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

Why you should be using CQRS almost everywhere…

Sunday, October 2nd, 2011.

grass… but differently than the way most people have been using it.

I think I’ve just about drove everybody crazy now with my apparent zigzagging on CQRS.

Some people heard about CQRS first from one of my presentations and got all excited about it. Then I did some blogging which further drove people to CQRS (as did Greg Young and some others). As CQRS was just about to hit its stride with the Early Adopters, I started pushing a more balanced view – CQRS not as an answer, but as one of many questions. More recently I’ve pushed more strongly back against CQRS saying that it should be used rarely.

So what’s the missing piece?

If you’re in the Domain-Driven Design camp (as many doing CQRS are), then it’s Bounded Contexts.

If you’re in the Event-Driven SOA camp (a much smaller camp to be sure), then it’s Services.

The problem is the naming, because the DDD guys have their kinds of services which do not fit the definition for Service of the Event-Driven SOA approach.

Let me propose the term Autonomous Business Component for the purposes of this blog post to describe that thing which is both a DDD Bounded Context (have the shared BC part of the acronym) and an SOA Autonomous Services. Resulting in the nice short form: ABC (and everyone knows you need to have a good acronym if you want something to catch on).

What does this have to do with CQRS?

Nothing just yet. Well, at least, nothing directly to do with CQRS.

Although some proponents of CQRS have stated that it can and should be used as the top-most architectural pattern, both myself and Greg Young (arguably the first two to talk about it and the two who ultimately collaborated on naming it – and now Google knows we didn’t means “cars”) always recommended it as a pattern to be used one level down.

Although Greg and I have had many long discussions on the topic and do agree very much about what the overall structure should look like, I’ll try to avoid putting words in his mouth from this point on.

Before talking more about ABCs, let’s discuss the principle upon which they rest: The Single Responsibility Principle (SRP).

What does SRP have to with CQRS?

Many developers are familiar with SRP and have seen good results from using it. What we’re going to do is take this principle to the next level.

In Object Orientation (OO), data is encapsulated in an object. A good object does not expose its data to other objects to do with as they wish. Rather, it exposes methods that other objects can invoke, and those methods operate on the internal data.

SRP would guide us to not have the same data exist in two objects. For example, if we saw the customer’s first name as an internal data member of two objects, we’d be right to question that kind of duplication and move to refactor it away. However, when we see two systems doing the exact same thing – somehow that gets excused.

“Of course we need to be able to see the customer’s first name in the front-end website as well as in the back-end fulfillment system. How could we NOT have the customer’s first name in both those code-bases?”

And there’s the catch.

Who said that a system should be a single code-base?

But what about integration?

Although many times we do need to integrate existing systems together, sometimes we have the ability to change those systems. More importantly, when going to create a new solution, we can avoid getting ourselves into the problems that integration tries to solve.

Integrating with a system that cannot be changed can be done also by composing multiple ABCs, but that’s a topic for another post.

It is better to think of integration as a necessary evil – kind of like regular expressions and multi-threading; things to be avoided unless absolutely necessary.

“If you have a problem that you decide to use a regular expression to solve, you now have 2 problems.” Or so the saying goes. With multi-threading, you have a non-deterministic number of problems to solve.

If you thought you had duplicate responsibilities with 2 systems operating on the same data, how will introducing a 3rd code base (also known as “integration”) help? Remember that Single Responsibility Principle – our goal is to get it down to one.

OK, so how do ABCs do that?

In order for us to get back into alignment with SRP, that would require us to have responsibility for a single piece of data exist in one code base. Note that SRP makes no statements about how many physical places a given code base can be deployed to. Nor does it state that only a single technology can be in play – code that emits HTML can be packaged at design time together with rich-client code in the same solution.

If an ABC is responsible for a piece of data, it is responsible for it everywhere, and forever. No other ABC should see that data. That data should not travel between ABCs via remote procedure call (RPC) or via publish/subscribe. It is the ultimate level of encapsulation – SRP applied at the highest level of granularity.

This results in systems which are the result of deploying the components of multiple ABCs to the same physical place. The ABC which owns the customer name would have the necessary web code to render it in the e-commerce front-end and in the shipping back-end for printing on labels. This would mean that practically every screen in any UI is a composite of widgets owned by their respective ABCs.

This is ultimately what keeps the complexity of each ABC’s code base to a minimum.

But why not just use CQRS as the top-level pattern? ABCs are weird.

Imagine trying to create a single denormalized view model for the entire Amazon.com product page – product name, price, inventory, editorial review, customer comments, other products that customers viewed, other products that customers bought, etc.

Pretty complex, right?

How much duplication would you have for the page shown after you add an item to a cart? Once again, you need to show other products that customers bought, their names, images, prices, and inventory.

And then on the home page – items you might be interested in, names, images, prices.

And that’s only in the front-end system.

It’s not just the duplication, but how complex the code is for each one.

Instead of the duplication that top-level CQRS would bring you, consider an ABC responsible for products names and images that has just about the same view model composed on each of the above screens. The same with another ABC responsible for price.

You may be thinking that this would result in more queries to get the data to show on a page, and you’d be right. But it isn’t necessarily a classical N+1 Select problem, as the queries are bounded to the number of ABCs. Secondly, consider the ability to have well-tuned caching at the granularity of an ABC – something that would be much more difficult when dealing with everything as a single monolithic view model. In short, not only will it not be a performance problem, often it will actually improve performance.

OK – that explains “everywhere”, what about “forever”?

Forever is where things get interesting – or more accurately, when they get interesting.

Let’s talk about things like invoices.

One of the requirements in this area is that immutability. If the customer’s name was Jane Smith when they made their purchase, it doesn’t matter that they’ve since changed their name to Jane Jones, the invoice should still show Jane Smith.

Often developers push these types of requirements on the data warehouse guys – that’s where history gets handled. The only thing is that if your ABC owns the customer’s name, then no other code base can deal with it. If it’s your data, you have to handle all historical representations of it.

On the one hand, this would seem to kill the data warehouse. On the other hand, it means that the principles of data warehouses are now core to every code-base.

This means you don’t ever delete data (see my previous blog post on the subject), and you definitely don’t overwrite it with an update – even if you think you’re in a simple CRUD domain. The only case where you can get away with traditional CRUD is if we’re talking about private data – data that is only ever acted on by a single actor.

This sounds like the collaboration you talk about with CQRS

It’s similar in principle but different in practice.

In a collaborative domain, an inherent property of the domain is that multiple actors operate in parallel on the same set of data. A reservation system for concerts would be a good example of a collaborative domain – everyone wants the “good seats” (although it might be better call that competitive rather than collaborative, it is effectively the same principle).

A customer’s name would not fall under that category. It isn’t an inherent property of the domain for multiple actors to operate on that data. While there can be multiple readers, one can easily enforce a single writer without any adverse effects. Doing that with a reservation system would cause the online system to behave as if users were lining up in front of a box office – not a desirable outcome.

Private data would be something like a user’s shopping cart. Until they make a purchase, that data doesn’t need to be visible anywhere. Here you could theoretically do simple CRUD – that is, until the business realizes that there’s extremely valuable information to be extracted from the historical record of things people do with their carts.

I think you’re ready to make your point, so just make it already

OK – so we now realize that Update and Delete don’t exist in their traditional form. Delete is really just a kind of update, and update is effectively an “upsert” – a combination of update and insert to retain history. This can be done by having ValidFrom and ValidTo columns for our data.

In which case, Create is really just a special case of Upsert, which looks like this:

UPDATE Something SET ValidTo = NOW() WHERE Id=@Id AND ValidTo = NULL; INSERT INTO Something SET { regular values }, Id=@Id, ValidTo = NULL;

And then we’d have 2 forms of Read – reading the current state (ValidTo = NULL), and reading history (ValidFrom <= Instant AND (ValidTo >= Instant OR ValidTo = NULL))

Here we don’t need fancy N-Tier architectures, data transfer objects, service layers, or domain models. A simple 2-Tier approach could probably suffice. We don’t need a task-based UI, events, denormalized view models, or any of that CQRS stuff. This was at the crux of my previous anti-CQRS post.

The only thing is that this is exactly CQRS.

Say what?

Have we not effectively separated the responsibility of commands/upserts and queries/reads?

As Greg Young has said before, “the creation of 2 objects where there previously was one”.

Effectively 2 paths through our ABC.

CQRS.

Let me give you a second to gather your thoughts.

*

You see, CQRS is an approach, a mind-set – not a cookie cutter solution. Frameworks that guide you to applying CQRS exactly the same way everywhere are taking you in the wrong direction. The fact is that you couldn’t possibly know what your Aggregate Roots were before you figured out how to break your system down into ABCs. Attempting to create commands and events for everything will make you overcomplicate your solution.

So the built-in history of this model is event-sourcing?

Well, it’s not event-sourcing in the sense that we don’t necessarily have events. It achieves many of the benefits of event-sourcing by giving us the full history of what happened.

On the whole issue of replaying events to fix bugs – that’s a bit problematic, logically, unless we have a closed system. A closed system is one that doesn’t interact with anything else – no other systems, no users, nothing. As such, closed systems aren’t that common.

In an open system, one with users, let’s say there was a bug. This bug could have caused the wrong data to be written and/or shown to users. As such, users could have submitted subsequent commands based on that erroneous data that they would not have submitted otherwise. There’s no way for us to know.

The problem with replaying events when we fix the bug is that we’re in essence rewriting history – making it as if the user didn’t see the wrong data. The only problem is that we can’t know which events not to replay – we can’t automatically come up with the right events that should have come afterwards. We could try to sit together with our users and have them try to revise history manually, but our organization often isn’t in a bubble. Our users interacted with customers and suppliers. It isn’t feasible to try to undo the real-world impacts of this situation.

Why didn’t you just tell us this from the very beginning?

I did, you just weren’t listening.

You wanted a cookie cutter, and until you tried CQRS out as cookie cutter (and saw it create a bunch of complexity) you wouldn’t listen to anything else.

As developers, we’re trained to solve problems – the faster the better. Unfortunately, this causes us to be blind to things that don’t immediately present themselves as solutions.

When applying CQRS with ABCs, the solutions you end up with are very simple, but the process of getting there is quite hard and takes practice. Finding the boundaries of ABCs such that data isn’t duplicated between them and that data doesn’t travel between them either via RPC or publish/subscribe – it may feel impossible the first several times you try. Keep at it – it is almost always possible.

We haven’t touched on the whole saga/aggregate-root thing yet, but that isn’t as important until you can successfully apply the principles described here.

Also, this post has already gotten long enough, so it looks like now would be a good time to stop.

Until next time…

  
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.

49 Comments

  1. Simon Says:

    Cool to see you and your mate Greg stating everything and the opposite of everything.


  2. Tomas Says:

    Great stuff. Hurry up with the next one ;-)


  3. Frank Quednau Says:

    An interesting read, as so often. I would expect all this to evolve with your experience as well, so the CQRS of 2 years ago may not be the very same CQRS as today’s one.

    thanks for posting on Sunday btw, so little content on that day!


  4. Shane Courtrille Says:

    One question that we came up with was how would you handle the following situation…

    ABC 1 is responsible for handling a persons name (could contain other info of course).

    ABC 2 is responsible for handling your security pass. If your name changes you should automatically be sent a new security pass.

    How does ABC 2 know to do its job if there is no pub/sub or communication?


  5. Sharas Says:

    Hi Udi,

    What an exquisite picture you’ve used for this post:D


  6. udidahan Says:

    Shane,

    I didn’t say that there wasn’t ANY pub/sub between ABCs, but rather that data (like the person’s name) wouldn’t be duplicated via that mechanism. You could have ABC1 publish an event saying PersonNameChanged(PersonId=123) and have ABC2 subscribed to that and do whatever it needs to do there.


  7. Scooletz Says:

    Speaking about “N+1″ generated by the split ABCs you’re writing about. I can imagine that it can be generated only in case where you don’t delegate showing some list to the ABC responsible for it, but rather querying it one by one, from the one of one-to-many relationship. It’d be non-straightforward anyway. Can you give an example of this possibility?

    Thank you for the counterexample for event sourcing. It’s vast. I still do find this technique useful, though. Better to have more info for resolving any kind of problematic situations. Even those, with wrongly chosen commands (by a user), than trying to examine sql tables.

    Upserting, or more: versioning can now be easily handled with a Raven’s bundle, can’t it?


  8. Arnon Rotem-Gal-Oz Says:

    Hi Udi, wazzup ? :)

    You kind of lost me here on the “everywhere/forever” issue
    1. In the example Shane gave and your answer – if the new person name will not be published/transmitted – how would you expect the new name tag to carry the new name. If it isn’t cached (a.k.a. duplicated) by second service how would it be autonomous (i.e. not depend on service 1 being online for its operation)
    2. Again regarding keeping data private from the DW perspective. I totally agree that services should retain the history and that data should be versioned (so that old invoice of version X would be to a name of version Y). I don’t agree that the data cannot be replicated into a datamart/OLAP cube/whatever for reporting purposes esp. (something I call “Aggregated Reporting” see http://www.infoq.com/articles/BI-and-SOA )
    Arnon


  9. Kevin Taylor Says:

    @Arnon Rotem-Gal-Oz

    Here’s my take on it, hopefully I’m not far off.

    1. You know the name change by the type of event, in that example the event is “PersonNameChanged” and all you need is the ID of the person in order to process the event.
    Part of the process may be to look up various pieces of information in order to finish it but that can be retrieved with Queries against the ABC. In that example I see the need to get the person’s email address and possibly the previous and new names, the second ABC that is responsible for the security pass doesn’t need to store the old or new name as long as it can query the other one when necessary.
    The process to generate and then send out a new security pass will end up becoming a saga because it depends on the availability of other services for querying data in order to finish. A saga works in this scenario because if one of the services is down it will wait for it to come back up then continue on with the next step.

    I know from personal experience it gets a bit confusing when you start trying to combine all of the pieces into something larger. I was recently reading an old programming classic that helped me get my head around it though “The Pragmatic Programmer From Journeyman to Master”. I highly recommend it if you haven’t already read it and there’s a section that describes how Unix based systems are built with all of their single purpose console applications. I think it’s a very good analogy for anyone familiar with the theory behind Unix and Linux and does a much better job of explaining it than I can here.


  10. Sean Says:

    Hi Udi, interesting read. I am curious about the ValidTo business. The assumption there seems to be that we might regularly need the “historical” data, but what if we don’t? Could the same not be accomplished with an archive? Basically a “copy values to the historical database (or tables) and update the current values” type thing.


  11. Arnon Rotem-Gal-Oz Says:

    @Kevin
    The kind of integration where you send an event that something changed and then you need to query the other service whenever you need the data is not a very good option performance wise. It can work well when you are within the same memory space but not in a distributed environment (see the fallacies of distributed computing http://nighthacks.com/roller/jag/resource/Fallacies.html).
    You create a temporal coupling between the two services which is why a lot of RPC based system fail miserably – Suggesting to add a saga (or orchestration which is another alternative here) to solve this adds a lot of complexity – Properly implementing Sagas requires coordination logic, compensations etc (see http://www.rgoarchitects.com/Files/SOAPatterns/Saga.pdf). Considering the alternative is to pass the data with the original event and caching the *immutable* versioned data it propagates demonstrates further why it is a faulty idea.
    Not to mention what would happen in reporting and BI scenarios which I mentioned in my point 2.

    Arnon


  12. nightwatch Says:

    Hi,
    I really like the idea of every application and service being equipped with a little web server for rendering views of their data, but I don’t think it will work. We don’t own every application that uses our data so we can’t implement a html view that will satisfy everyone. There’s the same problem with exposing the data in any other way so I don’t see how the html view is better than a web service or RPC?

    The next thing: ‘forever’ – any service that is responsible for some data is also responsible for making all previous versions of that data available. Why? Just because of the principle of not giving your data to anyone else – you own it and you’re the only source of that data therefore you have to keep all the previous versions because someone might ask for that. This is sick imho. Versioning is very hard to implement correctly (especially in a relational database) and could be easily avoided if you let other software keep a copy of the data – it’s immutable after all so I don’t see why not?

    Third – the ‘evil integration’ and your thesis that it will never help when there are duplicate responsibilities in two systems. Why? I can easily see how integrating these systems can help avoid duplicate responsibilities – for example you can use integration to transfer the responsibility for duplicate actions to just one system and make the other system a read-only view for the first one’s data. Isn’t it a CQRS? Even if the data is duplicated, the modifications are performed only by the owner system.

    I didn’t want to look like a nitpicker with all these questions, but this post as a whole is too abstract and too general for me to understand what’s it really about and so I concentrated only on more specific statements. I’d be very happy to see practical illustrations of these concepts in application architecture.


  13. Eben Roux Says:

    When an order is placed the customer name should be kept as the name the order was registered with ad-infinitum. This I agree with. However, it would seem strange to have to look up the customer name when, for instance, printing the order. The fact the the Customer Management BC (ABC) and the Order Management BC (ABC) are separate does *not*, IMHO, mean that the data is duplicated if the customer name is kept in the Order ABC. By *not* keeping the original data (customer name) on the order one would effectively complicate matters somewhat as we now need to request *historical* data from the customer ABC, and that is *totally* unnecessary.

    If I have misinterpreted what you meant then let me know :)

    Regards,
    Eben


  14. Sharas Says:

    Well… Since Udi is not responding… We have no choice but turn this into discussion thread :D

    @Kevin: I have a feeling that those ABC don’t rely on querying each other that much. They wouldn’t be autonomous in that case. They split coherent sets of data like User into chunks according to usage scenarios. ABCs then coordinate consistency of that coherent peace of data by an ID which serves as a correlationId.

    @Sean: Probably it has to do with autonomy of services and not having to cascade changes to all interested parties. If one ABC issues an event about something related to concept like User even if other ABCs’ representation of User changed it still knows what first ABC is talking about. Kind of data versioning I guess.

    @nightwatch: I guess standard replication of reference data and things like Master Data Management were too painful to solve. Maybe that’s the reason for Udi style SOA.


  15. Kevin Taylor Says:

    @Sharas I re-read my original comment just now and I see that is does sound like I’m suggesting ABC 2 is responsible for calling ABC 1 :)

    I agree an ABC shouldn’t be querying another one, once you have all your ABC’s set up you will end up using them together in some sort of composite way. That’s the ultimate point I was trying to get across, the code running the saga to handle the “security pass” scenario would probably be a composite application with some high level business process that makes use of more than one ABC.
    I suggested reading that book and thinking of it in terms of console apps, you can create a GUI that uses multiple console apps together to meet a business requirement but those individual apps wouldn’t make calls to each other (ideally).


  16. Jørn Wildt Says:

    @nightwatch – allow me to give you my personal take on some of this.

    Taking an immutable copy of the data does help one ABC being more autonomous at runtime. Unfortunately it makes it a lot more structural coupled to the other ABC since it will have to know exactly how the other ABC’s data is structured.

    Example:

    1) ABC-1 stores customer data (fullname, address line 1, address line 2)

    2) ABC-2 takes a copy of the customer data in order to show it on the screen.

    3) Time goes by and some day ABC-1 decides to remove the fullname field and use two fields firstname + lastname instead.

    4) Now ABC-2 has to update all of its customer name handling code. Had it only included a widget there would have been no problem.

    By encapsulating not only data, but also displaying of the data, completely in an ABC, removes structural coupling and thereby improves maintaince. This is how you avoid ending up having a one-big-ball-of-mud code base. But I will agree that it seems to have a price at runtime.

    If you don’t own an external system and cannot give it it’s own web-widgets, then its an integration issue and a composite UI won’t work. But … you could make the integration by building you own little integration widget in your own (new) ABC and use that one instead. In this way you prevent the integration “story” to creep into other ABCs.

    /Jørn


  17. nightwatch Says:

    Jørn, we have yet to see this concept work in a real application. I don’t believe it will. GUI is not everything, sometimes it could be built from independent widgets but it’s rarely the case.


  18. udidahan Says:

    Scooletz,

    Let’s take a shopping cart for example. The ABC which owns which product IDs are in the shopping cart would emit them. Then, the ABC which owns the names of those products would get a callback (jQuery) that the product IDs were loaded and emit their names. Another ABC that owns the prices would do the same.

    Finally, all of these would be composed into a table using Knockout.js as described here:

    http://blog.hansenfreddy.com/2011/07/07/tabular-data-composition-in-the-browser-for-soa/


  19. udidahan Says:

    Arnon,

    I’m good buddy – nice to see you chiming in here.

    Since ABCs can have their components deployed to multiple physical places/systems, any history they maintain internally can be visualized everywhere.

    While DW technology can be used *within* an ABC, that is an implementation detail.

    Reporting is done by building composite UIs, or by exporting data to excel for ad-hoc / research-centric work.

    Cheers.


  20. udidahan Says:

    Kevin,

    The ABC which receives the PersonNameChanged event can send a message to an integration endpoint to print a new badge. At that endpoint, all the necessary ABCs can be deployed where each emits its own information – just like a composite UI.

    No need for sagas here.


  21. udidahan Says:

    Sean,

    Most organizations require the history – it could be that you’re only looking at the context of a single system/app. ABCs span system boundaries.


  22. udidahan Says:

    nightwatch,

    Applications aren’t services/ABCs – they are the result of deploying components from multiple services/ABCs to the same process. Therefore, an ABC doesn’t have its own webserver, but if hosted in a web server, can emit html/xml/json either as a part of a web page, or as a web-service.

    You’re right that versioning is hard, so we want to avoid making it that much harder by having all sorts of copies of data without clear logical ownership. Note that within an ABC you can make as many physical copies of the data as you see fit, and structure them however you like. BTW, if relational databases are making matters more complicated – why use them?

    It sounds like you’re looking for irrefutable proof as to why my statements are true (as they are quite unconventional). It is unlikely that this medium of blog posts and comments will have high enough bandwidth for that to happen. If you can make it to one of my courses, there you’ll have the proof that you’re looking for.


  23. udidahan Says:

    Eben,

    Since the printing of an order is a physical thing, we can have multiple ABCs take part in it. An ABC is not a system. The customer care ABC will be responsible for emitting the customer name on the printed order, as well as on the web site, as well as in the back-end CRM system.


  24. nightwatch Says:

    Udi, I’d love to take your course but currently have no venture capital for that. Hope this changes some day. BTW I don’t think I’m looking for any ‘irrefutable proof’ – I’m a software guy old enough to know that technologies are like religion – there are many of them but none can offer an irrefutable proof. However, some religions are better at solving your earthly problems than others and especially in enterprise software you need some gurus to lead you.


  25. Werner Clausen Says:

    @Jørn,

    “4) Now ABC-2 has to update all of its customer name handling code. Had it only included a widget there would have been no problem.”

    I don’t understand this logic. If this was “standard” CQRS then ABC-2 would just have 1 (one) handler for that event. There is no “all of it’s customer name handling”!

    The problem would be equally large (or small). It would just be located somewhere else. Instead of fixing a handler you would be fixing a widget. The only difference is that with a widget in ABC you *could* be using that same widget in multiple UI’s which *could* be an advantage. However it could also be a bad thing. For example, instead of that widget showing 1 line, it is now showing 2. I can’t imagine any UI that could accept that change.


    Werner


  26. Steve Sheldon Says:

    I understand how this works from a viewmodel perspective. What about how would one ABC interact with another to calculate a result?

    Simple ex. We have an HR system. A payroll system needs information about the employee. pay rate, and address to calculate taxes. How would these work together if you aren’t sending data?


  27. Sharas Says:

    @Udi: Regarding forever. The example you gave about invoices and user names. So if ABC1 has InvoiceId stored against UserId and user name is defined in ABC2. When user name changes it’s an upsert in ABC2 but it is still the same user with same id right?

    Now when I want to show an invoice issued with old name in a composite app, how do I ask ABC2 to display previous name?

    Or is the answer that ABC1 should store name if it’s concerned with history and immutability? It seems kinda complex in that case to decide which ABC needs or deserves to have the name most. Come to think of it, maybe it becomes arbitrary which one has it, since you can compose them in any way you like in composite apps. In that case it’s hard to reason about meaning and responsibilities of an ABC.

    Good example would be much appreciated. Hope you’ll get a chance to answer.


  28. Kevin Taylor Says:

    Ah OK, I was thinking there was some offline process that needed to be done somewhere. I think I need to read this post a few more times along with all the comments :)


  29. Kevin Coks Says:

    What do you think to use CQRS to separate writers from readers of my domain?


  30. Lee Witherington Says:

    Hello Udi,

    What are your thoughts on “Progressive Enhancement” in regards to the javascript and the browsers hat have it disabled (albeit a smaller number these days) ?

    I really like the idea of composing the services (ABCs) on the client but some businesses stil wont accept some users not having a availability, which suggests composing the services (ABCs) on the server for some applications as the cost of maintaining both architectures would likely be to great. Would you still advocate this?

    Also would a service encapsulate a collection of ABCs? or would an ABC represent a single service (the level at which the versioning and contract is agreed at)?

    Many thanks, Lee

    Do you think it is feasible to compose the


  31. udidahan Says:

    nightwatch,

    I do offer discounts for people who don’t have their company paying for them.


  32. Jørn Wildt Says:

    @Werner,

    >> “4) Now ABC-2 has to update all of its customer name handling code. Had it only included a widget there would have been no problem.”

    > I don’t understand this logic. If this was “standard” CQRS then ABC-2 would just have 1 (one) handler for that event. There is no “all of it’s customer name handling”!

    Well, even though its only one handler, its still “all of its customer name handling”. Nothing wrong in that statement. The fact is that ABC-2 need to change its code because ABC-1 did. And maybe its only one handler – but it is also the UI code that displays the data from that handler.

    > Instead of fixing a handler you would be fixing a widget.
    > The problem would be equally large (or small).
    > It would just be located somewhere else.

    And that is in fact the whole point. It is only those who are responsible for the customer name that needs to modify their code – everybody else can happily go on with their job, totally ignorant of the fact that Customer Care just decided to change their data model.

    > The only difference is that with a widget in ABC you *could* be using that same widget in multiple UI’s which *could* be an advantage.

    Yes, that might, or might not, be an advantage, depending of how generic the widget is.

    > However it could also be a bad thing. For example, instead of that widget showing 1 line, it is now showing 2. I can’t imagine any UI that could accept that change.

    Yes, and if they suddenly decide that all addresses must include a photo of the person then it won’t work either. So ABC-1 is not allowed to break the layout without coordination – but it can at any time change its internal data model and code base without affecting others.

    In short: a widget based approach reduces coupling but doesn’t eliminate coordination completely.

    I do admit that the cost up front is bigger than with a more direct “copy-the-data” approach since you need to invent widget/plugin-mechanisms in various places where you would normally just hard code something. It’s a long term investment.

    /Jørn


  33. udidahan Says:

    @Steve Sheldon,

    You wouldn’t use systems as ABCs – there would be one ABC responsible for the employee’s pay rate, and that would cut across both your HR and Payroll systems.


  34. Aaron Torgerson Says:

    Interesting post. This sounds quite similar to an approach we are experimenting with at work. I have an example of what I think would be two ABCs, where one would have to share data with the other, and I would like your take on it.

    Take, for example, a service (X) for managing financial stocks and quote data; and a service (Y) for investment portfolio tracking and reporting.

    In order to generate an investment performance report, data belonging to both X and Y would need to be combined and complex calculations performed. It would seem that either Y would need to query X for quote data and use it to do the calculations, or Y would supply X with its trade data and allow X to do the calculations (which seems weird, but for the sake of argument). I can’t seem to envision a design where Y would not be a client of X in some way, or some component (Z) compose the two and combine the data itself.

    What do you think? Thanks!


  35. Joshua Ramirez Says:

    Great blog post, great discussions. I’d like to point out that the discussion in the comments thus far has been heavily focused on the write side of things. I have a question on the read side. I’m having a hard time wrapping my mind around the concept of what seems to be in ABCs a grand unified set of ui composite widgets that work for every presentation scenario. The only way it makes sense to me right now is if I imagine an ABC for each field… Is that too granular? I don’t know, you use granular fields in your example, but was that just for the sake of example? Composing widgets together for any number of views suggests in my mind that I shift the complexity of “all that CQRS stuff” to some kind of universal presentation engine that glues together shape-shifting widgets. Explained in the abstract, it seems like I’d be normalizing many permutations in denormalized views of the data I’d be working with in each ABC. Yea, my head is spinning right now. Anyway, without going to one of your classes (which I’d sure like to do) could you tell me how close I am to the point?


  36. Joshua Ramirez Says:

    (edit for above comment) ixnay the, “in each ABC” at the end of, “it seems like I’d be normalizing many permutations in denormalized views of the data I’d be working with (in each ABC)”

    Guess I’m saying, it seems like I’d be normalizing all the little pieces of denormalized data… “Ok, so here’s a first name. I display it as a 12pt font here, it’s displayed in drop boxes on these views, and I let people edit it here…” etc…


  37. udidahan Says:

    Aaron,

    Then maybe those aren’t the ABCs you need – maybe you need something that will cut across both those systems, that can then be composed back again.


  38. udidahan Says:

    Joshua,

    How something gets presented (12pt font) is actually the responsibility of a different ABC.


  39. Steve Sheldon Says:

    @Udi,

    >”You wouldn’t use systems as ABCs – there would be one ABC responsible for the employee’s pay rate, and that would cut across both your HR and Payroll systems.”

    Ok, I see where this is going. I like this concept, being modular, compositional and yet with some thought towards performance.

    I have to think about this a bit in terms of how we’d manage things with development.


  40. udidahan Says:

    Steve,

    Yes, this really takes you way down the rabbit hole :)


  41. Sean Says:

    I don’t see a lot of difference between coordinating widget changes with widget consumers versus coordinating public data representations such as service contracts or ETL packages with consumers that rely on them.

    In either implementation the data owner is free to “at any time change its internal data model and code base without affecting others” as you suggest. It’s the public representation of that data that matters to consumers.

    In my experience the internal data model of line of business systems do not change so often as to demand this sort of architecture. Rebranding, and UI changes are a far more common requirement and the widget approach dramatically breaks down here. It is similarly unsuited for corporate activity such as mergers or acquisitions as any new solution would require a massive data migration and system consolidation to align with this architecture.

    Replication and data warehousing are more suitable solutions to these problems so it’s little wonder why they are ubiquitous and this approach isn’t.


  42. udidahan Says:

    Sean,

    In the “widget approach”, all the styling is kept separate from the widget itself so there isn’t a problem with rebranding. This architecture has similarly weathered mergers and acquisitions so I’m not exactly sure what your concerns are there.


  43. Phil Says:

    Maybe a silly question but is the VM in MVVM similar to an ABC?


  44. udidahan Says:

    Phil – no, the VM stands for View Model and wouldn’t be considered autonomous.


  45. Piers Lawson Says:

    Udi, in response to Araon you suggested that perhaps his ABCs are wrong. Could you expand on how he might deal with his example? Does the following sound sensible:

    A Portfolio ABC that knows what investments a client has. A Stock ABC that knows the details of individual stocks (such as name and historical price). He might then have an Investment Tracking System that uses both of these ABCs to allow a client to maintain his portfolio and look at its current value. If this system should then provide some significant functionality such as the creation of a detailed performance report, where might that report be generated? Would a third ABC be introduced… Performance Report ABC?

    If Performance Report ABC is required, would you expect the system to ask for a report based on a whole set of data that the system passes in (meaning the system needs to know what data is relevant to the Performance Report ABC) or would you expect the system to pass in a set of identifiers and expect the Performance Report ABC to retrieve the data it needs from the other ABCs (which would mean the Performance Report ABC is not so autonomous)…. OR does this linkage indicate you have not got the ABC divide correct?


  46. udidahan Says:

    Piers,

    There’s always a problem in dealing with a very shallow description of a problem domain. Details matter and should influence the breakdown.

    More generally, reporting is usually handled in 2 ways. The first is via a composite UI (things usually not thought of as reports). The second is via a data dump to excel where users can do whatever they want with the data – ad-hoc reporting.

    A third, and interesting alternative to reporting, is to think of what real world event users are trying to see by using reports and then make the system surface that explicitly.


  47. Piers Lawson Says:

    I agree Udi it is difficult to work with such shallow detail… so perhaps going shallower is an alternative ;-) The question I have, which may not be Aaron’s, is:

    Say you are analysing a business and you have identified two candidate ABCs that own distinct data and provide unique functionality that is reused by a number of systems (not necessarily the same set of systems). You then identify a third ABC, that would manage some data of its own, encapsulate significant business logic and could be used by another set of systems. However, you realise this new ABC consumes some of the data currently assigned to those other ABCs. So we appear to have one ABC that is reliant on the detailed data that is managed by other ABCs (not just the correlation ids exposed by the other ABCs). Is this situation symptomatic of a poor ABC split or quite normal?

    If it is quite normal, have you any suggestions as to how the third ABC retains its autonomy? If it is only passed correlation Ids, then it will be reliant on the other ABCs in order to retrieve the detail it needs, or it could access their data stores directly or it could keep a read only copy of the data it needs. The alternative whereby, the systems using the new ABC have to pass in the extra detail when they want some function performed, means that those systems need to know what data to pass in.

    In Araon’s case this could be a Portfolio ABC, Stocks ABC and a Financial Forecasting ABC where the Finacial Forecast ABC will consume the detail from a portfolio along with detailed stock market data, apply some complex business rules (such as tax treatments, growth assumptions etc. etc.) and generate a forecast.


  48. Jeremy Rosenberg Says:

    Udi, in #20, you said the ABC receiving the PersonNameChanged event sends to an integration endpoint. Why wouldn’t the PersonNameChanged event handler be deployed directly to the integration endpoint, which ultimately queries ABC1 (asks to emit, whatever) and commands ABC2? Why the extra middle-man? Either way, who owns the handler at the integration endpoint?


  49. udidahan Says:

    Jeremy,

    The reason to have an ABC subscribed to the PersonNameChanged event which then sends a PrintBadge command is that it is not the integration ABC’s responsibility to know that PersonNameChanged requires that a new badge be printed.


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
 
Locations of visitors to this page

Recommendations

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

97 Things Every Software Architect Should Know



Creative Commons License  © Copyright 2005-2011, Udi Dahan. email@UdiDahan.com    Freely hosted by Weblogs.us