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DDD & Many to Many Object Relational Mapping

Saturday, January 24th, 2009.

many to many The ability to map entity relationships is broadly supported by many O/RM tools. For some reason, though, many developers run into issues when trying to map a many-to-many relationship between entities. Although much has already been written about the technological aspects of it, I thought I’d take more of an architectural / DDD perspective on it here.

Value Objects Don’t Count

While the canonical example presented is Customer -> Address, and has a good treatment here for nHibernate, it isn’t architecturally representative.

Addresses are value objects. What this means is that if we have to instance of the Address class, and they both have the same business data, they are semantically equivalent. Customers, on the other had, are not value objects – they’re entities. If we have two customers with the same business data (both of them called Bob Smith), that does not mean they are semantically equivalent – they are not the same person.

All Entities

Therefore, for our purposes here we’ll use something different. Say we have an entity called Job which is something that a company wants to hire for. It has a title, description, skill level, and a bunch of other data. Say we also have another entity called Job Board which is where the company posts jobs so that applicants can see them, like Monster.com. A job board has a name, description, web site, referral fee, and a bunch of other data.

A job can be posted to multiple job boards. And a job board can have multiple jobs posted. A regular many to many relationship. At this point, we’re not even going to complicate the association.

This is simply represented in the DB with an association table containing two columns for each of the entity tables’ ids.

In the domain model, developers can also represent this with the Job class containing a list of JobBoard instances, and the JobBoard class containing a list of jobs.

It’s intuitive. Simple. Easy to implement. And wrong.

In order to make intelligent DDD choices, we’re going to first take what may seem to be a tangential course, but I assure you that your aggregate roots depend on it.

Moving forward with our example

Let’s say the user picks a job, and then ticks off the job boards where they want the job posted, and clicks submit.

For simplicity’s sake, at this point, let’s ignore the communication with the actual job sites, assuming that if we can get the association into the DB, magic will happen later causing the job to appear on all the sites.

Our well-intentioned developer takes the job ID, and all the job board IDs, opens a transaction, gets the job object, gets the job board objects, adds all the job board objects to the job, and commits, as follows:

   1:          public void PostJobToBoards(Guid jobId, params Guid[] boardIds)
   2:          {
   3:              using (ISession s = this.SessionFactory.OpenSession())
   4:              using (ITransaction tx = s.BeginTransaction())
   5:              {
   6:                  var job = s.Get<Job>(jobId);
   7:                  var boards = new List<JobBoard>();
   8:   
   9:                  foreach(Guid id in boardIds)
  10:                      boards.Add(s.Get<JobBoard>(id));
  11:   
  12:                  job.PostTo(boards);
  13:   
  14:                  tx.Commit();
  15:              }
  16:          }

In this code, Job is our aggregate root. You can see that is the case since Job is the entry point that the service layer code uses to interact with the domain model. Soon we’ll see why this is wrong.

** Notice that in this service layer code, our well-intentioned developer is following the rule that while you can get as many objects as you like, you are only allowed one method call on one domain object. The code called in line 12 is what you’d pretty much expect:

   1:          public void PostTo(IList<JobBoard> boards)
   2:          {
   3:              foreach(JobBoard jb in boards)
   4:              {
   5:                  this.JobBoards.Add(jb);
   6:                  jb.Jobs.Add(this);
   7:              }
   8:          }

Only that as we were committing, someone deleted one of the job boards just then. Or that someone updated the job board causing a concurrency conflict. Or anything that would cause one single association to not be created.

That would cause the whole transaction to fail and all changes to roll back.

Rightly so, thinks our well-intentioned developer.

But users don’t think like well-intentioned developers.

Partial Failures

If I were to go to the grocery store with the list my wife gave me, finding that they’re out of hazelnuts (the last item on the list), would NOT buy all the other groceries and go home empty handed, what do you think would happen?

Right. That’s how users look at us developers. Before running off and writing a bunch of code, we need to understand the business semantics of users actions, including asking about partial failures.

The list isn’t a unit of work that needs to succeed or rollback atomically. It’s actually many units of work. I mean, I wouldn’t want my wife to send me to the store 10 times to buy 10 items, so the list is really just a kind of user shortcut. Therefore, in the job board scenario, each job to job board connection is its own transaction.

This is more common than you might think.

Once you go looking for cases where the domain is forgiving of partial failures, you may start seeing more and more of them.

Aggregate Roots

In the original transaction where we tried to connect many job boards to a single job, we saw that the single job is the aggregate root. However, once we have multiple transactions, each connecting one job and one job board, the job board is just as likely an aggregate root as the job.

We can do   jobBoard.Post(job);    or     job.PostTo(jobBoard);

But we need just a bit more analysis to come to the right decision.

While we could just leave the bi-directional/circular dependency between them, it would be preferable if we could make it uni-directional instead. To do that, we need to understand their relationship:

If there was no such thing as “job”, would there be meaning to “job board” ? Probably not.

If there was no such thing as “job board”, would there be meaning to “job” ? Probably. Yes. Our company can handle the hiring process of a job regardless of whether the candidate came in through Monster.com or not.

From this we understand that the uni-directional relationship can be modelled as one-to-many from job board to job. The Job class would no longer have a collection of Job Board objects. In fact, it could even be in an assembly separate from Job Board and not reference Job Board in any way. Job Board, on the other hand, would still have a collection of Job objects.

Going back to the code above we see that the right choice is   jobBoard.Post(job);   

Job Board is the aggregate root in this case. Also, the many-to-many mapping has now dissolved, leaving behind a single one-to-many mapping.

Let that sink in a second.

But Wait…

While the GUI showing which jobs are posted on a given job board are well served by the above decision (simply traversing the object graph from Job Board to its collection of Jobs), that’s not the whole story. Another GUI needs to show administrative users which Job Boards a given Job has been posted to. Since we no longer have the domain-level connection, we can’t traverse myJob.JobBoards.

Our only option is to perform a query. That’s not so bad, but not as pretty as object traversal.

The real benefit is in chopping apart the Gordian M-to-N mapping knot and getting a cleaner, more well factored domain model.

That gives us much greater leverage for bigger, system-level decomposition.

We’re now all set to move up to a pub/sub solution between these aggregate roots, effectively upgrading them to Bounded Contexts. From there, we can move to full-blown internet-scale caching with REST for extra scalability on showing a job board with all its jobs.

In Closing

We often look at many-to-many relationships just like any other relationship. And from a purely technical perspective, we’re not wrong. However, the business reality around these relationships is often very different – forgiving of partial failures, to the point of actually requiring them.

Since the business folks who provide us with requirements rarely think of failure scenarios, they don’t specify that “between these two entities here, I don’t want transactional atomicity” (rolling our technical eyes – the idiots [sarcasm, just to make sure you don't misread me]).

Yet, if we were to spell out what the system will do under failure conditions when transactionally atomic, those same business folks will be rolling our eyes back to us.

What I’ve found surprises some DDD practitioners is how critical this issue really is to arriving at the correct aggregate roots and bounded contexts.

It’s also simple, and practical, so you won’t be offending the YAGNI police.


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28 Comments

  1. Andy Palmer Says:

    Another fantastic blog post, thanks Udi.


  2. Ayende Rahien Says:

    If I understand you correctly, Job Board is the aggregated root. But Job doesn’t know about Job Board.

    The problem with that is that now you have Job taking part in several aggregated roots. Doesn’t this defeat the whole purpose of aggregated roots?


  3. Tapio Kulmala Says:

    If the Job entity has a meaning outside of the context of the Job Board, it’s not part of the Job Board aggregate. A Job Board will have a reference to it but it does not make it part of the aggregate.

    Tapio


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  7. Sean Chambers Says:

    Excellent example, but you seem to have removed one of the requirements that you originally stated to get it to fit into your example:

    “A job can be posted to multiple job boards. And a job board can have multiple jobs posted.”

    If thats the case, how can you still achieve that with a Job only belonging to one job board? It seems like your example works, but violates the requirements that were originally stated.


  8. udidahan Says:

    Ayende, Tapio, Sean,

    For me, “aggregate root” is the DDD term for “only calling one method on one object in your service layer”. In other words, it’s use case specific, and not an inherent structural property of the domain model.

    Hope that helps.


  9. Michael Hart Says:

    @Sean – I assume that Udi meant the shift from many-to-many to one-to-many only happened in the mapping itself (ie, the Job class no longer has a property relating to JobBoard at all – whether singular or plural). A many-to-many is really like two one-to-manys and in this case one of those has been hidden at the object level. The database tables could still be mapped in the original many-to-many fashion.


  10. Marc Brooks Says:

    So, can a job “be” until it’s been posted to a board? Is a board useful (in an non-admin sense / more than a lookup table) until some job is posted to it?

    Perhaps the REAL root is the Job Posting, which is queried for matching jobs or boards. I’ve found that most output/report pages (which is what most software is about–getting the data out in useful projections) is presented in some form of list, so defining the aggregate root “property” as a collection of JobBoardPosting is the clearest approach to me.


  11. Andy Hitchman Says:

    I’d suggest that Job is probably more important to the users managing recruitment. The domain around Jobs is likely much richer than this example has so far explored. This would lead me to infer that Job is the aggregate root.

    If you wish to abstract the association of Job Board to Job, then perhaps a service to control the publication of Jobs to Job Boards is more appropriate.


  12. udidahan Says:

    Marc, Andy,

    You’re both right.
    In one bounded context, Job is an aggregate root, in another, it’s Job Board.


  13. Nuno Lopes Says:

    I liked the mention of partial failures and its often forgotten in batch business transactions scenarios. Nevertheless I fail to understand how that particular business requirement actually mandates who the root should be.

    Whoever read Streamlined Object Modelling (wich I strongly advise) basically understand that the collaboration director between two objects should have the business function (your aggregate root), the other is optional. Now the collaboration director depends on the complexitiy of business rules within that collaboration.

    * Does the JobBoard imposes any constraints over inlisting Job Vancacies?

    * Does the Job (or a Job with status vacant) imposes any constraints over advertising on specific JobBoards (one would not publish a Job Vacancy of Finantial Director of a top 5 company on an IT Job Boards)?

    In a situation like this Many-To-Many usually the collaboration director should be the one who carries the most complex business constraints.

    I’m always very, very suspicious about Many to Many relationships. In most cases it means that the domain is not well understood as roles are not in place (Modeling in Color & Streamlined Object Modeling).

    Let me explain.

    You are absolutely right, we should not couple what does not need to be coupled. That is, if the Job class represents mostely anything else then a Job Vacancy ready to be advertised then a new Class JobVacancyPost is needed. This JobVacancyPost is the Role of a Job within this collaboration (read Modeling in Color or serch for the Domain Neutral Componentt) modelled as a separate class, not even part of the core HR domain of a Company (where Job and JobSpecification are modeled).

    Job{1:N} JobVacancyPost (Role+MI) {N:1} JobBoard

    JobVacancyPost is a first order object in the domain (as it workds as a Role of Job and a Post in a Job Board) so it does not hurt anyone to state that it should be located by a Repository.

    “That’s not so bad, but not as pretty as object traversal.”

    There is no such thing IMHO. if the natural way would be through object transversal means that the domain is not well understood by the developer (aka as not well mapped).

    In other words we can most of the times reduce a M:N relationship to either {1:N} or to {1:N:1} only if the domain allows from pure DDD (othwerwise Domain Drive = Model Driven IMHO), nothing else nothing more. If not more sooner then later you will face problems, hacks and turns.

    Execuse my ramblings in my first post and I’m learning.

    Nuno Lopes
    PS: If people don’t know what DNC is look for this http://www.step-10.com/SoftwareDesign/ModellingInColour/TheDomainNeutralComponent.htmlor this http://www.petercoad.com/download/bookpdfs/jmcuch01.pdf


  14. Nuno Lopes Says:

    I failed to mention interactions my explanation so here it is on the service layer:

    // begin transaction
    ….
    JobVacancyPost aJVP = new JobVancncyPost(Job); // usually this is how roles are created
    aJVP.Send(aJobBoard)
    tx.Commit();

    // end transaction

    IMHO there is no such thing as partial failures. If there is business rule stating that all Job Vacancies should be advertised or not at all, then most probabably a JobVancancySheet is in order. If this the case we would have:

    ….
    try catch()
    {
    tx.begin()
    var job = s.Get(jobId);
    JobVacancySheet aJVS = new JobVacancySheet();
    foreach(Guid id in boardIds)
    {
    aJVS.Add(aJob, session.Get(id));
    }
    JobVacancySheet.Send();
    tx.Commit()
    }
    catch {Exception ex)
    {
    // depending on the expcetion it would postpone sending to later (say external services are down) or fail completely advising the user to correct the sheet.
    }

    Nuno Lopes
    PS: This are my personal views, in no way whatsover they are absolute.


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  16. udidahan Says:

    Nuno,

    It’s not that the partial failures dictate which object is the aggregate root, but they indicate that the intuitive answer isn’t necessarily correct.

    > IMHO there is no such thing as partial failures

    Well, that’s a question that should be put to the business. If we include in the discussion the time within which the system will be consistent one way or the other (say, 1 minute), you may find that partial failures and eventual consistency are quite acceptable.

    Also, it’s usually easier to scale a system built around partial failures than one founded in 100% consistency.

    And thanks for sharing your personal views – it’s really valuable seeing other perspectives.


  17. Ramin Says:

    If the board not only posts jobs but other information, would you then have 3 aggregates roots: job, info-board and info-board-job-posting? Would the info-board-job-posting aggregate root then subscribe to the two others in a publish/subscribe solution? How would that be best modeled?


  18. udidahan Says:

    Ramin,

    I’d need a fair bit more information about the other use cases and relevant data to say anything about the scenario you’re alluding to.


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  22. Christian Says:

    Just a stupid question. Why is an address a value object???


  23. p47l4f0n [3D noob] Says:

    Hello, many thanks to you for this very valuable post and also to others for their comments.
    I’d know if this coul’d be the solution for that problem:
    – there are 2 really independent entities.
    – there are 4 points of view at the relationship between them
    1) Job is independent, doesn’t know about any JobBoard AND
    JobBoard is independent, doesn’t know about any Job (it’s just a PostBoard) [potentional Employee's view]
    2) Job knows about JobBoard, but “PostBoard” doesn’t know about any Jobs [HR manager's view]
    3) Job doesn’t know, that it’s been posted anywhere, but JobBoard knows Jobs [JobBoard manager's view]
    4) Job and JobBoard know them both. [Programmers view]

    If we are implementing just one point of view (is it bounded context?), there is one or two agreggate root(s)
    If we are implementing more than one PoV, there are more agregate roots up to four. Here I feel that the max value of combinations could be get by some kind of math formula…
    Could this simply help us to check quickly, if we’ve been right for one aggregate root, if we expected two PoV?


  24. udidahan Says:

    3D noob,

    I’m not exactly sure how to respond to you. While multiple users may want to view the same information slightly differently, that doesn’t always translate to different bounded contexts – often its just different query screens. When working with CQRS, these queries aren’t served off of the domain model so you can do almost anything you like there.


  25. chandra Says:

    You have said that, query should be used for fetching data from job’s aggregate.

    But is that defeat ddd perspective. Why query ?

    Do we avoid m to m always and convert that in to 1 to m ?


  26. udidahan Says:

    Chandra,

    I believe that I said that we would use a separate query to get the job boards by job ID, rather than navigating the collection. That doesn’t defeat the purpose of DDD. I suggest reading up on some of my CQRS material to understand more about how queries should be handled by different objects than commands.


  27. Didier Says:

    Are you suggesting that we should never have a many-to-many relationship in our domain model, because they make the mapping to a database cumbersome and difficult?

    And then are you justifying this change in your domain model based on your persistence system by implying that it was a bad modeling in the first place?

    Or are you saying some of those many-to-many relationships would be better modeled in a one-to-many, but it’s ok to still have some many-to-many if that’s what makes more sense to the model?


  28. udidahan Says:

    Didier,

    In many of my later blog posts I talk about the concept of Bounded Contexts (or SOA Services) which encapsulate strong business responsibilities.

    It’s been my experience that since entities on either side of a many-to-many relationship have the ability to exist independently of each other, the often belong in different Bounded Contexts.

    As such, since a domain model sits entirely inside a Bounded Context, we would not expect to see many-to-many relationships there.


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“Udi has provided us with guidance in system architecture and supports our implementation of NServiceBus in our core business application.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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


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

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

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

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

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

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

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

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

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

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

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

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

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

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

Consult with Udi

Guest Authored Books
Chapter: Introduction to SOA    Article: The Enterprise Service Bus and Your SOA

97 Things Every Software Architect Should Know



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