Udi Dahan   Udi Dahan – The Software Simplist
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The Known Unknowns of SOA

Monday, November 15th, 2010.

rumsfeldOne of the better known analysts in the enterprise software area, JP Morgenthal, wrote this post about the relationship between SOA, BPM, and EA. In it he defines SOA as follows:

“SOA is a practice that focuses on modeling the entities, and relationships between entities, that comprise the business as a set of services. This can be done on a small or large scale. Typically, the relationships in this model represent consumer/provider relationships.”

I have some serious concerns about the ramifications of this definition/description.

First of all, when reading “entities”, many people will interpret that to mean the entities found in Entity Relationship Diagrams [ERD] or in Object Oriented Analysis & Design [OOAD]. In both, these entities are identified as the “nouns” of the domain. Examples of these ERD/OOAD-type entities include things like Customer, Order, and Product.

These are almost always the wrong place to start for identifying services in SOA.

Second, on the consumer/provider relationship: on the one had, this fits very well with how web services can consume (or call) other web services. However, the downsides of using web services as services in SOA is becoming well enough known that even in the same post we see this warning:

“Web Services is not SOA, it is merely a standardized approach to accessing functionality on remote systems.”

But the question remains, if a producer/consumer relationship is OK for SOA-type services, why doesn’t that hold for web services? And the answer is… it depends on the type of producer/consumer relationship. The typical relationship is one of synchronous calls from consumer to producer, this is not OK for SOA-type services either.

You see, this synchronous producer/consumer implies a model where services are not able to fulfill their objectives without calling other services. In order for us to achieve the IT/Business alignment promised by SOA, we need services which are autonomous, ie. able to fulfill their objectives without that kind of external help.

Instead, we need to look for a more loosely coupled producer/consumer relationship – like publish/subscribe, where the producer emits events, and the consumer subscribes and handles those events. The reason that this kind of relationship doesn’t hurt autonomy is that it disconnects services on the dimension of time. In order for a service to be able to make a decision autonomously without synchronously calling any other service, using only information provided by events it received in the past, it must be strongly aligned with the business.

Most projects which bandy about the SOA acronym aren’t actually made up of services – they’re made up of XML over HTTP functions calling other XML over HTTP functions, eventually calling XML over HTTP databases. You can layer as much XML and HTTP as you want on top of it, but at the end of the day, most projects are just functions calling functions calling databases – in other words, procedural programming in the large, and no amount of SOAP will wash away the stink.

Here’s a different definition of services for SOA that may communicate a bit better what it’s all about:

A service is the technical authority for a specific business capability.
Any piece of data or rule must be owned by only one service.

What this means is that even when services are publishing and subscribing to each other’s events, we always know what the authoritative source of truth is for every piece of data and rule.

Also, when looking at services from the lense of business capabilities, what we see is that many user interfaces present information belonging to different capabilities – a product’s price alongside whether or not it’s in stock. In order for us to comply with the above definition of services, this leads us to an understanding that such user interfaces are actually a mashup – with each service having the fragment of the UI dealing with its particular data.

Ultimately, process boundaries like web apps, back-end, batch-processing are very poor indicators of service boundaries. We’d expect to see multiple business capabilities manifested in each of those processes.

I know that this may be more confusing than the traditional web services approach but, to paraphrase Donald Rumsfeld, it is better to know that you don’t know, than to not know that you don’t know :-)

  
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11 Comments

  1. Ward Bell Says:

    Perceptive as always. Cutting right through the Service == Web Service nonsense. Two quotes that stand-out for me:

    “A service is the technical authority for a specific business capability”

    “process boundaries … are … poor indicators of service boundaries”

    Words to code by. Thx.


  2. Elliot Says:

    Udi

    I understand what you’re saying and I agree with the principle but I don’t understand how the implementation would work. My exposure to composite applications is limited so I may well be missing something here.

    “a product’s price alongside whether or not it’s in stock”

    A product’s price is a separate concern from it’s stock level and as such both pieces of information are retrieved separately from different services and mashed up in the UI. However what if we wanted to display a list of the 10 most popular products, ordered by their name, in an admin screen of some sort. How would we know what queries to issue to the separate contexts so that we can retrieve the correct information from each and ensure that it can be reliably correlated in the UI?

    Thanks


  3. Alex Vilela Says:

    This is one of the easiest reading about SOA I’ve seen so far.
    It is a very good post indeed, thanks.


  4. Paul Says:

    Great post as usual, and I learned a new word.. “Bandy”


  5. Frank Quednau Says:

    Having seen Greg Young at oredev about CQRS, it looks like such architectures just play well with what you are writing here.

    On a sidenote, mentioning known unknowns seems to be impossible these days without referencing Rumsfeld. His “Old Europe” is also a very memorable quote. The guy was probably a philosoph.


  6. Scooletz Says:

    It’s good to see it state clearly what SOA is not. I don’t know how many years it will take to make it clear/obvious/published;-) The SOA wrap around procedural systems is still alive, and still considered as a good practice in some environments. I hope not much longer.


  7. Basharat Wani Says:

    Saying
    “Any piece of data or rule must be owned by only one service” is easy to say but very hard to implement or work with, we have to be pragmatic.

    Your example of Product price and its availability in stock is a good example; I better wrap them together if my business needs are asking me the same set of info again and again (Scale and Performance reasons).

    Now If I need to display the Product Price in the UI, I will still try to use the same service for getting that data also provided if I code the service correctly.

    Basharat


  8. udidahan Says:

    Basharat,

    I don’t want to be harsh here, but pragmatism is a common excuse for avoiding change – just because something is hard to do, doesn’t mean that it isn’t the right thing to do.


  9. Roberto dell'Oglio Says:

    We are on the right track thanks to person like you Udi.
    Only through knowledge we can improve our artifact.

    That’s why someone wrote about “The five orders of ignorance”

    http://www.amazon.com/Laws-Software-Process-Production-Management/dp/0849314895


  10. David Tildesley Says:

    Hi Guys,

    It is wrong (perhaps intellectually lazy at best) to dismiss the role of OOD in Service design by harping on about a particular fetish – entity modelling or the dreaded “BOM” from RUP.

    It’s not individual business objects (or entities if you prefer) like Customer, Product, Order etc that are the starting point for defining your services. Instead, it is the OO domain model which if modeled well, will be specific to your business problem domain and be loosely coupled between business components that have high value (to your business) business object(s) at their core. If the domain model follows what Peter Coad et al called the DNC (Domain Neutral Component) shape, you will have loosely coupled components with significant behavior that you can model your services on. Don’t underestimate the required effort or the benefit in doing this step.

    It’s rather ironic: Those who don’t bother (or simply don’t realize they need to, or were told there is not enough time) doing up front domain modelling for the design of significant business applications usually end up with hard to maintain and often procedural code rather than an easy to understand, easy to maintain, clean implementation of OO.

    And so it is the same for SOA services – history repeating itself.

    David.


  11. udidahan Says:

    David,

    I think that we can agree that the prerequisite of deeply understanding the business (which is often achieved through modeling processes) is critical for the success of most large software projects.

    Those who only superficially scratched the surface would end up in trouble no matter whether they did (or said they were doing) OOD or SOA or any other TLA.


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