
“I have known Udi for many years having attended his workshops and having several personal interactions including working with him when we were building our Composite Application Guidance in patterns & practices. What impresses me about Udi is his deep insight into how to address business problems through sound architecture. Backed by many years of building mission critical real world distributed systems it is no wonder that Udi is the best at what he does. When customers have deep issues with their system design, I point them Udi's way.”

Karl Wannenmacher Karl Wannenmacher, Senior Lead Expert at Frequentis AG
“I have been following Udi’s blog and podcasts since 2007. I’m convinced that he is one of the most knowledgeable and experienced people in the field of SOA, EDA and large scale systems.
Udi helped Frequentis to design a major subsystem of a large mission critical system with a nationwide deployment based on NServiceBus. It was impressive to see how he took the initial architecture and turned it upside down leading to a very flexible and scalable yet simple system without knowing the details of the business domain. I highly recommend consulting with Udi when it comes to large scale mission critical systems in any domain.”

Simon Segal Simon Segal, Independent Consultant
“Udi is one of the outstanding software development minds in the world today, his vast insights into Service Oriented Architectures and Smart Clients in particular are indeed a rare commodity. Udi is also an exceptional teacher and can help lead teams to fall into the pit of success. I would recommend Udi to anyone considering some Architecural guidance and support in their next project.”

Ohad Israeli Ohad Israeli, Chief Architect at Hewlett-Packard, Indigo Division
“When you need a man to do the job Udi is your man! No matter if you are facing near deadline deadlock or at the early stages of your development, if you have a problem Udi is the one who will probably be able to solve it, with his large experience at the industry and his widely horizons of thinking , he is always full of just in place great architectural ideas.
I am honored to have Udi as a colleague and a friend (plus having his cell phone on my speed dial).”

Ward Bell Ward Bell, VP Product Development at IdeaBlade
“Everyone will tell you how smart and knowledgable Udi is ... and they are oh-so-right. Let me add that Udi is a smart LISTENER. He's always calibrating what he has to offer with your needs and your experience ... looking for the fit. He has strongly held views ... and the ability to temper them with the nuances of the situation.
I trust Udi to tell me what I need to hear, even if I don't want to hear it, ... in a way that I can hear it. That's a rare skill to go along with his command and intelligence.”

Eli Brin, Program Manager at RISCO Group
“We hired Udi as a SOA specialist for a large scale project. The development is outsourced to India. SOA is a buzzword used almost for anything today. We wanted to understand what SOA really is, and what is the meaning and practice to develop a SOA based system.
We identified Udi as the one that can put some sense and order in our minds. We started with a private customized SOA training for the entire team in Israel. After that I had several focused sessions regarding our architecture and design.
I will summarize it simply (as he is the software simplist): We are very happy to have Udi in our project. It has a great benefit. We feel good and assured with the knowledge and practice he brings. He doesn’t talk over our heads. We assimilated nServicebus as the ESB of the project. I highly recommend you to bring Udi into your project.”

Catherine Hole Catherine Hole, Senior Project Manager at the Norwegian Health Network
“My colleagues and I have spent five interesting days with Udi - diving into the many aspects of SOA. Udi has shown impressive abilities of understanding organizational challenges, and has brought the business perspective into our way of looking at services. He has an excellent understanding of the many layers from business at the top to the technical infrstructure at the bottom. He is a great listener, and manages to simplify challenges in a way that is understandable both for developers and CEOs, and all the specialists in between.”

Yoel Arnon Yoel Arnon, MSMQ Expert
“Udi has a unique, in depth understanding of service oriented architecture and how it should be used in the real world, combined with excellent presentation skills. I think Udi should be a premier choice for a consultant or architect of distributed systems.”

Vadim Mesonzhnik, Development Project Lead at Polycom
“When we were faced with a task of creating a high performance server for a video-tele conferencing domain we decided to opt for a stateless cluster with SQL server approach. In order to confirm our decision we invited Udi.

After carefully listening for 2 hours he said: "With your kind of high availability and performance requirements you don’t want to go with stateless architecture."

One simple sentence saved us from implementing a wrong product and finding that out after years of development. No matter whether our former decisions were confirmed or altered, it gave us great confidence to move forward relying on the experience, industry best-practices and time-proven techniques that Udi shared with us.
It was a distinct pleasure and a unique opportunity to learn from someone who is among the best at what he does.”

Jack Van Hoof Jack Van Hoof, Enterprise Integration Architect at Dutch Railways
“Udi is a respected visionary on SOA and EDA, whose opinion I most of the time (if not always) highly agree with. The nice thing about Udi is that he is able to explain architectural concepts in terms of practical code-level examples.”

Neil Robbins Neil Robbins, Applications Architect at Brit Insurance
“Having followed Udi's blog and other writings for a number of years I attended Udi's two day course on 'Loosely Coupled Messaging with NServiceBus' at SkillsMatter, London.

I would strongly recommend this course to anyone with an interest in how to develop IT systems which provide immediate and future fitness for purpose. An influential and innovative thought leader and practitioner in his field, Udi demonstrates and shares a phenomenally in depth knowledge that proves his position as one of the premier experts in his field globally.

The course has enhanced my knowledge and skills in ways that I am able to immediately apply to provide benefits to my employer. Additionally though I will be able to build upon what I learned in my 2 days with Udi and have no doubt that it will only enhance my future career.

I cannot recommend Udi, and his courses, highly enough.”

Nick Malik Nick Malik, Enterprise Architect at Microsoft Corporation
You are an excellent speaker and trainer, Udi, and I've had the fortunate experience of having attended one of your presentations. I believe that you are a knowledgable and intelligent man.”

Sean Farmar Sean Farmar, Chief Technical Architect at Candidate Manager Ltd
“Udi has provided us with guidance in system architecture and supports our implementation of NServiceBus in our core business application.

He accompanied us in all stages of our development cycle and helped us put vision into real life distributed scalable software. He brought fresh thinking, great in depth of understanding software, and ongoing support that proved as valuable and cost effective.

Udi has the unique ability to analyze the business problem and come up with a simple and elegant solution for the code and the business alike.
With Udi's attention to details, and knowledge we avoided pit falls that would cost us dearly.”

Børge Hansen Børge Hansen, Architect Advisor at Microsoft
“Udi delivered a 5 hour long workshop on SOA for aspiring architects in Norway. While keeping everyone awake and excited Udi gave us some great insights and really delivered on making complex software challenges simple. Truly the software simplist.”

Motty Cohen, SW Manager at KorenTec Technologies
“I know Udi very well from our mutual work at KorenTec. During the analysis and design of a complex, distributed C4I system - where the basic concepts of NServiceBus start to emerge - I gained a lot of "Udi's hours" so I can surely say that he is a professional, skilled architect with fresh ideas and unique perspective for solving complex architecture challenges. His ideas, concepts and parts of the artifacts are the basis of several state-of-the-art C4I systems that I was involved in their architecture design.”

Aaron Jensen Aaron Jensen, VP of Engineering at Eleutian Technology
Awesome. Just awesome.

We’d been meaning to delve into messaging at Eleutian after multiple discussions with and blog posts from Greg Young and Udi Dahan in the past. We weren’t entirely sure where to start, how to start, what tools to use, how to use them, etc. Being able to sit in a room with Udi for an entire week while he described exactly how, why and what he does to tackle a massive enterprise system was invaluable to say the least.

We now have a much better direction and, more importantly, have the confidence we need to start introducing these powerful concepts into production at Eleutian.”

Gad Rosenthal Gad Rosenthal, Department Manager at Retalix
“A thinking person. Brought fresh and valuable ideas that helped us in architecting our product. When recommending a solution he supports it with evidence and detail so you can successfully act based on it. Udi's support "comes on all levels" - As the solution architect through to the detailed class design. Trustworthy!”

Chris Bilson Chris Bilson, Developer at Russell Investment Group
“I had the pleasure of attending a workshop Udi led at the Seattle ALT.NET conference in February 2009. I have been reading Udi's articles and listening to his podcasts for a long time and have always looked to him as a source of advice on software architecture.
When I actually met him and talked to him I was even more impressed. Not only is Udi an extremely likable person, he's got that rare gift of being able to explain complex concepts and ideas in a way that is easy to understand.
All the attendees of the workshop greatly appreciate the time he spent with us and the amazing insights into service oriented architecture he shared with us.”

Alexey Shestialtynov Alexey Shestialtynov, Senior .Net Developer at Candidate Manager
“I met Udi at Candidate Manager where he was brought in part-time as a consultant to help the company make its flagship product more scalable. For me, even after 30 years in software development, working with Udi was a great learning experience. I simply love his fresh ideas and architecture insights.
As we all know it is not enough to be armed with best tools and technologies to be successful in software - there is still human factor involved. When, as it happens, the project got in trouble, management asked Udi to step into a leadership role and bring it back on track. This he did in the span of a month. I can only wish that things had been done this way from the very beginning.
I look forward to working with Udi again in the future.”

Christopher Bennage Christopher Bennage, President at Blue Spire Consulting, Inc.
“My company was hired to be the primary development team for a large scale and highly distributed application. Since these are not necessarily everyday requirements, we wanted to bring in some additional expertise. We chose Udi because of his blogging, podcasting, and speaking. We asked him to to review our architectural strategy as well as the overall viability of project.
I was very impressed, as Udi demonstrated a broad understanding of the sorts of problems we would face. His advice was honest and unbiased and very pragmatic. Whenever I questioned him on particular points, he was able to backup his opinion with real life examples. I was also impressed with his clarity and precision. He was very careful to untangle the meaning of words that might be overloaded or otherwise confusing. While Udi's hourly rate may not be the cheapest, the ROI is undoubtedly a deal. I would highly recommend consulting with Udi.”

Robert Lewkovich, Product / Development Manager at Eggs Overnight
“Udi's advice and consulting were a huge time saver for the project I'm responsible for. The $ spent were well worth it and provided me with a more complete understanding of nServiceBus and most importantly in helping make the correct architectural decisions earlier thereby reducing later, and more expensive, rework.”

Ray Houston Ray Houston, Director of Development at TOPAZ Technologies
“Udi's SOA class made me smart - it was awesome.

The class was very well put together. The materials were clear and concise and Udi did a fantastic job presenting it. It was a good mixture of lecture, coding, and question and answer. I fully expected that I would be taking notes like crazy, but it was so well laid out that the only thing I wrote down the entire course was what I wanted for lunch. Udi provided us with all the lecture materials and everyone has access to all of the samples which are in the nServiceBus trunk.

Now I know why Udi is the "Software Simplist." I was amazed to find that all the code and solutions were indeed very simple. The patterns that Udi presented keep things simple by isolating complexity so that it doesn't creep into your day to day code. The domain code looks the same if it's running in a single process or if it's running in 100 processes.”

Ian Cooper Ian Cooper, Team Lead at Beazley
“Udi is one of the leaders in the .Net development community, one of the truly smart guys who do not just get best architectural practice well enough to educate others but drives innovation. Udi consistently challenges my thinking in ways that make me better at what I do.”

Liron Levy, Team Leader at Rafael
“I've met Udi when I worked as a team leader in Rafael. One of the most senior managers there knew Udi because he was doing superb architecture job in another Rafael project and he recommended bringing him on board to help the project I was leading.
Udi brought with him fresh solutions and invaluable deep architecture insights. He is an authority on SOA (service oriented architecture) and this was a tremendous help in our project.
On the personal level - Udi is a great communicator and can persuade even the most difficult audiences (I was part of such an audience myself..) by bringing sound explanations that draw on his extensive knowledge in the software business. Working with Udi was a great learning experience for me, and I'll be happy to work with him again in the future.”

Adam Dymitruk Adam Dymitruk, Director of IT at Apara Systems
“I met Udi for the first time at DevTeach in Montreal back in early 2007. While Udi is usually involved in SOA subjects, his knowledge spans all of a software development company's concerns. I would not hesitate to recommend Udi for any company that needs excellent leadership, mentoring, problem solving, application of patterns, implementation of methodologies and straight out solution development.
There are very few people in the world that are as dedicated to their craft as Udi is to his. At ALT.NET Seattle, Udi explained many core ideas about SOA. The team that I brought with me found his workshop and other talks the highlight of the event and provided the most value to us and our organization. I am thrilled to have the opportunity to recommend him.”

Eytan Michaeli Eytan Michaeli, CTO Korentec
“Udi was responsible for a major project in the company, and as a chief architect designed a complex multi server C4I system with many innovations and excellent performance.”

Carl Kenne Carl Kenne, .Net Consultant at Dotway AB
“Udi's session "DDD in Enterprise apps" was truly an eye opener. Udi has a great ability to explain complex enterprise designs in a very comprehensive and inspiring way. I've seen several sessions on both DDD and SOA in the past, but Udi puts it in a completly new perspective and makes us understand what it's all really about. If you ever have a chance to see any of Udi's sessions in the future, take it!”

Avi Nehama, R&D Project Manager at Retalix
“Not only that Udi is a briliant software architecture consultant, he also has remarkable abilities to present complex ideas in a simple and concise manner, and...
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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