11 June 2004I recently had a reader of my business objects book ask about combining
CSLA .NET and SOA concepts into a single application.
<?xml:namespace prefix = o ns = “urn:schemas-microsoft-com:office:office” />
But the reader put forth the following, which I wanted to discuss in more detail. The text in italics is the reader’s view of Microsoft’s “standard” SOA architecture. My response follows in normal text:
1. Here is the simplified MS’s “standard” layering,
(a) UI Components
(b) UI Process Components [UI never talk to BSI/BC directly, must via UIP]
(c) Service Interfaces
(d) Business Components [UIP never talk to D directly, must via BC]
(e) Business Entities
(f) Data Access Logic Components
I’m aware of its “transaction script” and “anemic domain model” tendency, but it is perhaps easier for integration and using tools.
To be clear, I think this portrayal of ‘SOA’ is misleading and dangerous. SOA describes the interactions between separate applications, while this would lead one into thinking that SOA is used inside an application between tiers. I believe that is entirely wrong – and if you pay attention to what noted experts like Pat Helland are saying, you’ll see that this approach is invalid.
SOA describes a loosely-coupled, message-based set of interactions between applications or services. But in this context, the word ‘service’ is synonymous with ‘application’, since services must stand alone and be entirely self-sufficient.
Under no circumstance can you view a ‘tier’ as a ‘service’ in the context of SOA. A tier is part of an application, and implements part of the application’s overall functionality. Tiers are not designed to stand alone or be used by arbitrary clients – they are designed for use by the rest of their application. I
discussed this earlier in my post on trust boundaries, and how services inherently create new trust boundaries by their very nature.
So in my view you are describing at least two separate applications. You have an application with a GUI or web UI (a and b or ab), and an application with an XML interface (c, d, e and f or cdef).
To fit into the SOA model, each of these applications (ab and cdef) is separate and must stand alone. They are not tiers in a single application, but rather are two separate applications that interact. The XML interface from cdef must be designed to service not only the ab application, but any other applications that need its functionality. That’s the whole point of an SOA-based model – to make this type of functionality broadly available and reusable.
Likewise, application ab should be viewed as a full-blown application. The design of ab shouldn’t necessarily eschew business logic – especially validation, but likely also including some calculations or other data manipulation. After all, it is the job of the designer of ab to provide the user with a satisfactory experience based on their requirements – which is similar, but not the same, as the set of requirements for cdef.
Yes, this does mean that you’ll have duplicate logic in ab and cdef. That’s one of the side-effects of SOA. If you don’t like it, don’t use SOA.
But there’s nothing to stop you from using CSLA .NET as a model to create either ab or cdef or both.
To create ab with CSLA .NET, you’d design a set of business objects (with as little or much logic as you felt was required). Since ab doesn’t have a data store of its own, but rather uses cdef as a data store, the DataPortal_xyz methods in the business objects would call the services exposed by cdef to retrieve and update data as needed.
To create cdef with CSLA .NET, you’d follow the basic concepts I discuss in Chapter 10 in my
VB.NET and C# Business Objects books. The basic concept is that you create a set of XML web services as an interface that provides the outside world with data and/or functionality. The functionality and data is provided from a set of CSLA .NET objects, which are used by the web services themselves.
Note that the CSLA-style business objects are not directly exposed to the outside world – there is a formal interface definition for the web services which is separate from the CSLA-style business objects themselves. This provides separation of interface (the web services) from implementation (the CSLA-style business objects).