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When to avoid CQRS

Friday, April 22nd, 2011.

which way?It looks like that CQRS has finally “made it” as a full blown “best practice”.

Please accept my apologies for my part in the overly-complex software being created because of it.

I’ve tried to do what I could to provide a balanced view on the topic with posts like Clarified CQRS and Race Conditions Don’t Exist.

It looks like that wasn’t enough, so I’ll go right out and say it:

Most people using CQRS (and Event Sourcing too) shouldn’t have done so.

Should we really go back to N-Tier?

When not using CQRS (which is the majority of the time), you don’t need N-Tier either.

You see, if you’re not in a collaborative domain then you don’t have multiple writers to the same logical set of data as an inherent property of your domain. As such, having a single database where all data lives isn’t really necessary.

Data is inherently partitioned by who owns it.

Let’s take the online shopping cart as an example. There aren’t any use cases where users operate on each others’ carts – ergo, not collaborative, therefore not a good candidate for CQRS. Same goes for user profiles, and tons of other cases.

So why is it that we need a separate tier to run our business logic?

Originally, the application server tier was introduced for improved scalability, but specifically around managing the connection pool to the database. Increasing numbers of clients (when each had its own user/account for connecting to the database) caused problems. Luckily, most web applications side-step this problem – that is, until someone got the idea that the web server was only supposed to run the UI layer, and the Business Logic layer would be on a separate application server tier.

Rubbish – see Fowler’s First Law of Distribution: Don’t.

Keep it all on one tier. Same goes for smart clients.
No, Silverlight, you don’t count – architecturally speaking, you’re a glorified browser.

But what about scalability?

In a non-collaborative domain, where you can horizontally add more database servers to support more users/requests/data at the same time you’re adding web servers – there is no real scalability problem (caveat, until you’re Amazon/Google/Facebook scale).

Database servers can be cheap – if using MySQL/SQL Express/others.

But what about the built-in event-log CQRS/ES gives us?

Architectural gold-plating / stealing from the business.

Who put you in a position to decide that development time and resources should be diverted from short-term business-value-adding features to support a non-functional requirement that the business didn’t ask for?

If you sat down with them, explaining the long-term value of having an archive of all actions in the system, and they said OK, build this into the system from the beginning, that would be fine. Most people who ask me about CQRS and/or Event Sourcing skip this step.

Finally, you can usually implement this specific requirement with some simple interception and logging. Don’t over-engineer the solution. If using messaging, you can get this by turning on journaling, or if you want to centralize this archive, NServiceBus can forward all messages to a specific queue.

Don’t forget that this storage has a cost – including administration. Nothing is free.

What about the “proof of correctness” in Event Sourcing

I’ve heard statements made that when you use the events that flowed into/through your system AS your system’s data, rather than transforming those events to some other schema (relational or otherwise) and storing the result – you can prove that your system behaves correctly.

Let me put it this way:

No programming technique used by humans will prevent those same humans from creating bugs.
No testing technique used by humans will prevent those same humans from not catching those bugs.
* Automated tests – see programming technique.

While having a full archive of all events can allow us to roll the system back to some state, fix a bug, and roll forwards, that assumes that we’re in a closed system. We have users which are outside the system. If a user made a decision based on data influenced by the bug, there’s no automated way for us to know that, or correct for it as we roll forwards.

In short, we’re interested in the business’ behavior – as composed of user and system behavior. No proof can exist.

Umm, so where should we use it

If you’ve uncovered a scenario where you’re wondering “first-one-wins, or last-one-wins”, that’s often a good candidate for a place where CQRS could make sense. Then re-read my Race Conditions Don’t Exist post.

Also, CQRS should not be your top-level architectural pattern – that would be SOA.
CQRS, if used at all, would be used inside a service boundary only.

Given that SOA guides us away from having a given 3rd normal form entity exist in any one service, it is unlikely that the building blocks of your CQRS design will be those kinds of entities. Most 3rd normal form one-to-many and many-to-many relationships simply do not exist when doing SOA and CQRS properly.

Therefore, I’m sorry to say that most sample application you’ll see online that show CQRS are architecturally wrong. I’d also be extremely wary of frameworks that guide you towards an entity-style aggregate root CQRS model.

In Summary

So, when should you avoid CQRS?

The answer is most of the time.

Here’s the strongest indication I can give you to know that you’re doing CQRS correctly: Your aggregate roots are sagas.

And the biggest caveat – the above are generalizations, and can’t necessarily be true for every specific scenario. If you’re Greg Young, then you probably can (and will) decide on your own on these matters. For everybody else, please take these warnings to heart. There have been far too many clients that have come to me all mixed up with their use CQRS in areas where it wasn’t warranted.

If you want to know everything you need to know to apply CQRS appropriately, please come to my course – there is so much unlearning to do first that just can’t happen via a series of blog posts.

  
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38 Comments

  1. Mark Seemann Says:

    Based on the amount of experience I’ve had so far with CQRS (without event sourcing) is that what I really like about it is that it makes complexity explicit.

    The fact that we explicitly choose to reduce all operations with side effects to pure Commands is very beneficial because that means we can use a Pipes and Filter architecture to handle them. Commands are also the most composable form of abstraction: http://blog.ploeh.dk/2011/03/22/CommandsAreComposable.aspx

    That all complexity is handled by autonomous components is, IMO, very attractive because it tends to be very easy to unit test them. Reads are just direct queries, so need almost no testing.

    While I find the scalability promises of CQRS attractive, I really don’t think it’s the key benefit. Complexity management is.


  2. Stacy Says:

    Wow – this is the most shocking article I’ve read here. Almost feels like April Fools day.

    I must say that I find CQRS liberating as hell from that big monolithic domain morass I use for commands AND queries. I never saw it for collaboration concerns because my web apps are just like you said, non-collaborative. Nor do I need snapshots and concurrency checking. So I’m left with something far simpler than what I use to do. Unit testing with events is far simpler than per class, brittle TDD. Throw in codegen, replaying events to get another view, messaging, and I don’t feel the complexity you speak about here. A real-time event log is simply invaluable to a startup, as well as a head start on scaling built right in.

    But I can understand your point about confusion as I learn about this stuff. When I see long complex questions-answers on the cqrs group, most seem to be rooted in poor DDD modeling, IMHO. If you don’t get the bounded contexts and aggregate roots properly identified, it’s just massive confusion and making up new rules to shoe-horn a solution into cqrs-es-read model-ui type pattern.

    Udi I have learned sooooo much from you about cqrs. Thank you! And I do hope you’ll continue to help us figure things out. Sometimes the value unexpectantly falls further from the tree :)


  3. udidahan Says:

    Mark,

    I’d say that SOA is the primary way to mitigate complexity (when done right). Often there is so little complexity left after that that CQRS (or any other technique for that matter) is unnecessary.


  4. Mark Seemann Says:

    I know that I need to attend your course (one day I will), but the way you present SOA and the way you present CQRS they seem very closely related to me.

    That wasn’t a criticism – it’s an observation that makes lots of sense to me.


  5. Adam D. Says:

    Multiple models, cqrs or not, within domains, is something that is not used often enough. Complexity arises often from trying to cram too much into one thing to “get the design right”. Cqrs may overshoot what the ideal is, but this is needed at this time of the love-in with the active record pattern. Perhaps this post is a warning to those getting in over their heads too early in their careers.


  6. udidahan Says:

    Mark,

    While there are similar foundational principles to both SOA and CQRS, their usage is intended for very different circumstances.


  7. Colin Jack Says:

    “Here’s the strongest indication I can give you to know that you’re doing CQRS correctly: Your aggregate roots are sagas.”

    Not sure I follow, have you written about this elsewhere?


  8. udidahan Says:

    Adam,

    I don’t think the purpose of CQRS was to pull the industry away from monolithic active-record hell (even if it did that for some). But maybe the devil you know is better than the one you don’t.


  9. udidahan Says:

    Stacy,

    Sometimes people mistaken the use of the technical elements of CQRS (messaging etc) for CQRS. Also, many try to apply it without the division of bounded contexts (a.k.a SOA services).

    There is no such thing as an unequivocal ubiquitous best practice.


  10. udidahan Says:

    Colin,

    That was my post on race conditions.


  11. Łukasz Podolak Says:

    Udi,

    “Therefore, I’m sorry to say that most sample application you’ll see online that show CQRS are architecturally wrong. I’d also be extremely wary of frameworks that guide you towards an entity-style aggregate root CQRS model.”

    Could you be more explicit with this? Which example and which framework should we be wary and why ?


  12. John Teague Says:

    Udi,
    Could you elaborate on what extra complexity adds to an application, excluding event sourcings as the persistence strategy.


  13. John Teague Says:

    Sorry couldn’t edit and I wanted to clarify. Please elaborate on the extra complexity CQRS would add to an application.


  14. Scooletz Says:

    I’m glad you added the part about logging. It’s often used as one of the core advantages of CQRS. The truth is, that using a reasonable data access, you can do it in 200 LOC.
    I do like the style of the whole article, especially the line: “Most 3rd normal form one-to-many and many-to-many relationships simply do not exist when doing SOA and CQRS properly.” It could be a base for another series of articles.


  15. udidahan Says:

    Łukasz,

    All of them :)


  16. udidahan Says:

    John,

    The move to separate stale query stores updated in an eventually consistent manner using pub/sub.


  17. udidahan Says:

    Scooletz,

    Glad you liked it – and yes, there is a lot more material to be written.


  18. Anon Says:

    I actually had to leave my last job at an otherwise fantastic company because the architects applied loads of distributed systems cqrs split tier high scalability stuff they read on this blog, and I got sick of telling everyone not to overcomplicate our web app. It’s now made of umpteen different websites and backend services and no-one can understand it. I wish you had written this years ago.


  19. Steve Sheldon Says:

    Ok, I think with your comment on 16 this is starting to make sense and I agree.

    What we’ve been calling CQRS here internally probably isn’t, it’s more just following some of the patterns with nServiceBus, or a more message driven architecture. From that standpoint I think we see some substantial benefit in reducing complexity.

    I’ve been pushing to get a couple of our developers to attend your class.

    Anon> In post 18 you talk about the complex mess. You can create a complex mess doing any number of different patterns. Good architects should understand when to apply patterns.


  20. Zilvinas Says:

    I’m a big fan of your blog posts but I think that you’re confusing others in what CQRS is.

    The definition of CQRS by Greg Young himself is:

    “CQRS is the recognition that there are differing architectural properties when looking at the paths for reads and writes of a system. CQRS allows the specialization of the paths to better provide an optimal solution.”

    By definition it has nothing to do with Domain Model, DDD, Aggregate Roots, Scalability, SOA, Event Sourcing or Sagas. These things are often used or mentioned together with CQRS, or synergize with CQRS.

    I can assure you that applications that exercise pure CQRS are completely architecturally valid because it ONLY means that queries are separated from commands. This can be as simple as objects which do not contain reporting query methods and only contain methods which change behaviour.

    That by itself is very valuable practice as it helps developers to think about objects as things which have state and behaviour. I’ve seen so much code where objects are nothing but stateless containers of procedural code.

    What I think you do mean with your post and what I think you should have said very explicitly is that people use CQRS, Domain Model, DDD, ES, ED, Sagas all together where it wasn’t needed. Please do not name all these things as CQRS architecture.


  21. John Teague Says:

    Udi,
    Leaving Eventual Consistency aside for the moment, since there is no requirement to use eventual consistency in CQRS. I don’t see how it add complexity to an application, it just moves things around alot (which does take some getting used too).

    In my current project (cqrs in process), command is created, validated, then published as an event and then saved in a denormalized table.

    All of this would have had to happen in a “standard architecture”, but just put into the same place.

    If there is any complexity added, it usually offset with the simplicity of our read model, which is just projections off one of our few view tables (insanely easy).


  22. gazarsgo Says:

    I find it interesting that nobody took issue with what reads, to me, as the most debatable point of all: the very first sentence.

    “It looks like that CQRS has finally “made it” as a full blown “best practice”.”

    Which leads me with great curiosity to ask: what prompted this blog posting?

    For what it’s worth, you should be able to map any architecture to a solution implementation for your business case, and if you can’t, 99% of the time it’s not the architecture that’s at fault, or we’d all be stuck using C/C++. Learn the rules, and immediately start exploring the exceptions to the rules. At the end of the day it’s the right abstraction at the right time that leads us to elegant solutions, not the ability to tout the number of acronyms implemented.

    Personally, I find that messaging systems don’t meet my standards of maintainability, but there are still plenty of concepts I use from CQRS, primarily the ideas of eventual consistency and creating denormalized caches of data for reads.


  23. The Morning Brew - Chris Alcock » The Morning Brew #840 Says:

    [...] When to avoid CQRS – Udi Dahan discusses when you should not use CQRS type architectures, and how even when using CQRS correctly, it should not be your overall architecture. [...]


  24. Ian Cooper Says:

    Hi Udi,

    I always think its important to seperate the pattern of CQRS (i.e. seperating reads from writes) from Event Sourcing; CQRS is an enabling pattern for event sourcing, but there is a lot of pain from a layered architecture that can be avoided by adopting a clean seperation of command and query. The most common issue, which I know you are aware of, is the differences between the read and write model. The write model is the ‘rules engine’, the read model the ‘menu’ of choices and using the same model in both contexts is often overcomplicated and unperformant.

    I’d agree that ES requires a domain model that *needs* a full transactional history, but to suggest that there are few such domains is to miss the extent to which business has employed transactional approaches them in a wide variety of domains already.

    I accept the intent of your post: let’s not treat CQRS as a blueprint for anything but a pattern to be applied in a context. But that’s true of everything.

    Ian


  25. Roy Dictus Says:

    Udi,

    I think, like many other commentators here, that a CQRS architecture offers many advantages, and for many apps CQRS would be a good choice even if they’re not collaborative.

    Personally, I like the idea of separating read models from write models, and would use CQRS for this reason alone. Mind you: I’m not talking about DDD or Event Sourcing per se, just about pure CQRS.

    An advantage of DDD then is that, when the Domain is properly designed, the business logic becomes very easy to apply, read and modify. This in itself is not related to CQRS, in my view.

    And while Event Sourcing offers the audit trail for free out of the box, for me that is not the “compelling event” needed to choose for this option. I like to use Event Sourcing because it makes persisting any domain model easy and consistent.

    I do use my own CQRS+DDD+ES framework here, which is taking shape, currently for small back-end Web Service projects only, and I like how you can hide complexity in the framework and focus on the important logic in the service code itself.


  26. Chris Nicola Says:

    Ah the old architecture astronaut argument. Honestly I can’t think of too many decent software developers who aren’t for keeping things simple, I can however think of far more who don’t always know enough at any give point to always achieve that (in fact I’d probably say that’s every developer when starting into something new). Knowing when not to use a pattern or architecture is well known to be more valuable than knowing how to use it, but it isn’t a substitute for knowing how. Knowledge of the later is always necessary to know the former.

    Anyone currently confused (as anyone should be on first glance) about CQRS will take this article as a direct argument against ever considering it, as if CQRS is a technology that one might choose whether or not to buy rather than a concept and a tool to be understood and wielded. On the other hand your caution is valid and sensible, albeit I personally feel a bit overstated.

    True, simple systems don’t need CQRS and heck they don’t need n-tier at all. Simple systems also probably don’t need messaging, or SOA, or even anything resembling a traditional domain model at all. But what do these “simple” systems actually look like. How do we recognized and separate what a simple system looks like from a complex one? I don’t agree that it is only based on the need for collaboration. Obviously collaboration will generally be sufficient but I don’t feel it’s necessary.

    Instead I would probably argue that in any system that requires a proper DDD domain model you will find that CQRS offers some noticable degree of simplification when applying DDD principles.

    That said, great post ;-)


  27. Nuno Lopes Says:

    Hi Udi,

    I found the article to be difficult to read as disparate concerns where put together. Quite often that means good things …

    1) Personally I don’t remember anyone saying that we should put business logic in a separate tier. Is that a requirement of CQRS? On the other hand if we we put business logic behind say a Web Service aren’t we putting logic on a different tier? It seams to me that SOA by definitions makes that point you seam to argue against CQRS so we should argue the same for SOA.

    2) I think overall what you are saying is that from a design stand point most solutions don’t require to separate the data model for reads from the model for writes. One data model for read and write is enough. Current Database Technology allows one specify a set of replicas and have one database for writes and others used for reads We can even state how consistent we want the replicas to be. In this context I fully agree with your assessment.

    3) On Event Sourcing. I agree that event sourcing is not a requirement for CQRS. Yet you seam to justify this with article “There is no Race Conditions” pointing to a way of thinking that side effects don’t exist on CQRS, and in particular to SOA done right. Which I find puzzling considering that the underlying technology over which SOA is built is usually state full.

    4) CQRS is not the only way to separate read models from write models at the data level, at least conceptually. I done that in several occasions with simple non blocking Data Transfer Scheme in production.

    Cheers,

    Nuno


  28. Julien Letrouit Says:

    Humm, I agree with Zilvinas. It seems different people are putting different things in the “CQRS” basket. Depending on what your definition is, you can advocate wider or narrower use.

    So which fuzzy keyword could we use to describe the DDD + CQRS + ES + Messaging + Sagas?


  29. Neil Freier Says:

    Hi Udi

    My team and I have been learning as much as we can about CQRS in the recent months. We inherited the maintenaince and ongoing development of a large financial application and over the last year or so we introduced various changes like TDD, a more Domain focussed approaced, various architectural refatorings and a massise amount of new features. We don’t feel that CQRS is something that we could implement overnight but we did get lots of ideas by watching videos (yours and Greg’s) as well as reading various blogs and articles.

    CQRS inspired us to seperate Queries/Commands (at a data access level) and do queries via stored proces instead of proprietary ORM stuff. I feel this simplified our project a lot.

    We also started thinking about task based UI’s when doing new work and it really feels a lot cleaner.

    In terms of events we did our own little basic implementation of a command pattern which we currently only used for async stuff but again we feel that it was a great success and resulted in something thats simpler to maintain and even performs/scales better.

    Even with our limited experience with CQRS and DDD I feel that we took a big mess and made it better only by applying some of the ideas that we got by learning about it.

    In some ways this blog confirms what I suspected and in other ways it almost demotivates me. :)

    I’ll be moving on to my next project soon and I looked forward to perhaps doing a CQRS implementation from the start by using nCQRS.

    Suddenly I’m no longer sure that it’s a good idea.


  30. udidahan Says:

    Commenter 20-29,

    I also joined the discussion on the DDD/CQRS group where I wrote up my response to their reactions there. I believe that it addresses many of your concerns. The link is here, but content copied below:

    If you have separated your code into commands and queries but you
    still have a single underlying data source, I’d call that CQS (or an
    extension thereof) but not CQRS.
    CQRS adds the separation at a data level as well, therefore requiring
    some kind of synchronization mechanism.

    One of the things that did not appear to get much reaction on this
    thread was the statement about CQRS not being the top-level
    architectural pattern (that being SOA). Some recalled my posts on SOA/
    EDA combination (which is correct), others equated SOA with web
    services (which isn’t correct). I’m afraid we might be comparing
    apples to oranges when comparing CQRS (as a full arch pattern) and SOA/
    EDA with some CQRS inside services on an as needed basis.

    One of the big differences is that when applying SOA/EDA at the top
    level, entities tend to get vertically partitioned between services.
    For example, you wouldn’t see a customer’s email address and their
    shipping addresses on the same entity/class. Correspondingly, a screen
    in the UI would also be a composition/layout of “widgets” belonging to
    different services.

    Very complex domains may thus be broken up into multiple small pieces
    to the point where “naive” solutions (as Greg calls them) may be the
    simplest thing that could work (for some of them). It is quite likely
    that you don’t even need a Fowler Domain Model for them. For the rest,
    one characteristic that is similar between them (in my experience with
    many clients/domains over the years) is that of collaboration. When
    you start considering first-one-wins/last-one-wins concurrency is an
    indication that you have a collaborative domain. In these, CQRS can
    indeed make a lot of sense (as I wrote under the “where should we use
    it” heading).

    You can think of this as a rule of thumb – prescriptive, not
    definitive; just like the saga/AR thing:

    “Here’s the strongest indication I can give you to know that you’re
    doing CQRS correctly: Your aggregate roots are sagas.”

    I did *not* say that all sagas are ARs or that all ARs are sagas. This
    indicates that you’re using long-running processes to govern the
    collaboration in the collaborative domains and that you are avoiding
    the use of the domain model pattern in the simple/naive places that
    don’t warrant CQRS.

    One area where the above prescriptive guidance doesn’t hold *as
    strongly* is when developing *platforms* rather than solving a set of
    defined business problems.

    I might say (generalizing, of course) that often people use CQRS where
    they shouldn’t, but don’t use it (properly) where they should.


  31. Eben Says:

    Well, as with any technique or technology: if you are go to do it wrong you are setting yourself up for failure.

    I have seen systems that claim to be SOA that simply aren’t, and I can guarantee you that the next vendor approaching the client with a ‘SOA’ solution will probably be shown the door. Why? Because the previous ‘SOA’ bunch sold them a lie.

    I have used CQRS in very simple systems (100% consistency) and it works just fine.

    So either we have a problem in objectively / adequately defining what CQRS *is* or some folks are trying to pluck the pond from beneath the duck!


  32. Nuno Lopes Says:

    I wrote a rather long post on DDD CQRS within the scope of this article:

    http://groups.google.com/group/dddcqrs/msg/d68d74a6122fa224?

    After re-reading I noticed some errors of explanation, but hopefully is enough to understand my point.

    On your initial article:

    “Collaboration refers to circumstances under which multiple actors will be using/modifying the same set of data – whether or not the intention of the actors is actually to collaborate with each other.”

    In most business apps we have potentially multiple actors “playing” with the same data. It might be two or 3, it might be a rare occasion but still is within the scope of the word, multiple.

    So still defining the need for CQRS in such terms is a bit vague. You see, if I only have potentially say 3 people interacting with a set of data in some rare occasions why would I even need to separate my reads from writes?

    Considering RDBMS tech that exist today, why such need? For instance we can configure a RDBMS to maintain multiple replicas of the same data model so much as to have one instance just used for transactions and the other instances-replicas use just to serve queries. So why all the pumping? SOA?

    I try to provide a different answer in the post.

    Cheers,

    Nuno


  33. udidahan Says:

    Nuno,

    I differentiate between cases where users may occasionally collaborate/conflict and those where it is an inherent property of the domain.


  34. Tarek Nabil Says:

    Have you written more about this concept of partitioning an entity among services in other articles?


  35. ben Says:

    Spot on , pity so many people stuff up SOA with chatty distributed services instead of chunky messages and hence have a poisoned view.


  36. Tony Says:

    This article doesn’t make any sense to me. Confusing, vague, more like a marketing “scary” move to sell his courses?

    The simple fact of calling “wrong” all the code out there showing the use of CQRS simply shows a very unattractive arrogance not welcome in this community. Are you relative to this Ravendb guy Ayende? You just sound alike :)


  37. udidahan Says:

    Tony,

    Registration for my courses has been going very well – I appreciate your concern.

    While Ayende and I are both from Israel, there is no family relation there (that we know of). If you would hear us both speak (in English), you’d notice quite a difference – he has quite a heavy Israeli accent while I have a mid-West Canadian accent.

    Thank you for your comments.


  38. ben Says:

    Tony sorry he is spot on .. and the dev community has a habit of overdesigning solutions so should be shocked.

    Agree on most but im not sure of

    1) SOA as the top architecture .. unless your a large organization and you are sharing services amongst developement teams I would say the top should be 2 Tier !. Use assemblies and interfaces for reuse

    2) All AR should be Sagas , i like this but its a pain bringing them into memory


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"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
Chapter: Introduction to SOA    Article: The Enterprise Service Bus and Your SOA

97 Things Every Software Architect Should Know



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