Udi Dahan   Udi Dahan – The Software Simplist
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SOA, EDA, and CEP a winning combo

Saturday, November 1st, 2008.

jump in There’s been some discussion on the SOA yahoo group around the connection between SOA, EDA, and CEP (complex event processing) since Jack’s original post on the topic. I’ve been waiting for the right opportunity to jump in and it seems to have come.

Dennis asked this:

There are different design choices in a SOA, even when you already have identified the services. I have a simple example that I would like to share:

Imagine a order-to-cash process. One part of that process is to register an order. Suppose we have two services, Order Service and Inventory Service. The task is to register the order and make a corresponding reservation of the stock level. I would be pleased to have the groups view on the following 3 design options (A, B, C):

A.
1. The “process/application” sends a message (sync or async) to “registerOrder” on the Order Service.
2. The “process/application” sends another message (sync or async) to “reserveStock” on the the Inventory Service.

B.
1. The “process/application” sends a message (sync or async) to “registerOrder” on the Order Service.
2. The Order Service sends a message (sync or async) to “reserveStock” on the the Inventory Service.

C.
1. The “process/application” sends a message (sync or async) to “registerOrder” on the Order Service.
2. The Order Service publishes an “orderReceived” event.
3. The Inventory Service subscribes to the “orderReceived” event .

On the whole “already identified the services” thing – naming a service doesn’t mean much. It’s all about allocating responsibility, and until that’s been done, those “services” don’t give us very much information.

 

Business Services

If we were to view this example in light of business services, and look at the business events that make up this process, maybe we’d get a different perspective.

Three business services: Sales, Inventory, and Shipping.

In Sales, many applications and people may operate, including the person and the application he used to submit the order. When the order is submitted and goes through all the internal validation stuff, Sales raises an OrderTentativelyAccepted event.

Inventory and Orders

Inventory, which is subscribed to this event, checks if it has everything in stock for the order. For every item in the order on stock, it allocates that stock to the order and publishes the InventoryAllocatedToOrder event for it. For items/quantities not in stock, it starts a long running process which watches for inventory changes.

When an InventoryChanged event occurs, it matches that against orders requiring allocation – if it finds one that requires stock, based on some logic to choose which order gets precedence, it publishes the InventoryAllocatedToOrder event.

Sales, which is subscribed to the InventoryAllocatedToOrder event, upon receiving all events pertaining to the order tentatively accepted, will publish an OrderAccepted event.

Orders and Shipping

When Inventory receives the OrderAccepted event, it generates the pick list to bring all the stock from the warehouses to the loading docks, finally publishing the PickListGenerated event containing target docks.

When Shipping receives the PickListGenerated event, it starts the yard management necessary to bring the needed kinds of trucks to the docks.

 

What else is possible

I could go on, talking about things like the maximum amount of time stock of various kinds can wait to be loaded on trucks, subscribing to earlier events to employ all kinds of optimization and prediction algorithms, having a Customer Care service notifying the customer about what’s going on with their order (probably different for different kinds of customers and preferred communication definitions). Obviously, we’d need a Billing service to handle the various kinds of billing procedures, whether or not the customer has credit, pays upon delivery, etc.

It turns out that many business domains map very well to this join of SOA and EDA.

 

What an ESB is for

When we have these kinds of business services primarily publishing events and subscribing to those of other services, you don’t need much else from your “enterprise service bus”. All sorts of transformation, routing, and orchestration capabilities don’t come into play at all.

In all truthfullness, those bits of functionality are really just a historical artifact of their broker heritage.

Don’t get me wrong, sometimes a broker is a nice thing to have – behind a service boundary in order to perform some complex integration between existing legacy applications.

Just keep that stuff in its place – not between services.

 

Complex Event Processing

We can look at how Sales transitions an order from being tentatively accepted to being accepted as requiring event correlation around InventoryAllocatedToOrder events. This isn’t exactly “complex” in its own right. If there were some kind of CEP engine that did this for us out of the box, it might be a possible technology choice for implementing this logic within our service.

As we add more concerns, like time, we may find new ways to make use of this engine. For instance, if the time to provide the order to the customer is approaching, we may choose to split the order into two – accepting one for which we have all the stock allocated, and leaving the second as tentatively accepted.

 

Summary

While it is difficult to move forward on service responsibility without discussing the events it raises and those it subscribe to, the whole issue of CEP can be postponed for a while.

Although there aren’t many who would say that EDA is necessary for driving down coupling in SOA, or that SOA won’t likely provide much value without EDA, or that SOA is necessary for providing the right boundaries for EDA, it’s been my experience that that is exactly the case.

CEP, while being a challenging engineering field, and managing the technical risks around it necessary for a project to succeed in some circumstances, and really shines when used under the SOA/EDA umbrella, it should not be taken by itself and used at the topmost architectural levels.

 


Related Content

SOA and Enterprise Processes

How client interaction fits with SOA

Time and SOA

Durable Messaging for Fault-Tolerant Services

And if you’re wondering about how to handle all that complexity inside services (different kinds of billing, periodic tests for electronics inventory, etc), you might like listening to this podcast about business components.

  
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16 Comments

  1. Mark Nijhof Says:

    Hi Udi,

    Good post! I totally Agree with this idea. SOA is a good pattern to clearly separate the different responsibilities within the application Then use EDA to have these Services communicate with each other. This gives a very high grade of louse coupling and with that all the nice abilities to make changes easier. I have been thinking about combining EDA with DIP as a technical pattern with the main purpose to make change easy (http://blog.fohjin.com/2008/09/no-dependency-architecture-nda.html). Adding SOA to this seems like a logical next step, I will definitely look more into this.

    Thanks,

    -Mark


  2. Jack van Hoof Says:

    Now you really are my friend, Udi. Excellent post!!

    -Jack


  3. udidahan Says:

    Mark,

    Glad you liked it.


  4. udidahan Says:

    Jack,

    We need more people fighting the good fight :)


  5. Peter Rajsky Says:

    Nice article. I follow Jack fight :)
    What I do not like on it is following:
    You call Sales, Inventory and Shipping “business services”. Actually there are three different component types: Services, Event producers and Event reaction processes (these are not reusable and catalogized in the same way as services or as events; http://it.toolbox.com/blogs/system-integration-theory/eda-soa-crossproject-documentation-26215).
    ESB is still needed – for implementation of reaction processes and simple composite services (e.g. routing/transformation). Again ESB tools should differentiate between these three component types
    (http://it.toolbox.com/blogs/system-integration-theory/what-does-the-letter-s-in-jbossesb-mean-27069)
    We need to differentiate when to use event-driven communication for your example and not one-way command? It is not about loosely coupling only probably… (I tried to find out answer in http://it.toolbox.com/blogs/system-integration-theory/would-you-like-service-or-event-sir-26243; but it is not so easy…)


  6. udidahan Says:

    Peter,

    I accept that you have a different opinion on the definition of “service” than I do. Apparently, it’s not just the two of us – everyone has their own definition :)

    For me, I’ve given up on the whole reuse thing. I think it’s a crock. Has been since OO.

    Also, as the example in my post describes, we can see the same thing being both an event producer and “reaction process” as you call it for different kinds of events.

    I’ve read the posts you linked and it appears that you take an integration perspective on SOA. I perceive integration as something that occurs within the context of a single business service.

    Anyway, glad you commented and I’m looking to hearing more from you.


  7. Peter Rajsky Says:

    It is strange that European union standardize what is banana (and has exact criteria)… They should standardize notion of integration service :)


  8. Mário Rui da Costa Says:

    Hello,

    Congratulations, your post is the clearest view relating the combined use of SOA and EDA principles for Information systems specification I have ever seen. I am a “lonesome cowboy” fighting in the arena of a Telco operator and talking alone about EDA in the last years. Your clear objective view of the benefits of EDA + SOA and their complementary role will be determinant to open some minds around here. Thanks.
    Mário Rui da Costa
    of


  9. udidahan Says:

    Mário,

    Glad you liked it.
    And never underestimate the power of the lone ranger :)


  10. Lost Notifications? No Problem. Says:

    [...] More in depth example on events and pub/sub between services [...]


  11. mfathur Says:

    Hi, Udi
    I Love This!

    Many thanks


  12. sandeep Says:

    Hello,
    I downloaded the NserviceBus and opened up samples. The main method is quite puzzlin with too many extension methods. Can you please demystify this .

    thanks in advance for wonderful contributions


  13. udidahan Says:

    Sandeep,

    Glad you like nServiceBus. The configuration approach does take some getting used to – I agree. I would suggest that you bring these kinds of questions to the discussion group, though, where more people can see both the question and answer.

    Best regards,


  14. Alex Says:

    Hi Udi, I liked your article and it has provided me with some insights. I am working on a project where I faced some of these dilemmas, and captured my version of a pattern that is able to support SOA. You can find it here if you are interested:

    http://www.communities.hp.com/online/blogs/nextbigthingeds/archive/2009/09/25/managing-enterprise-attributes-and-events-using-a-new-soa-provisioning-and-synchronisation-pattern.aspx


  15. udidahan Says:

    Thanks Alex – looks interesting. I would say that while it may be valuable to technically keep rules separate (rather than hard coded), I still see them inside the logical service boundary, not outside.


  16. Maninder Says:

    Udi,
    In EDA, how do we handle exceptions that span service/component boundaries?
    Say the requirements are to create order and if customer does not exist, create customer. The UI captured order, new customer and billing information. Should the UI
    1. Invoke 3 services, create order, create customer, bill customer by sending order, customer and billing information respectively. That is classic process service invoking entity services and handling any compensation required.

    2. Invoke create order with all the 3 entities and submit to order service, even though order service just needs order info. The order service creates the order and emits, order created event, along with new customer info and billing info.
    The customer info component subscribes to the order created event and creates a new customer and emits a customer created event and billing service subscribes to the order event and creates billing done event.
    But lets say customer component fails to create a new customer because address validation failed, but order service and billing component worked. How do you handle this scenario. Say the business rule is to rollback all changes and send notification to customer.
    First it seems odd that i am sending more information to order, customer and billing than necessary. Second it seems like i am still thinking in a process oriented way, where some component tracks all events and handle compensation, which is equivalent to process service.


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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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