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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);


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);


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:


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.


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:

Interfaces solve visibility and testing issues

Monday, June 4th, 2007

Jimmy’s recent post called out some of the insights on the advantages of Ruby from Niclas’ keynote at DevSummit 2007. Jimmy writes:

In Ruby it’s easy to redefine the visibility of a method from private to public for testing purposes. This was just one small detail in his talk of course, but I started to think about how much pain I quite often go through regarding this in legacy code.

Let me just start out by saying that dealing with legacy code is far from easy. However, when writing new code, or as a part of refactoring that legacy, adding interfaces to your design can help you get around those visibility and testing issues.

For instance, if you only let “client” code access the interface, and not the implementation (probably using some kind of dependency injection), then you could leave all sorts of methods public on your concrete class without worry that the client will call them since the interface doesn’t expose them. Now that the methods on the concrete class are public, you can easily test that class.

The way to package your code to make sure this occurs follows a very simple design principle. This is much easier to put in practice on new development, but you’ll find that it isn’t that hard for legacy either. Since legacy code often doesn’t make use of interfaces, and the implementation is already packaged, and your new client code will already be separately packaged, you’re 90% there!

If you can change the legacy code, you’re actually 95% there. Just create a new package for the interface that the client code will use. Then change the legacy code to implement that interface – since the methods will be pretty much the same, it’s not that much work.

If you can’t change the legacy code, create a “wrapper” class that implements the interface and delegates the calls to the legacy code.

Finally, let me sum up by saying that I think Ruby is great. However, I think that often many advantages are attributed to anything new that may have already been possible/easy with what we already have today. Sometimes, the new stuff helps raise awareness on important issues – and then we can have great discussions on how to solve those issues with today’s (yesterday’s ?) technology 🙂

Layering – too simplistic to actually work

Sunday, June 3rd, 2007

After seeing Mark’s post on Reasons for Isolation describing the ways Layered Architectures break down, and the ways making it more testable can change it, I’ve got to wonder – is Layering just too simplistic to actually work?

Just the other day I was doing a design review for a fairly simple Smart Client whose design was layered. In order to stay away from interfaces that accepted dozens of ints, strings, and dates, they wanted to have each layer talk to the other using “entities”. So where are these entities defined – oh, in a “vertical layer” that all the horizontal layers talk to.

OK, so we’ve taken the simplistic one-dimensional layered architecture and added a dimension. What now?

Well, it seems that having the business logic and the entities in separate layers goes against one of the most basic Object Oriented principles – encapsulation. So, let’s put the entities back in the Business Logic Layer. But then how will the Data Access Layer accept those objects as parameters?

So, that is solved by keeping Entity Interfaces in the “vertical” shared “layer”, and having the entities in the business logic layer implement those interfaces. That way, the data access layer can still accept parameters corresponding to those interfaces:

void InsertCustomer(Shared.Entities.ICustomer customer);

So far so good. Now, we want more testable UI layer code – so we use Model-View-Controller (MVC) – of whichever flavor suits your fancy. I’d say that Supervising Controller is a must. You could also add another presenter for more complex screens as in Passive View, but I’d be less strict on that. So, in which layer do these Controllers/Presenters sit? And is the Business Logic Layer the Model? Or is the Model just part of it?

Well, our Supervising Controllers are those who decide what action to do and when, where to get the data from, etc. That sounds like business logic to me. So let’s put them in the BLL. Presenters for the Passive View are much more UI centered, so let’s put them in the Presentation Layer. But we don’t want them tied to the implementation of the view, so we’ll put them in a separate package, and have them depend only on the view’s interface. So we’ll put the view interfaces in a package separate from the view implementation as well.

If it wasn’t clear up to this point, all the questions raised in this post are architectural in nature – as in they have a substantial impact on the structure and flow of the system, and will definitely have a profound effect on its maintainability. In other words, if you think that Layer Diagram covers your design – you’re probably deluding yourself. Personally, I think that’s why many developers consider architects to be “out of touch with the real world”.

When you have a design that answers these, and other architectural concerns, you’ll find that layering is of little importance. The specific constraints on each package are what counts. The fact that the Presentation Layer can talk to the Business Logic Layer doesn’t mean that the classes in your Views Implementation Package can. A large part of an architects work is to specify these constraints, and communicate them to the team. Tools like FxCop may help in terms of enforcing these constraints, but I believe that getting the team to actually “buy-in” is more effective.

Single-dimensional layered architectures don’t work. They violate Einstein’s maxim:

Make everything as simple as possible, but not simpler.

Layering – “simpler” to the point of simplistic.

Testing services the hard way with WCF

Thursday, May 17th, 2007

I just read the kind of hoops you have to go through in order to test your WCF service implementations. Oh. My. GOD.

If that’s the best there is, NOBODY is going to be testing their WCF services. That’s scary.

When using this message-based design, your service implementations tend to look like this:

public class WorkflowMessageHandler : IMessageHandler
	public void Handle(Stage1Msg msg)
		using (IDBScope scope = this.DbServices.GetScope(TransactionOption.On))
			IWorkflow wf = this.DbServices.Get(msg.WorkFlowID);


And it’s this easy to test them:

public void TestWorkflowMessageHandler()
	WorkflowMessageHandler handler = new WorkflowMessageHandler();
	handler.Bus = this.MockBus;

	// set up expectations on bus in terms of messages returned

	Stage1Msg msg = new Stage1Msg();
	// fill msg with data


	// verify expectations on bus

That’s right, it’s just plain old unit testing the way we test everything else these days.

I’m beginning to get the impression that the new suite of technologies that is coming out of Microsoft is making things more complicated than they need to be.

Well, maybe that’s just me.

Frictionless Design

Monday, May 14th, 2007

From Ryan’s post on scaling Rails with multiple databases, he raises a point near and dear to my heart around the use of well designed frameworks (apparently sourced by Joe):

When referring to framework and tooling, “friction” is a (subjective) measure of how much the tooling gets in your way when trying to solve a specific-case problem. I’ve come to evaluate frameworks based on two rough metrics: how far the framework goes in solving the general case problem out of the box and how little friction the framework creates when you have to solve the specific-case problem yourself. When a framework finds a balance between these two areas, we call it “well designed.”

It has been my experience that when I follow my first principle of design, a good separation of logical packages occurs. These packages enable an appropriate level of flexibility by interacting via interfaces and supporting Dependency Injection. Of course, a fierce focus on assigning responsibility correctly probably makes quite a difference too. I suppose this is why I shoot for multiple smaller frameworks, than one large one. Beyond being easier to manage in terms of version control and supporting across multiple projects and products, they’re also quite a bit easier to develop.

Finally, consider that the code you’re writing is someone else’s framework/API. For people looking for more advanced API design techniques, check out fluent interfaces as well.

Sketch a little design for me

Monday, May 14th, 2007

So it’s my first visit with this client, and we’re discussing how the current system works and in what ways they want to expand it, what the new requirements are, etc. I ask Isaac, the development manager, if they have any diagrams or documents with this information and he proudly answered that they do. After going through the powerpoints, visios, and word documents describing everything but what I was looking for, I had a few more questions.

“I appear to have missed the architecture diagrams. Can you show me where they are?”

“Of course. Here it is.”, Isaac proclaimed pointing to something that looked like a deployment diagram.

“I see, but which logical pieces talk to each other? I couldn’t find very much information about the software itself.”, I asked.

Isaac went into an hour long monologue where each of the little boxes ornamenting the servers, clients, and databases was described. A couple of times, he corrected himself mid-way through:

“Um, no. Strike that. I wanted it to work that way, but it can’t because of the XYZ. I think that the team implemented it like this…”

After thoroughly soaking things up, I went and talked to the team about the new requirements. Five minutes into our conversation, they stopped me and Daniel, the technical lead said:

“Yah, I know that’s what Isaac thought, but we couldn’t get that working either. Here’s what we did…”

And not two minutes into that did Jackie, the UI lead cut in. “There were threading problems with that. We did something else…”

And, of course, a few minutes after that, Simon, one of the UI developers spoke up sheepishly. “Well… The product catalog doesn’t work like that. I reused quite a lot of code from (company they merged with) which worked a bit different. It was practically done, all I had to do was fiddle with the interfaces a bit.”

The first thing I suggested we do on that project was to sketch up some diagrams explaining the way things really worked. And not just one single diagram with everything in it, but a bunch of smaller diagrams each describing one perspective of the system; one showing how it would scale out, another showing basic messaging patterns, and another showing dependencies between packages, etc. After we finished that exercise (took a little under a week all things considered), the result was a little (or a lot) different from what each of the team thought. After that we were able to have very productive conversations about how to expand on the system.

Here’s the bottom line: you don’t need to invest an enormous amount of resources on “Architecture” and “Design”. Just sketch up what you think the solution should look like. Don’t let the tools dictate too much – you’re not going to be generating code from this. These drawings are just a platform on which you can have design discussions. Keep the sketches as simple as possible without hurting their ability to communicate your design intent.

The purpose of these exercises is to get everyone pulling in the same direction. It’s hard enough making progress without having people coding to conflicting design objectives.

First principle of design refined

Saturday, May 12th, 2007

I’ve been trying over time to distill core principles that when followed lead to high quality designs. So far, I’ve only got one that I’ve been polishing. Here’s a reminder:

For any classes A and B in a system, A should interact with B through an interface.

The interface which separates two concrete classes should be packaged separately from either of those classes.

OK, so where do I go around these principles?

Well, concrete MessageHandler classes directly use concrete message classes. I don’t think of this as a problem because message classes are really just Data Transfer Objects – classes with no behavior.

So here’s the refined principle:

For any classes A and B in a system where both A and B have behavior, A should interact with B through an interface.

The interface which separates two concrete classes should be packaged separately from either of those classes.

That feels better. It holds for Views interacting with Controllers, MessageHandlers with Messages, even interactions with Domain Classes.

What do you think? Does this hold for the way you design your systems? Are there scenarios where you think this principle is too general? Let me know.

Tasks and Spaces versus Messages and Handlers

Thursday, May 10th, 2007

While going through the JavaSpace presentation I found on Owen Taylor’s blog, I kept saying to myself, “well, I can do that without a space”, until I got to one part of it.

The ability to introduce a new task at runtime without restarting any servers, and have new clients be able to send those tasks, and existing servers perform them. I never did that before.

This is very important when you’ve already got a system running and you want to expand on it. I think that this covers at least half of all the software work being done on the planet.

The interesting thing is that this ability comes in two parts – one design, the other technology.

In terms of design, in order for existing servers to be able to do the work of the new task without any kind of restart, we need the code for performing the work of the task to reside in the task itself, possibly in some kind of “execute” method. The server would simply take a task out of the “queue” of pending tasks and call that execute method.

Now, those of us trying to do this with Microsoft technology know that if the assembly containing the code for that task was not available on that machine, this wouldn’t work. Technologically speaking, we’d receive some kind of deserialization exception that resulted from a TypeNotFoundException. In other words, in order to support the new task, we’d need to “install” all of its participating assemblies on all our servers.

For those of us who know and use Jini with Java, we get this behavior automatically – the bytecode of the task is downloaded automatically. This is an advantage in terms of operations, but I’m not an operations guy so I can’t say how huge this really is.

The interesting thing for me in this design is that it’s somewhat different from the messaging paradigm I’ve been so successful with. Here we don’t use messages as simple Data Transfer Objects. Tasks contain both data and behavior. This behavior used to belong to message handlers. I like having separate message handlers, it enables me to create very flexible pipelines.

Here’s how I’d trade it off. When using Jini, the tasks style gives me zero footprint deployment. When using .NET, if I’m already installing things on my existing servers, I can deploy new messages and handlers just as well as the task assemblies. The problem is that I’d need some way for the server to pick up on the new handlers – a background thread scanning the deployment directory, or the FileSystemWatcher (given the fact that it sometimes misses things).

Well, it looks like both design styles are feasible on both platforms. The ability to have code download automatically on the Java platform is a plus that is most felt when using tasks.

Bottom line so far, there’s a lot to learn from JavaSpaces, and even if you don’t use Java, Jini, or Space technologies (like this), the design patterns employed there are extremely valuable.

Generic Validation

Monday, April 30th, 2007

Ayende brought up the topic of Input & Business Rule Validation and I wanted to post how I solve this issue.

On kind of input validation is something you do as close to the user as possible for performance reasons. This includes all sorts of smart stuff you can do with JavaScript in Web scenarios. When in a Smart Client environment, you usually have greater capabilities.

When I look at the issue of validation, I see that it centers around the entity. Sometimes, it is also affected by other things, like what process are we in (as described in the comments on Ayende’s post).

So, we can model the thing that validates an entity with an interface, say, IValidator<T> where T : IEntity. This interface will have one main method: bool IsValid(T entity); and one main property: string ErrorDescription { get; }

What this allows us to do is to separate out different validation concerns into different classes, yet have all of them implement the same interface.

The next thing we’ll need is to be able to get an instance for each of the classes that is a validator for a specific kind of entity. For instance, when the NewCustomerView raises an event proclaiming that it has a Customer object ready to be saved, the Controller will want to find all classes that implement IValidator<Customer> so that it can run all the validation rules.

Luckily, the generics patch I put out for the Spring.Net Framework allows us to do this in one simple line of code:

IList<IValidator<Customer>> validators = spring.GetObjectsOfType(typeof(IValidator<Customer>));

and quite simply perform the validation as follows:

foreach(IValidator<Customer> v in validators)
  if (!v.IsValid(myCustomer))
    // notify user with v.ErrorDescription, write to log, whatever

Now, when using this in Smart Client scenarios, you will often have views that allow the user to enter in a single entity which you will then want to validate. If you have those views implement a generic interface like: IEntityView<T> where T : IEntity, then you could have a single base class implement a “Validate” method, which would perform the work above generically like so:

foreach(IValidator<T> v in spring.GetObjectsOfType(typeof(IValidator<T>)))
  if (!v.IsValid(this.Entity))
    // notify user with v.ErrorDescription

and just have your specific view call that method on the button click.

This enables all entity views to activate all the relevant custom validation logic without being tied to it. It also enables you to extend your system by adding new classes implementing the IValidator<T> interface, and have them automatically run without even changing a config file. How’s that for loose coupling?

Finally, on the issue of tying validation rules to specific processes, this can be done by extending the interface to: IValidator<T, P> where T : IEntity, P : IProcess; You then model each process with a marker interface. You then have your specific validation classes implement the above generic interface for each specific process interface. For instance, say we have a validation rule that needs to run for processes P1 (marked by IP1), and P2 (marked by IP2), but not P3 (marked by IP3), which validates entities of type E. This would be done by defining the class like so:

public class MyValidator : IValidator<E, IP1>, IValidator<E, IP2> {}

and the Controller class that would request validators for process 2 would just call:

spring.GetObjectsOfType(typeof(IValidator<T, IP2>));

That’s it. The basic principles are simple, but, as you can see, can create very powerful structures. I’ve got to say that this exemplifies one of the reasons why I love generics so much. When used with Dependency Injection, and/or Delegates, and/or Anonymous Methods, you get such a power of expression just by defining an interface. This is one of things that make coding fun for me. Or maybe I’m just wierd that way 🙂

Service-Layer Separation of Concerns

Sunday, April 1st, 2007

The idea of having a generic bus object that dispatches message objects to their appropriate message handler objects seems to have struck a chord with Ayende (aka Oren). It is definitely one way of doing the Unit-of-Work Pattern that also partly goes along with the idea of Consumer-Driven Contracts, in that the consumer “defines” the transaction boundary. These two topics are directions that the Bus API was not originally intended to cover, but I’ve found them useful in many cases.

One of the strengths of this approach that isn’t immediately apparent is that you can have multiple classes that handle the same message type. These classes are then configured in the Bus to run in a certain order – very similar to the channel model in WCF, or the HTTP Handlers in ASP.NET. Each message handler in the chain can stop processing of the message if it should so choose.

The use of this strength is that it allows for a strong separation of concerns in the message handling logic. Need to do some pessimistic lock checking first? No problem – have a separate message handler class that does that. Want to add some custom auditing before and after all other processing, configure in a couple more message handlers. Have some complex validation logic that you’d like to keep separate from the rest of the business logic? Put it in its own message handler class.

For those thinking about more advanced messaging scenarios, you could have each message handler in the chain do some Content Enrichment and have that data available in down-stream handlers.

I’ll address the issue of sending messages using this pattern in a coming post, so stay tuned J


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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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