Components of Web Services. The components of XML web services include applications, UDDI, WSDL, and SOAP [16]. Applications may be clients of XML web services provided by another application, providers of XML web services to other applications, or both [31]. UDDI, Universal Description Discovery and Integration, provides a registry of available XML web services. The Web Service Description Language (WSDL) provides an XML based description of XML web services and how to interact with them [14]. The WSDL is the interface definition language of XML web services. Finally, SOAP, or Simple Object Access Protocol, is a lightweight remote-procedure-call protocol that uses XML to format messages and HTTP to transmit messages [3]. The general procedure of using a web service starts when a client searches for the web service from an UDDI registry. The UDDI registry in some sense is equivalent to the yellow pages in a phone book. However, the UDDI registry is not a completely secure registry and in fact is a public registry. Many government officials are very reluctant to populate a public registry of web services using the UDDI standard [29]. As a result, government agencies that wish to share their services with each other will share the service description information through a secure communication channel. In either way, the client will be able to obtain a description of the web service, which is written in WSDL, through a private or public registry. Then, the client can retrieve the description (written in WSDL), parse the XML data contained in the description, and learn the URL of the web service and the right ways to call the web service using SOAP. At the web server end, each SOAP request will be first received by the SOAP listener then forwarded to a specific request processor, called the WSDL server, which will parse the XML data contained in the SOAP request. A set of service requests which can be understood by the server application will be generated after the XML data are parsed, and the set of service requests will be sent to the server application for processing.
