Describing Web Services: WSDL
Web services expose a software-oriented view of a business or consumer function with which applications may interact over the network. To successfully enable such an interaction using a Web service, it must be described and advertised to its potential consumers. Furthermore, users must be able to find out about how to interact with the service: what data it expects to receive, whether it delivers any results, and what communication protocol or transport it supports.
In a somewhat primitive form, Web services have been around for a long time. It's possible to send a search string or a stock quote request by appending information to URLs. However, this format is of limited usefulness and scope for many reasons, not the least of which is the size limit and the usability of a URL for passing data. It's also difficult to program and to understand, and no consistent, standard approach has been defined for describing it.
Web services have been around for a long time in primitive form
The standard form of Web services puts the request and the reply into XML documents, that is, into the resources that are referenced by the URLs rather than within parameters of the URLs themselves. The emerging standards for Web services also facilitate a more universal approach, so that every Web site is not deciding how to use extended URLs individually. The following example illustrates the most primitive type of Web service:
Here, the input data is carried as a parameter to the URL address of the Web page implementing the service—in this case, an employee search function that returns an e-mail name and a telephone number.
The Web Services Description Language (WSDL) specification was created to describe and publish the formats and protocols of a Web service in a standard way. Web service interface standards are needed to ensure that you don't have to create special interactions with each server on the Web, as you would today, using the extended URL approach from a browser.
WSDL establishes a common format for describing and publishing Web service information
WSDL elements contain a description of the data, typically using one or more XML schemas, to be passed to the Web service so that both the sender and the receiver understand the data being exchanged. The WSDL elements also contain a description of the operations to be performed on that data, so that the receiver of a message knows how to process it, and a binding to a protocol or transport, so that the sender knows how to send it. Typically, WSDL is used with SOAP, and the WSDL specification includes a SOAP binding.
WSDL elements describe data and operations on it
WSDL was developed by Microsoft, Ariba, and IBM, and v1.1 of the specification was submitted to the W3C, which accepted WSDL as a note and published it on the W3C Web site.1 Twenty-two other companies joined the submission, comprising at that time the largest number of W3C members ever to support a joint submission. WSDL therefore already enjoys broad-based support, and many companies offer implementations of WSDL in their Web services products. In fact, with such near unanimity within the vendor community, it could be said that the WSDL specification provides the de facto definition of a Web service description. However, it is very likely that a W3C working group will nonetheless make significant improvements and changes.
WSDL was developed collaboratively by IBM, Microsoft, and Ariba
Both parties that participate in a Web Services “conversation” or interaction must have access to the same WSDL to be able to understand each other.2 In other words, both the sender and the receiver of a message involved in a Web service interaction must have access to the same XML schema. The sender needs to know how to format the output message correctly, and the receiver needs to understand how to interpret the input message correctly. As long as both parties to the interaction have the same WSDL file, the implementations behind the Web services can be anything. This is the magic of WSDL: It provides a common format to encode and to decode messages to and from virtually any back-end application, such as CORBA, COM, EJB, JMS, MQ Series, ERP systems, and so on.
Both parties to a Web service interaction need copies of the same WSDL file
As shown in Figure 3-1, Web services typically are implemented using programming languages designed for interaction with the Web, such as Java servlets or Application Server Pages (ASPs) that call a back-end program or object. These Web service implementations are also typically based on WSDL or represented using WSDL. That is, either new services can be generated from WSDL, or existing services can be described using WSDL. It's likely that both approaches will catch on, as designing and exposing Web services is bound to be an iterative process. Neither approach by itself is going to provide the best solution in all cases.
Figure 3-1. A business can use WSDL to advertise its Web services.
Web services are implemented using Web-oriented languages
Figure 3-1 shows an EJB described using a WSDL file and exported using a servlet over the network. The servlet listens to the network through either a Web server or a custom HTTP listener. In the example, the servlet accepts a SOAP message and forwards it to the beans that are represented by the WSDL file. The beans in turn implement, or connect to, the specific business applications being exposed to the network, providing the information that is carried in the SOAP body as input, if any, and returning any results back through the beans to the servlet. If results need to be sent back across the network, data is returned to the beans through output arguments and back to the servlet from which the reply goes back to the servlet. The servlet that provides the implementation of the WSDL file packages the data into a SOAP message reply to send back across the network.
In this way, simple extensions to existing Internet infrastructure can implement Web services for interaction via browsers or directly within an application, such as the one in Figure 3-1. As well as an EJB, the application behind the servlet could be implemented using .NET, JMS, CORBA, COBOL, or any number of proprietary integration solutions. Furthermore, Web services can represent B2B document-exchange interactions.
Web services definitions can be mapped to any language, object model, or messaging system
Exposing Objects and Beans Directly
It may not always make sense to expose a bean or an object directly as a Web service; more often, it seems likely that a bean or other program will be written specifically for the purpose of exposing a Web service. In other words, it's unlikely that existing programs will be a good fit or be built according to a design that fits Web service requirements, and special wrapper programs may need to be written to expose the level of granularity appropriate to a Web service.