Udi Dahan   Udi Dahan – The Software Simplist
Enterprise Development Expert & SOA Specialist
 
   
    Blog Consulting Training Articles Speaking About
  

Performant and Explicit Domain Models

Monday, June 4th, 2007.

Some Technical Difficulties

Ayende and I had an email conversation that started with me asking what would happen if I added an Order to a Customer’s “Orders” collection, when that collection was lazy loaded. My question was whether the addition of an element would result in NHibernate hitting the database to fill that collection. His answer was a simple “yes”. In the case where a customer can have many (millions) of Orders, that’s just not a feasible solution. The technical solution was simple – just define the Orders collection on the Customer as “inverse=true”, and then to save a new Order, just write:

session.Save( new Order(myCustomer) );

Although it works, it’s not “DDD compliant” :)

In Ayende’s post Architecting for Performance he quoted a part of our email conversation. The conclusion I reached was that in order to design performant domain models, you need to know the kinds of data volumes you’re dealing with. It affects both internals and the API of the model – when can you assume cascade, and when not. It’s important to make these kinds of things explicit in the Domain Model’s API.

How do you make “transparent persistence” explicit?

The problem occurs around “transparent persistence”. If we were to assume that the Customer object added the Order object to its Orders collection, then we wouldn’t have to explicitly save orders it creates, so we would write service layer code like this:

using (IDBScope scope = this.DbServices.GetScope(TransactionOption.On))
{
IOrderCreatingCustomer c = this.DbServices.Get<IOrderCreatingCustomer>(msg.CustomerId);
c.CreateOrder(message.OrderAmount);

scope.Complete();
}

On the other hand, if we designed our Domain Model around the million orders constraint, we would need to explicitly save the order, so we would write service layer code like this:

using (IDBScope scope = this.DbServices.GetScope(TransactionOption.On))
{
IOrderCreatingCustomer c = this.DbServices.Get<IOrderCreatingCustomer>(msg.CustomerId);
IOrder o = c.CreateOrder(message.OrderAmount);
this.DbServices.Save(o);

scope.Complete();
}

But the question remains, how do we communicate these guidelines to service layer developers from the Domain Model? There are a number of ways, but it’s important to decide on one and use it consistently. Performance and correctness require it.

Solution 1: Explicitness via Return Type

The first way is a little subtle, but you can do it with the return type of the “CreateOrder” method call. In the case where the Domain Model wishes to communicate that it handles transparent persistence by itself, have the method return “void”. Where the Domain Model wishes to communicate that it will not handle transparent persistence, have the method return the Order object created.

Another way to communicate the fact that an Order has been created that needs to be saved is with events. There are two sub-ways to do so:

Solution 2: Explicitness via Events on Domain Objects

The first is to just define the event on the customer object and have the service layer subscribe to it. It’s pretty clear that when the service layer receives a “OrderCreatedThatRequiresSaving” event, it should save the order passed in the event arguments.

The second realizes that the call to the customer object may come from some other domain object and that the service layer doesn’t necessarily know what can happen as the result of calling some method on the aggregate root. The change of state as the result of that method call may permeate the entire object graph. If each object in the graph raises its own events, its calling object will have to propagate that event to its parent – resulting in defining the same events in multiple places, and each object being aware of all things possible with its great-grandchild objects. That is clearly bad.

What [ThreadStatic] is for

So, the solution is to use thread-static events.

[Sidebar] Thread-static events are just static events defined on a static class, where each event has the ThreadStaticAttribute applied to it. This attribute is important for server-side scenarios where multiple threads will be running through the Domain Model at the same time. The easiest thread-safe way to use static data is to apply the ThreadStaticAttribute.

Solution 3: Explicitness via Static Events

Each object raises the appropriate static event according to its logic. In our example, Customer would call:

DomainModelEvents.RaiseOrderCreatedThatRequiresSavingEvent(newOrder);

And the service layer would write:

DomainModelEvents.OrderCreatedThatRequiresSaving +=
delegate(object sender, OrderEventArgs e) { this.DbServices.Save(e.Order); };

The advantage of this solution is that it requires minimal knowledge of the Domain Model for the Service Layer to correctly work with it. It also communicates that anything that doesn’t raise an event will be persisted transparently behind the appropriate root object.

Statics and Testability

I know that many of you are wondering if I am really advocating the use of statics. The problem with most static classes is that they hurt testability because they are difficult to mock out. Often statics are used as Facades to hide some technological implementation detail. In this case, the static class is an inherent part of the Domain Model and does not serve as a Facade for anything.

When it comes to testing the Domain Model, we don’t have to mock anything out since the Domain Model is independent of all other concerns. This leaves us with unit testing at the single Domain Class level, which is pretty useless unless we’re TDD-ing the design of the Domain Model, in which case we’ll still be fiddling around with a bunch of classes at a time. Domain Models are best tested using State-Based Testing; get the objects into a given state, call a method on one of them, assert the resulting state. The static events don’t impede that kind of testing at all.

What if we used Injection instead of Statics?

Also, you’ll find that each Service Layer class will need to subscribe to all the Domain Model’s events, something that is easily handled by a base class. I will state that I have tried doing this without a static class, and injecting that singleton object into the Service Layer classes, and in that setter having them subscribe to its events. This was also pulled into a base class. The main difference was that the Dependency Injection solution required injecting that object into Domain Objects as well. Personally, I’m against injection for domain objects. So all in all, the static solution comes with less overhead than that based on injection.

Summary

In summary, beyond the “technical basics” of being aware of your data volumes and designing your Domain Model to handle each use case performantly, I’ve found these techniques useful for designing its API as well as communicating my intent around persistence transparency. So give it a try. I’d be grateful to hear your thoughts on the matter as well as what else you’ve found that works.

Related posts:

  
If you liked this article, you might also like articles in these categories:


Check out my Best Content If you've got a minute, you might enjoy taking a look at some of my best articles.
I've gone through the hundreds of articles I've written over the past 6 years and put together a list of the best ones as ranked by my 5000+ readers.
You won't be disappointed.

Subscribe to my feedIf you'd like to get new articles sent to you when they're published, it's easy and free.
Subscribe right here.

Follow me on Twitter Follow me on Twitter @UdiDahan.



Something on your mind? Got a question? I'd be thrilled to hear it.
Leave a comment below or email me, whatever works for you.

  1. thesoftwaresimplist Says:

    It appears that NHibernate Bags don’t have this drawback:

    http://www.hibernate.org/hib_docs/nhibernate/1.2/reference/en/html/performance.html#performance-collections-mostefficentinverse


  2. Frederik Says:

    Another thing one might ask is, why should your Customer Entity contain a collection of Order objects ?
    This means that, whenever you do something with your Customer, you drag all his orders (maybe 1000) with you. In which case(s) would it be necessary to have the Orders of a Customer ? In which cases do you want to have an overview of all the Orders of a particular Customer ?
    I always ask myself this question in such scenario’s, and very often, I decide to not have a collection of (in this example) Orders in my Customer entity. Instead, I opt for a method in my OrderRepository which is called ‘GetAllOrdersForCustomer( Customer c )’. Then, in the cases where I do need the Orders of a particular Customer, I just have to call this method …
    In my opinion -and in this example- it is far more usefull to know the Customer to which an Order belongs, then to having a collection of Orders in the customer entity.


  3. Colin Jack Says:

    Interesting topic. Personally I don’t like the event idea, you only really want there to be one subscriber and they absolutely should be subscribing so events don’t seem to me to be appropriate.

    Equally I don’t like being forced to use relationships from the one end (e.g. Order to Client) just for performance. Designing your associations like that is going to result in a mess in some cases, it might work for Order/Customer but in complex cases where the classes are in seperate modules it can be messy. I’ve posted about this sort of issue here:

    http://colinjack.blogspot.com/2007/03/persistence-ignorance-and-associations.html

    One way I handle this is to have these sorts of calls go through a Coordination layer that will ensue the new Orders are saved:

    http://colinjack.blogspot.com/2007/02/domain-coordination-layer.html

    Having said all that we use bags with NHibernate so it is not really an issue for us.

    Any thoughts?


  4. thesoftwaresimplist Says:

    Frederik,

    > Another thing one might ask is, why should your Customer Entity contain a collection of Order objects ? In which case(s) would it be necessary to have the Orders of a Customer ?

    We may have business rules that impact whether the customer is preferred (and thus gets a discount) or not based on some calculation on their order history.

    > This means that, whenever you do something with your Customer, you drag all his orders (maybe 1000) with you.

    That was my point – I don’t want to do that. My problem was that even if the collection of orders was lazy loaded, adding a new order would cause all those orders to be loaded. This turns out to be a non-problem with NHibernate “bags” which don’t load when an object is added.

    > I decide to not have a collection of (in this example) Orders in my Customer entity. Instead, I opt for a method in my OrderRepository which is called ‘GetAllOrdersForCustomer( Customer c )’. Then, in the cases where I do need the Orders of a particular Customer, I just have to call this method.

    Since I implement my business rules within the Domain Model, the problem I have with that is that I don’t want my Domain Objects calling into Repositories. That’s not to say that such a query would not be valuable on its own.

    Does that answer your questions?


  5. thesoftwaresimplist Says:

    Colin,

    > Interesting topic.

    Glad you liked it.

    > Personally I don’t like the event idea, you only really want there to be one subscriber and they absolutely should be subscribing so events don’t seem to me to be appropriate.

    Events / delegates are used to keep the logic about when things need to occur in the Domain Model, while keeping it indpendent of how the wiring occurs. It’s all about dependency management, but you’re entitled to your own taste.

    > Equally I don’t like being forced to use relationships from the one end (e.g. Order to Client) just for performance.

    Me neither. That’s why I make use of interfaces for hiding these sorts of implementation details.

    > Designing your associations like that is going to result in a mess in some cases, it might work for Order/Customer but in complex cases where the classes are in seperate modules it can be messy.

    I’ve done the multi-module thing and it’s usually doable by using IList of IEntity for holding cross module collections.

    > One way I handle this is to have these sorts of calls go through a Coordination layer that will ensue the new Orders are saved.

    Personally, I prefer events to coupling the Domain Model to some other Coordination Layer, even if that’s behind some interface, it forces me to inject that dependency into my Domain Objects. That kind of complexity I don’t want.

    I think that we can agree to disagree on this one :)


  6. Frederik Says:

    Hmm, why don’t you want to call Repositories from your Domain Entities ? As I read in Evans’ book: repositories are first class citizens of the domain layer; they are part of the domain.

    I understand that you might have some business logic which decides wether a customer is a gold customer or not, based on the orders that this customer has made, but then, it might be necessary that you iteratoe over all the Orders in the OrderCollection of that Customer. When this customer has a lot of Orders, this can put a weight on performance. Wouldn’t it then be better to use an injected OrderRepository to get the Orders you’re really interested in to perform this logic out of the persistence store ?


  7. thesoftwaresimplist Says:

    Frederik,

    > Hmm, why don’t you want to call Repositories from your Domain Entities?

    Separation of Concerns – Domain Objects deal with business logic only and know nothing about persistence, and thus are highly testable while Repositories are all about persistence.

    > Wouldn’t it then be better to use an injected OrderRepository to get the Orders you’re really interested in to perform this logic out of the persistence store ?

    I actually “denormalize” both my Domain Objects and database to handle these kinds of performance requirements. By having the running total of all orders (if that’s what the rules state) as a persistent member on my Customer class, and make sure that value is consistent, I won’t need to iterate. Injecting an OrderRepository wouldn’t solve the performance issue.

    Does that make sense?


  8. Frederik Says:

    Hmm, well, then we have a different view on repositories. In my opinion, they’re part of the Domain and as such, they can contain specialized methods that are required to do some Business Logic. (For Instance ‘GetOrderTotalForCustomerInPeriod’.
    For me, a repository is more then just a way of retrieving / storing data.

    About denormalizing: sometimes this is indeed necessary; however, I do not like to ‘sacrifice’ my DB model just because it is necessary if I want to keep all my logic in the Domain Entities.
    Your domain layer might make sure that everything is kept consistent, but it is maybe also possible that other (legacy) applications use the same database. Then, the consistency is not guaranteed.


  9. thesoftwaresimplist Says:

    Frederik,

    I prefer handling ‘GetOrderTotalForCustomerInPeriod’ using the Query Object Pattern, as I wrote about here:

    http://udidahan.weblogs.us/2007/03/28/query-objects-vs-methods-on-a-repository/

    The necessity comes out of performance. Keeping as much of my logic as possible in the Domain Model has been important for me in terms of testability.

    I agree that the issue of using the database as the integration platform creates problems that we may not be able to do away with in one shot. However, I try not to propogate this anti-pattern and phase things out over time. As long as we’re going in the right direction, I’m willing to make some sacrifices on correctness in the short term.

    I’m going to be putting an article out in the next month or so on the use of DBs as integration platforms and how that’s changing with SOA. I’d be interested in hearing more about your experiences in that area.


  10. Colin Jack Says:

    Just re-read this post and got this bit:

    “When it comes to testing the Domain Model, we don’t have to mock anything out since the Domain Model is independent of all other concerns.”

    I agree with you but wondered how you go about it, do you use any mocking at all? Do you test at the aggregate level? What do you do about testing encapsulated classes (e.g. state objects or strategies), do you just test them through the effects they have on the public methods?

    Just interested as its a topic thats not discussed enough.

    > Personally, I prefer events to coupling the Domain Model to some
    > other Coordination Layer, even if that’s behind some interface, it > forces me to inject that dependency into my Domain Objects. That
    > kind of complexity I don’t want.

    I should also say, on the coordination layer front its the coordination layer that calls the domain classes not the other way round. The domain classes only know about other domain classes, not about any layers above or at the same level.


  11. thesoftwaresimplist Says:

    Colin,

    For testing the correctness of the Domain Model (unit testing), no mocking is done. I test according to interfaces, which may or may not be at the aggregate level. For instance, Orders belong to a Customer, the aggregate – I do test Orders directly.

    About encapsulated classes, I test through the external interface – focusing on externally visible effects.

    About coordination, Domain Objects can call back into Coordination Layer classes – without depending on them, using events. Does that make sense?


  12. Nuno Lopes Says:

    Again late to the party so sorry to bring this up again as I think it is crucial.

    Although Bags resolve the problem at hand, and sometimes are necessary, prior to it we have another problem (actually I have been seeing this problem in a lot of examples).

    Again aggregates. We have found more then handful types of aggregations depending on the type of collaboration (Composite-item, Container-item, Group-Member, Party-Transaction, Template-Item, etc etc etc etc etc).

    The collaboration pattern of choice have specific rules about who directs what in a collaboration. This in contrast of one recepee fits all for any kind of aggregation (just put add remove in the aggregator and it rules everything about creating its “items”). The problem is that say the object being aggregated (The Order) can also be agregated by some other object at the time of creation (Place?).

    For instance:

    “A Customer making an Order of products on a Place”.

    So who does create the Order? The customer (or its Role) that makes it, or the place (or its Role) that accepts it?

    I know that everyone will say that is the Customer that makes it (good OO). But this question makes sense in a lot contexts (too many).

    I’ve found that in situations such as this it is preferable to do something like this on the service:

    Order aOrder = new Order(aCustomer, aPlace);
    // add products.
    tx.Commit.

    Actually I tend to do this everytime except for aggregations that imply ownership … aka composition (which is different for assembly-part).

    An example of this we have right here. The Order and its OrderLines. An Order is Composed by OrderLines. Which is not the case of Customer and Order (A customer is not composed by its orders). In most business cases an Order (Transaction) can last longer then then any info put in the Customer entity as it can change (Name, Delivery Address, Payment etc etc etc should be all be within the Order that can refer the customer for completness but it is not essential).

    In my experience, I’ve found also that in most Composite collaborations actually the number of items in the composite (Say Building-Floor-Room, or Organization-Department-Subdepartement) is quite small or hierarchical.

    In other words, except for the composites objects usually aggregators don’t create it’s “items”. That is easy to explain to any team member coding the service layer.

    So the problem is solved at the Domain Model and OOP practice rather then technology and it becomes natural. Furthermore the technical solution given by Ayende works quite well.

    Notice that I’m not proposing that the Customer shouldn’t be able have and provide a list of its Orders, it should. Nevertheless if it is “him” solely governs the list to the point it creates the items in it is another matter.

    Does this make sense?

    Nuno


  13. Nuno Lopes Says:

    By the way I liked the article about Domain Fluence.

    One might argue that this approach is not Domain Fluent, but I can equally argue that it is.

    Nuno


  14. Nuno Lopes Says:

    For completeness of my reasoning above.

    There might be a business rule that there “must” be no Customer without at least one order. Ideally we would do:

    Customer aCustomer = new Customer(aOrder);

    But the approach I gave required that the Customer to be “born” first.

    This problem reminds me a riddle: “Who was born first, the chicken or the Egg?”

    So how do we validate this? Well we use Validators much the explained in some powerpoint of Udi (where an event is trapped right before the object is to be stored).

    Validators very helpfull for invariants. That is rules that once true are required to be true as long as the object exists in the system.

    Nuno


Your comment...



If this is your first time commenting, it may take a while to show up.
I'm working to make that better.

Subscribe here to receive updates on comments.
  
   


Don't miss my best content
 

Recommendations

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.”

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



Creative Commons License  © Copyright 2005-2011, Udi Dahan. email@UdiDahan.com