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A CQRS Journey – with and without Microsoft

Thursday, March 29th, 2012.

Update – clarification post here.

circlesI was on a call recently with the Advisory Board for the Microsoft Patterns & Practices (P&P) CQRS Journey project where they were showing the current state of their development. Towards the middle of the call, I mentioned that I found there to be too many concerns in one place and that I had expected there to be a division into multiple sub-domains/bounded contexts/business components (BCs). The answer was that they hadn’t gotten to the other areas yet and that’s why at that point in time there was only one BC.

The conversation got a bit derailed at that point, and I was asked how I would do it (though not quite as politely), ultimately leading to my tweeting this:

I think I got over 50 people who wanted in on this, while some of them urged me to work with P&P rather than separately. I think I’ll do both, hopefully resulting in two implementations that can be compared – one based on Azure (done by P&P) and the other based on NServiceBus (done by my guys). Who do you think is more worried 😉

But first things first

The fundamental flaw that I see happening with many software projects (including the P&P CQRS effort) is that not enough time is spent to understand the underlying business objectives – the thinking behind the use cases / user stories. Developers assume behavior is “like” that of another/similar domain – when the difference in the details matter a lot. That often leads to software boundaries that aren’t properly aligned with those of the business.

The effects of this lack of alignment may be felt only much later in the project, when we get a requirement that just doesn’t fit the architecture we’ve set up. I’ve blogged about the symptoms of this problem about 2 years ago in my post Non-functional architectural woes.

We need to get into the nitty-gritty of our problem domain to find out what makes it special.

Not all e-commerce is equal

Anytime somebody is going to make a purchase online, developers immediately create some kind of “order” entity with a bunch of “order lines”, just like they read about in all the blog posts and books. Then, all sorts of other behavior are shoe-horned around those entities and… voila, a working system.

The domain of conferences is different – we don’t actually ship products when people register so payment concerns are very different. If our company is purchasing 5 tickets to the conference, the number of people (and which specific people) that eventually go to the conference may be very different than the people we had originally registered – there doesn’t tend to be that kind of volatility in traditional B2C retail (like selling books to people online).

It’s also quite likely that if a company is sending many people to a conference that they wouldn’t be paying by credit card – invoicing and payment may happen much later. That is no reason to block registration from completing.

Not all registration systems are equal

I understand how people can look at systems like TicketMaster and use that as a model for this system but, once again, the differences in the domain matter.

First of all, most people don’t purchase movie tickets weeks in advance – conference tickets do go on sale that far in advance. Second, if the movie you want to go to is sold out this week, no big deal, you’ll see it next week – conferences are more of a one-time/yearly deal. Third, you usually go to the movies with family/friends – if you can’t get tickets for everyone, you’ll go next week. When it comes to conferences, there is no “next week”, so whoever can go, does. Also, attendees going to a conference together are usually coworkers, not family – there are less qualms about leaving someone behind.

This is already leading us to a model where we should not view a group registration as a single success or failure affair. This will have an impact on the commands, events, and transactions that flow through our system.

In any case where people are reserving something far in advance, there is a high likelihood of cancellations. This is similar to the domain of hotels/hospitality where you can cancel your reservation up to N days before your arrival at no charge. This also tends to influence the payment structure – we’d rather not have to return people’s money as there can be per-transaction charges for that, instead delaying payment can make sense.

Similar to how hotels overbook by a certain amount (to offset cancellations), our conference might look at doing something similar. The difference is that in the case of a hotel, the guest will likely just book a room in a different hotel in the case the first hotel was fully booked. This probably won’t happen with a conference.

For that reason, we want to remember who wanted to come to our conference even when we thought we were full. You see, our best chance of filling a seat that opened up due to a cancellation is by a person who wanted to register before. What we need here is a waiting list – something that doesn’t make the same kind of sense for hotels or airlines (although airlines do use waiting lists, just that that is usually exposed to travel agents and not to travelers booking online).

First-come, first-served – fairness

The traditional developer thinking about systems is rooted in synchronous and sequential processes. In attempting to give a good user experience, developers want to give the user final confirmation as quickly as possible – whether that’s success or failure.

This results in a first-come, first-served user interaction model – whichever user registers in our conference management system first, the better the chance they’ll get what they want. That sounds like a pretty fair system, the only thing is that fairness was not a requirement.

In the real world, if people are standing in line for tickets, they’d get really upset if the tellers decided somewhat arbitrarily to serve people in the back of the line before those in the front. The great thing about online systems is that nobody can see the “virtual line” – the system can be as unfair as we like and there isn’t a real way for the users to know that this is happening.

Why be unfair?

While conferences, theaters, and airlines all want to have all seats filled, the difference between the ongoing models of airlines and theaters and the once-a-year model of the conference influence how sales are done. Some companies send a lot of employees to our conference so we want to give them preference in registration. This is area that we have the most leverage over – when it comes to the masses who arrive in ones and twos, there’s not very much we can do. It makes sense to bend over backwards for a large group, but not for a small one. A commitment from a large company tends to mean more than that from a small one.

If Boeing has already registered 70 people to your conference and now wants to send 5 more, are you really going to tell them “sorry, we’re fully booked”, or are you going to do everything in your power to keep them happy so that next year they’ll want to keep working with you? Wouldn’t it be nice if you could “unregister” some people to make room for the Beoing guys.

Now, you can’t necessarily do this up until the last minute, but potentially 2 weeks (or whatever) before the event could be reasonable, leaving people the ability to cancel flights and hotels without charges (assuming we tell them during registration that they should buy refundable plane tickets).

The easiest way to “unregister” someone is to not tell them that their registration was confirmed. In short, 2 weeks before the start of the event we finalize all registrations deciding (based on our internal priority) who gets in and who doesn’t. We may have logic that decides to immediately finalize registration from Boeing (and other select customers) without waiting until 2 weeks before the event.

Just don’t look TOO unfair

Appearance is everything. Perception matters. You don’t want to get a reputation for being unfair.

So when we open registration, we can allow the first N people to bypass our waiting list and get accepted right away (payment still needing to be handled later). At that point, you can start moving new registrations through the waiting list.

The thing is, nobody knows that you aren’t actually full at that point 🙂

Influences on architecture

I hope you’re getting the impression that this collection of scenarios is going to have a big impact on the design. It indicates to us which parts of the business need to be 100% consistent with each other and which parts can be eventually consistent – ultimately defining where one bounded context stops and another begins. This has a direct impact on the events that we’d end up with – who would publish what, and how many others would subscribe to it.

I know some people will look at the above scenarios and say “but what if the requirements were different?”. The thing is that not all requirements are created equal. In working with our business stakeholders, we need to identify which elements are stable and which are potentially volatile and, yes, that’ll be different in each project. We want to align the main boundaries of our software with the stable business elements.

And don’t even try to create a system so flexible that it could handle any new requirement without any architectural changes – down that path lies madness. User-defined custom fields used in user-defined custom workflows, all of it appearing in reports with sorting, filtering, and grouping. You might as well give your users Visual Studio.

Back to P&P

I don’t know if P&P will adopt this set of requirements for their CQRS Journey. The thing here is that we can see the collaborative nature of the domain quite clearly – multiple actors working in parallel where the decisions of one affect the outcomes of another.

The requirements that I’ve seen being handled in the CQRS Journey so far don’t seem complicated enough to justify anything more than a 2-tier architecture – it’s feeling somewhat over-engineered right now. I know that people in the community see other benefits to CQRS but I’ll have to put up a separate blog post describing why there are other better solutions than CQRS most of the time.

Anyway, I’m willing to see how things progress and tweak these requirements (up to a point) so that both the NServiceBus solution and the Azure solution are addressing the same problem.

In closing

Occasionally I hear people still raising the agile mantra against Big Design/Requirements Up Front. The thing is that Agile Manifesto never said to intentionally bury your head in the sand with regards to the purpose of the system. It was a push-back against spending months in analysis without anything but documents coming out, but the goal was to reach a middle ground. Nobody ever said “no design up-front” or “no requirements up front”.

I’m going to try to work with both P&P and the alumni of my Advanced Distributed Systems Design course to come up with simplest possible solution that addresses the requirements (functional and non-functional).

Hope you’ll find this journey interesting.

Update – clarification post here.

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  1. Steve Sheldon Says:

    Actually, I think this unregister story is an example of something that wouldn’t ever be done. The conferences I’ve been involved with this is handled by holding back blocks of tickets to handle priority requests and then release these to the waiting list.

    This generally highlights why it’s important to have a domain expert involved. Developers will often start arguing what the requirements are without really understanding the domain.

  2. Mads Kiso Says:

    One problem that I have and I haven’t found in any explanation about CQRS is if I need to distribute physically my sides (write/read) or if I can use these sides just to facilitate the domain (black-box) access and use “flat queries” to display data on UI.

  3. Clemens Vasters Says:

    Hi Udi,

    since I was the one trying to get you to give an answer on how you would solve the complexity issue you rightfully pointed out, let me try that again. I think you’re doing a good job framing the business problem and highlighting that the chosen business problem is – indeed – complex as you’ve already done on the call.

    That is, if you allow and chose the problem to be complex. For a lot (if not most) of instances of the problem at hand, it’s actually fairly reasonable to print 500 paper tickets, sell them, and call the show sold out once they’re all gone. That’s not particularly complicated, that’s just a queue.

    What you’re unfortunately not doing is to answer the question on how you would, in very concrete terms, go about dealing with the complexity challenge in the implementation that was up on the screen when you raised the concern.

    I was hoping that if you’re proposing to expand the scope of the business requirements as you seem to be doing here, you’d also come around with a proposal on how to concretely build a reservation system that’s flexible enough to manage and handle policies like the ones you explain here.

    Presumably there are policies around block reservations and automatism for a point in time when excess allocations from the pools free up and are released into the global pool that then satisfies the top of a prioritized wait list. There might be policies for reserved VIP space, discounts, and all other things. I think you’re right to point out that this is too much complexity for a single artifact and that this calls for a division into services/modules/subsystems/BC or whatever the term-du-jour is.

    The question was – given your chosen complexity scope, how would you divide that up and how would information flow across and between those aspects of the system.

    I do work at MS, but I don’t run that effort and I’m just an advisor like anyone else, so I am genuinely curious what your code will look like.

    Best Regards

  4. udidahan Says:


    For the problem definition as presented on the call, I would have gone with a simple 2-tier solution – no CQRS, no messaging, and used a NoSql storage that can scale out across multiple machines without requiring sharding (Cassandra for example).

  5. udidahan Says:

    Mads – no, you don’t necessarily need to physically distribute the sides.

  6. udidahan Says:


    I totally agree with your statement: “This generally highlights why it’s important to have a domain expert involved. Developers will often start arguing what the requirements are without really understanding the domain.”

  7. Daniel Cazzulino Says:

    > In short, 2 weeks before the start of the event we finalize all registrations deciding (based on our internal priority) who gets in and who doesn’t.

    As a user, I wouldn’t register on a site that doesn’t confirm to me that I’m attending MONTHS ahead of time (that is, the time I’m freakin’ registering!). See, I have kids that go to school, there are events that I don’t want to miss that I might be able to bend to a different date given enough anticipation, ditto with cheap airplane tickets, etc. etc. This “requirement” seems completely off to me. Sure, if you invent requirements that make little sense to the business owner, you can bend the architecture any way you want so that you can build the thing YOU want (not necessarily what the business owner wanted).

    > For the problem definition as presented on the call, I would have gone with a simple 2-tier solution

    Well, that’s the part I understand clearly. I *know* the requirements will include much more complex stuff, but you have to start somewhere. And we’re mere weeks into coding. So yes, given that I foresee the complexity that will come, I can start with CQRS right from the start, instead of saying “for the requirements so far, I’ll build this little thingy, and if gets too complex, I’ll just start over”. All the advisors (and yourself) seem to agree the problem domain merits CQRS, so I kept wondering how come you focused on *this sprint* requirements to discard it?

    That’s the deal with Agile. You know what’s ahead, so you make sure there’s room to accommodate it, but only as a general pattern/path you follow, not starting with the whole thing. Otherwise, your first deliverable will be quite far away… I’d much rather start with the simple scenarios at hand (necessarily the first ones for the first couple sprints), and refactor mercilessly as new requirements get in the picture. Otherwise, you’re constantly trying to guess the future design/code/requirements.

    It may very well come the day when the business owner says “you know, we’ve done the 80% and it’s serving us well, we’ll wait to bring those esoteric ‘unregister’ ideas, so no need to complicate everything with that right now”. You’re implying that you MUST make that call early on. Thing is, business requirements evolve. By the time you DO get to implement that esoteric feature, the customer may have already changed his mind on how it should work, and now you ended up with logic/flexibility/architecture that is useless.

    But I see how it could beneficial to someone doing up-front by-the-hour consulting on the ‘architecture/design’ of a system if the customer was convinced of what you say. I’m not convinced, and doesn’t seem agile at all to me. You have to live through the entire duration of a years-long project to see how the house of cards you envisioned up-front falls apart to really appreciate the value of agile and delaying decisions until the LAST responsible moment.

    You seem to imply that you have to decide on ALL the features the system will have, up front, before you can make a coding decision. I don’t think that’s responsible. More like: http://www.codinghorror.com/blog/2006/10/the-last-responsible-moment.html

  8. P&P CQRS Journey Post Clarification Says:

    […] appears that yesterday’s post has made some […]

  9. Will Rogers Says:

    In the States, TicketMaster is primarily known as the top concert ticket vendor; they’re not a vendor for movie tickets that I’m aware of. For this reason the mention of them and the subsequent description of the requirements for a movie ticket system rings a bit discordant.

    Perhaps you should consider replacing the mention of them with a vendor such as MovieTickets, Fandango, or MoviePhone, whose primary business is movie tickets. Otherwise, excellent post, as always.

  10. Eugen Says:

    Hi Udi,

    I am wondering if you started work on “NServiceBus (done by my guys)” (taken from the post) ? I mean P&P implemented on NServiceBus.
    If yes where can we find it ?


  11. Eduardo Says:

    I wonder it Udi’s CQRS Journay has been published in Github.

    I am reading the book and I would love to see another perspective about the same topic.

  12. udidahan Says:


    I’m afraid I wasn’t able to find the time to get this done. We’ve start drifting towards creating more guidance on UI composition but it will still take some time until it all gets stitched together into a comprehensive “journey” like the one from Microsoft.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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