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Understanding the Definition and Philosophy of Web Services and SOA Wikipedia offers the following definition of Web services:4 The W3C defines aWeb service as a software system designed to support interoperable machine-to-machine interaction over a network. This definition encompasses many different systems, but in common usage the term refers to those services that use SOAPformatted XML envelopes and have their interfaces described by WSDL. For example, WS-I only recognizes Web services in the context of these specifications. Interestingly, like the pure Ajax definition, aWeb service is defined to a large degree using technical terms such as Simple Object Access Protocol (SOAP), Web Services Description Language (WSDL), and so on. It leads you to believe that in order to build aWeb service, you must use SOAP and WSDL. What is misleading is that aWeb service is directly related to the technology being used. For example, the REST way of building Web services may not involve XML, WSDL, or SOAP. Thus, is REST aWeb service? The answer is that REST is indeed aWeb service if the following more succinct definition5 is used: Web services [instead] share business logic, data and processes through a programmatic interface across a network. What is preferable with this definition is the reference to business logic, data, and processes and the exposing of those items using a programmatic interface. With this definition, Web services need not be a machine-to-machine interaction, as aWeb browser in the context of Ajax has the ability to call aWeb service. It’s important to realize that in the context of Ajax, the programmatic interface may generate an interface definition that is intended to be processed by a human for example, a link or button that is pressed to generate new content. With a generalized definition of Web services, let’s look at a definition of service-oriented architecture (SOA):6 In computing, the term service-oriented architecture (SOA) expresses a perspective of software architecture that defines the use of loosely coupled software services to support the requirements of business processes and software users. In an SOA environment, resources on a network are made as independent services that can be accessed without knowledge of their underlying platform implementation. This time, instead of a definition that uses technical terms, abstract terminology is used to describe an SOA. Looking at the definition of SOA, you might consider a network printer as an SOA. Yet, is that what the definition of SOA intends? Is aWeb service an SOA, and is an SOA aWeb service? JP Morgenthal7 says it best: An SOA is a service with a contract. Morgenthal’s comment is simple, succinct, and expresses exactly what an SOA is: An SOA is a service with a contract. What makes an SOA unique is that somebody who has no knowledge of a system can ask an SOA, “What services do you offer?” and the SOA will respond, “Here is what I offer and here is how you call me.” Therefore, since Web services provide a description of their interface, aWeb service is an SOA. A file server is an SOA, if a client is able to query the file server for its contract in order to ask for data. Keep the following facts in mind when trying to understand the philosophy and definition of Web services and SOA: • An SOA can be aWeb service, and aWeb service can be an SOA. • When building robust, scalable, and extendable Ajax applications, write the client code to only make Web service calls. Don’t use the traditional Web application architecture, where pieces of HTML are cobbled together to make a functioning HTML page. • Don’t get too caught up with the details of the definition of a true Web service or a true SOA. Theory is good, but pragmatics solves problems. • A Web service is a programmatic interface to business logic, data, or processes across a network. • An SOA is a service (a programmatic interface to business logic, data, or processes across a network) with a contract. |
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