What's the difference between document/literal and rpc/encoded Web Services? What's the history behind them?
There are two SOAP message styles, called document and rpc. Document style indicates that the SOAP body simply contains an XML document. The sender and receiver must agree on the format of the document ahead of time, such as traditional messaging systems like Microsoft® Message Queue (MSMQ), MQSeries, and so on. The agreement between the sender and receiver is typically negotiated through literal XML Schema definitions. Hence, this combination is referred to as document/literal.
RPC (Remote Procedure Call) style, on the other hand, indicates that the SOAP body contains an XML representation of a method call such as the traditional distributed component technologies of DCOM, Corba, and others. RPC style uses the names of the method and its parameters to generate structures that represent a method's call stack (see section 7 of the SOAP 1.1 specification at http://www.w3c.org/TR/SOAP). These structures can then be serialized into the SOAP message according to a set of encoding rules. The SOAP specification defines a standard set of encoding rules for this purpose (see section 5 of the SOAP 1.1 spec) that codify how to map the most common programmatic data structures, such as structs and arrays, into an XML 1.0 format. Since RPC is traditionally used in conjunction with the SOAP encoding rules, the combination is referred to as rpc/encoded.
The document/literal approach is more straightforward and easier for toolkits to get right because it simply relies on XML Schema to describe exactly what the message looks like on the wire. SOAP, however, was created before XML Schema existed. And back then they were focused primarily on objects (hence the O in SOAP), which led to the rpc/encoded way of doing things. Since universal description languages such as XML Schema or Web Services Description Language (WSDL) weren't available back then, the rpc/encoded style had to assume that additional metadata would be available for describing the method call (such as a type library, Microsoft .NET Framework assembly, or Java class file).
