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What Is Soap?
SOAP (Simple Object Access Protocol)
Soap is the communications protocol for XML Web services. When SOAP is described as a communications protocol, most people think of DCOM or CORBA and start asking things like, "How does SOAP do object activation?" or "What naming service does SOAP use?" While a SOAP implementation will probably include these things, the SOAP standard doesn't specify them. SOAP is a specification that defines the XML format for messages—and that's about it for the required parts of the spec. If you have a well-formed XML fragment enclosed in a couple of SOAP elements, you have a SOAP message. Simple isn't it?
Parts Of SOAP:
There are other parts of the SOAP specification that describe how to represent program data as XML and how to use SOAP to do Remote Procedure Calls. These optional parts of the specification are used to implement RPC-style applications where a SOAP message containing a callable function, and the parameters to pass to the function, is sent from the client, and the server returns a message with the results of the executed function. Most current implementations of SOAP support RPC applications because programmers who are used to doing COM or CORBA applications understand the RPC style. SOAP also supports document style applications where the SOAP message is just a wrapper around an XML document. Document-style SOAP applications are very flexible and many new XML Web services take advantage of this flexibility to build services that would be difficult to implement using RPC.
The last optional part of the SOAP specification defines what an HTTP message that contains a SOAP message looks like. This HTTP binding is important because HTTP is supported by almost all current OS's (and many not-so-current OS's). The HTTP binding is optional, but almost all SOAP implementations support it because it's the only standardized protocol for SOAP. For this reason, there's a common misconception that SOAP requires HTTP.
Some implementations support MSMQ, MQ Series, SMTP, or TCP/IP transports, but almost all current XML Web services use HTTP because it is ubiquitous. Since HTTP is a core protocol of the Web, most organizations have a network infrastructure that supports HTTP and people who understand how to manage it already. The security, monitoring, and load-balancing infrastructure for HTTP are readily available today.
A major source of confusion when getting started with SOAP is the difference between the SOAP specification and the many implementations of the SOAP specification. Most people who use SOAP don't write SOAP messages directly but use a SOAP toolkit to create and parse the SOAP messages. These toolkits generally translate function calls from some kind of language to a SOAP message. For example, the Microsoft SOAP Toolkit 2.0 translates COM function calls to SOAP and the Apache Toolkit translates JAVA function calls to SOAP. The types of function calls and the datatypes of the parameters supported vary with each SOAP implementation so a function that works with one toolkit may not work with another. This isn't a limitation of SOAP but rather of the particular implementation you are using.
By far the most compelling feature of SOAP is that it has been implemented on many different hardware and software platforms. This means that SOAP can be used to link disparate systems within and without your organization. Many attempts have been made in the past to come up with a common communications protocol that could be used for systems integration, but none of them have had the widespread adoption that SOAP has. Why is this?
Because SOAP is much smaller and simpler to implement than many of the previous protocols. DCE and CORBA for example took years to implement, so only a few implementations were ever released. SOAP, however, can use existing XML Parsers and HTTP libraries to do most of the hard work, so a SOAP implementation can be completed in a matter of months. This is why there are more than 70 SOAP implementations available. SOAP obviously doesn't do everything that DCE or CORBA do, but the lack of complexity in exchange for features is what makes SOAP so readily available.
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