Web Services and SOAP

Whengiven any XML parser, such as a Microsoft’s XML DOM component, anyone can passa function name embedded into an XML message. Therefore, you do not need SOAPor any formal protocol around XML Web services to accomplish this. The designersof such a system could provide a specification for the outlay of the functionparameters and even return XML after the function is called if they wanted.Standard XML Web services just make it easier. SOAP simply standardizes thistype of RPC mechanism. SOAP is the communication protocol used by XML Webservices. Is SOAP required? No. Is it recommended? Yes. Only because SOAPprovides a standardized means of describing the data elements that make up themessages sent to SOAP servers, including .NET servers hosting Web services.Along with WSDL and UDDI, SOAP is another element in the distributed arm of.NET and the Web services it hosts. Besides being a mechanism for invokingRPCs, it can also be used as a wrapper around entire XML documents or binarydata, as is the case with the DIME protocol from Microsoft. This greatlyexpands what can be done with this protocol and will continue to expand as XMLWeb services evolve.

Asmentioned, when using Web services, SOAP is not the only protocol that can beused to call Web services. For example, a simple HTTP GET request can be madeas long as it is passing and receiving simple data types, such as strings.However, when you begin to work with more complex types, such as ADO Datasets,it becomes necessary to use a protocol such as SOAP and more specifically, XMLschemas to describe such data as referenced in the SOAP message. SOAP wasdesigned for simplicity and elegance, and it does not do some of things youmight expect. Some of the elements not covered (at this time) are:

1. Message batching - You must create your means of batching messages/functions calls.
2. Distributed garbage collection - There are no means of telling the server that the client has suddenly disappeared.
3. Object references - Holding state from the client and server is an implementation-specific issue.
4. Reliability protocols - This really lies at the application protocol level and can be designed into your application, especially if HTTP is being used as the communication protocol.

SOAPcurrently does not outline these specific elements like other remote protocols,such as DCOM and CORBA. But it does what it was intended to do—provide us witha simple, standard specification to drive the description of the data types andmethods for using and producing Web services. It is important to note, however,that a SOAP “message” only specifies the XML format for the package sent acrossthe wire. As long as you have a well-formed XML document encapsulated in thenecessary SOAP tags, you have a valid SOAP message. Any compliant SOAPimplementation on the server should understand the request and should serviceit. In fact, although the default application protocol for SOAP is HTTP, itdoes not require it. That’s the simplicity of it. Other protocols, such asSMTP, FTP, and even asynchronous messaging protocols, will soon be supported.Because HTTP is the Internet protocol, it is also the default for SOAP. This iswhy when you hear about XML Web services, it is usually assumed that there willbe a one-to-one correspondence with both SOAP and HTTP.

To trulyunderstand SOAP, you first have to understand a few basic elements of thelanguage that makes up a SOAP message. Once again, that language is XML. Mostinteraction with SOAP usually takes place through a toolkit or service of somekind. For example, the Microsoft SOAP Toolkit (Java has its own) hides theexact format of a SOAP message by providing high-level programmatic clientobjects to use. These clients provide the code to serialize, de-serialize,send, and receive SOAP messages to and from a corresponding SOAP server, suchas a .NET Web service. This simplifies developers’ lives but it also sheltersthem from understanding the format in which their messages are bundled. Knowingsuch details will help your debugging and designing efforts in the future andit is recommended that you become at least slightly acquainted with suchdetails.