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UI Composition vs. Server-side Orchestration

Monday, July 9th, 2012.

orchestra_compositionFollowing on my last post called UI composition techniques for correct service boundaries, one commentor didn’t seem to like the approach I described saying:

“I’m sorry, but with all due respect I must strongly disagree. You haven’t avoided any orchestration work at all, you’ve just moved it in to client side script!

How are you going to deal with the scenario that one of the service calls fails? Say a failed credit card payment, or no more rooms left? In more javascript??

I would much rather take the less brittle approach of introducing an orchestration service. Like it or not, however trivial it may be, there is a relationship between these services, if one call fails, they both fail. This should be reflected in the architecture, not hidden in javascript. With an orchestration service you also either get transactions for free provided by infrastructure, or alternatively if the underlying service doesnt support this, explicit and unit testable control over recovery.”

Since this is a common point of view, I thought I’d take the time to explain a bit more.

Let’s start at a fairly high level.

On failures

I’ve talked many times in the past about how to handle technical causes for failure like server crashes, database deadlocks, and even deserialization exceptions. Messaging and queuing solutions like NServiceBus can help overcome these issues such that things don’t actually fail – they just take a little longer to succeed.

On the logical side of things, the CQRS patterns I talk about describe an approach where aggressive client-side validation is done to prevent almost all logical causes for failure. The only thing that can’t be mitigated client-side are race conditions resulting in actions taken by other users at the same time.

In short, it really is uncommon for things to fail when being processed server-side.

Back to the specific example

The concerns raised in the comment specifically talked about a failed credit card payment or no rooms left in the hotel, so let’s start with the credit card thing:

In my last post I talked about collecting guest and credit card information from the user as a part of the “checkout” process when making a reservation for a hotel room. Just to be clear – there is a final “confirm your reservation” step that happens after all information has been collected.

What this means is that we aren’t actually charging the customer’s card when we collect that data, therefore there is no real issue with a failed credit card payment that needs to be handled by the client-side javascript. When the customer confirms their reservation, yes, there might be a failure when charging the card though there are only some specific types of rates for which the hotel charges your card when you make a reservation.

In general, failed credit card payments are handled pretty much the same way for all ecommerce – an email is sent to the customer asking for an alternative form of payment, also saying that their purchase won’t be processed until payment is made.

In any case, it is only after the reservation is placed that the responsible service would publish an event about that. The service which collected the credit card information would be subscribed to that event and initiate the charge of the card when that event arrives (or not, depending on the rate rules mentioned).

With regards to there not being any rooms left, well, first of all, there’s overbooking – hotels accept more reservations than rooms available because they know that customers sometimes need to cancel, and some just don’t show up. Secondly, there is a manual compensation process if more people show up than there are actual rooms to put them in. In some cases, a hotel will bump you up to a higher class of room (assuming there aren’t too many reservations for those), and in others they will call a “partner” hotel nearby and put you up there instead.

In summary

While arguments can be made that yes, these issues have been addressed in this specific example, there may be other domains where it is not possible to do these kinds of “tricks”. Although I do agree with that in theory, I’ve spent the better part of 5 years travelling around the world talking to hundreds of people in quite a few business domains, and every single time I’ve found it possible to apply these principles.

In short, the use of UI composition allows services to collect their own data, making it so anything outside that service doesn’t depend on those data structures which makes both development and versioning much easier. Technical failure conditions can be mitigated at infrastructure levels in most cases and other business logic concerns can be addressed asynchronously with respect to the data collection.

Give it a try.

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  1. Chris Says:

    All your arguments are compelling, but I still can’t see the advantage of composing in the UI with Javascript over composing in a tiny piece of server-side code that’s coupled to the UI.

    The obvious problem with this approach you have not mentioned… What do you do about non-Javascript users – do you just ignore them? I would find amazing if you have been using these techniques for 5 years and not considered the non-Javascript case.

    The other thing that springs to mind with this approach is that you have to expose all your services. Does this not mean that a cunning user could in theory book a hotel room without sending any card details. A malicious user could in fact fill up a hotel with reservations by making direct calls to the relevant service? Just thinking out loud here…

  2. Josh Says:

    “In general, failed credit card payments are handled pretty much the same way for all ecommerce – an email is sent to the customer asking for an alternative form of payment, also saying that their purchase won’t be processed until payment is made.”

    I’m pretty sure this is a false statement. I have seen, and worked on, plenty of ecommerce sites where if the payment is not accepted it errors out on the screen you are on. The only time that has been different is when the price of the item was high and the shipping address was not my billing but that was a manual intervention and a phone call.

  3. Josh Kodroff Says:


    As I understand it, there are 2 phases of obtaining payment via a credit card:

    1. Authorization, where the funds to pay for the transaction are reserved (i.e. a “hold”).
    2. Capture, where the credit card is actually charged (typically when the order ships).

    When you say “In general, failed credit card payments are handled pretty much the same way for all ecommerce”, are you referring specifically to authorization or capture (or either)?

  4. Robert Young Says:

    This is just COBOL/VSAM/3270 screen edits from the 1960s. Codd devised the RM just to avoid client side messes. While OO folk preen about data and method being encapsulated, when it’s pointed out that transactional methods (semantics) *belong* with the data, they get all huffy about losing control of *their* application. If one wishes to continue with 1960s siloed application development, then go for it. But do know that you’ll be writing orders of magnitude more LoC. Perhaps that’s the point? Permanent employment??

    Perhaps a rear-guard action against code generation from the database schema? More of those frameworks are coming into existence. After all, with a 5NF database, all constraints are defined in the schema/catalog as plain text, and so can be used to generate client screens just as some do with messy xml files.

  5. udidahan Says:


    First of all, the proportion of the population without JS is dropping all the time. Second, the risk that you’re calling out is easily handled by the event-processing mechanisms in the back – users would only be hurting themselves.

  6. udidahan Says:

    Josh (2+3 assuming you’re the same person),

    I shouldn’t have spoken in such broad strokes – yes, I guess there are plenty of ecommerce systems that don’t work the way I described, but that doesn’t mean they should.

    The issue is that we can’t assume perfect connectivity between our systems and that of our credit card processor. It doesn’t make sense to tie up resources on our system, limiting our ability to accept more orders and handle more customers, by performing a blocking call here.

    This is true for both payments and authorizations.

  7. udidahan Says:


    I’ve seen the size of code bases decrease dramatically when building systems this way (as compared to the traditional layered/tiered architectures). Now, it could be that you’re against those as well, in which case we have something in common.

    On the issue of the primacy of the relational model, in my work with business stakeholders in many domains, I’ve found that they don’t require “up to the microsecond” referential integrity in many cases and can live quite happily with Eventual Consistency.

    With regards to generating code (a.k.a behavior) off of the constraints defined in a database, I’d be interested to see how one would model temporal rules like: When an order is placed, wait with its processing for 30 min (in case it gets cancelled during that time), or N minutes based on other rules (high priced item, preferred customer, etc).

    We also tend to need to be able to run multiple versions of the same long-running processes side-by-side, something that most databases don’t tend to make very easy. An example of this is a mortgage application process – we should be able to upgrade the system at any time such that existing applications continue behaving according to the rules that were in place at the start of the application.

  8. Josh Kodroff Says:


    We’re actually not the same person. I only wrote #3.

    And your reply makes perfect sense. Thanks!

  9. Dmitry Says:

    There are still quite a few users who are running browser add-ons like NoScript or using Blackberries that have limited JavaScript support.

    Your approach sounds really good for a lot of scenarios but I would still have a fallback plan if a submit button goes through without triggering JS/AJAX.

  10. Ra-el Says:

    Hi, awesome set of posts. Judging by the hostility in these comments you’re really challenging some people’s world view, which is great.

    A very smart developer once told me: “If you look at a design and think that it is stupid you have to take a moment and think that it may be you who is stupid.”

    To Chris: You’re making a fool of yourself. I would seriously suggest taking Udi’s “Advanced Distributed Systems Design with SOA” course, if you can. I took it a couple of years ago and it really put this kind of thing into perspective. But its when you start implementing these concepts that it really starts to fall into place.

    These approaches don’t make design, development or deployment easier in the short term. However they are easier to support, scale and add features to.

    Disclosure: I have no link to Udi. Other than taking the course and using his consulting services.

  11. Darran Says:


    I agree completely with what you are saying and I love the idea of the approach. But I just want to be able to do it server-side rather than client-side, or at least know there is an option to do so.

    On the server side, can’t we just call RenderAction() for each component (in the cases where we do not need to compose from multiple services into a table), and merge the html? We keep every aspect of our controllers de-coupled from each other, including our views.

    Tabular data composition:
    The example of tabular data composition using knockout.js shows that all we have to do is create a new viewmodel, composed from the various services, and which is then rendered with a view/template on the client side.

    The remaining problem is where we have to compose into a table, server side. In this case don’t we just aggregate the data into a new view model and RenderPartial from one controller – this would be one of a few places where a single controller and a single View interacts with multiple services, but currently, I cannot fathom any way around this minimal coupling.

    Or do you have another suggestion for solving this problem? It seems that many of us want to be able to do this server-side after all…


  12. Josh Kodroff Says:


    If someone is running NoScript, I say *bleep*-em. This is 2012.

    For stuff like ecommerce, the requirement I’ve most often seen is that you need to provide a path through purchasing and checkout, but it doesn’t have to be pleasant.

  13. Darran Says:


    So what do you do when you are building software for government services (tax payments, online health care, etc.) and you have accessibility requirements to meet? What if you are building software for a company or charity who actually cares about people with disabilities and where a “pleasant path through…” can make a real difference?

    Do you just say *bleep*-em then? What do you do next out of interest, I’m genuinely curious? Surely writing good software can be an inclusive pursuit rather than one where we just raise the bird to anyone who doesn’t fit into our neatly packaged world view whether by choice or circumstance?

    Or do you actually try to find a good solution? I like to find solutions to problems, personally, but in this domain (SOA) I’m a little out of my depth (for now) so I’ll take the time to ask the experts while mulling things over myself.

  14. Gary Stonerock II Says:


    Josh’s statement is derived from the “general” e-commerce attitude that it is 2012 and it might not be worth the extra effort to support non-js clients. If the particular business cared a lot about these clients, then a different implementation might be required.

    If you follow Udi and read his blog posts, one of the biggest messages he makes is the fact there is no cookie cutter solution. No solution that you can apply to everything. If something doesn’t work in your context, do what makes sense for the context, business, and project.

  15. Darran Says:


    Thanks, I agree completely that it comes down to what the business wants/needs and that there’s no one size fits all. It’s one of the biggest messages from Udi that I’m taking on.

    What I found disagreeable on this thread however was that there is a clearly stated desire to learn about how to maintain our service boundaries on all tiers of our architectures. Many of us have asked the question: how do you do this server-side with tables? We’re asking the guys that have the most experience…

    And, I think that on the one hand: screw the client, and on the other hand: abandon that desire to maintain boundaries because you can’t use Javascript, is a bit, disappointing.

  16. udidahan Says:


    Microsoft has been making it gradually easier to do more involved server-side composition with things like MVC Razor using Sections, but it still has a ways to go.

    There also isn’t a good built-in event model for loading things server side – ie when the product IDs have been found for a certain search string, that another service will get a callback and be able to bring in other product info for those IDs.

    In short, we’re missing quite a lot of infrastructure. This means either building it ourselves or allowing some coupling to come creeping back in there.

  17. Slawek Says:

    Thanks for UI Composition article series. It seems to me that problem of the UI design is quite often overlooked in the context of DDD and complex systems.

    Could you explain how to use Razor sections to compose view on the server-side. I wonder how to provide data for each section.

  18. Darran Says:

    Thanks Udi. I’m happy to accept a little server-side coupling for the case where I’m merging data from multiple services into a table. It seems largely avoidable in most other use-cases. Cheers.

  19. Justin Says:

    I actually appreciate the questions that Chris asked. I think the weakest link in “Udi SOA” has been the lack of a clarified client side technique implemented with real code and a real stack instead of just abstract hand waving.

  20. Paul Says:

    Just a quick note regarding the JavaScript vs server side. In ‘SOA Patterns – Manning’ they talk about having server side composition to support legacy systems. I.e. the point is about keeping the responsibility separate, if you can do that with server side components then great. If you can do that with JavaScript then great too. In my experience I am using a combination of both. For services that I do not care too much about I use server side composition as it’s faster for my .net team to implement. For more important/sensitive/fragile parts these are implemented with JS as it provides more opportunity for async, resistance to server failure, etc. NB it doesn’t have to be web, rich native apps also have the same pros/cons of js wrt composition.

  21. nick Says:

    How do you handle server-side orchestration when you have to do it, for example a REST API?

    Make the endpoint belong to a service, and duplicate any data it needs inside that service, or, have the web application do the orchestration and call multiple services?

  22. Gleb Says:

    I don’t quite follow – what do you mean by saying that NServiceBus helps to mitigate server-side failures? I thought it’s just a message-passing layer. Or does it have some sort of retry-after-exception policy?

  23. udidahan Says:


    Yes – NServiceBus is what invokes your code, catches any exceptions you throw, retries (with a back-off policy), logs, moves to a separate queue, and provides tooling for returning the message back to the original queue for reprocessing as well.

    And that’s just the simple stuff 🙂

  24. Terrence Joe Says:

    What happens if you want to display a server side validation message to the client after they have clicked submit? For example, say the hotel has a no overbooking policy, and upon clicking submit, the service in charge of booking rooms returns a “sorry, that room is now sold out” message and this needs to be displayed on the client. We might also want that validation failure to stop the submission to all other services as well.

    I understand that we could just accept all bookings perform a compensation action later like sending an email, but sometimes, ui’s may demand immediate feedback for the user that can only be performed from the server side. Any idea’s on how to tackle this with a composite ui?

  25. udidahan Says:


    If that is a requirement, we can model things differently.

    For example, implementing the client-side logic such that only after Service1 completes its server-side validation – returning a response to the client-side code of Service1 which will in turn raise a client-side event indicating that everything’s good, and that will call back to the client-side code of Service2 which is subscribed to it, which will kick off the server-side logic of Service2.

  26. Rajesh Says:

    Hi Udi,

    One thing i have not been able to figure out for myself (nor find code snippets for) is how UI composition would work within a table kind of display. This essentially will require correlation between responses received from different services. So suppose in a vanilla case, i want to display a few products and their prices in a table. If I assume that product name and description come from the Marketing service, and price from the Finance service, how does the correlation between the data received from these two services happen? Assuming that both services will return data in the same order (as was done in one sample code you had referred to on some other site) seems to me not a production-ready algorithm.

    UI composition in other scenarios where no such correlation is needed is, as you say, not a big deal to achieve. But this table thing has me beat.

    Warm Regards

  27. udidahan Says:


    There would be a Branding Service which owns the table layout, and has a ViewModel with properties for each of the service – each property being a dictionary from the product ID to the relevant value. Each service would set the property for which it has the data, and the ViewModel, when all properties would be set, would trigger the rendering logic.

    Does that answer your question?

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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.”

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