Also commonly preﬁxed with "SOAP-based," Web services represent an established and common medium for sophisticated, Web-based service logic. Along with XML, the core technologies behind Web services are represented by the following industry standards:
- Web Service Description Language (WSDL) - This markup language is used to create a WSDL deﬁnition that deﬁnes the application programming interface (API) of a Web service, including its individual operations (functions) and each operation's input and output messages.
- XML Schema Definition Language (XML Schema) - Messages exchanged by Web services must be expressed using XML. XML schemas are created to deﬁ ne the data structure of the XML-based input and output messages exchanged by Web services. XML schemas can be directly linked to or embedded within WSDL deﬁnitions.
- SOAP - Formerly known as the Simple Object Access Protocol, this standard deﬁnes a common messaging format used for request and response messages exchanged by Web services. SOAP messages are comprised of body and header sections. The former houses the main message content and the latter is used to contain metadata that can be processed at runtime.
- Universal Description, Discovery, and Integration (UDDI) - This standard regulates service registries in which WSDL deﬁnitions can be published as part of a service catalog for discovery purposes.
These four technologies collectively form the ﬁrst generation of Web service technologies (Figure 1). A comprehensive set of second-generation Web service technologies (commonly referred to as WS-*) has been developed to address various additional functional areas, such as security, reliability, transactions, routing, and business process automation.
To learn more about Web service technologies, read Web Service Contract Design & Versioning for SOA from the Prentice Hall Service Technology Series from Thomas Erl. This title covers first and second-generation Web service standards in technical detail. See www.servicetechbooks.com/wsc for more information.
Figure 1 - An overview of how first-generation Web service technologies commonly relate to each other.