Udi Dahan   Udi Dahan – The Software Simplist
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From CRUD to Domain-Driven Fluency

Friday, February 15th, 2008

I got a question about how to stay away from CRUD based service interfaces when the logic itself is like that, and I’ve found that this shift in thinking really needs more examples, so I’ve decided to put this out there:

For instance, in an HR system, the process of interviewing candidates – wouldn’t you just insert, update, and delete these Appointment objects?

If I were to put on my domain-driven hat, I would describe those requirements differently – interview appointments have a lifecycle: proposed, accepted, cancelled, etc. It seems that only a user of the role HR Interviewer should be able to make appointments for themselves, so the service layer code would probably look something like this:

using (ISession session = SessionFactory.OpenSession())
using (ITransaction tx = session.BeginTransaction())
    ICandidateInterviewer interviewer = session.Get<ICandidateInterviewer>(message.InterviewerId);
    ICandidate candidate = session.Get<ICandidate>(message.CandidateId);


The “ScheduleInterviewWith” method accepts an ICandidate and returns an IAppointment. IAppointment has a method “At” which accepts a DateTime parameter and returns void – just changes the data of the appointment. The state of the appointment at creation time would probably be proposed. The appointment object would probably be added to the list of appointments for that interviewer – that’s what will cause it to be persisted automatically.

Later, when the candidate accepts the meeting, we could have the following method on ICandidate – void Accept(IAppointment); that would obviously check that the candidate is the right person for that interview, the appointment’s current state (not cancelled), etc – finally updating its state. What part of this looks like create, update, delete? If that’s what your service layer to domain interaction looks like, do you now know what your messages will be looking like?CRUD seems to be what most of us are familiar with. Moving to domain-driven thinking takes time and practice, but is well worth it. Contrast this with a more traditional O/R mapping solution:

using (ISession session = SessionFactory.OpenSession())
using (ITransaction tx = session.BeginTransaction())
    ICandidateInterviewer interviewer = session.Get<ICandidateInterviewer>(message.InterviewerId);
    ICandidate candidate = session.Get<ICandidate>(message.CandidateId); 

    Appointment a = new Appointment(); 

    a.Interviewer = interviewer; 

    a.Candidate = candidate;

    a.Time = message.RequestedTime; 



As you can see, we’ve got simpler, more expressive, and more testable code when employing the domain model pattern, than using “just” O/R mapping. I’m not saying that the domain model pattern doesn’t need O/R mapping in the background for it to work. But that’s just it – the persistence gunk needs to be in the background and the business logic needs to be encapsulated.

So, while I’ll agree with Dave that the Domain Model is more lifestyle than pattern, I would argue against these conclusions:

If this post had a point, it’s only to share the idea that Domain Model is a big, big thing. It’s probably overkill in a lot of cases where you have simple applications that have very simple purposes.

As you just saw in the example above, there is no “overkill” to be seen. The domain model in the example wasn’t “a big, big thing”.

The domain model. Use it.

Why not have a better lifestyle?   ;-)

Fetching Strategy NHibernate Implementation Available

Sunday, September 16th, 2007

A couple of months ago I put out a post discussing one way to implement custom fetching strategies. Anyway, I finally got around to putting my money where my mouth was…

So, I’ve implemented the pattern in NHibernate, adding the following method to ISession:

T Create<T>();

As well as adding the following interface to the NHibernate package:

public interface IFetchingStrategy<T>
ICriteria AddFetchJoinTo(ICriteria criteria);

All this enables you to have a stronger separation between your service layer classes and your domain model class, as well as for you to express each service-level use case as a domain concept – an interface.

Once you have such an interface, you can create a fetching strategy for that use case and define exactly how deep of an object graph you want to load so that you only hit the DB once for that use case.

The nice thing is that its all configured with Spring. In other words, if you the entry for your fetching strategy class exists, you get the improved performance, if it doesn’t, you don’t. All without touching your service layer classes.

Just as an example, when I’m in the use case modeled by “ICustomer”, I want to get all the customer’s orders, and their orderlines. This would be done by having a class like this:

public class CustomerFetchingStrategy : IFetchingStrategy<ICustomer>
public ICriteria AddFetchJoinTo(ICriteria criteria)
criteria.SetFetchMode(“orders”, FetchMode.Eager).
SetFetchMode(“orders.orderLines”, FetchMode.Eager);

return criteria;

And the configuration would look like this (as a part of the regular spring template):

<object id=”CustomerFetchingStrategy” type=”Domain.Persistence.CustomerFetchingStrategy, Domain.Persistence” />

If you want to take a look at the full solution, you can find it here. For some reason, the combined file was too big for the upload on my blog so it’s split into two. Unzip both packages into the same directory. You’ll find a file called “db_scripts.sql” which contains the schema for the DB. Don’t forget to update your connection string in the “hibernate.cfg.xml”. If you’re looking for the changes I made to the NHibernate source, you can find it in the “Updated NHibernate Files” directory. The only real change is to the “SessionImpl.cs” file.

Relevant NHibernate and Spring binaries.

Source code of example.

BTW, there is some intelligent thread-safe caching going on in SessionImpl now so that you get a much smaller performance hit (in terms of code that uses reflection) on subsequent usages of the same interfaces.

Let me know what you think.

DevHawk not thrilled about Astoria either

Friday, May 25th, 2007

It appears that more people are coming to the same conclusions that I have about Astoria (Astoria, SDO, and Irrelevance), Microsoft’s new “Data Services for the Web” initiative.

DevHawk (aka Harry Pierson) writes:

As you might guess then, I’m not a fan of Astoria. I believe the sweet spot for so called “data services” will be read only (because they don’t need transactions, natch). I’m sure there are some read/write scenarios Astoria will be useful for, but I think they will be limited – at least within the enterprise.

After catching up to Pablo Castro (the man behind Astoria) at DevTeach, and chatting with him and Tim Mall about Astoria, I heard something interesting. In order to provide business context for updates, you will be able to use something like a WS-Action header.

So, it’s based on REST, but for updates, it’s like WS-*. Hmm… Best of both worlds, or worst? What do you think?

NHibernate will rule, because Ayende already does

Sunday, May 20th, 2007

First I find out that NHibernate does support “Persistence by Reachability”, even though the docs say it doesn’t. Next, Ayende makes it support multiple queries in a single DB roundtrip, something I’ve been asking all the other O/R mappers out there to do. To top it off, he’s got his sights set on solving the issues I raised in my talk on Complex Business Logic with DDD and O/R Mapping at DevTeach. That’s right, he’s going to give me my decorators and state machines.

I love you, Oren.

I know that the ADO.NET Entity Framework guys are open to this as well, but I’m pretty sure that the “Entity Model” thinking will hold them back. You just can’t divorce data and behavior – not when employing state machines or decorators.

I’m sold.

[Podcast] Occasionally Connected Smart Clients and ADO.NET Sync Services

Tuesday, April 17th, 2007

This week’s question comes from Oran who asks:

Hi Udi,

I’m enjoying the recent discussion on Entity Framework, disconnected clients, Unit of Work, and messaging. A few weeks ago I wrote a note to self to “Ask Udi” about the new ADO.NET Sync Services coming out of Microsoft, and this seems to be the perfect timing.

It really seems that Sync Services is meant to address the disconnected problem raised by Andres, but it appears to be primarily targeted at enabling DataSet-centric, grid-oriented UIs, not task-oriented UIs. They are spinning Sync Services as being an “SOA” way of doing data synchronization because “it uses WCF” and ” supports n-tier”. But under the hood it’s all about DataSet-style deltas. See Steve Lasker’s first Q&A for the SOA spin, and the rest of his blog for more info.

I’m curious what your thoughts are on the subject. Do you see Sync Services as a valid service-oriented solution for occasionally-connected clients, or is it just another attempt by the ADO.NET team to keep us all hooked on DataSet-driven development?


Get it via the Dr. Dobb’s site here.

Or download directly here.

Additional References

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

Tasks, Messages, & Transactions – the holy trinity

Saturday, March 31st, 2007

The discussion is picking up around disconnected Web Service interaction scenarios. Here’s a summary of what’s going on for those just joining us, but I would suggest reading the full posts as well:

Andres: “Basically, there is no disconnected mode… if you plan to build a multi-tier application with ADO.NET Orcas in the middle tier, you will need to hack your own change tracking mechanism in the client, send the whole changeset, and apply it in the middle tier.”

Udi: “[In] the UI, … tasks often corresponded very well to the coarse-grained messages we employed in terms of SOA.”

Jesse: “I have to disagree with Andres and agree with Udi.”

(Sorry, I just couldn’t resist. Here’s the important part)

“Andres rightly points out that you don’t want 5 different methods for updating an order. Although you technically could do it that way, you probably don’t want to be ignoring the transactional context of the conversation or locking tables for significant time periods.”

And, “The backend will still have all the update methods you are used to using, so you’re not writing a bunch of extra code in the DB. However, this approach is far more powerful. One reason why is that now you have an operation defined and you can control everything about that operation. You know when the call comes in through that method that you have a customer service rep trying to update an order and can build in logic based off of that.”

Andres: “Now, we need to send those changes to the server, together, because I want them executed in a single transaction. I cannot have a service called ‘ModifyOrderCustomerInformation’, another ‘AddALineToOrder’ and ‘DeleteLineFromOrder’. I need an ‘UpdateOrder’ service. You can use a diffgram to do it, or you could build your own change-serialization mechanism.”

And, “The only way I see is to map the UI to the service interface, so the user can ‘Change Address’ or ‘Update Marital Status’ as different operations in the UI layer, but I can’t let the user ‘Modify the Customer’. It’s a lot of work, and I seriously doubt that the users will like it.”

OK, now we’re all set.

Just to get something small out of the way, every Human-Computer Interaction (HCI) professional I have worked with has been in favor of task-based UIs. Every user that I have met that has used both styles of UI, task based and “grid” based, has reported that they were more productive when using the task based UI for “interactive work”. Data entry is not interactive work, so grids might be more suitable there, although improvements in bulk-loading technology and OCR have decreased the amount of “plain” data entry that I’m encountering. It also may be that I’m working less on systems where data entry is of significant importance.

First of all let’s agree that writing business logic for something like “DeleteLineFromOrder” or “UpdateMaritalStatus” is easier than for “UpdateOrder”, given that it fit with the overall system design.

Second, let’s agree that the business logic for something generic like “UpdateOrder” is composed of the various specific cases like “DeleteLineFromOrder” and “AddLineToOrder”.

Third, let’s agree that if the UI was task based, it would be easy for the client side to activate the correct web service/method – seeing as there is a high correlation between the tasks and the service methods.

Therefore, the question becomes how do we do all the above specific work in a single transaction? One answer may be to make the service statefull and using something like WS-Atomic Transaction to tie the various calls together. The problems in this approach are already well known. Another solution uses a messaging paradigm, something I wrote about before.

If we represent each of the tasks not as “web methods”, but rather as messages we could send all these messages together in one envelope to the server, and have it process all of them in a single transaction, activating the specific business logic pieces one after the other.

I’ll be using a code-first approach to describe the solution instead of an XML-first one since it abstracts away the technology, but it is still a contract-first approach.

Each message type is just a class that implements the “IMessage” interface. So we’d have a class called “DeleteLineFromOrderMessage” which is a Data-Transfer Object (DTO) whose members/properties are the data needed for processing – in this case, the order Id and the order line number. The same would be true for the class “AddOrderLineMessage”, it would contain the order Id and some other member for the order line data quite probably a DTO itself (so that we can use it in other places as well).

The client would generate an instance of the appropriate message class as the user finishes each sub-task, saving them up. When the user would click “confirm”, the client would send all these message objects to the server in one go with this API,

void IBus.Send(params IMessage[] messages);

Like so:

myBus.Send(addOrderLineMsg, delOrderLineMsg);

The bus would take all these objects and wrap them in a single SOAP envelope, and send them to the server, probably on a transactional channel, but that’s a configuration issue. At the server side, the bus would activate the transaction (because of the transactional channel) and start dispatching each of the specific messages to its message handler, one at a time, in the same thread.

So as you can see, there is no “transactional conversation”, and therefore we’re not locking tables in between calls, because we’ve gotten rid of “in between”. It’s just like the generic update in terms of transaction time and network hops, just with specific, simple business logic.

Andres might retort to that, “Even if Udi is right, and that’s the way applications should be built, I doubt most mere mortals in Earth will want to do it.” In fact, he did J If the frameworks supporting this style of development were supplied by Microsoft, I’m sure that most developers wouldn’t have a problem with it. When provided with such a framework, every developer I’ve worked with said that they wouldn’t want to go back to the “old way” (would that make Orcas outdated?). I’ll be putting up my frameworks soon, as well as examples on how to use them, it’s just taking longer than I expected.

Before closing, I just wanted to address the points Andres raised about concurrency:

“You could also need to know the previous values for optimistic concurrency checkings.”

I’ve written about how to do this before in regards to Realistic Concurrency, and the best example I have in terms of code is Better Domain-Driven Design Implementation.

This has gone a little longer than I planned, but I still don’t think I’ve covered everything in enough depth. I’d most appreciate investigative questions to help me shed light on the murkier parts of this, either as comments, posts on your own blog, or even via email. Let’s continue the conversation.

Entity Framework: Disconnected Problems & Solutions

Friday, March 30th, 2007

Andres Aguiar points out an enormous change to the behavior of data access in tiered architectures that is coming with the ADO.NET Orcas Entity Framework. He sums it up nicely, “Basically, there is no disconnected mode.” He then goes on to talk about the ramifications of this:

“This basically means that if you plan to build a multi-tier application with ADO.NET Orcas in the middle tier, you will need to hack your own change tracking mechanism in the client, send the whole changeset, and apply it in the middle tier. From this point of view, it’s a huge step backwards, as that’s something we already have with DataSets today.”

I do quite a bit of work on large-scale distributed systems, some that could even be called “multi-tier”, but I really don’t like that term. I’ve got to tell you, I’m not that worried. I could say that it’s because I’m using an object/relational mapping tool, but then again, the Entity Framework is supposed to be that too. It also might be because I don’t use DataSets that much today anyway, but that begs Andres’ question – did I develop my own change-tracking mechanism on the client?

The answer lies in the second part of Andres’ question, I don’t “send the whole changeset, and apply it in the middle tier”. I have an entire podcast up on why I don’t think that that’s a good idea, specifically around Web Services and SOA, but it’s broadly applicable. However, the question remains, what do we do about the client side?

Just over a year ago, me and Arjen Poutsma (of Spring fame) had a discussion about this exact topic at the Software Architecture Workshop in beautiful Cortina, Italy. Here’s the important part:

I think it started by me saying that services should not expose CRUD style operations. Arjen countered by mentioning that most user interfaces in line-of-business applications exposed the same model to the user by having them fill out data in grids. My retort to that was that while humans can get used to almost anything as long as it’s consistent, that doesn’t mean that it is a good solution. In the systems that I work on there is usually an HCI (human-computer interaction) person on the project who designs the UI, mostly around the tasks they perform. These tasks often corresponded very well to the coarse-grained messages we employed in terms of SOA. We finally agreed that the successor of SOA would be TOA (Task-Oriented Architecture) in its aggregation of client-side aspects to the already server-centric principles of SOA.

As a concrete example, instead of updating customer data in a grid showing everything laid out flat, users would activate tasks like “Change address”, or “Update marital status”. The form shown would contain only the relevant data to be updated – no need to perform “change tracking”, it is perfectly clear what data needs to be sent. Furthermore, the intent behind the changed data would be sent as well – the server wouldn’t apply the data generically “in the middle tier”. Rather, the current customer data would be retrieved as a part of the customer object, and the appropriate method called upon it (ChangeAddress, or UpdateMaritalStatus) with the relevant data.

I actually view this change in the Entity Framework as a step forward, it’s like taking off the training wheels. I’m positive that most developers will be able to make the change and move to these more advanced architectures.

This article has also been translated to Serbo-Croatian by Anja Skrba.

Dataset – O/R mapping rumble at TechEd MVP Dinner

Thursday, November 9th, 2006

So, last night I was at the MVP dinner in TechEd and everything was nice. We had a nice meal, conversation was nice, weather was… nice. And then the volume started to rise, slowly at first, so as you don’t quite notice it. After a bit, you kind of stop talking and look around. And then I hear it…

<WWF announcer voice>
Are you ready… to RUMBLE !?!?
</WWF announcer voice>

It was Datasets vs. O/R mapping, a slight twist on the infamous datasets vs. custom objects debate, all over again. They pulled me in, kicking and screaming, I swear, I really do. The lines were drawn, maintainability, performance, all the things that architects like to philosophize about in terms of other people’s work.

Anyway, I won’t give you the play-by-play ‘cause we were there almost all night. I’ll just cut to the chase.

First things first – any comparison of solutions without the context of a problem leads nowhere, fast, and stays there. So the first question I asked (when I got the chance to speak) was “are we talking about querying/reporting here?” and the answer was something like “well, yeah, but a lot of other things too”. So my suggestion was that we discuss the solutions in terms of two contexts – querying/reporting and OLTP.

What I mean by OLTP is the data-updating kind of work that you do on certain items. Examples of this include “insert order”, “change customer address”, and “discount product”. Querying/reporting doesn’t change data, and often involves dealing with large sets of data pulled from different kinds of entities (in ERD terms).

Luckily, my suggestion to deal with them separately was accepted. Secondly, I proposed that an object model (specifically implementing the Domain Model pattern) designed for OLTP would perform poorly when used for querying/reporting – simply because it wasn’t designed for it. The structure of a domain model is such that it makes it possible to define / implement business rules in one place. That’s possible, not easy.

Well, the dataset people weren’t going to just hand me the OLTP side of the equation without a fight, so they mentioned how easy it was to just “AcceptChanges”, and that my way was much more complex. My rebuttal came in the form of a question (are you seeing a pattern here?): Do you just swallow DbConcurrencyExceptions are do you throw all the user’s changes away when it happens? I didn’t quite make out the answer since there was a lot of mumbling going on, but I’m pretty sure they had one. I mean, you can’t develop multi-user systems using datasets without running into this situation.

The example that clinched OLTP was this. Two users perform a change to the same entity at the same time – one updates the customer’s marital status, the other changes their address. At the business level, there is no concurrency problem here. Both changes should go through. When using datasets, and those changes are bundled up with a bunch of other changes, and the whole snapshot is sent together from each user, you get a DbConcurrencyException. Like I said, I’m sure there’s a solution to it, I just haven’t heard it yet.

Now, here’s where things get interesting. I didn’t say that using a domain model automatically solves this problem. Rather, I described how each client could send a specific message, one a ChangeMaritalStatusMessage, the other a ChangeAddressMessage, to the server – in essence, giving the server the context in which each bit of data is relevant. The server could just open a transaction, get the customer object based on its Id, call a method on the customer (ChangeMaritalStatus or ChangeAddress), and commit the transaction. If two of these messages got to the server at the same time, the transactions would just cause them to be performed serially, and both transactions would succeed. The important part here is not losing the context of the changes.

When we talked about querying/reporting, things seemed quite a bit clearer. Datasets, or rather datatables seemed like a fine solution – most 3rd party controls support them out of the box. One guy mentioned that datasets performed poorly for large sets of data and that by designing custom entities for the result set, he could improve performance and memory utilization by, like, 70%. To tell you the truth, I think that if you need the performance, do it, if not, just use datasets. There isn’t much of an issue of correctness.

Just as an ending comment, in response to something someone said about scalability, I asked if they were reporting against the live OLTP data. The response was “yes”. Well, there’s a database scalability problem if I ever saw one. OLTP works most correctly when employing transactions that have an isolation level of serializable. The problem with them is that they lock up the whole table, or get blocked when a table scan is going on. Querying often results in a table scan. You can see the problem. Anyway, a common solution to this problem is to just reduce the isolation level, a quick fix that improves performance almost immediately. You take one hit in that your reports may be showing incorrect data, especially if they do aggregate type work. You might take another hit if your OLTP transactions need to do aggregate type work themselves. That second hit is pretty much unacceptable. A different solution is to accept the fact that the heaviest querying can usually show data that isn’t up to date up to the second.

In such a solution, you would have another database for reporting. It wouldn’t be just a replica of the OLTP database, but rather a lot more denormalized – which is a really not nice way of saying designed for reporting. You could then move the data from your OLTP database to the reporting database in some way (more to come on this topic) and you increase the scalability of your database. Just to define that a bit better – your OLTP database will be able to handle more transactions per unit of time, and reports will run faster, meaning that you will both improve their latency and the number of queries that can be handled per unit of time.

Anyway, I was pretty tired after all that, but if I had to sum it up I’d say something like this: before debating solutions, define the problem, you get a lot more insight into the solutions and you get it faster. That’s just win-win all around.

To map, or not to map?

Saturday, March 11th, 2006

I had this discussion with Clemens when he was last here in Israel – his position, as was so eloquently stated here, was against, while I was pro. The question he raised “To map, or not to map?” he himself answered, “to map”. The question remaining was how to map; write the sql yourself, or let some tool write/generate it for you.

The overarching question is: what do you REALLY gain by O/R mapping?

(As an aside, among the comments of his post are those using the acronym ORM. Please stop – that acronym is already taken by Object Role Modeling.)

I’ve been using O/R mapping techniques on mission critical projects for some time now, and if I wanted to compare it to what I did before, it would not be to writing all the sql by hand. I don’t remember ever doing that – there was always code generation involved. Because, let’s face it – there’s a lot of drudgery involved for things that aren’t performance critical. No reason to do THAT by hand.

So, for me, the ONE THING that O/R mapping gives me better than what I did before is this:

O/R mapping gets me better object-oriented business logic.

That’s it.

Like Clemens said, if you “don’t know SQL and RDBMS technology in any reasonable depth”, don’t expect to get good performance. Obviously this is true for any technique. But I guess that empirically speaking, the percentage of people without said knowledge is larger in the group where you don’t HAVE to write sql.

So, I’ll bet you’re asking yourself, “if that’s all Udi gets from O/R mapping, why does he keep doing it?” Or maybe you’re asking yourself “should I get a beer? This is getting long…”

The fact of the matter is that I don’t know a better way to write business logic than by using OO techniques. I grant that data is important, but the reason that many applications are built is business logic – there’s something that this new system can/should do, that the old systems couldn’t (often using the same data).

If I could sum up my understanding of Clemens position, it would be this:

A lot of developers probably aren’t experienced, or knowledgeable enough the use O/R mapping well. Therefore writing the sql yourself is better.

While I agree with the first statement, and I think that the same could be said about communication, threading, .net and many other things, I don’t think that the conclusion logically follows.

So, I guess I would sum up my position like this:

If you would like to develop a persistent domain model, O/R mapping techniques will probably help you. If you would like your solution to perform well, you should probably learn how databases work, as well as what the O/R mapping tool does under the covers.

Uniqueness across the world

Saturday, July 31st, 2004

Roy talks about the “The SQL Server usability advantage”, and begins to rumble with the Oracle zealots.

I just wanted to answer the question he asked in his post “BTW: one thing I’d really like in SQL server that I know Oracle has: Identities that are unique across multiple tables (I can’t remember the right word for that now ).” – why not just use GUIDs? There’s really great support for them in Sql 2k and in .net – and you get uniqueness not only across tables in a single DB, but globally (at least as close as you can do it today).

Just have as your primary key a column of type “uniqueidentifier” and give it a default value of NEWID() – this will take the column behaviour quite similar to IDENTITY – you don’t have to fill it in your code. When you need to take the data from the DB and into a data structure in your code, be it a dataset or custom class, all you need is System.Guid (a struct).

I’ve done some performance tests and it appears that guids are slower by a factor of 10-20% – but you do get global uniqueness, so it just might be worth the price, especially if you need replication capabilities. These numbers, of course, are by no means a valid benchmark. I suggest checking for yourself under loads that simulate your production environment.

One thing to note, for best performance on retrieval, it makes sense to add an identity column, not as a part of the primary key, but rather as your clustered index – keep the “uniqueidentifier” column as a non-clustered index. This makes a big difference when it comes to insertion times.

I’ve met a lot of people who haven’t thought that using GUIDs was so easy, and were left to suffer through replication with IDENTITY columns. I think it might be soon that the IDENTITY column will be replaced as the defacto standard for primary keys in tables. If you know better, well, don’t hesitate to prove me wrong 🙂


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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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