Architecture Category for Udi Dahan's Blog

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Fear those Tiers [IASA]

Saturday, March 24th, 2007

The International Association of Software Architects (IASA) has just completed an ambitious project which I am very proud to have taken a part of. As quoted by BusinessWire:

The industry’s first IT architecture skills library has been unveiled by the International Association of Software Architects (IASA), a 5,000-member association focused on defining and supporting the professional duties of IT architects. Consisting of more than 600 pages of information, the library is an invaluable resource for each primary skill of the practicing IT architect.

Besides information specific to information technology, software and infrastructure specializations, the library includes the fundamental skills necessary for all successful architects. The material is freely available to the community and will provide a solid foundation for an aspiring architect to understand the capabilities of a senior architect, regardless of specialization. It can also be invaluable in communicating the critical nature of architects to management, clients, and other decision-makers.

A collaborative effort of 75 practicing architects from around the world and commissioned by Microsoft, the library can be accessed for free at: www.IASAhome.org/web/home/skillset
.

My part in this effort was an article called “Fear those Tiers”, for the topic of Solutions Architecture – here’s a teaser:

—

I was horrified. My beautiful tiered architecture was buckling under the stress tests – at one tenth of the expected load of the system. And this was quality hardware, not the regular junk we scrapped together in most test labs, after all, this was the company’s flagship project. And I began seeing my once-bright career flashing before my eyes. The only redeeming thing about this situation was that I had learned the hard way to do performance tests early in the project, and that there might still be enough time to save the situation. A couple months later things were back on track but I’ll never forget the feeling I had back then – “All the big vendors are pushing this exact architecture. How could this be happening?” Since then, I’ve learned that tiers and networks are not to be taken lightly in any architecture and this article chronicles my journey, and how I got to a robust and scalable architecture.

—

Read the full article here.

Posted in Architecture, Articles | No Comments »

Errors, Exceptions, and Asynchronous Web Services

Friday, March 23rd, 2007

When a client initiates an action on a service by sending it a command message, it is reasonable to assume that that client will want to know about the success, or failure of that action. When working with today’s Web Service technologies, many developers perform these Web Service calls synchronously and have the service throw an exception to provide details about the specific failure. This is especially common when the Web Service call needs to return some other data, for instance the ID of the entity just added. Scalability issues around these kinds of synchronous interactions are leading the industry to more asynchronous message-based communications. The only problem is that the stack-based exception model for dealing with failure breaks in the asynchronous model.

Let’s see some code that demonstrates simple, synchronous communications and handles some error case:

public class UserController
{
	public void AddUser(string username)
	{
		try
		{
			int userId = myServer.MyService.AddUser(username);
			// add user to list of users
		}
		catch(DuplicateUsernameException)
		{
			// notify user and ask to choose a different user name
		}
	}
}

Once we move to an asynchronous interaction model, the thread that makes the Web Service call is no longer waiting for a response – in other words, the call to “AddUser” in the above example has a “void” return type. This begs the question how would the user ID be returned, let alone information about the success, or failure of the call.

The pattern which handles this scenario is called Asynchronous Completion Token. This pattern suggests having the client pass an additional object as a part of its call to the Web Service. This object would contain both the logic and data needed to handle the result of the call. In .NET, asynchronous invocations are supported with the AsyncCallback delegate and its corresponding IAsyncResult interface for getting the data – let’s take a look at the resulting code:

public class UserController
{
	public void AddUser(string username)
	{
		myServer.MyService.AddUser(
			username,
			(AsyncCallback)delegate(IAsyncResult result) {
				// remember that this code runs on a background thread
				int userId = Convert.ToInt32(result.AsyncState);
				// add user to list of users
			}
		);
	}
}

The code defined as the asynchronous callback will be run by a background thread at some time after the original thread that called “AddUser” had already exited the method. The next question is “what happened to the exception?”, or how do we use exceptions in the asynchronous case.

The simple answer is that we can’t use exceptions to handle these errors – simply because there is no thread that is waiting to catch such an exception even if it were thrown. This is strictly a client-side limitation, on the server-side there is no problem in throwing an exception – since it will be caught by the Web Services infrastructure and marshaled to the client. What this means is that we need an alternative mechanism for returning error information to the client. At the most basic level, this means using return codes, integer values representing the various error conditions. Going one small step up, we can use an enumeration to communicate the intent of each value like so:

public enum MyServiceErrorCodes { NoSuchUser, DuplicateUserName, ... }

Unfortunately, we find that the basic .NET types are too generic to support this scenario in that we need to support both the information returned by the Web Service and the error code. If we assume an integer based error code, we can create our own implementation of the Asynchronous Completion Token and use it like this:

public delegate void AsyncCompletionToken(object state, int errorCode);

public class UserController
{
	public void AddUser(string username)
	{
		myServer.MyService.AddUser(
			username,
			(AsyncCompletionToken)delegate(object state, int errorCode) {
				// remember that this code runs on a background thread
				if (errorCode == (int)MyServiceErrorCodes.DuplicateUserName)
					//notify user
				else
				{
					int userId = Convert.ToInt32(state);
					// add user to list of users
				}
			}
		);
	}
}

In order to make this code work, we need to change some of the infrastructure-level behaviors of the Web Services stack, something that is much easier to do with Microsoft’s Windows Communication Foundation (WCF) than with previous implementations (like the Web Service Enhancements and the basic XML Web Services of .NET). A coming article will show exactly how to do this.

The important thing to understand when moving to asynchronous communications between clients and Web Services is that the old exception-based models for communicating errors no longer work. This impacts the way client-code is written to a large degree; server code is affected as well – returning error codes rather than throwing exceptions, but this is a relatively minor change. One final note, in both the synchronous and asynchronous cases the errors returned by the Web Service are a part of its contract and need to be versioned carefully. The move to increasingly scalable Web Services comes with an increase in complexity as well. Hopefully this article has helped you better understand the details of how to make asynchronous Web Services work for you.

Posted in Architecture, Development, Web Services | No Comments »

Fear those Tiers

Tuesday, March 20th, 2007

Published as a part of the International Association of Software Architects (IASA) Skills Library.

Summary: I was horrified. My beautiful tiered architecture was buckling under the stress tests – at one tenth of the expected load of the system. And this was quality hardware, not the regular junk we scrapped together in most test labs, after all, this was the company’s flagship project. And I began seeing my once-bright career flashing before my eyes. The only redeeming thing about this situation was that I had learned the hard way to do performance tests early in the project, and that there might still be enough time to save the situation. A couple months later things were back on track but I’ll never forget the feeling I had back then – “All the big vendors are pushing this exact architecture. How could this be happening?” Since then, I’ve learned that tiers and networks are not to be taken lightly in any architecture and this article chronicles my journey, and how I got to a robust and scalable architecture.

Posted in Architecture, Performance, Scalability, Web Services | No Comments »

Communication, Risk, and Your Career

Tuesday, March 13th, 2007

Over on the Real-World Software Architecture blog I just read a great post, “I want what I want when I say I want it”. Does this sound like one of your customers/managers?

I wonder if these people go into Burger King, order a # 5, and finish the order by saying, and I would like that done in 60 seconds. Then do they proceed to ask every 10 seconds if the order is done? When they hit the 60 second mark do they start screaming at the top of their lungs “I told you I wanted that order in 60 seconds. If you don’t have it to me in the next 15 seconds I want my money back and the order for free.”?

I recently had the pleasure of working with a customer who “got it”. First of all, they understood that they couldn’t really set scope, time, and cost. Or rather, if they did, then quality – the main property that they had the least power over, would suffer. This could manifest itself in terms of an ugly or unusable user interface, or poor scalability – resulting in sky-rocketing operations costs. In short, they understood that in order to get what they really wanted, they had to give up “control”, or rather the illusion of control. By working collaboratively with the development team, they were able to get a good looking and workable prototype out in time for a tradeshow, as well as do a load test early in the project to see that the architecture was scalable.

Risk is an inherent part of all projects, to a lesser or greater degree. We need to manage risk by including risk-reduction activities (like load tests) into the project schedule. Some customers don’t understand that. It is therefore our responsibility to explain that, although this isn’t a value-creating activity, it is still necessary – or rather what cost could manifest later as a result of ignoring the risk (sky-rocketing operations costs).

But at the end of the day, the buck doesn’t stop here, I’m afraid. It’s not the development team’s money. If the customer chooses to make decisions that increase project risk, it is their choice. It is our responsibility to make them aware of the ramifications of their choices. It is good practice to document these decisions so that if (when) they come back to bite us, we can show that we communicated the possible outcomes ahead of time. I can tell you that it is a huge relief when you have an email record to counter “This system is slow. I thought you knew how to make a system that could handle this many users! Now you’re going to tell me that you have to take down the whole system to fix it – do you know how much that costs?!”

I don’t know why customers behave these ways, I just know that they do. I also know that I have to protect myself and my team from it. But, so as not to end on a doom-and-gloom note, I have found that by communicating more with my customers, I find out all sorts of interesting things. For instance, it’s not that they don’t want us to load test, it’s just that they don’t want us to do it now – they need to get something ready for the tradeshow. They just didn’t think it was important to tell us that.

So, reach out. I know I was surprised by the amount of illogical behaviour that disappeared once I started communicating, despite the customer’s apparent inapproachability. It just might save your project, or your career.

Posted in Architecture, Management | No Comments »

Better Domain-Driven Design Implementation

Tuesday, March 6th, 2007

I just ran into a great example for explaining DDD in Ayende’s post “Entities, Services, and what goes between them”. Let’s just jump right into the code:

public class Order
{
       public virtual Money CalculateCost()
       {
              Money cost = Money.Zero;
              foreach (OrderLine line in OrderLines)
                     cost = cost.Add(line.Cost);

              cost = cost.Add(this.ShippingCosts);
              return ApplyTaxes(cost);
       }
}

public class OrderService
{
       public virtual Money CalculateCostForOrder(int orderId)
       {
              Order order = Repository<Order>.FindOne(
                     (Where.Order.Id == orderId).ToDetachedCriteria()
                           .SetFetchMode(“OrderLines”, FetchMode.Eager));

return order.CalculateCost();
}
}

I’d like to improve this already great code.

First of all, the fact that the OrderService just calls the CalculateCost method on the Order object is great. What is interesting is how it knows that the implementation of that method requires the OrderLines to be fetched. From a performance perspective, this is absolutely correct. We want to hit the database only once, so eager fetching is good. What I would like to do is to take this knowledge a collocate it with who should know it.

Well, the class that absolutely knows about the fact that OrderLines are required for the CalculateCost method is the Order class. However, the OrderService is the one that needs to make use of the fetching strategy, so there needs to be some way for us to communicate this. I usually use an interface to represent a role that (9 times out of 10) has a single fetching strategy.

The two common cases where fetching strategy differs is when fetching the object “for read” – in other words, to show data to the user, and the other is “for write” – in essence calling methods on the object which change its state. The third, somewhat less common case is “for calculation” which is described above. We could model these as IOrderInfo, IOrder, and IOrderCalculator respectively – with each of them exposing the relevant properties, methods, and events. By and large, we would have one class implement all these interfaces, but this isn’t a hard and fast rule.

Now, imagine that there was an interface called IFetchingStrategy. Also, imagine we had a factory which created objects implementing it based on some type we gave them – let’s call that IFetchingStrategyFactory. In it we could find that under the type IOrderCalculator was a fetching strategy describing eager loading for OrderLines. Our generic repository could go to the IFetchingStrategyFactory automatically, without the service getting involved. This would simplify the service code:

public class OrderService
{
       public virtual Money CalculateCostForOrder(int orderId)
       {
              IOrderCalculator order = Repository<IOrderCalculator>.FindOne(
                     (Where.Order.Id == orderId));

              return order.CalculateCost();
       }
}

But the issue here isn’t just simplicity, but the Single Responsibility Principle – or that each class should have only one reason to change. Should the fetching strategy change, the OrderService class would not have to change.Moving forward with this, we realize that when OrderLines are loaded for an IOrderCalculator, they don’t need their connection to the Product. In which case, it might make sense for us to have a separate class, OrderCalculator, and not just Order. OrderCalculator would contain a list of OrderLines, but not as IList, but rather IList<IOrderLineForCalculation>. Using a sufficiently intelligent O/R mapper, we could map the class OrderLine with a fetching strategy as well, so that when it is requested as IOrderLineForCalculation, the Product association would not be traversed. Of course, when using simpler O/R mappers we might have to create a new class.

This may seem like a lot of trouble to go to. It actually isn’t. It’s pretty much just a different way to package the code you already wrote. Yes, there are more interfaces, and probably more classes, but the amount of business logic code is the same. I’ve been able to keep performance high with this design, but increase its maintainability. I measure maintainability as both the amount of time, and number of changes that need to be made by a programmer familiar with the design. Learnability (?) is often called maintainability, but I think that it’s something else. This design may not be as learnable– meaning it would take a given programmer longer to learn this design than the previous one. I submit that the increased maintainability outweighs the increased learnability substantially.

I would love to see some standardization around these principles, making it easier to change O/R mapping tools and decreasing the learning curve for developers changing tools. In my opinion, these principles are key to moving the implementation and adoption of Domain-Driven Design and O/R mapping forwards, specifically in handling the problematic performance perception in data-driven environments like most enterprises.

Posted in Architecture, DDD, Development, NHibernate, Simplicity | 23 Comments »

Paratechnological value

Tuesday, March 6th, 2007

Technological churn – it’s a killer. It appears that as time goes by, the deluge just increases. I almost went into management because of it. Well, to be more precise, I do (kinda) manage, but I mean that I almost left the whole technology/engineering side of things. Luckily there’s more to software than that. Specifically the things around technology that aren’t technology dependent – paratechnological (from paralegal) if you will.

The most interesting thing is that the knowledge investment you make in paratechnology has a (much) longer lifespan than that in technology directly. Those of us who have perfected our skills in VB6 found that .NET obseleted much of that. The same is true for Microsoft’s Web Service Enhancements, EJBs, etc. Even the languages themselves are changing substantially – as are their runtimes. However, object-orientation seems to be holding its own well into its 3rd decade. I suspect Domain-Driven Design (DDD) to enjoy a long life as well. Beyond just being a lifetime issue, these paratechnological entities just seem to build on each other more and more. In order to understand higher order patterns, lower one need to be understood first. These patterns increase the scale of the problems that can be solved repeatably.

It has been my experience that those developers with similar experience-in-years, but only technological knowledge, are less productive than their peers with more paratechnological knowledge. I measure productivity in terms of number of feature points per unit of time. I understand that this is biased against infrastructure developers in application development teams – but I haven’t figured out how to measure them yet (at least, not well enough to say anything about it).

Therefore, it is my thesis that developers should spend at least the same amount of time investing in paratechnological learning as technological learning. It is my suggestion that a 3-to-1 ratio would be even better – as in only a quarter of your time on technological study. I submit that the on-the-job work done with a given technology, including the figuring out how to do something that you haven’t done before, is enough. When I’m talking about “study”, I’m referring to courses, conferences, books read (not when looking sometime up).

For me, the way that I find out (most) of what I need to know about a technology (not necessarily new), is to grill an expert on it. For instance, at TechEd Developers Barcelona 2006, I had the good fortune to sit with the Workflow Foundation guys for at least an hour, the WCF guys for about 3 hours total, WPF for half an hour, and the CAB guys for almost 4 hours – not all in one sitting 🙂 Anyway, it was mostly me asking how to solve thorny issues I’m having difficulty with in my projects, and them trying to explain to me, in a way I’d understand, that most people don’t have my problems. Personally, I think that, if that’s true, “most people” just haven’t noticed yet 🙂 There were also numerous new ways of doing things that I didn’t consider to be an issue, so I tried to focus my learning on other things.

And I guess that that’s my bottom line. When new technology comes along, you shouldn’t need to “start over”. The way that I design my systems, most technology is hidden away. WPF changes how my views are implemented – but that’s tucked behind an interface so that my supervising controllers don’t care. WCF changes how messages are transferred over the network, but that’s tucked away behind an “IBus” interface; message dispatch is also abstracted with “IMessageHandler” and “IMessage”. By the way, I find WPF to be very interesting in improving the human-to-human interface between Human-Computer Interaction (HCI) professionals and developers.

So… Invest wisely. Compound interest is your friend.

Posted in Architecture, DDD, Development, General, TDD | No Comments »

SOA requires a clear information ownership model

Wednesday, February 28th, 2007

Natan Gur posted a “lessons learned” threesome from some SOA projects he’s been on. These jive very much with my own experiences. His point about a clear information ownership model at the business level is something I have found to have a significant impact on the result software architecture.

A service is the software representation of the entity which is in charge of a given set of information in the enterprise. This means that any changes to that set of data have to be done by that service. It also makes it much clearer about who is in charge of versioning messages containing that data – who owns the message schema.

In those cases where I’ve seen projects ignore this issue, the resulting spaghetti-oriented architecture became a nightmare in terms of development, deployment, and operations management.

Posted in Architecture, SOA | No Comments »

Product-Driven Architecture and O/R Mapping

Thursday, February 22nd, 2007

Published in Developer.com

Summary: The single, most important trap to watch out for when choosing an object-relational mapping tool is this: “Product-Driven Architecture”. PDA is a term I use to describe a set of symptoms that I’ve seen in projects over the years. In these projects, entire teams would spend months agonizing over product comparison tables, debating the importance and rankings of various features, all without a coherent architecture.

Posted in Architecture, NHibernate | No Comments »

Autonomous Services and Enterprise Entity Aggregation

Tuesday, February 20th, 2007

Published in Issue 8 of the Microsoft Architecture Journal.

Summary: Enterprises today depend on heterogeneous systems and applications to function. Each of these systems manages its own data and often doesn’t explicitly expose it for external consumption. Many of these systems depend on the same basic concepts like customer and employee and, as a result, these entities have been defined in multiple places in slightly different ways. Entity aggregation embodies the business need to get a 360-degree view of those entities in one place. However, this business need is only one symptom of the larger issue: business/IT alignment. Service-oriented architectures (SOAs) have been hailed as the glue that would bring IT closer to business, yet the hype is already fading. We’ll take a look at concrete ways that autonomous services can be used to transform the way we develop systems to more closely match business processes and solve immediate entity aggregation needs.

Posted in Architecture, Autonomous Services, EDA, ESB, Master Data Management, SOA, Web Services | 7 Comments »

Request/Service state affinity – don’t.

Sunday, February 18th, 2007

I saw this question today on the one of the blogs I follow, and seeing that it’s a question that variations of it pop up all the time, I thought that I’d chip in with my 2 cents.

How do I store some state about the current request so that I can use it later during the same service operation?

One analysis I read came at it from a technological angle – how to do this with WCF. I want to take a look at two other angles here.

The first has to do with one interpretation of the question – in the course of handling that request, there are numerous objects involved. How can we make it so that all of them have access to the request data? From a technological perspective, the answer is simple – make it thread-static (in .net this is done by applying the ThreadStaticAttribute to it), or store it in some thread-local-storage. From a design perspective, though, things aren’t all that clear. Which property of which class contains the request data, so that we can mark it with such an attribute, or under what key is the data stored in the thread-local-storage?

What I usually do is use my “IBus” interface, which exposes a “MessageBeingHandled” thread-static property. Any object that needs state about the current request makes sure to get an instance of “IBus” injected. The classic example of objects that need this data include message handlers (implementing the “IMessageHandler” interface). For more information about this design, take a look at this.

The second interpretation looks at having that request data available to the service on subsequent invocations. Personally, I don’t like the idea of having this data in-memory on the object that serviced the original request. One reason I don’t like it is that it creates an affinity between the client and the specific server handling its requests. Those of you who know me already are expecting this…

What if the server restarts?

Will that client’s state be lost? Well, not if we persisted it somewhere durable instead of just in memory. Will we stop servicing requests from that client until the original server becomes available again? Well, if the data was durably persisted, then any server could pick it up. And this is exactly what BizTalk does. You don’t want to implement BizTalk again, do you?

I can tell that some of you are surprised to hear me say this. Such a small requirement, and already we need BizTalk? Did Udi really say that?

Well, there is another, simpler way. If what you have is some kind of back-and-forth between the client and the service, you could use the Message History pattern and pack up the previous request data into the messages being sent. Although we’re increasing the message size, we’ve made it so that any server can handle any request and have access to all the previous data without creating some sort of durable contention area within the service like a database. Another option is to look at long-running workflow to model these interactions.

Finally, when it comes to ultra-scalable systems, I strongly suggest keeping the network dumb and pushing the smarts out to the edges – the clients. If you don’t need to have one client pick up where another client left off, this could be the ultimate solution. It combines with the Message History pattern and ends up sending only the data necessary on subsequent requests, thus keeping message size to a minimum. Also, your service doesn’t have to handle the state any more making it capable of handling more concurrent clients.

State management is the heart of any distributed systems development effort. Unfortunately, there aren’t any easy answers to it, but it’s important not to gloss over it if you want to have any hope of scalability in the future. Patterns help, but eventually we have to make the tradeoffs ourselves. Just don’t go running to one product or another in the hopes that it will make everything magically better.

Posted in Architecture, BizTalk, NServiceBus, Simplicity | No Comments »

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“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.

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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.”

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“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.”

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“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, 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, 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, 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, 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, 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, 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, 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, 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, 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, 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, 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, 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, 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, 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, .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, 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, 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, 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, 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, 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, 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, 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, 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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Guest Authored Books

Article: The Enterprise Service Bus and Your SOA

97 Things Every Software Architect Should Know

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