Recently I have done a few architecture reviews at customers struggling to get their SOA efforts going. The common problem of these attempts on service-enabling legacy systems to support agile business processes have been - except from system designers not grasping that SOA is in fact a paradigm shift from invoking-operations composition thinking - that they focus solely on business process management (BPM) modeling. From this model they try to identify services without having methodology to classify services into different kinds of services (entity, activity, process), struggling with service granularity and how to identify what goes into the message and data contracts.
The problem is that modeling your business processes typically misses out on the central building block of service-oriented solutions: the data and the semantics of the data as related to the business processes. This is typical for workflow models like the 'UML Activity Diagram'.
The data should represented by a Common Information Model (CIM). The CIM should be the starting point of your modeling efforts, in combination with Business Process Modeling Notation (BPMN) diagrams, as BPMN encompass process, events, messages and business documents. Then you could follow the SOA mantra coined by David Linthicum: data-service-process (much like the TDD red-green-refactor).
Last week Jean-Jacques Dubray posted an article about 'The Seven Fallacies of Business Process Execution'. The article contains a lot of good points about the 'case-tool grail' dreams about business people doing agile modeling in BPMN and then just generating systems that automates the business processes.
The really interesting part of the article is in 'Fallacy #5: Business Process Execution' where Jean-Jacques differentiates business objects (resources) and their lifecycle from the (set of variations of) business processes that drives the lifecycle of the resources. The article explains how the data aspects are missing from BPMN even if it is the most important part of the business - just the problem I see so often.
Another point made in the article is that it is the resource lifecycle events that are important when trying to identify the services that your business process will depend on. The business events advance the business data through a state machine until the end of the business process. This is what Jack van Hoof, Nick Malik, myself and others have blogged a lot about this year: it is the business events and their message and data (documents) that are the key artefacts in your modeling efforts when creating a service-oriented architecture.
The article also shows how process service composition (service execution and resource lifecycle management; BPEL) is different from user-centric business processes (tasks; BPMN, WS-HumanTask, BPEL4People). It is important to understand that there are processes at different levels: composition of activity/entity services into 'resource' processes, and the composition of tasks into business process. The former can be thought of as orchestration, while the latter can be thought of as human workflow services.
The article goes on to discuss how development is the glue that joins the different artifacts that constitutes a SOA. Developers are needed to actually implement the BPMN using the provided services. I strongly agree. Even if you could do BPMN to BPEL code generation, professional testing and QA in a development-staging-production environment is still a must. I will only go as far as to allow business people manage a set of goverened business rules, never directly implement any process (i.e. declaratively configure the diamond shapes in flow charts). Read more about business rules/decision service at James Taylor's blog.
Finally, the need for a 'composite service container' for the orchestrations is also something that is often disputed during my SOA reviews. "Command-and-control" developers rarely consider how the availability of autonomous services affects the fault-tolerance in the overall SOA solution. The question I always ask is: "What is the availability of a business process that composes five services that each have 99% availability? Is it 99%?" The answer is of course less: 95%. This is where a container for providing long running + reliable processes and 'resource' state machines comes in handy.
I recommend that you read and, most importantly, understand the concepts and the architecture blueprint presented in the article. I have to give Jean-Jacques a big credit for the good figures he has made to illustrate the taxonomy and ontology presented in the article.