Here I describe XML and XML Schema, which are foundation technology of an SOA.
XML and XML Schema
XML (a data formating and representation industry standard) is a foundation technology for any SOA implementation, as it establishes in combination with XML Schema (XSD) the structure and format of messages, which are sent between services and inside service compositions. XSDs are part of the technical contract and the data representation layer relies on their validation.
The service layer is built upon the data representation layer, a series of standardized data models shared by the service and modeled as XSDs.