What is the difference between Document style and RPC style communication?

Can somebody explain to me the differences between Document and RPC style webservices? Apart from JAX-RPC, the next version is JAX-WS, which supports both Document and RPC styles. I also understand document style webservices are meant for Asynchronous communication where a client would not block until the response is received.

Either way, using JAX-WS I currently annotate the service with @Webservice, generate the WSDL and from that WSDL I generate the client side artifacts.

Once the artifacts are received, in both styles, I invoke the method on the port. Now, this does not differ in RPC style and Document style. So what is the difference and where is that difference visible?

Similarly, in what way does SOAP over HTTP differ from XML over HTTP? After all SOAP is also XML document with SOAP namespace.

> Can some body explain me the differences between a Document style and > RPC style webservices?

There are two communication style models that are used to translate a WSDL binding to a SOAP message body. They are: Document & RPC

The advantage of using a Document style model is that you can structure the SOAP body any way you want it as long as the content of the SOAP message body is any arbitrary XML instance. The Document style is also referred to as Message-Oriented style.

However, with an RPC style model, the structure of the SOAP request body must contain both the operation name and the set of method parameters. The RPC style model assumes a specific structure to the XML instance contained in the message body.

Furthermore, there are two encoding use models that are used to translate a WSDL binding to a SOAP message. They are: literal, and encoded

When using a literal use model, the body contents should conform to a user-defined XML-schema(XSD) structure. The advantage is two-fold. For one, you can validate the message body with the user-defined XML-schema, moreover, you can also transform the message using a transformation language like XSLT.

With a (SOAP) encoded use model, the message has to use XSD datatypes, but the structure of the message need not conform to any user-defined XML schema. This makes it difficult to validate the message body or use XSLT based transformations on the message body.

The combination of the different style and use models give us four different ways to translate a WSDL binding to a SOAP message.

Document/literal
Document/encoded
RPC/literal
RPC/encoded

I would recommend that you read this article entitled Which style of WSDL should I use? by Russell Butek which has a nice discussion of the different style and use models to translate a WSDL binding to a SOAP message, and their relative strengths and weaknesses.

> Once the artifacts are received, in both styles of communication, I > invoke the method on the port. Now, this does not differ in RPC style > and Document style. So what is the difference and where is that > difference visible?

The place where you can find the difference is the "RESPONSE"!

RPC Style:

package com.sample;
 
import java.util.ArrayList;
import javax.jws.WebService;
import javax.jws.soap.SOAPBinding;
import javax.jws.soap.SOAPBinding.Style;

@WebService
@SOAPBinding(style=Style.RPC)
public interface StockPrice { 

    public String getStockPrice(String stockName); 

    public ArrayList getStockPriceList(ArrayList stockNameList); 
}

The SOAP message for second operation will have empty output and will look like:

RPC Style Response:

<ns2:getStockPriceListResponse 
       xmlns:ns2="http://sample.com/">
    <return/>
</ns2:getStockPriceListResponse>
</S:Body>
</S:Envelope>

Document Style:

package com.sample;
 
import java.util.ArrayList;
import javax.jws.WebService;
import javax.jws.soap.SOAPBinding;
import javax.jws.soap.SOAPBinding.Style;
 
@WebService
@SOAPBinding(style=Style.DOCUMENT)
public interface StockPrice {
 
    public String getStockPrice(String stockName);
 
    public ArrayList getStockPriceList(ArrayList stockNameList);
}

If we run the client for the above SEI, the output is:

123 [123, 456]

This output shows that ArrayList elements are getting exchanged between the web service and client. This change has been done only by the changing the style attribute of SOAPBinding annotation. The SOAP message for the second method with richer data type is shown below for reference:

Document Style Response:

<ns2:getStockPriceListResponse 
       xmlns:ns2="http://sample.com/">
<return xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"  
        xmlns:xs="http://www.w3.org/2001/XMLSchema"
        xsi:type="xs:string">123</return>
<return xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
        xmlns:xs="http://www.w3.org/2001/XMLSchema"
        xsi:type="xs:string">456</return>
</ns2:getStockPriceListResponse>
</S:Body>
</S:Envelope>

Conclusion

• As you would have noticed in the two SOAP response messages that it is possible to validate the SOAP response message in case of DOCUMENT style but not in RPC style web services.
• The basic disadvantage of using RPC style is that it doesn’t support richer data types and that of using Document style is that it brings some complexity in the form of XSD for defining the richer data types.
• The choice of using one out of these depends upon the operation/method requirements and the expected clients.

> Similarly, in what way SOAP over HTTP differ from XML over HTTP? After > all SOAP is also XML document with SOAP namespace. So what is the > difference here?

Why do we need a standard like SOAP? By exchanging XML documents over HTTP, two programs can exchange rich, structured information without the introduction of an additional standard such as SOAP to explicitly describe a message envelope format and a way to encode structured content.

SOAP provides a standard so that developers do not have to invent a custom XML message format for every service they want to make available. Given the signature of the service method to be invoked, the SOAP specification prescribes an unambiguous XML message format. Any developer familiar with the SOAP specification, working in any programming language, can formulate a correct SOAP XML request for a particular service and understand the response from the service by obtaining the following service details.

• Service name
• Method names implemented by the service
• Method signature of each method
• Address of the service implementation (expressed as a URI)

Using SOAP streamlines the process for exposing an existing software component as a Web service since the method signature of the service identifies the XML document structure used for both the request and the response.

An RPC style web service uses the names of the method and its parameters to generate XML structures representing a method’s call stack. Document style indicates the SOAP body contains an XML document which can be validated against pre-defined XML schema document.

A good starting point : SOAP Binding: Difference between Document and RPC Style Web Services

In WSDL definition, bindings contain operations, here comes style for each operation.

Document : In WSDL file, it specifies types details either having inline Or imports XSD document, which describes the structure(i.e. schema) of the complex data types being exchanged by those service methods which makes loosely coupled. Document style is default.

• Advantage:
  • Using this Document style, we can validate SOAP messages against predefined schema. It supports xml datatypes and patterns.
  • loosely coupled.
• Disadvantage: It is a little bit hard to get understand.

In WSDL types element looks as follows:

<types>
 <xsd:schema>
  <xsd:import schemaLocation="http://localhost:9999/ws/hello?xsd=1" namespace="http://ws.peter.com/"/>
 </xsd:schema>
</types>

The schema is importing from external reference.

RPC :In WSDL file, it does not creates types schema, within message elements it defines name and type attributes which makes tightly coupled.

<types/>  
<message name="getHelloWorldAsString">  
<part name="arg0" type="xsd:string"/>  
</message>  
<message name="getHelloWorldAsStringResponse">  
<part name="return" type="xsd:string"/>  
</message>  

• Advantage: Easy to understand.

• Disadvantage:

• we can not validate SOAP messages.

• tightly coupled

RPC : No types in WSDL
Document: Types section would be available in WSDL

The main scenario where JAX-WS RPC and Document style are used as follows:

• The Remote Procedure Call (RPC) pattern is used when the consumer views the web service as a single logical application or component with encapsulated data. The request and response messages map directly to the input and output parameters of the procedure call.

  Examples of this type the RPC pattern might include a payment service or a stock quote service.

• The document-based pattern is used in situations where the consumer views the web service as a longer running business process where the request document represents a complete unit of information. This type of web service may involve human interaction for example as with a credit application request document with a response document containing bids from lending institutions. Because longer running business processes may not be able to return the requested document immediately, the document-based pattern is more commonly found in asynchronous communication architectures. The Document/literal variation of SOAP is used to implement the document-based web service pattern.

I think what you are asking is the difference between RPC Literal, Document Literal and Document Wrapped SOAP web services.

Note that Document web services are delineated into literal and wrapped as well and they are different - one of the primary difference is that the latter is BP 1.1 compliant and the former is not.

Also, in Document Literal the operation to be invoked is not specified in terms of its name whereas in Wrapped, it is. This, I think, is a significant difference in terms of easily figuring out the operation name that the request is for.

In terms of RPC literal versus Document Wrapped, the Document Wrapped request can be easily vetted / validated against the schema in the WSDL - one big advantage.

I would suggest using Document Wrapped as the web service type of choice due to its advantages.

SOAP on HTTP is the SOAP protocol bound to HTTP as the carrier. SOAP could be over SMTP or XXX as well. SOAP provides a way of interaction between entities (client and servers, for example) and both entities can marshal operation arguments / return values as per the semantics of the protocol.

If you were using XML over HTTP (and you can), it is simply understood to be XML payload on HTTP request / response. You would need to provide the framework to marshal / unmarshal, error handling and so on.

A detailed tutorial with examples of WSDL and code with emphasis on Java: SOAP and JAX-WS, RPC versus Document Web Services

Document
Document style messages can be validated against predefined schema. In document style, SOAP message is sent as a single document. Example of schema:

  <types>  
   <xsd:schema> <xsd:import namespace="http://example.com/" 
    schemaLocation="http://localhost:8080/ws/hello?xsd=1"/>  
   </xsd:schema>  
  </types>

Example of document style soap body message

  <message name="getHelloWorldAsString">   
     <part name="parameters" element="tns:getHelloWorldAsString"/>   
  </message> 
  <message name="getHelloWorldAsStringResponse">  
     <part name="parameters"> element="tns:getHelloWorldAsStringResponse"/>   
  </message>

Document style message is loosely coupled.

RPC RPC style messages use method name and parameters to generate XML structure. messages are difficult to be validated against schema. In RPC style, SOAP message is sent as many elements.

  <message name="getHelloWorldAsString">
    <part name="arg0"> type="xsd:string"/>   
   </message> 
  <message name="getHelloWorldAsStringResponse">   
    <part name="return"
   > type="xsd:string"/>   
  </message>

Here each parameters are discretely specified, RPC style message is tightly coupled, is typically static, requiring changes to the client when the method signature changes The rpc style is limited to very simple XSD types such as String and Integer, and the resulting WSDL will not even have a types section to define and constrain the parameters

Literal By default style. Data is serialized according to a schema, data type not specified in messages but a reference to schema(namespace) is used to build soap messages.

   <soap:body>
     <myMethod>
        <x>5</x>
        <y>5.0</y>
     </myMethod>
   </soap:body>

Encoded Datatype specified in each parameter

   <soap:body>
     <myMethod>
         <x xsi:type="xsd:int">5</x>
         <y xsi:type="xsd:float">5.0</y>
     </myMethod>
   </soap:body>

Schema free