Udi Dahan   Udi Dahan – The Software Simplist
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Archive for the ‘Simplicity’ Category

Are DSLs the cure to OO’s ails?

Tuesday, March 27th, 2007

I very much enjoy the podcasts put out by No Fluff, Just Stuff and while I may not agree with all that is said, it is always a pleasant way to spend my commute. While listening to the recent conversation with Neal Ford (blog), I found myself wanting to respond. This further strengthened itself today after coming out of a design review where objects were used, but no object-orientation was to be found. I’m beginning to think the solutions being presented by Domain Specific Languages (DSL) are for the same problems that OO solves. The fact that OO is practiced so little, even when using languages that support its concepts well, makes me think that DSLs might suffer a similar fate.

Around the basis of OO, many other design elements have sprouted and grown. Ideas like “once, and only once”, also known as Don’t Repeat Yourself (DRY – by the pragmatic programmers) have crystallized into principles like the Single Responsibility Principle. Yardsticks of design like the Law of Demeter, used in conjunction with “Tell, Don’t Ask” (see IEEE Software,Jan./Feb. 2003, p. 10) have served practitioners well. Larger efforts like Domain-Driven Design are taking us to the next level in terms of IT/Business alignment. Maybe the most interesting thing is that all this has been occurring in a highly disconnected way from developments in code generation, one of the ancestors of DSLs. New, exciting developments are still growing in the shadow of the great OO oak tree – Behavior-Driven Development (BDD) is building on Test-Driven Development (TDD) which, itself, is still in the “early adopter” phase.

[Full Disclosure] I authored a chapter in this book.

I am worried that in the rush to embrace DSLs as the new “silver bullet”, the baby will be thrown out with the bathwater – and 2 to 3 decades of learning will have to be re-learned. It’s either that or jumping to the next silver bullet that comes along.

Of the two kinds of DSL, internal and external, I believe internal DSLs, described by Paul Graham as Programming Bottom-Up, and Fluent Interfaces to continue in the tradition of OO evolution.

I don’t think there will be any one practice that will bring about a dramatic jump in productivity as I look 10 years down the road. I do believe that by embracing our collective lessons learned we can do just that. The best programmers aren’t 10, 20, 100 times more productive than the average because they type faster, if anything, they type a lot less. An ounce of prevention is worth a pound of cure, so too with design and debugging.

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.

Design & Testability – Sense & Sensibility

Sunday, March 4th, 2007

A discussion has been growing (primarily) between the Israeli bloggers Eli Lopian, Roy Osherove, and Ayende Rahien (aka Oren Eini) around the issue of OO design and testability. Mark Seemann quoted me on the topic (although I don’t quite remember saying it – but my memory isn’t what it used to be) so I guess that I’m now compelled to join in.

First of all, let me say that design does not necessarily begin and end with OO. The quality of a given design needs to be measured against both functional and non-functional requirements. Beyond the original OO, other principles have emerged that positively affect design – Arnon blogged about these a while ago. The Single Responsibility Principle being one that keeps you thinking, “should that code go here?”. I also couldn’t design without leaning heavily on the Interface Segregation Principle and the Dependency Inversion Principle. Together with OO, I have found that these principles lead to more modular, flexible, loosely-coupled, and (yes) testable designs – all good things. Furthermore, we are continuing to uncover more areas where the original OO thinking does not serve us too well – distribution is a big one, so is persistence.

On the topic of unit testing, it is OK to do state-based testing instead of directly calling methods on the class under test. I find this happening quite often with my “controller” classes which do most of their work in private event handlers. I could make those methods public for testability’s sake, not that it would change the design much since no other class interacts directly with the controller. I prefer to activate the controller as it would in production, and that’s by having my mocks raise events. The assertions I make are done upon the state the controller changes  – in my (mock) repository. I believe that this style of testing does nothing to compromise the “unit-ness” of the tests and addresses many of the points Eli brought up in his post.

Anyway, that’s my 2 cents and I hope they make sense, and bring some down-to-earth sensibility to those wondering “are we asking the right question?”.

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.

It’s poison, I tell you. Poison!

Thursday, February 15th, 2007

When developing systems that handle messages that arrive on queues or other asynchronous transport mechanisms, broader failure scenarios need can be handled than in RPC-style communications. One of these failure scenarios that’s been getting some recent play on the blogosphere is that of poison messages. Microsoft’s Nicholas Allen does a great job describing what defines a poison message and the common ways of handling it. Thomas Restrepo adds another facet to the whole issue of poison message handling that is a must-read.

Let’s take a rather simple case, but definitely a common one when we start versioning our services – that of the receipt of a malformed message. It could be that we were unable to deserialize the data or something more complicated, but at the most basic level, there’s nothing the service can do with the message. This is often expressed in terms of some sort of exception that reaches all the way down to the messaging infrastructure. If we were using the queue in a transaction, we’d see a rollback and the message return to the queue, causing the service to loop endlessly on that message. This sort of failure case is best handled by having the messaging infrastructure just move the message to an error queue.

Other kinds of failure conditions include having our DB transactions fail because they were chosen as the victim of a deadlock. Matts does a good job of describing why these things occur and how to handle and minimize their occurrence. The appropriate system level error handling is to just retry the transaction a bit later. Once again, if we were using our queues within the context of a transaction scope that included that of the database, the message would return to the queue when that deadlock exception bubbled through. This is exactly the behavior that we want in this case, although having the message go to the end of the queue instead of the front would also be just fine.

Other kinds of failure conditions we might run into include things like unique-constraint violations, attempting to perform actions on entities that no longer exist, and other activities where trying them over again probably won’t yield any different behavior. In these cases, if an exception were to cause the perpetrating message to return to the input queue we’d get exactly the wrong behavior. At the very least we would want to maybe log the exception information, but putting the message in the error queue doesn’t seem right. I mean, there was nothing wrong with the message, it’s just that the application state had moved on or that some other logic failed. One of the reasons for putting the message in the error queue is so that a person can manually fix what’s wrong with the message and send it back to the input queue for reprocessing. In these failure conditions, that hardly seems likely.

What I usually suggest for handling failures when messaging is involved goes something like this. First of all, have an error queue/topic – they’re exceedingly useful. Second, don’t automatically roll messages back. If the service wants to retry, they should do so after handling all the other messages already in the queue – unless (!) you’ve got message ordering issues like Thomas mentioned. This means having the service request the messaging infrastructure send the message back to itself. When it comes to exceptions around the logic handling the message, just write them to a log and be done with it. The service logic will probably log anything else that’s of particular interest to it. This leaves us with the malformed message scenario. In that case, the communications layer can know that the problem occurred before any service logic (like deserialization exceptions) and therefore can (rightly) pass the message along to the error queue.

Of course, for the above guidance to be valid, we need to educate service logic developers as to when it is appropriate for them to throw exceptions and when they actually have to send the message back themselves. It is often the case that by taking on certain constraints, we can greatly simplify the handling of scenarios which are difficult in the general case.

So, how many machines/CPUs do we need?

Monday, February 12th, 2007

Regu posted an interesting question recently: “Is scalability a factor of the number of machines/CPUs?”. His answer can ultimately be summed up as “yes, but…” – it was qualified in terms of threads: “… scalability in a well designed system is a factor of number of threads that can be efficiently executed in parallel”. The word “efficiently” meaning that the threads are actually doing work and not just waiting. However, the question of how many machines do we need is a hard one. Nick calls out a very important point on this, “An asymmetric farm, with machines of varying capabilities, is really hard to tune.” In all cases we find that load-leveling mechanisms like queues are good for scalability.

Just as a slight sidebar for anybody who deals with systems where work needs to be divided up and run in parallel to achieve required latency requirements, we have to deal with all the above problems and more. For instance, if we have to process images, finishing the processing on each image in one minute. Now, we have an algorithm that can do part of an image that runs at a speed of 1MB per second, single threaded on a dedicated machine with a standard 3GHz processor. So, how can we process a 1GB image in 60 seconds? Simple, get 17 processors right? Well, if you were running a 16 or 32 way SMP machine then probably yes. But what if you want to scale out, say, because you’re receiving one image every 2 seconds on average? Well, once we scale out, time is impacted quite significantly by the cost of just moving data between servers – one of the fallacies of distributed computing. It becomes a much more difficult problem – the kind that I just love sinking my teeth into 🙂

Anyway, a lot of us aren’t dealing in these massively parallel problem spaces but are just looking for good scalability advice. Well, one of the characteristics of a scalable system is that load is evenly distributed between machines (up to a point – if we have more machines than work that needs to be done, some will be idle). Load can be broken up in terms of resource usage – CPU, memory, disk, network, etc and we should be looking at all parameters. I’ve noticed a tendency of people to focus only on CPU usage. One case I consulted on was a system that was having performance problems although average CPU utilization was around 50%. They did a costly hardware upgrade at the time from single-CPU machines to all double-CPU, hoping to drive down the utilization and improve performance. They only succeeded half way – CPU utilization did drop, but performance (in terms of response time and throughput) didn’t improve – quite simply because the network was the bottleneck, and not processor power. As Dan so eloquently states: “Latency exists, Cope!”

If you use the Pipeline architectural pattern (page 5) that is so well known in the embedded/real-time space at the macro level (inside the service, not between services – that’s SOA), and SEDA (Staged Event-Driven Architecture) at the micro level you can create an environment where you can know the amount of resources you need to buy/provision for the expected load at a high degree of accuracy. An additional, maybe even more important benefit has to do with the resiliency of such a system. If there is a degradation in resource performance or availability, the system won’t come crashing down but rather “limp along”. Conversely, if load continues to increase beyond expected maxima, the performance (in terms of throughput) of such a system would not degrade. By monitoring response time per request, you could notice the upward trend and provision more resources. If you were working with a grid-like infrastructure, you could set these rules up so that they would be executed automatically. These are the building blocks for building “self healing” systems – one of my current favorite areas of interest.

Bottom line, I’ve found that the layered-architecture/tiered-distribution pair to be rather limited in terms of scalability (in terms of load). I would say that the solution isn’t necessarily to move to a Space-Based Architecture, as Guy mentions in this post, although many of the event-based concepts are definitely broadly applicable. Werners Vogels (Amazon’s CTO) mentions the CAP (consistency, availability, partitioning – choose 2) model for distributed systems in this podcast which I think is critical in analyzing the different parts of a complex system. On the flip side, Patrick does an excellent job of warning about the dangers of other appealing, siren-esque paths – follow them at your peril.

I’m afraid that there aren’t any easy answers, but at least we have some models that have proven themselves viable in the most strenuous scenarios. These models sometimes contradict popular architectural styles and it’s good to be aware of that. At the end of the day, it is our job to make the difficult technical tradeoffs.


Check out the “Ask Udi” podcast for this topic: Space-Based Architectures for the Web.

"…the intensity of thought you desire"

Saturday, February 10th, 2007

Every once in a while I read something which resonates deeply with my inner being, and it seems fitting to have come from Maeda’s SIMPLICITY blog this time:

“In Hamming’s lecture, he quotes another famous technologist besides himself, Hendrik Bode:

“The more you know, the more you learn; the more you learn, the more you can do; the more you can do, the more the opportunity — it is very much like compound interest. I don’t want to give you a rate, but it is a very high rate. Given two people with exactly the same ability, the one person who manages day in and day out to get in one more hour of thiking will be tremendously more productive over a lifetime.”

Yet in the same breath, Hamming adds that this means that inevitably you must neglect your loved ones to achieve the intensity of thought you desire.

Finding balance is certainly the ultimate challenge in life. Luckily you have a entire lifetime to take on this challenge.”


[Podcast] SOA, ESB, and Events — No 2.0

Tuesday, June 13th, 2006

Despite the 2.0 moniker that’s been slapped on events, they really are necessary for loosely coupled services. Join us for a deep-dive on how to enable events with messages, handlers, and today’s technologies.

Get it via the Dr. Dobb’s site here.

Or download directly here.

Want more? Go to the “Ask Udi” archives.

Specialists vs Generalists

Saturday, April 1st, 2006

Ralf brings up a lot of intersting points in his last post – some I agree with, and some points I don’t. In the context of XP, Ralf counters the requirement that members of an XP team be “jacks of all trades” (which I think is a bit of a misunderstanding – from what I’ve learned, (and I’m far from being a XP master) XP and the agile methodologies “generalizing specialists”). Anyway, that’s not the point I wanted to bring up. Ralf states:

“Software quality requires specialization: Number and complexity of software technologies and tools will continue to increase. I think it´s obvious, that it becomes ever harder to stay on top of current tools and technologies. 20 years ago database programming was plain simple and required knowledge of one API. Today it´s several APIs, languages, programming models (e.g. ADO.NET, O/R Mapping, SQL XML, SQL), plus the intricacies of database products like SQL Server 2005 (e.g. SQL CLR, T-SQL, Web service interface, SQL Service Broker, SQL Reporting Services). And what about (G)UI programming? There is at least WinForms, WebForms, AJAX, Flash, and WPF around the corner. Add to that a couple of new Smart Client frontend options VSTO and IBF and I guess you see what I mean.”

Yes, there are lots of technologies out there. No you don’t necessarily have to know them all, XP teams included. If you’re a UI developer, most of your coding takes place behind the technology specific facade. MVP and “the humble dialog” make client-side programming more resilient to technological churn. On the web, things are different – the paradigm for UI programming there is still solidfying, but most data-driven web UIs where user experience is not the primary requirement can use many of the winclient techniques as well.

The same is true for data access, or rather will be true once you take the O/R mapping pill.

A well designed architecture will isolate technology behind domain-driven interfaces in such a way that the vast majority of a system can be understood without getting down and dirty on MSMQ vs Rendezvous vs MQ – a queue’s a queue (more or less).

However, one sentence in particular struck a chord with me:

“That´s also one reason why software architecture is on the rise. There needs to be someone with an overview of all this and coordinating the weaving: the architect.”

Personally, I disagree with the view that architecture can be handled by a team without any one person grasping the end-to-end view. That person may not need to delve down into every last detail (depending on system size), but the system level flows need to be understood, fully, in a (at least) one brain.

To sum up:

Every problem in computing can be solved by adding yet another level of indirection – well, besides the problem of having too many levels of indirection.

Focusing on Value

Wednesday, February 8th, 2006

I almost skipped over the required “back from…” post in my rush to put out all the content building up in my head after the software architecture workshop in Cortina – so this will be that post.

Anyway, after reading “Innovation Stifled by Dogma”, I was reminded of one of the session we had at Cortina – about IOC and DI. What reminded me was our attempt to focus on the Value produced by these practices, rather than getting too bogged down in techniques or philosophy.

The goals we identified that drive us to use DI, or at its more basic level, dependency on interfaces instead of implementation (DI or service locater being refinements), were simple, and important:

Testability and Runtime flexibility.

By introducing interfaces, we can test classes in isolation, creating tests that will give us fewer false negatives – tests that fail even though the class under test behaved alright. Higher testability often leads to fewer defects.

As for runtime flexibility, the ability to vary implementation without affecting system structure is interesting. Beyond the ability to fix bugs in a given implementation without adversely affecting other parts of the system (a worthy goal in and of itself) I hadn’t really thought this one through. The idea of so-drastically changing an implementation, not with mocks/stubs, but actually changing one with something really different, seems a bit out of touch with reality to me – but maybe its just me.

Anyway, from that group of IOC and DI practitioners I gained much deeper understanding about “why” which has really helped me change the “how” of my work – but I’ll save that for a later post.


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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Avi Nehama, R&D Project Manager at Retalix
“Not only that Udi is a briliant software architecture consultant, he also has remarkable abilities to present complex ideas in a simple and concise manner, and...
always with a smile. Udi is indeed a top-league professional!”

Ben Scheirman Ben Scheirman, Lead Developer at CenterPoint Energy
“Udi is one of those rare people who not only deeply understands SOA and domain driven design, but also eloquently conveys that in an easy to grasp way. He is patient, polite, and easy to talk to. I'm extremely glad I came to his workshop on SOA.”

Scott C. Reynolds Scott C. Reynolds, Director of Software Engineering at CBLPath
“Udi is consistently advancing the state of thought in software architecture, service orientation, and domain modeling.
His mastery of the technologies and techniques is second to none, but he pairs that with a singular ability to listen and communicate effectively with all parties, technical and non, to help people arrive at context-appropriate solutions. Every time I have worked with Udi, or attended a talk of his, or just had a conversation with him I have come away from it enriched with new understanding about the ideas discussed.”

Evgeny-Hen Osipow, Head of R&D at PCLine
“Udi has helped PCLine on projects by implementing architectural blueprints demonstrating the value of simple design and code.”

Rhys Campbell Rhys Campbell, Owner at Artemis West
“For many years I have been following the works of Udi. His explanation of often complex design and architectural concepts are so cleanly broken down that even the most junior of architects can begin to understand these concepts. These concepts however tend to typify the "real world" problems we face daily so even the most experienced software expert will find himself in an "Aha!" moment when following Udi teachings.
It was a pleasure to finally meet Udi in Seattle Alt.Net OpenSpaces 2008, where I was pleasantly surprised at how down-to-earth and approachable he was. His depth and breadth of software knowledge also became apparent when discussion with his peers quickly dove deep in to the problems we current face. If given the opportunity to work with or recommend Udi I would quickly take that chance. When I think .Net Architecture, I think Udi.”

Sverre Hundeide Sverre Hundeide, Senior Consultant at Objectware
“Udi had been hired to present the third LEAP master class in Oslo. He is an well known international expert on enterprise software architecture and design, and is the author of the open source messaging framework nServiceBus. The entire class was based on discussion and interaction with the audience, and the only Power Point slide used was the one showing the agenda.
He started out with sketching a naive traditional n-tier application (big ball of mud), and based on suggestions from the audience we explored different solutions which might improve the solution. Whatever suggestions we threw at him, he always had a thoroughly considered answer describing pros and cons with the suggested solution. He obviously has a lot of experience with real world enterprise SOA applications.”

Raphaël Wouters Raphaël Wouters, Owner/Managing Partner at Medinternals
“I attended Udi's excellent course 'Advanced Distributed System Design with SOA and DDD' at Skillsmatter. Few people can truly claim such a high skill and expertise level, present it using a pragmatic, concrete no-nonsense approach and still stay reachable.”

Nimrod Peleg Nimrod Peleg, Lab Engineer at Technion IIT
“One of the best programmers and software engineer I've ever met, creative, knows how to design and implemet, very collaborative and finally - the applications he designed implemeted work for many years without any problems!

Jose Manuel Beas
“When I attended Udi's SOA Workshop, then it suddenly changed my view of what Service Oriented Architectures were all about. Udi explained complex concepts very clearly and created a very productive discussion environment where all the attendees could learn a lot. I strongly recommend hiring Udi.”

Daniel Jin Daniel Jin, Senior Lead Developer at PJM Interconnection
“Udi is one of the top SOA guru in the .NET space. He is always eager to help others by sharing his knowledge and experiences. His blog articles often offer deep insights and is a invaluable resource. I highly recommend him.”

Pasi Taive Pasi Taive, Chief Architect at Tieto
“I attended both of Udi's "UI Composition Key to SOA Success" and "DDD in Enterprise Apps" sessions and they were exceptionally good. I will definitely participate in his sessions again. Udi is a great presenter and has the ability to explain complex issues in a manner that everyone understands.”

Eran Sagi, Software Architect at HP
“So far, I heard about Service Oriented architecture all over. Everyone mentions it – the big buzz word. But, when I actually asked someone for what does it really mean, no one managed to give me a complete satisfied answer. Finally in his excellent course “Advanced Distributed Systems”, I got the answers I was looking for. Udi went over the different motivations (principles) of Services Oriented, explained them well one by one, and showed how each one could be technically addressed using NService bus. In his course, Udi also explain the way of thinking when coming to design a Service Oriented system. What are the questions you need to ask yourself in order to shape your system, place the logic in the right places for best Service Oriented system.

I would recommend this course for any architect or developer who deals with distributed system, but not only. In my work we do not have a real distributed system, but one PC which host both the UI application and the different services inside, all communicating via WCF. I found that many of the architecture principles and motivations of SOA apply for our system as well. Enough that you have SW partitioned into components and most of the principles becomes relevant to you as well. Bottom line – an excellent course recommended to any SW Architect, or any developer dealing with distributed system.”

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