WS-CDL

WS-CDL "is an XML-based language that describes peer-to-peer collaborations of participants by defining, from a global viewpoint, their common and complementary observable behavior; where ordered message exchanges result in accomplishing a common business goal."

The WS-CDL W3C Candidate Recommendation was published by the work group on 2005-11-09 and the WS-CDL: Primer was published on 2006-06-19.

History

There was a criticism particuarly from ebXML group that the standards such as BPEL had a conceptual problem that it can only decribe business collaborations as one-sided RPC calls, than desribing it as a collaborative peer-to-peer process. It seems that this standard has been developed to address such a problem from around 2004.

Orchestration vs Choreography

BPEL focused on making it a buisness "process" execution lanugage, which can "orchestrate" required web services to form a "business process". The workflow aspects of BPEL are required in this respect.

However, a typical business transaction is often made up of related interactions which have to be collaboratively managed between two business entities till the conclusion of the unit of transaction involved in the exchange. This level is called "choreography" to distinguish this level of interactinons from that of the "orchestration", the latter of which is handed by e.g. BPEL. Orchestration is an interaction between the lower web services within a busniess process decription.

XML elements proposed

XML Schema from the WS-CDL: Primer.

Example case

An example diagram from WS-CDL: Primer for an intermediate level choreography of interactions. The interactions indicated by surrounding rectangles are to be defined as <workunit>.

XML example

Example: WS-CDL Complete Example from: WS-CDL: Primerfor showing a simple choreography to handle 3 messages, e.g. request, response and fault. As you can see all roles, variables and messages are named. It describe the two party interactions as a whole. There is no indicatino of directions of message flows (e.g. from Party A to Party B). Without a sollaboration diagram, it will be hard to understand the interactins taking place.

<?xml version="1.0" encoding="UTF-8"?>
<package
    name="DegenerateExample"
    author="Stephen Ross-Talbot"
    version="1.0"
    targetNamespace="http://www.w3.org/2002/ws/chor/primer"
    xmlns:tns="http://www.w3.org/2002/ws/chor/primer"
    xmlns="http://www.w3.org/2005/10/cdl"
    xmlns:xsd="http://www.w3.org/2001/XMLSchema"
    xmlns:primer="http://www.w3.org/2002/ws/chor/primer">
    <description type="documentation">
        Degenerate Example for Primer
    </description>

    <informationType name="QuoteRequestType" type="primer:QuoteRequestMsg">
        <description type="documentation">
            Quote Request Message
        </description>
    </informationType>
    <informationType name="QuoteResponseType" type="primer:QuoteResponseMsg">
        <description type="documentation">
            Quote Response Message
        </description>
    </informationType>
    <informationType name="QuoteResponseFaultType" type="primer:QuoteResponseFaultMsg">
        <description type="documentation">
            Quote Response Fault Message
        </description>
    </informationType>
    <informationType name="IdentityType" type="xsd:string">
        <description type="documentation">
            Identity Attribute
        </description>
    </informationType>
    <informationType name="URI" type="xsd:uri">
        <description type="documentation">
            Reference Token For Channels
        </description>
    </informationType>

    <token name="id" informationType="tns:IdentityType">
        <description type="documentation">
            Identity token
        </description>
    </token>
    <token name="URI" informationType="tns:URI">
        <description type="documentation">
            Reference Token for Channels
        </description>
    </token>
    <tokenLocator tokenName="tns:id" informationType="tns:QuoteRequestType" query="/quote/@id">
        <description type="documentation">
            Identity for Quote Request
        </description>
    </tokenLocator>
    <tokenLocator tokenName="tns:id" informationType="tns:QuoteResponseType" query="/quote/@key">
        <description type="documentation">
            Identity for Quote Response
        </description>
    </tokenLocator>
    <tokenLocator tokenName="tns:id" informationType="tns:QuoteResponseFaultType" query="/quote/@key">
        <description type="documentation">
            Identity for Quote Response Fault
        </description>
    </tokenLocator>

    <roleType name="BuyerRole">
        <description type="documentation">
            Role for Buyer
        </description>
        <behavior name="BuyerBehavior" interface="BuyerBehaviorInterface">
            <description type="documentation">
                Behavior for Buyer Role
            </description>
        </behavior>
    </roleType>
    <roleType name="SellerRole">
        <description type="documentation">
            Role for Seller
        </description>
        <behavior name="SellerBehavior" interface="SellerBehaviorInterface">
            <description type="documentation">
                Behavior for Seller
            </description>
        </behavior>
    </roleType>

    <relationshipType name="Buyer2Seller">
        <description type="documentation">
            Buyer Seller Relationship
        </description>
        <roleType typeRef="tns:BuyerRole"/>
        <roleType typeRef="tns:SellerRole"/>
    </relationshipType>

    <participantType name="Seller">
        <description type="documentation">
            Seller Participant
        </description>
        <roleType typeRef="tns:SellerRole"/>
    </participantType>
    <participantType name="Buyer">
        <description type="documentation">
            Buyer Participant
        </description>
        <roleType typeRef="tns:BuyerRole"/>
    </participantType>

    <channelType name="Buyer2SellerChannel">
        <description type="documentation">
            Buyer to Seller Channel Type
        </description>
        <roleType typeRef="tns:SellerRole"/>
        <reference>
            <token name="tns:URI"/>
        </reference>
        <identity type="primary">
            <token name="tns:id"/>
        </identity>
    </channelType>

    <choreography name="DegenerateChoreography" root="true">
        <description type="documentation">
            The Choreography for the degenerate use case
        </description><relationship type="tns:Buyer2Seller"/>
        <variableDefinitions>
            <variable name="Buyer2SellerC"
                channelType="tns:Buyer2SellerChannel"
                roleTypes="tns:BuyerRole tns:SellerRole">
                <description type="documentation">
                    Channel Variable
                </description>
            </variable>
            <variable name="quoteRequest"
                informationType="tns:QuoteRequestType"
                roleTypes="tns:BuyerRole tns:SellerRole">
                <description type="documentation">
                    Request Message
                </description>
            </variable>
            <variable name="quoteResponse"
                informationType="tns:QuoteResponseType"
                roleTypes="tns:BuyerRole tns:SellerRole">
                <description type="documentation">
                    Response Message
                </description>
            </variable>
            <variable name="faultResponse"
                informationType="tns:QuoteResponseFaultType"
                roleTypes="tns:BuyerRole tns:SellerRole">
                <description type="documentation">
                    Fault Message
                </description>
            </variable>
        </variableDefinitions>

	<sequence>
	        <interaction name="QuoteElicitation" operation="getQuote" channelVariable="tns:Buyer2SellerC">
	            <description type="documentation">
	                Quote Elicitation
	            </description>
	            <participate relationshipType="tns:Buyer2Seller" fromRoleTypeRef="tns:BuyerRole" toRoleTypeRef="tns:SellerRole"/>
	            <exchange name="QuoteRequest" informationType="tns:QuoteRequestType" action="request">
	                <description type="documentation">
	                    Quote Request Message Exchange
	                </description>
	                <send variable="cdl:getVariable('quoteRequest','','')"/>
	                <receive variable="cdl:getVariable('quoteRequest','','')"/>
	            </exchange>
	            <exchange name="QuoteResponse" informationType="tns:QuoteResponseType" action="respond">
	                <description type="documentation">
	                    Quote Response Message Exchange
	                </description>
	                <send variable="cdl:getVariable('quoteResponse','','')"/>
	                <receive variable="cdl:getVariable('quoteResponse','','')"/>
	            </exchange>
	            <exchange name="QuoteResponseFault" informationType="tns:QuoteResponseFaultType" action="respond" faultName="InvalidProductFault">
	                <description type="documentation">
	                    Quote Response Fault Exchange
	                </description>
	                <send variable="cdl:getVariable('faultResponse','','')"/>
	                <receive variable="cdl:getVariable('faultResponse','','')"/>
	            </exchange>
	        </interaction>
	</sequence>
    </choreography>
</package>

WS-CDL as a workflow, relationship to BPEL

WS-CDL have only <sequence>, <parallel> and <choice> to encase <interaction>. The expressive power of the language as a workflow is limited since it is not desinged as a generic workflow modelling language but a special purpose interaction/collaboration describing language.

On the other hand, the definition is lanuguage neutral, i.e. you can define new types freely inside the specification itself. Therefore, it is not dependant on SOAP or WSDL while BPEL is. Although it is a good aspect of the specification, it also means that it is not immediately executable, whereas BPEL is. To make a WS-CDL instance executable, it must be mapped to e.g. some web services in an implementing system at the lower level, e.g. to use web service end-point and varaibles described with WSDL.

WS-CDL (Web Services Choreography Description Launguage)