Udi Dahan   Udi Dahan – The Software Simplist
Enterprise Development Expert & SOA Specialist
    Blog Consulting Training Articles Speaking About

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>();
   9:                  foreach(Guid id in boardIds)
  10:                      boards.Add(s.Get<JobBoard>(id));
  12:                  job.PostTo(boards);
  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.

Related Content

From CRUD to Domain-Driven Fluency

[Podcast] Domain Models, SOA, and The Single Version of the Truth

If you liked this article, you might also like articles in these categories:

Check out my Best Content If you've got a minute, you might enjoy taking a look at some of my best articles.
I've gone through the hundreds of articles I've written over the past 6 years and put together a list of the best ones as ranked by my 5000+ readers.
You won't be disappointed.

Subscribe to my feedIf you'd like to get new articles sent to you when they're published, it's easy and free.
Subscribe right here.

Follow me on Twitter Follow me on Twitter @UdiDahan.

Something on your mind? Got a question? I'd be thrilled to hear it.
Leave a comment below or email me, whatever works for you.


  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.


  4. New and Notable 283 : Sam Gentile's Blog (if (DeveloperTask == Communication && OS == Windows) Says:

    […] Udi on DDD & Many to Many Object Relational Mapping […]

  5. Reflective Perspective - Chris Alcock » The Morning Brew #272 Says:

    […] DDD & Many to Many Object Relational Mapping – Udi Dahan takes a look at the complexities of Many To Many Mappings using ORM technology from an architectural / DDD standpoint. […]

  6. BizTalk Linkflood, January 26, 2009 « ADA ICT Devsquad’s Blog Says:

    […] DDD & Many to Many Object Relational Mapping  […]

  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

    // 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()
    var job = s.Get(jobId);
    JobVacancySheet aJVS = new JobVacancySheet();
    foreach(Guid id in boardIds)
    aJVS.Add(aJob, session.Get(id));
    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.

  15. Dew Drop - February 1, 2009 | Alvin Ashcraft's Morning Dew Says:

    […] DDD & Many to Many Object Relational Mapping and ALT.NET DDD Podcast (Udi Dahan) […]

  16. udidahan Says:


    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:


    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.

  19. Part 9: NHibernate transactions « BASICly everything Says:

    […] Partial failures can be good. The programmer in all of us sees that and screams atomicity. Transactions  should be all-or-nothing. Anything less is just wrong. Right? In real life, there are instances where we allow, and even prefer partial failures of business transactions. Udi gives us a great example. Would you leave the grocery store empty handed simply because they were out of one item on your list? Probably not. When you’re gathering requirements, be sure to ask questions about the proper way to fail. “Roll it all back” isn’t the only option. […]

  20. Part 9: NHibernate transactions - NHibernate blog - NHibernate Forge Says:

    […] Partial failures can be good. The programmer in all of us sees that and screams atomicity. Transactions  should be all-or-nothing. Anything less is just wrong. Right? In real life, there are instances where we allow, and even prefer partial failures of business transactions. Udi gives us a great example. Would you leave the grocery store empty handed simply because they were out of one item on your list? Probably not. When you’re gathering requirements, be sure to ask questions about the proper way to fail. “Roll it all back” isn’t the only option. […]

  21. Elegant Code » Tips for ORM Data Access Says:

    […] Learn how to identify the right aggregate […]

  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:


    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:


    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.

  29. adi Says:

    I have relation like this:
    Vacancy – {1:N} – Assignment – {N:1} – Candidate

    Assignment has property “IsPrimary”

    And business rule:
    Candidate can be primary only on 1 vacancy at the same time
    and vacancy can has only 1 primary candidate.

    Then I need to check these conditions before I can set IsPrimary property:
    1. For candidate: assignments.Any(x => x.IsPrimary) (IsPrimaryCandidate)
    2. For vacancy: assignments.Any(x => x.IsPrimary) (HasPrimaryCandidate)

    When I try to unit test that behavior of the domain model

    public void SetPrimaryCandidateOnVacancy()
    var candidate = new Candidate(“Some Candidate”);
    var vacancy = new Vacancy(“Some Vacancy”);



    It will fail.

    To fix it I need to populate assignments on both candidate and vacancy,
    inside vacancy.AddAssignment method I need to call candidate.AddAssignment(assignment).

    But if I will try to do integration test and perform
    AddAssignment and MakeAssignmentPrimary in different sessions (NHibernate)
    then I don’t need to call candidate.AddAssignment(assignment)
    it will be persisted using cascade configured in mapping.

    I don’t like adding another method.

    What you can suggest in such case?

    Thank You.

  30. udidahan Says:


    I suggest creating bigger methods that encapsulate those sorts of behaviors on your objects.

    Additionally, if the rule for “at most one primary candidate” is stable, you should model it as such. Meaning having a PrimaryCandidate property on the Vacancy, or having a PrimaryVacancy property on the Candidate – keeping that separate from the collection.

    In any case, I think that the rule as specified probably won’t hold up longer-term in the real world. The world is much too messy.

  31. adi Says:

    Thank You for reply Udi.

    But creating PrimaryCandidate property wouldn’t help.
    I need also to create PrimaryVacancy on Candidate.

    It will be same situations but without collections.

    On vacancy:
    public virtual void MakeCandidatePrimary(Candidate candidate)
    if (IsClosed)
    throw new InvalidOperationException(“Vacancy is closed.”);

    if (primaryCandidate != null)
    throw new InvalidOperationException(“Vacancy already has primary candidate.”);

    // Here is the problem when I try to test pure domain model without ORM cascade
    if (candidate.PrimaryVacancy != null)
    throw new InvalidOperationException(“Candidate is already primary on vacancy.”);

    primaryCandidate = candidate;

    // This is the fix to allow testing domain model “in-memory”, but I it looks wrong.

    On candidate:
    public virtual void MakePrimaryOnVacancy(Vacancy vacancy)
    if (vacancy.IsClosed)
    throw new InvalidOperationException(“Vacancy is closed.”);

    if (vacancy.PrimaryCandidate != null)
    throw new InvalidOperationException(“Vacancy already has primary candidate.”);

    if (primaryVacancy != null)
    throw new InvalidOperationException(“Candidate is already primary on vacancy.”);

    primaryVacancy = vacancy;

    I can’t figure out how can I avoid this bidirectional relation,
    because I need to validate business rule that candidate is not primary on some vacancy.
    Same situation if I try to do it vice-versa, I need to validate vacancy if it contains some primary candidate.

  32. adi Says:

    I ended up with the following solution:

    On Vacancy:
    public virtual void AddCandidate(Candidate candidate)
    if (IsClosed)
    … “Cannot add assignment to closed vacancy.”

    if (assignments.Any(x => x.Candidate == candidate))
    … “Candidate is already assigned to this vacancy.”

    var assignment = new VacancyAssignment(this, candidate);


    On Candidate:
    protected internal virtual void AssignVacancy(Vacancy vacancy)
    var assignment = vacancy.Assignments.FirstOrDefault(x => x.Candidate == this);
    if (assignment != null)

    Now AssignVacancy is hidden for outside of domain assembly.
    Even if some one inside the domain will try to add assignment
    from Candidate side then AssignVacancy can handle this situation.

    And now I can use vacancy.HasPrimaryCandidate and candidate.IsPrimaryCandidate
    in test without ORM.

  33. Israel Pereira Says:

    Hi Sir, first of all Thanks for the article.
    I am on this situation
    “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.”

    And I also post the job to Job Boards on this GUI.
    Is it nice or is bad idea ? where will be the method PostJobToBoards?

    Where will be the query method to get the Job Boards ? in the Job Boards service or Job Service ?

    Thank you

  34. udidahan Says:


    I suggest performing queries without going through the same domain objects. You can put this query on a separate API.

    With regards to posting a job to multiple boards, I’d recommend sending multiple messages over a queue where each message is posting a job to a single board.

Your comment...

If this is your first time commenting, it may take a while to show up.
I'm working to make that better.

Subscribe here to receive updates on comments.

Don't miss my best content


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.”

Consult with Udi

Guest Authored Books

Creative Commons License  © Copyright 2005-2011, Udi Dahan. email@UdiDahan.com