What exactly do you mean by open source SOA? My understanding is that SOA is a pattern that is followed for designing interfaces to existing applications, irrespective of technology. So in my enterprise, we have mainframes, midranges, Windows, Unix, you name it - and one of my jobs in promoting SOA is to help developers create re-usable, standardized interfaces into their systems. SOA does imply a common declarative medium, usually XML formatted in a REST or SOAP style, but I don't consider this kind of effort open-source, just standards based.
What I am referring by "Open Source SOA" is how to create a SOA technology platform (you are correct, SOA is more than technology, obviously) using completely open source products. The product categories I use, such as BPM, service component frameworks, ESB etc., when surrounded by the proper governance and policies, either can contribute towards helping you implement SOA or can benefit significantly by adopting SOA. For example, I provide some use cases on how a business rule implementation using JBoss Rules/Drools can be exposed as "decision services". Thus, while business rules themselves may not be directly related to SOA, adoption of business rules can be advanced by implementing them in a fashion consistent with SOA best practices.
We actually debated on whether the book might be more aptly titled "Open Source Enterprise", because it describes how enterprise-caliber open source products can be integrated together using the underlying principles of SOA as the foundation.
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