Understanding Web Services

Web services provide interactivity through a number of protocols, including the following:

• Simple Object Access Protocol (SOAP)

• Disco

• Universal Description, Discovery, and Integration (UDDI)

• Web Services Description Language (WSDL)

By default, communications between Web services and their clients is accomplished using XML messages over Hypertext Transfer Protocol (HTTP) connectivity. This allows access to Web services through most firewall configurations, although it is possible to configure a firewall to prevent HTTP access.

Simple Object Access Protocol (SOAP)

The Simple Object Access Protocol (SOAP) provides a method to translate objects, along with their methods and properties, into XML that may be sent via HTTP, the encrypted and more secure HTTPS, or other protocols. Because SOAP is XML based, Web services may be used by many different vendors and clients.

It is possible to transmit SOAP messages over protocols other than HTTP. SOAP messages may also be sent via asynchronous methods such as File Transfer Protocol (FTP) or email using Simple Mail Transfer Protocol (SMTP).

All SOAP messages have an envelope and a body. The envelope portion of the SOAP message contains information on its destination, and each parameter of a method is transmitted as an XML element in the body of the message.

Disco

Disco is a Microsoft standard for the creation of Web service discovery documents. Disco documents are kept on a Web Services server, containing paths and other information including the path to the Web Services Description Language (WSDL) file that describes the service.

Universal Description, Discovery, and Integration (UDDI)

Universal Description, Discovery, and Integration (UDDI) describes a method for finding Web services, URLs for information, or other details of Web services available on a particular server. UDDI relies on a central directory that can be searched for details on available services. Public UDDI registries (such as http://uddi.microsoft.com) provide information to any client requesting the data, whereas private UDDI registries provide information only to authorized clients or those within a restricted intranet. You can think of UDDI as a sort of electronic white pages that lets you search for Web service addresses.

The UDDI standard is being developed by a joint group of industry partners including Microsoft and IBM.

Web Services Description Language (WSDL)

The Web Services Description Language (WSDL) file provides details about the SOAP messages that a Web service can receive and send. The WSDL file is an XML file maintained on the Web Services server that details the data types, methods, and URLs that may be used to interact with each Web service.

It is not necessary for a Web service to have a UDDI registration or WSDL file. These are only necessary to allow clients to automatically discover the message formats and locations used with the Web service. Clients may be configured to use the proper format without exposing the Web service to discovery.