1. Introduction
In the last years Service oriented architectures (SOA) [9] have been proposed to create distributed applications and computing environments. The service is generally represented by means of the cooperation between web services. Web services
Standards of the W3C are available in http://www.w3.org/2002/ws/
represent an alternative to distributed computing and are being strongly used instead of distributed object middleware such as Java RMIRemote Method Innvocation: http://java.sun.com/rmi
and CORBA [8].