Udi Dahan   Udi Dahan – The Software Simplist
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Tasks, Messages, & Transactions – the holy trinity

Saturday, March 31st, 2007.

The discussion is picking up around disconnected Web Service interaction scenarios. Here’s a summary of what’s going on for those just joining us, but I would suggest reading the full posts as well:

Andres: “Basically, there is no disconnected mode… if you plan to build a multi-tier application with ADO.NET Orcas in the middle tier, you will need to hack your own change tracking mechanism in the client, send the whole changeset, and apply it in the middle tier.”

Udi: “[In] the UI, … tasks often corresponded very well to the coarse-grained messages we employed in terms of SOA.”

Jesse: “I have to disagree with Andres and agree with Udi.”

(Sorry, I just couldn’t resist. Here’s the important part)

“Andres rightly points out that you don’t want 5 different methods for updating an order. Although you technically could do it that way, you probably don’t want to be ignoring the transactional context of the conversation or locking tables for significant time periods.”

And, “The backend will still have all the update methods you are used to using, so you’re not writing a bunch of extra code in the DB. However, this approach is far more powerful. One reason why is that now you have an operation defined and you can control everything about that operation. You know when the call comes in through that method that you have a customer service rep trying to update an order and can build in logic based off of that.”

Andres: “Now, we need to send those changes to the server, together, because I want them executed in a single transaction. I cannot have a service called ‘ModifyOrderCustomerInformation’, another ‘AddALineToOrder’ and ‘DeleteLineFromOrder’. I need an ‘UpdateOrder’ service. You can use a diffgram to do it, or you could build your own change-serialization mechanism.”

And, “The only way I see is to map the UI to the service interface, so the user can ‘Change Address’ or ‘Update Marital Status’ as different operations in the UI layer, but I can’t let the user ‘Modify the Customer’. It’s a lot of work, and I seriously doubt that the users will like it.”

OK, now we’re all set.

Just to get something small out of the way, every Human-Computer Interaction (HCI) professional I have worked with has been in favor of task-based UIs. Every user that I have met that has used both styles of UI, task based and “grid” based, has reported that they were more productive when using the task based UI for “interactive work”. Data entry is not interactive work, so grids might be more suitable there, although improvements in bulk-loading technology and OCR have decreased the amount of “plain” data entry that I’m encountering. It also may be that I’m working less on systems where data entry is of significant importance.

First of all let’s agree that writing business logic for something like “DeleteLineFromOrder” or “UpdateMaritalStatus” is easier than for “UpdateOrder”, given that it fit with the overall system design.

Second, let’s agree that the business logic for something generic like “UpdateOrder” is composed of the various specific cases like “DeleteLineFromOrder” and “AddLineToOrder”.

Third, let’s agree that if the UI was task based, it would be easy for the client side to activate the correct web service/method – seeing as there is a high correlation between the tasks and the service methods.

Therefore, the question becomes how do we do all the above specific work in a single transaction? One answer may be to make the service statefull and using something like WS-Atomic Transaction to tie the various calls together. The problems in this approach are already well known. Another solution uses a messaging paradigm, something I wrote about before.

If we represent each of the tasks not as “web methods”, but rather as messages we could send all these messages together in one envelope to the server, and have it process all of them in a single transaction, activating the specific business logic pieces one after the other.

I’ll be using a code-first approach to describe the solution instead of an XML-first one since it abstracts away the technology, but it is still a contract-first approach.

Each message type is just a class that implements the “IMessage” interface. So we’d have a class called “DeleteLineFromOrderMessage” which is a Data-Transfer Object (DTO) whose members/properties are the data needed for processing – in this case, the order Id and the order line number. The same would be true for the class “AddOrderLineMessage”, it would contain the order Id and some other member for the order line data quite probably a DTO itself (so that we can use it in other places as well).

The client would generate an instance of the appropriate message class as the user finishes each sub-task, saving them up. When the user would click “confirm”, the client would send all these message objects to the server in one go with this API,

void IBus.Send(params IMessage[] messages);

Like so:

myBus.Send(addOrderLineMsg, delOrderLineMsg);

The bus would take all these objects and wrap them in a single SOAP envelope, and send them to the server, probably on a transactional channel, but that’s a configuration issue. At the server side, the bus would activate the transaction (because of the transactional channel) and start dispatching each of the specific messages to its message handler, one at a time, in the same thread.

So as you can see, there is no “transactional conversation”, and therefore we’re not locking tables in between calls, because we’ve gotten rid of “in between”. It’s just like the generic update in terms of transaction time and network hops, just with specific, simple business logic.

Andres might retort to that, “Even if Udi is right, and that’s the way applications should be built, I doubt most mere mortals in Earth will want to do it.” In fact, he did J If the frameworks supporting this style of development were supplied by Microsoft, I’m sure that most developers wouldn’t have a problem with it. When provided with such a framework, every developer I’ve worked with said that they wouldn’t want to go back to the “old way” (would that make Orcas outdated?). I’ll be putting up my frameworks soon, as well as examples on how to use them, it’s just taking longer than I expected.

Before closing, I just wanted to address the points Andres raised about concurrency:

“You could also need to know the previous values for optimistic concurrency checkings.”

I’ve written about how to do this before in regards to Realistic Concurrency, and the best example I have in terms of code is Better Domain-Driven Design Implementation.

This has gone a little longer than I planned, but I still don’t think I’ve covered everything in enough depth. I’d most appreciate investigative questions to help me shed light on the murkier parts of this, either as comments, posts on your own blog, or even via email. Let’s continue the conversation.

  
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15 Comments

  1. » From Disconnected Scenarios to SOA Says:

    [...] no way did I intend to discuss SOA related issues in my previous posts. Since I talk so much about SOA, and the use of the Bus Pattern and Asynchronous Messaging, I [...]


  2. Frans Bouma's blog : Why change-tracking has to be part of an entity object Says:

    [...] inside the entity objects itself and gave some examples why that’s unfortunate. Udi pulled the discussion into SOA land, and I think that was unfortunate as well, because there’s a much wider used example which will [...]


  3. » Can Indigo be my bus? Says:

    [...] How the Bus API handles the connection between messages and transactions [...]


  4. » [Podcast] Occasionally Connected Smart Clients and ADO.NET Sync Services Says:

    [...] enjoying the recent discussion on Entity Framework, disconnected clients, Unit of Work, and messaging. A few weeks ago I wrote a note to self to “Ask Udi” about the new ADO.NET Sync [...]


  5. Darrel Miller Says:

    Just to add another voice to the “Change Address” position instead of “Modify Customer”. I would go one step further to try and identify why the address is being changed. Are we “Correcting the Address” due to a clerical error, or changing the address because “Customer has Moved”? The actions taken based on those two different scenario’s could be significantly different.
    Now… how you actually implement a UI in this style is a very different question. I look forward to someone trying.


  6. thesoftwaresimplist Says:

    Darrel,

    At the very simplest level, when right-clicking a customer, one of the options available is “Customer Moved”.

    If there are relatively few actions that could be done on customer, some kind of outlook 2003 bar could be used.


  7. John "Z-Bo" Zabroski Says:

    Udi,

    What is a grid-based UI?

    Task-based UI – you give examples.

    Grid-based UI – you throw in there like it is a common term.

    Quite simply, Google Top 10 Search Results says the term is own you made up and didn’t define. I’d appreciate an explanation of what sort of symptoms Grid-based UIs have that you don’t like.

    Look at it this way: You say, “let’s agree that if the UI was task based, it would be easy for the client side to activate the correct web service/method – seeing as there is a high correlation between the tasks and the service methods.”

    This sets up a strawman for dismissing “Grid-based UI” (whatever that is). You actually never explain why Grid-based UI has a LOW correlation, or why correlation makes programmer’s life more difficult.

    Also,

    @Data entry is not interactive work, so grids might be more suitable there, although improvements in bulk-loading technology and OCR have decreased the amount of “plain” data entry that I’m encountering.

    I don’t understand this, either. There is always a Suzy Creamcheese on the phone with a customer who needs to enter information somewhere about the call. Even if you have tasks that map to her cold and hot call scripts, at some point along the task she will be punching in numbers and names. If Suzy is a development personnel in a fundraising department working during a telethon, she needs a fast way to enter data.


  8. udidahan Says:

    John,

    Sorry for not defining it – I did make an assumption there about people understanding its intent.

    A grid-based UI is one where the user performs most/all activities in the app through a grid – specifically editing data. In this kind of user interaction model, the intent captured is a fairly technical “I want to edit this entity”, without the business context of “why” – like Darrel’s comment above describes.

    Relating to “she will be punching in numbers and names”, I’m not arguing that that occurs, but that having additional business context is invaluable.

    Some systems really are pure data entry in that the system doesn’t really have any validating to do, as the data coming in is being typed from a file of sorts which is already the authoritative source of information – nothing really that a computer could refute.

    Does that make more sense?


  9. John "Z-Bo" Zabroski Says:

    You seem to be suggesting that there is no way for this “grid-based UI” to communicate a “change of address” message has been sent on UPDATE. I guess if your Complex Event Processor can only handle blobs of messages, then you need that level of granularity?

    Also, what if the application doesn’t capture the business context. Instead, the business context is tied to a virtual event only known to complex event processing. Such is the case in a federated system controlled by messaging. You can see the messages flying back and forth, but without the ability to recognize events in a human understandable way, it’s just mud and you can’t actually see anything insightful.

    The business context would be that there is a service waiting for a customer’s correct address.

    So again, still I see a sort of strawman. Sorry. :(


  10. John "Z-Bo" Zabroski Says:

    Actually, I think I know what you are trying to say.

    I also think I can now better articulate what I am put-off by.

    Sometimes users do need a “green screen terminal” do to their jobs well.

    The scenarios you are talking about, with tasks like “Change XXX”, only make definitive sense when there is a service waiting on something. The service has a human dependency on XXX, and therefore the UI is Change XXX.

    A good example is Amazon.com. Usually, they annoy me for two reasons.

    1) A book I preordered has been delayed, and they want to confirm I am still interested.
    2) A credit card I have on file with them is no longer valid, but my order has already taken place (but is now on lay-away until I change my CC#).

    In both these scenarios, I get an email w/ a hyperlink to a secure URL (requiring login) that plants me on a page where I can take action to move the business process along. I don’t have to go looking for my order. They directly put the action item in front of me, and the screen tells me what my primary task is (”fix billing info” or “confirm you will wait until hell freezes over to get this book”).

    I don’t like the phrase task-based UI here. Task-based UI means the user’s workspace is organized to improve completion of tasks, but doesn’t communicate the real usability concept: the UI *knows* what you need to do.


  11. udidahan Says:

    John,

    Re: “the UI *knows* what you need to do.”

    Exactly.

    If you can find a better term to describe this, giving it a good contrast from what I described as “grid-based UIs”, I’d be most happy to hear it.


  12. John "Z-Bo" Zabroski Says:

    In UX/Interaction Design circles, the operative phrase is “workflow-centric”.

    However, let me be very clear that technical jargon is virtually meaningless without the presence of examples (like the Amazon ones). I realize this is simply a blog, but the major failing of most blog entries on most blogs is a very dangerous myth: the myth that all programmers share the same psychology, when they do not.

    Not only do programmers have different psychologies, but they also don’t share the same vocabulary.

    Workflow-centric UIs are less common, in part because (a) you can’t simply use a code generation tool to build the UI for you (b) workflow is inherently hard to describe using human language, and as such there is no format for a compiler writer to use to help automate workflow-centric UI creation (c) workflows can often change, and so in workplaces where employee turnover is also high, human resources training officers have to work double duty to make sure people know what the hell their job is

    With workflow UIs, you want to be sure that you only invest in them when you have a very stable description of the problem domain. If you properly design your application based on the real world, then any changes that occur in the real world should be not too dramatic, because people rarely dramatically change their workflow. Human beings, ya know, we’re risk adverse and don’t like change, and when we do change we tend to change in ways creating only subtle differences in the problem domain’s structure.


  13. udidahan Says:

    John,

    Workflow-centric UI sounds good.


  14. Sly Gryphon Says:

    Allowing multiple messages to be submitted is a good idea.

    I have designed a system that way before, and it works very well. To indicate a transaction is required across multiple messages you can easily include the details in a wrapper (or header). (Or to get something working quickly even just initially assume multiple messages === transaction).

    From memory the design was based on the IBM IFW (Information Framework, a framework for financial services), which uses a similar architecture (multiple messages can be sent in one transaction).

    Sly


  15. udidahan Says:

    Sly,

    Thanks for your comments, I’ve found it a good option as well.
    The thing I like is that the server can change its mind and process them one at a time.


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


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

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

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

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

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

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

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

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

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

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

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

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

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

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

Consult with Udi

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

97 Things Every Software Architect Should Know



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