XML Reference Guide

Mar 14, 2003

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We first meet the Web Services Resource Framework in the context of Grid Computing. Grid Computing, designed as it is to spread the work of computation over multiple geographically separated computers, seems a perfect application for web services, but herein we find a problem.

By nature, web services are stateless. Like a HTTP request, once the request is made and the response returned, there is no connection between the requestor and the requestee. If you want to create a "conversational" type service, in which one request/response builds on the previous request/response, you have to build it into your application, adding information such as session ids or other information.

Grid Computing, and thus Web Services Resource Framework, requires a slightly different look at the issue of state.

The white paper Modeling Stateful Resources with Web Services, which explains much of the philosophy involved, describes "A service that acts upon stateful resources" as one that "provides access to, or manipulates a set of logical stateful resources (documents) based on messages it sends and receives". In this case, we're talking about a service in which the "state" that comes into play is the state of the resource itself, as opposed to the state of the conversation.

Being able to find these "resources" is vital in a distributed environment such as Grid Computing, where the same resource might be acted on by multiple clients on multiple computers. To do that, we need two pieces of information. First, we need to identify the web service to call as an endpoint, typically as a URL. Second, we need some kind of identifier for the "stateful resource" itself. Fortunately, we can get that.

Consider this WS-Addressing message:

<wsa:EndpointReference>
   <wsa:Address>
        http://www.example.com/services/someService
   </wsa:Address>
   <wsa:ReferenceProperties>
        <tns:resourceID>DataChunk42</tns:resourceID>

   </wsa:ReferenceProperties>
</wsa:EndpointReference>

The resourceID identifies the actual resource to be acted on by the web service. The web service gets that information because the WS-Addressing specification states that it goes into the header of the SOAP message, as in:

<SOAP-ENV:Envelope>
     <SOAP-ENV:Header>

          <tns:resourceID>DataChunk42</tns:resourceID>
     </SOAP-ENV:Header>
     <SOAP-ENV:Body>
        ...
     </SOAP-ENV:Body>
</SOAP-ENV:Envelope>

Note that the web service "requestor" is specifically forbidden from trying to interpret the resourceID -- you must leave that for the web service itself -- but that it's permissible to create a web service request that retrieves information about the resource. For example, if you wanted to find out the current processed value of DataChunk42 and how many iterations it's been through, you could use the WS-Resource Property namespace and its GetMultipleResourceProperties request:

<SOAP-ENV:Envelope>
    <SOAP-ENV:Header>

        <tns:resourceID>DataChunk42</tns:resourceID>
    </SOAP-ENV:Header>
    <SOAP-ENV:Body>
        <wsrp:GetMultipleResourceProperty>
            <wsrp:ResourceProperty>currentValue</wsrp:ResourceProperty>

            <wsrp:ResourceProperty>numIterations</wsrp:ResourceProperty>
        </wsrp:GetMultipleResourceProperty>
    </SOAP-ENV:Body>
</SOAP-ENV:Envelope>

(You can use GetMultipleResourceProperty for one or more properties.)

OK, so how do you know those properties are available? The answer is that those properties are defined according to the WS-Resource Properties specification, and then associated with the web service itself. For example, we can use XML Schema to define our properties:

<xs:element name="DataChunkProperties">
    <xs:complexType>
        <xs:sequence>
            <xs:element name="currentValue" type="string" />
            <xs:element name="numIterations" type="int" />

        </xs:sequence>
    </xs:complexType>
</xs:element>

We can then associate it with the web service by including this definition in the WSDL file and then referencing it in the portType:

<wsdl:definitions targetNamespace="http://example.com/services/resources/"
    xmlns:wsdl="http://schemas.xmlsoap.org/wsdl/"
    xmlns:xs="http://www.w3.org/2001/XMLSchema"
    xmlns:wsrp="http://www.ibm.com/xmlns/stdwip/web-services/ws-resourceProperties"
    xmlns:tns="http://example.com/services/resources/">

<wsdl:types>
    <xs:schema>
        <xs:import namespace="http://example.com/services/resources/" 
                schemaLocation="http://example.com/services/resources/definitions.xsd"/>
    </xs:schema>
</wsdl:types>
…
<wsdl:portType name="ChunkProcessor" wsrp:ResourceProperties="tns:DataChunkProperties">
...

</wsdl:portType>
…
</wsdl:definitions>

So that's the basics of what a resource is, and how it's integrated with a web service. The actual Web Services Resource Framework is a collection of several specifications. I'll be covering them individually in later guide entries when they're a little more fully cooked, but for now, check out the resources for direct links to them in their current states.

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