Cpanel web hosting - Writing Session Bean Web Services 109 Again, there
Writing Session Bean Web Services 109 Again, there is the HTTP header and an XML document that contains a SOAP envelope. This time, the SOAP body represents the result of the operation call, which is a single result element of type string. The two messages reproduced here serve to illustrate another key term that is often used in the context of Web Services. The SOAP protocol is extremely lightweight in the sense that it is very simple to use and does not make many assumptions about the behavior of clients and services. The SOAP protocol is not lightweight in terms of compactness and high performance. If uncompressed, there is a large transmission overhead when compared to binary representations, for example in CORBA s IIOP protocol. The XML parsing required to marshal and demarshal messages can also become CPU-intensive for larger messages. But this is beside the point: Web Services are not designed to deliver hitherto unknown performance but to enable integration where high-performance middleware is much less useful than lightweight protocols that can be implemented easily by simple scripts. (For an interesting discussion of scripting languages as middleware, refer to Steve Vinoski s article on Middleware Dark Matter available at www.iona.com/hyplan/vinoski/.) XML Artifacts and Platform Independence Web Services help with the integration of heterogeneous, distributed systems by using standardized XML documents for many different aspects of service design, deployment, lookup, and usage that leverages a broad array of open standards and sophisticated tools that are widely available. Many of the tools, like Apache Axis SOAP, IBM WSDL4J toolkit, and JBoss Application Server, are also in the open source arena. In a sense, the XML usage that we just looked at is perhaps the biggest technological advantage here because many of the practical virtues, like loose coupling and platform independence, follow from XML itself and the way the different XML technologies are combined. XML documents are also self- describing in that they contain a description of their structure in their markup tags. This does not mean that you will be able to understand arbitrary documents without any prior knowledge. What it does mean is that you can easily skip parts of a message that you are not concerned with and don t understand, and just deal with those parts that do concern you. This may sound trivial at first, but it has important consequences in that this enables the decoupling of applications and middleware. To understand this point, recall that clients of your beans have to use a fixed component interface. If that interface changes because a parameter is added to a method signature, you will have to rebuild, reassemble, and redeploy not only your beans, but your clients will also have to be recompiled. This is not loose coupling because you cannot develop the different components of your application individually. If one piece changes, the others have to change, too.
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