A Web service is an independent, modular, and self-describing application function or service. Based on XML standards, these application functions can be described, made available, located, transformed, or called using standard Internet protocols.
Each Web service therefore encapsulates a function, which is used, for example, to forward a price query to a provider, check the availability of an item in a retail system, locate a telephone number, or run credit card checks, convert currencies, or execute payroll functions.
Web Service Paradigm
A service provider provides access to a service. If a service is a Web service, the service provider has a corresponding XML-based description that is a Web Services Description Language (WSDL) document. Any programming language can be used to implement the Web service. In a client/server relationship, the service provider can be regarded as the server.
When publishing a service, the service provider transmits information about itself and a description of the service it offers to the Service Registry. A Service Registry is a type of yellow pages for Web services. It provides, among other things, information about calling the Web service. The Service Registry therefore provides only a description of the Web service. This description forms an abstraction layer, independent of the corresponding implementation. The Web service itself is hosted by the service provider.
A Web service user is referred to as a service requester. A service requester can, for example, be someone who locates a Web service using a Web browser and then uses the service. In most cases, however, the service requester is an application that accesses the Web service. The application can also bind to the service dynamically if required, that is, the application can dynamically create a Web service client proxy at runtime and use this proxy to access the Web service. The application obtains the necessary information for accessing the Web service from the service description.
This information is then stored in the Service Registry. However, if the application knows the provider and the call details, it can use the Web service without having to access the Service Registry. In a client/server relationship, the service requester is the application client.
Standards for Web Service
Web services can exist in any implementation. Therefore, a standardized description is required if Web services are to be called from any application. The WSDL best meets this demand. However, a Web services description in WSDL alone is not sufficient. To find the right business partner and corresponding service quotation, you need a company registry to help you to find the required service.
The Web service provider must also be able to make its offer publicly available as easily as possible. The Universal Description, Discovery and Integration (UDDI) offers a solution. For more information, see http://uddi.xml.org.
The following list explains some standards that are used with Web services:
UDDI (Universal Description, Discovery and Integration) Registry: With its UDDI Registry and UDDI specification, UDDI provides the necessary tools for making services public. The specification provides a detailed description of how to locate and register services. The UDDI Registry contains a list of registered Web services in WSDL format. The UDDI Registry does not store documents or specifications, but only references them.
SOAP (originally Simple Object Access Protocol): To call Web services based on Internet technologies, a suitable protocol definition is needed. SOAP has created a simple standard that allows Web services to be accessed in decentralized, distributed landscapes. SOAP specifies a package of XML documents for transport via Internet protocols such HTTP(S), SMTP, or FTP. SOAP defines a so-called envelope. In this envelope, you will find the actual XML-based message with a header and a body, as well as further information about how the message should be processed.
WSDL (Web Service Description Language): WSDL is an XML-based description language for Web services. The WSDL Service definition includes the description of distributed systems and provides instructions for automating data exchange between applications. The Web Service Description Language is used to describe Web services or electronic services in XML format. A service is defined as a collection of endpoints (ports) and the messages they work with. By using WSDL, a service provider can describe the requirements and features of their Web service so that a potential customer understands them and interacts properly with the service.
