[Originally posted on my old BEA Dev2Dev blog on July 31, 2007]
In SOAP, there are two possible styles:
- RPC. Implies a SOAP body structure which indicates service name, and multiple parameters and return values
- Document. Implies a SOAP body which is a complex message document
In SOAP, there are two possible uses:
- Encoded. Adheres to a set of rules for serialising a graph of typed objects using basic XML schema data-types, but as a whole, does not conform to a schema
- Literal. Body content conforms to a specific XML schema
In most SOAP toolkits, the most common combinations of Style and Use are RPC-Encoded and Document-Literal. Additionally, RPC-Literal is becoming more prevalent although it is currently a lot less common. Document-Encoded doesn't really make sense and as a result I doubt you'll find it implemented in your favourite SOAP toolkit.
RPC-Encoded was the initial message format for SOAP, when SOAP was originally aimed at just the Remote Procedure Call programming model. Document-Literal was incorporated into the SOAP standard in time for SOAP 1.0. It was intended to enable XML documents (messages) to be passed as the full content of the SOAP body, usually with one input message part and one return message part.
Like most J2EE Application Servers, the core of WebLogic's Web Services support is based on the JAX-RPC 1.1 specification. JAX-RPC defines a Remote Procedure Call based programming model and API for developers who want to expose a set of Java methods remotely (JAX-RPC does not offer much in the way of support for adopting the alternative distributed computing model of "Messaging").
Given that JAX-RPC is based on the RPC programming model, then in terms of best practices, it's obvious then that we should be using RPC-Encoded (or RPC-Literal) as the preferred SOAP Style/Use for creating and exposing newly developed Web Services, isn't it?
Well, not necessarily....
The terminology of RPC versus Document SOAP Styles is very unfortunate when we start to consider Remote Procedure Call versus Document/Messaging distributed programming models. These terms imply that the RPC Style should be used for RPC programming models and that the Document style should be used for Document (Messaging) programming models. That is not the case at all. In practice, the SOAP Style has nothing to do with a programming model, it merely dictates how to translate a WSDL binding to a SOAP message. For example, WebLogic's JAX-RPC toolkit equally supports exposing the same Java methods remotely via either style. You can use either style with either programming model.
A SOAP Style/Use of Document-Literal provides two distinct advantages over RPC-Encoded:
- WS-I Basic Profile precludes the use of "Encoded" as the SOAP Use. So, if promoting interoperability and openness is your concern then you wouldn't choose RPC-Encoded over Document-Literal* (and why else would you be using SOAP other than for interoperability?).
- RPC-Encoded provides no real separation between the format of the SOAP body (eg. that could be defined by a Schema) and the transport protocol and invocation format of a SOAP operation (eg. defined by a WSDL). For Document-Literal, the SOAP body content conforms to one or more Schemas which can optionally be externalised from the WSDL (and then included into the WSDL via an 'import' statement). Why is this important? Well, Document-Literal can promote the re-use of the same XML Schemas across the many different Web Services you may need to expose and throughout the rest of your distributed application logic which may need to deal with the same XML data formats. As we know, re-use reduces development effort, helps avoid errors and promotes consistency.
In summary, the decision of using RPC-Encoded or Document-Literal really doesn't have a direct relation to whether one is adopting a Remote Procedure Call programming model rather than a Document/Message-passing programming model. In practice, regardless of programming model, Document-Literal offers practical advantages.
* In fact, use WebLogic's 'Wrapped' option with "Document-Literal" to help further promote interoperability with both Microsoft based Web Services toolkits (which traditionally prefer the Document-Literal-Wrapped style) and Remote Procedure Call oriented client Web Services toolkits (which often expect to be able to include the 'remote operation' name within the SOAP requests they send, rather than using another mechanism such as WS-Addressing to identify the operation to invoke).
Soundtrack for today: Off To One Side by Come