This book provides the best background on service interface design and presents the culmination of knowledge on the subject.  As a senior SOA architect for many years on many projects and with 20+ years total in distributed (SOA/OO) systems, I undersand that all of the principles presented here must be fully understood and applied at the same time to correctly design services.  The background theory is the best and case studies are provided to make them clear.  This book is targeted to those who design at the business service layer.  It may be too high level for entry level developers and too detailed for project managers.  But all the princples must be understood by architects and designers and are very well organized and partitioned.  These SOA principles are generally applicable to modern software design and are not specific to Web services.Thank you to Thomas Erl for providing such an excellent work.  All of these books are and have been on my shelf and have been highly recommended to others.

SOA Principles of Service Design PDF

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