Semantic Annotations for WSDL — Usage Guide

1. Introduction

As the set of available Web Services expands, it becomes increasingly important to have automated tools to help identify services that match a requester's requirements. Finding suitable Web services automatically depends on the facilities available for service providers to describe the capabilities of their services and for service requesters to describe their requirements in an unambiguous and ideally, machine-interpretable form. Adding semantics to represent the requirements and capabilities of Web services is essential for achieving this unambiguity and machine-interpretability.

Semantics play an important role in all aspects of the lifecycle of Web services. During development, a service provider can explicate the intended semantics by annotating the appropriate parts of the Web service with concepts from a richer semantic model. Since semantic models provide agreement on the terms and intended use of terms, and may provide formal and informal definitions of the entities, there will be less ambiguity in the intended semantics of the provider. During discovery, a service requestor can describe the service requirements using terms from the semantic model. Reasoning techniques can be used to find service descriptions that match the request. During composition, these annotations can be used to aggregate the functionality of multiple services to create useful service compositions. Also, semantics based schema mappings can facilitate data transformations from which mediation code can be generated to enable Web services invocation. Therefore, once represented, semantics can be leveraged by tools to automate service discovery, mediation, composition and monitoring.

The World-Wide Web Consortium (W3C) standard Web Services Description Language (WSDL) specifies a standard way to describe the interfaces of a Web Service at a syntactic level and how to invoke it. However, WSDL does not explicitly provide mechanisms to specify the semantics of a Web service. Semantic Annotations for WSDL (SAWSDL) is an effort to define mechanisms by which semantic annotations can be added to WSDL components. This usage guide is an accompanying document to SAWSDL specification. It presents examples to illustrate how to associate semantic annotations with a Web service that could be used for classifying, discovering, matching, composing, and invoking Web services.

The sections in this document are organized to show how to associate semantic annotations with a WSDL document for use in service classification, discovery, matching, composition and invocation in that order.

1.1 Namespaces

The XML namespace names URIs used by this specification are as follows:

Prefix	Namespace name
xs	http://www.w3.org/2001/XMLSchema#
sawsdl	http://www.w3.org/2002/ws/sawsdl/spec/sawsdl#
wsdl	http://www.w3.org/2006/01/wsdl
rdf	http://www.w3.org/1999/02/22-rdf-syntax-ns
rdfs	http://www.w3.org/2000/01/rdf-schema
owl	http://www.w3.org/2002/07/owl

1.2 Running Examples

Throughout this document, we use two Web services and several variations on these Web services to illustrate how semantic annoations on a WSDL can be used to do Web service interface matching and composition. One of these two Web services is presented as a request of a desirable Web service while the other is used as an advertisement or a service being offered by a service provider. Some readers may be new to the concept of representing the requirements of a client as a Web service - WSDL. While there may be different ways of representing the requirements of a client, in this document, for illustrative purposes we adopted the notion of a request WSDL where the requirements of a client are modeled as a Web service - WSDL. The inputs and outputs of this request WSDL codify the available inputs that a client can provide and the expected outputs from a service respectively. We then use the concepts of annotating a request similar to the way that one can annotate a service provider's WSDL and illustrate how a request WSDL interface can be matched, composed and transformed to invoke a service provider's advertised service.

We present these two Web services here so as to allow the reader to get familiarized with the examples used in the rest of the document. These are 'vanilla' WSDL files with no semantic annotations. In the following sections, these WSDLs are shown with suitable semantic annotations that suit the specific illustration. First a WSDL representation of a request WSDL is given followed by a service provider's advertisement WSDL.

<wsdl:description
  targetNamespace="http://org1.example.com/wsdl/CheckAvailabilityRequestService/"
  xmlns="http://org1.example.com/wsdl/CheckAvailabilityRequestService/"
  xmlns:wsdl="http://schemas.xmlsoap.org/wsdl/"
  xmlns:xs="http://www.w3.org/2001/XMLSchema#">

  <wsdl:types>
   <xs:schema targetNamespace="http://org1.example.com/wsdl/CheckAvailabilityRequestService">
    <xs:element name="CheckAvailabilityRequestServiceRequest">
      <xs:complexType>
        <xs:sequence>
          <xs:element name="itemCode" type="xs:string"/>
          <xs:element name="date" type="xs:string"/>
          <xs:element name="qty" type="xs:float"/>
        </xs:sequence>
      </xs:complexType>
    </xs:element>
    <xs:element name="CheckAvailabilityRequestServiceResponse type="itemConfirmation"/>
    <xs:simpleType name="itemConfirmation">  
      <xs:restriction base:"xs:boolean"/>
    </xs:simpleType>
    </xs:schema>
  </wsdl:types>

 <wsdl:interface name="CheckAvailabilityRequestService">
   <wsdl:operation name="CheckAvailabilityRequestOperation" pattern="http://www.w3.org/2006/01/wsdl/in-out"
   <wsdl:input element="CheckAvailabilityRequestServiceRequest"/>
   <wsdl:output element="CheckAvailabilityRequestServiceResponse"/>
    </wsdl:operation>
  </wsdl:interface>
 </wsdl:description>

Listing 1.2-1: WSDL excerpt for CheckAvailabilityRequest service

<wsdl:description
  targetNamespace="http://org2.example.com/wsdl/CheckInventoryService/"
  xmlns="http://org2.example.com/wsdl/CheckInventoryService/"
  xmlns:wsdl="http://schemas.xmlsoap.org/wsdl/"
  xmlns:xs="http://www.w3.org/2001/XMLSchema#">

  <wsdl:types>
    <xs:schema targetNamespace="http://org2.example.com/wsdl/CheckInventoryService">
     <xs:element name="CheckInventoryServiceRequest">
      <xs:complexType>
        <xs:sequence>
          <xs:element name="SKU" type="xsd:string"/>
          <wsdl:element name="deliveryDate" type="xs:string"/>
          <wsdl:element name="numBundles" type="xs:float"/>
        </xs:sequence>
      </xs:complexType>
    </xs:element>
    <xs:element name="CheckInventoryServiceResponse">
      <xs:complexType>
        <xs:sequence>
          <xs:element name="conf" type="xsd:boolean"/>
          <xs:element name="numBundles_available" type="xsd:string"/>            
        </xs:sequence>
      </xs:complexType>
    </xs:element>
    </xs:schema>
  </wsdl:types>

  <wsdl:interface name="CheckInventoryService">
    <wsdl:operation name="checkInventoryService" pattern="http://www.w3.org/2006/01/wsdl/in-out">
      <wsdl:input element="CheckInventoryServiceRequest"/>
      <wsdl:output element="CheckInventoryServiceResponse"/>
    </wsdl:operation>
  </wsdl:interface>
</wsdl:description>

Listing 1.2-2: WSDL excerpt for CheckInventoryService

2. Defining Annotations to Publish a Web Service

A Web service can be semantically annotated to carry categorization information that could be used to publish it for instance in a service registry. SAWSDL extension mechanism modelReference can be used to add this categorization information to Web services. This categorization information could be used when automatically publishing services in registries such as UDDI [UDDI]. Below, we illustrate a couple of ways in which categorization information can be associated with a Web service.

2.1 Adding Categorization Information When a Taxonomy Already Exists

If a categorization semantic model already exists (for example a taxonomy), then a modelReference element could be defined either on an interface or on an operation of a Web service to point to a particular categorization in the taxonomy. For example, if a purchase order taxonomy was created as shown in Listing 2.1-1 below (shown in RDF Turtle [Turtle] format for readability), then the interface of an item availability check Web service (introduced in Listing 1.2-1) could be annotated with taxonomy information as shown in Listing 2.1-2.

@prefix rdf: <http://www.w3.org/1999/02/22-rdf-syntax-ns#> .
@prefix rdfs: <http://www.w3.org/2000/01/rdf-schema#> .
@prefix owl: <http://www.w3.org/2002/07/owl#> .
@prefix : <http://org1.example.com/taxonomies/POServiceClassification#> .

    <http://org1.example.com/taxonomies/POServiceClassification#> rdf:type owl:Ontology .
    :PurchaseOrderServices rdf:type owl:Class .
    :OrderModification rdf:type owl:Class;
       rdfs:subClassOf :PurchaseOrderServices .
    :ItemAvailabilityCheck rdf:type owl:Class;
       rdfs:subClassOf :PurchaseOrderServices .
    :OrderPlacement rdf:type owl:Class;
       rdfs:subClassOf :PurchaseOrderServices .
    :ProductInfoInquiry rdf:type owl:Class;
       rdfs:subClassOf :PurchaseOrderServices .
    :OrderTracking rdf:type owl:Class;
       rdfs:subClassOf :PurchaseOrderServices .

Listing 2.1-1: A taxonomy to organize Web services related to checking, placing, and tracking purchase orders

    ...
<wsdl:interface name="CheckItemAvailabilityRequestService"
  sawsdl:modelReference="http://org1.example.com/taxonomies/POServiceClassification#ItemAvailabilityCheck">
    ...
</wsdl:interface>
    ...

Listing 2.1-2: Shows how a WSDL interface could be annotated with categorization information that could be used in publishing a Web service (for example, into a service registry)

2.2 Adding Categorization Information by Defining a Taxonomy

Some taxonomies may not provide direct URIs for their categories and may require multiple pieces of information to identify the categories.In such a case, users can define such information as per the requirements and associate a modelReference to point to such user-defined taxonomic information. For example, if a particular taxonomy specifies three pieces of information namely, the name of the taxonomy, the specific categorization in the taxonomy and a unique code for that taxonomy, then, it can be defined as follows.

@prefix categorization: <http://org1.example.com/taxonomies/categorization#> .
@prefix rdfs: <http://www.w3.org/2000/01/rdf-schema#> .
@prefix xs: <http://www.w3.org/2001/XMLSchema#> .
@prefix rdf: <http://www.w3.org/1999/02/22-rdf-syntax-ns#> .

  categorization:Category rdf:type rdfs:Class .
  categorization:hasValue rdf:type rdf:Property;
     rdfs:domain categorization:Category;
     rdf:range rdfs:Literal .
  categorization:usesTaxonomy rdf:type rdfs:Property;
     rdfs:domain categorization:Category;
     rdf:range xs:anyUri .

Listing 2.2-1: Schema for defining a taxonomy with specific properties

Then, an instance such as the one below in listing 2.2-2 that adheres to the above schema in Listing 2.2-1 can be defined and either imported or inserted into the WSDL file whose interface (or operation) is to be annotated.

@prefix categorization: <http://org1.example.com/taxonomies/categorization#> .
@prefix rdfs: <http://www.w3.org/2000/01/rdf-schema#> .
@prefix xs: <http://www.w3.org/2001/XMLSchema#> .
@prefix rdf: <http://www.w3.org/1999/02/22-rdf-syntax-ns#> .

  categorization:Category rdf:type "urn:Electronics" ;
     categorization:hasValue "443112" ;
     categorization:usesTaxonomy "http://naics.com/" .

Listing 2.2-2: An instance of a taxonomy defined using the schema defined in Listing 2.2-1.

The annotation can be associated with the interface of a Web service as follows.

<wsdl:interface name="CheckAvailabilityRequestService"  
   sawsdl:modelReference="http://org1.example.com/taxonomies/categorization#Electronics">
  ...
</wsdl:interface>

Listing 2.2-3: A WSDL excerpt showing how a wsdl:interface could be annotated with taxonomy information from the listing defined in Listing 2.2-2.

Using similar modelReference annotation operations within an interface can also be annotated. SAWSDL does not specify any relationship between the categorization information specified at the level of an interface and the one that is specified on an operation that is contained within that interface. The example in listing 2.2-4 below shows how an operation within a Web service interface could be annotated using the same 'categorization' scheme.

<wsdl:operation name="CheckAvailabilityRequestOperation"   sawsdl:modelReference="http://org1.example.com/taxonomies/categorization#Electronics"
  ...
</wsdl:operation>

Listing 2.2-4: A WSDL excerpt showing how a wsdl:operation could be annotated with taxonomy information from the listing defined in Listing 2.2-2.

2.3 Using Categorization Annotations to Publish a Service in a UDDI Registry

In this subsection, we discuss how one can use the categorization annotation information defined by using the mechanisms described in section 2 to publish Web services in a UDDI registry. This is provided as an example for illustrative purpose. One can use similar mechanisms to publish the Web services in registries or repositories other than UDDI.

Following the example in Listing 2.2-2, one can define taxonomy references to point to a specific category defined in, say, NAICS [NAICS] taxonomy that is registered in a UDDI registry as shown in listing 2.3-1. This information can then be reused when a service (such as the CheckItemAvailability Service whose interface definition is shown in Listing 2.2-3) is published in the UDDI registry. In the following example we use the NAICS taxonomy registered in UDDI.

@prefix categorization: <http://org1.example.com/taxonomies/categorization#> .
@prefix rdfs: <http://www.w3.org/2000/01/rdf-schema#> .
@prefix xs: <http://www.w3.org/2001/XMLSchema#> .
@prefix rdf: <http://www.w3.org/1999/02/22-rdf-syntax-ns#> .

  categorization:Category rdf:type "urn:Electronics" ;
     categorization:hasValue "443112" ;
     categorization:usesTaxonomy "uddi:uddi.org:ubr:categorization:naics:2002" .

Listing 2.3-1: An instance of a taxonomy defined using the schema defined in Listing 2.2-1 that refers to NAICS taxonomy registered in UDDI registry.

To publish a Web service that is represented in SAWSDL in a UDDI registry, an automatic Web service publishing engine can dereference the taxonomy information from its interface and use that information to determine which category of what taxonomy in the registry to publish the service to. For example, when publishing the service in UDDI V3, a category bag can be created to classify the service using this information. Listing 2.3-2 below shows the UDDI categoryBag structure that could be created from the categorization information in Listing 2.3-1.

<categoryBag>
  <keyedReference
    tModelKey="uddi:uddi.org:ubr:categorization:naics:2002"
    keyName="Electronics"
    keyValue="443112"/>
</categoryBag>

Listing 2.3-2: UDDI Category bag structure derived from the taxonomy instance in Listing 2.3-1.

In this section we have discussed the usage of modelReference concepts on an interface and operation for associating categorization relation information for illustrative purposes. Users may choose to use modelReferences on interfaces and operations for associating other behavioral aspects with a servicel.

3. Defining Annotations for use in Matching and Composing Web services

One of the main motivations for SAWSDL specification is to provide mechanisms using which semantic annotations can be added to WSDL documents so that these semantics can be used to help automate the matching and composition of Web services. In this section we present some examples to show how to add such annotations for use during Web service interface matching and composition. First we present an example to illustrate the use of annotations in Web service interface matching.

3.1 Matching Web Service Interfaces using a Shared Ontology

Let's consider the the following scenario. A requestor would like to submit a request to verify the availability of an item from a preferred supplier. Say that this request is represented as a CheckAvailabilityRequest WSDL (introduced in Listing 1.2-1). Also suppose that a service provider has a service to offer that enables the requesters to check for the availability of items it offers. This is called CheckInventoryService and is also represented as a WSDL with the same name (also introduced in Listing 1.2-2). Specifically, the request WSDL codifies the inputs it can supply and the outputs it expects of an item availability check service by the service provider. The service WSDL specifies the interface of a service that allows requesters to check for the availability of an inventory item by exposing the inputs it requires in order to provide item availability confirmation. At a high level the service offered by the service provider should match the request. However, the differences in the vocabulary used by the two services to represent their interfaces may get in the way of making a match. For example, the term itemCode used by the requester and the term SKU (Stock Keeping Unit) used by the provider both are meant to uniquely identify the item in question. Also, the term qty used by the requester and the term numBundles used by the provider both in this context may refer to the number of items (in one case, it is the number of items being requested and in the other case, it is the number of items being offered). However, a matching engine may not have sufficient information to identify them as related terms unless explicitly specified. Semantic annotations, in cases such as these, could be quite helpful. In the simple case, if there were to be a common semantic model that one can use to annotate the WSDLs of the requester and the service provider as shown below (in Listings 3.1-1 and 3.1-2), then a semantic engine could use this information to match the two Web services. In this example, annotation is done using modelReference extensibility element defined in SAWSDL.

The WSDL excerpt shown below in listing 3.1-1 represents one of the ways of semantically annotating WSDL document of the request for checking item availability.

<wsdl:description
  targetNamespace="http://org1.example.com/wsdl/CheckAvailabilityRequestService/"
  xmlns="http://org1.example.com/wsdl/CheckAvailabilityRequestService/"
  xmlns:wsdl="http://schemas.xmlsoap.org/wsdl/"
  xmlns:xs="http://www.w3.org/2001/XMLSchema#"
  xmlns:sawsdl="http://www.w3.org/2002/sw/sawsdl/spec/sawsdl#"
  xmlns:SampleOntology="http://org1.example.com/ontologies/SampleOntology#>

  <wsdl:types>
    <xs:schema targetNamespace="http://org1.example.com/wsdl/CheckAvailabilityRequestService">
    <xs:element name="CheckAvailabilityRequestServiceRequest">
      <xs:complexType>
        <xs:sequence>
          <xs:element name="itemCode" type="xs:string"
            sawsdl:modelReference="http://org1.example.com/ontologies/SampleOntology#PartNumber"/>
          <wsdl:element name="date" type="xs:string"
            sawsdl:modelReference="http://org1.example.com/ontologies/SampleOntology#DueDate"/>
          <wsdl:element name="qty" type="xs:float"
            sawsdl:modelReference="http://org1.example.com/ontologies/SampleOntology#Quantity/>
        </xs:sequence>
      </xs:complexType>
    </xs:element>
    <xs:element name="CheckAvailabilityRequestServiceResponse type="itemConfirmation"/>
    <xs:simpleType name="itemConfirmation">
      sawsdl:modelReference="http://org1.example.com/ontologies/SampleOntology#AvailabilityConfirmation"
      <xs:restriction base:"xs:boolean"/>
    </xs:simpleType>
    </xs:schema>
  </wsdl:types>

  <wsdl:interface name="CheckAvailabilityRequestService">
      <wsdl:operation name="checkAvailabilityRequestOperation" pattern="http://www.w3.org/2006/01/wsdl/in-out">
      <wsdl:input element="CheckAvailabilityRequestServiceRequest"/>
      <wsdl:output element="CheckAvailabilityRequestServiceResponse"/>
    </wsdl:operation>
  </wsdl:interface>
</wsdl:description>

Listing 3.1-1: A WSDL excerpt of a semantically annotated service request that shows the usage of modelReference.

The WSDL excerpt in listing 3.1-2 represents a way of semantically annotating the WSDL document of the service offered by the service provider for checking item availability.

<wsdl:description
  targetNamespace="http://org2.example.com/wsdl/CheckInventoryService/"
  xmlns="http://org2.example.com/wsdl/CheckInventoryService/"
  xmlns:wsdl="http://schemas.xmlsoap.org/wsdl/"
  xmlns:xs="http://www.w3.org/2001/XMLSchema#"
  xmlns:sawsdl="http://www.w3.org/2002/sw/sawsdl/spec/sawsdl#"
  xmlns:SampleOntology="http://org1.example.com/ontologies/SampleOntology#>

  <wsdl:types>
    <xs:schema targetNamespace="http://org2.example.com/wsdl/CheckInventoryService">
    <xs:element name="CheckInventoryServiceRequest">
      <xs:complexType>
        <xs:sequence>
          <xs:element name="SKU" type="xsd:string"
            sawsdl:modelReference="http://org1.example.com/ontologies/SampleOntology#SKU"/>
          <wsdl:element name="deliveryDate" type="xs:string"
            sawsdl:modelReference="http://org1.example.com/ontologies/SampleOntology#DueDate"/>
          <wsdl:element name="numBundles" type="xs:float"
            sawsdl:modelReference="http://org1.example.com/ontologies/SampleOntology#Quantity/>
        </xs:sequence>
      </xs:complexType>
    </xs:element>
    <xs:element name="CheckInventoryServiceResponse">
      <xs:complexType>
        <xs:sequence>
          <xs:element name="conf" type="xsd:boolean"
            sawsdl:modelReference="http://org1.example.com/ontologies/SampleOntology#AvailabilityConfirmation"/>
          <xs:element name="numBundles_available" type="xsd:string"
            sawsdl:modelReference="http://org1.example.com/ontologies/SampleOntology#Quantity"/>
        </xs:sequence>
      </xs:complexType>
    </xs:element>
    </xs:schema>
  </wsdl:types>

  <wsdl:interface name="CheckInventoryService">
     <wsdl:operation name="checkInventoryOperation" pattern="http://www.w3.org/2006/01/wsdl/in-out">
       <wsdl:input element="CheckInventoryServiceRequest"/>
       <wsdl:output element="CheckInventoryServiceResponse"/>
    </wsdl:operation>
  </wsdl:interface>
</wsdl:description>

Listing 3.1-2: A WSDL excerpt of a semantically annotated service (advertisement) that shows the usage of modelReference.

As can be noted, in this example both WSDL documents in this example are annotated with concepts from the same semantic model – SampleOntology (shown in listing 3.1-3). In this example, the ontology contains the relationship between the concepts PartNumber and SKU i.e., a SKU Code is a subClassOf PartNumber. A semantic engine can infer this relationship during Web service interface matching by parsing and reasoning over this semantic model. Therefore, the WSDL elements 'itemCode' in the request WSDL and the 'SKU' in the service WSDL match with one another. In the case of the other input elements namely'qty' vs 'numBundles' and 'date' vs 'deliveryDate', since both these sets of elements are annotated with shared semantic concepts namely 'Quantity' and 'DueDate' respectively the semantic ambiguity can be resolved by direct matching of semantic annotations. The same applies to matching outputs as well. The ontology that represents the subClassOf relationship between the two concepts PartNumber and SKU can be modeled in OWL as follows.

@prefix rdfs: <http://www.w3.org/2000/01/rdf-schema#> .
@prefix rdf: <http://www.w3.org/1999/02/22-rdf-syntax-ns#> .
@prefix: <http://org1.example.com/ontologies/SampleOntology#> .
@prefix owl: <http://www.w3.org/2002/07/owl#> .

<http://org1.example.com/ontologies/SampleOntology#> rdf:type owl:Ontology .

  :PartNumber rdf:type owl:Class .
  :SKU rdf:type owl:Class;
    rdfs:subClassOf :PartNumber .
  :Quantity rdf:type owl:Class .
  :DueDate rdf:type owl:Class .
  :AvailabilityConfirmation rdf:type owl:Class .
     

Listing 3.1-3: A simple SampleOntology that captures the subClassOf relationship between PartNumber and SKU concepts.

3.2 Matching Web Service Interfaces with Ontology Mediation

In section 3.1 we made a shared ontology assumption between the requester and service provider domains. This assumption may not always hold good. The vocabulary differences may result in two different ontologies one for each side albeit for the same domain. In such a case one can create a mapping ontology by capturing the relationships between the concepts used in the different ontologies. Below we annoate the same two request and advertisement WSDLs using concepts from different ontologies. When a mapping ontology such as the one shown in listing 3.2-3 is available, then the semantic annotations extracted from the request and the advertisement WSDL can be matched using such a mapping ontology.

The WSDL excerpt shown below in listing 3.2-1 represents one of the ways of semantically annotating WSDL document of the request for checking item availability.

<wsdl:description
  targetNamespace="http://org1.example.com/wsdl/CheckAvailabilityRequestService/"
  .... 
  xmlns:Org1Ont="http://org1.example.com/ontologies/SampleOntologyOrg1#>

  <wsdl:types>
    <xs:schema targetNamespace="http://org1.example.com/wsdl/CheckAvailabilityRequestService">
    <xs:element name="CheckAvailabilityRequestServiceRequest">
      <xs:complexType>
        <xs:sequence>
          <xs:element name="itemCode" type="xs:string"
            sawsdl:modelReference="http://org1.example.com/ontologies/SampleOntologyOrg1#PartNumber"/>
          <wsdl:element name="date" type="xs:string"
            sawsdl:modelReference="http://org1.example.com/ontologies/SampleOntologyOrg1#DueDate"/>
          <wsdl:element name="qty" type="xs:float"
            sawsdl:modelReference="http://org1.example.com/ontologies/SampleOntologyOrg1#Quantity/>
        </xs:sequence>
      </xs:complexType>
    </xs:element>
    <xs:element name="CheckAvailabilityRequestServiceResponse type="itemConfirmation"/>
    <xs:simpleType name="itemConfirmation">
      sawsdl:modelReference="http://org1.example.com/ontologies/SampleOntologyOrg1#AvailabilityConfirmation"
      <xs:restriction base:"xs:boolean"/>
    </xs:simpleType>
    </xs:schema>
  </wsdl:types>

  <wsdl:interface name="CheckAvailabilityRequestService">
      <wsdl:operation name="checkAvailabilityRequestOperation" pattern="http://www.w3.org/2006/01/wsdl/in-out">
      <wsdl:input element="CheckAvailabilityRequestServiceRequest"/>
      <wsdl:output element="CheckAvailabilityRequestServiceResponse"/>
    </wsdl:operation>
  </wsdl:interface>
</wsdl:description>

Listing 3.2-1: A WSDL excerpt of a semantically annotated service request that shows the usage of modelReference.

The WSDL excerpt shown below in listing 3.2-2 represents a way of semantically annotating the WSDL document of the service offered by the service provider for checking item availability.

<wsdl:description
  targetNamespace="http://org2.example.com/wsdl/CheckInventoryService/"
  .....
  xmlns:Org2Ont="http://org2.example.com/ontologies/SampleOntologyOrg2#>

  <wsdl:types>
    <xs:schema targetNamespace="http://org2.example.com/wsdl/CheckInventoryService">
    <xs:element name="CheckInventoryServiceRequest">
      <xs:complexType>
        <xs:sequence>
          <xs:element name="SKU" type="xsd:string"
            sawsdl:modelReference="http://org2.example.com/ontologies/SampleOntologyOrg2#SKU"/>
          <wsdl:element name="deliveryDate" type="xs:string"
            sawsdl:modelReference="http://org2.example.com/ontologies/SampleOntologyOrg2#DeliveryDate"/>
          <wsdl:element name="numBundles" type="xs:float"
            sawsdl:modelReference="http://org2.example.com/ontologies/SampleOntologyOrg2#NumBundles/>
        </xs:sequence>
      </xs:complexType>
    </xs:element>
    <xs:element name="CheckInventoryServiceResponse">
      <xs:complexType>
        <xs:sequence>
          <xs:element name="conf" type="xsd:boolean"
            sawsdl:modelReference="http://org2.example.com/ontologies/SampleOntologyOrg2#AvailabilityConfirmation"/>
          <xs:element name="numBundles_available" type="xsd:string"
            sawsdl:modelReference="http://org2.example.com/ontologies/SampleOntologyOrg2#NumBundles"/>
        </xs:sequence>
      </xs:complexType>
    </xs:element>
    </xs:schema>
  </wsdl:types>

  <wsdl:interface name="CheckInventoryService">
     <wsdl:operation name="checkInventoryOperation" pattern="http://www.w3.org/2006/01/wsdl/in-out">
       <wsdl:input element="CheckInventoryServiceRequest"/>
       <wsdl:output element="CheckInventoryServiceResponse"/>
    </wsdl:operation>
  </wsdl:interface>
</wsdl:description>

Listing 3.2-2: A WSDL excerpt of a semantically annotated service (advertisement) that shows the usage of modelReference.

As noted, in this case the concepts originating from different ontologies represented in the two WSDLs shown above are mapped in a mapping ontology such as the one shown below in Listing 3.2-5. It contains relationships between concepts such as Quantity & NumBundles and Due Date & DeliveryDate and PartNumber & SKU. Just as in the example in section 3.1, a semantic matching engine can now be applied to reason over these relationships during Web service interface matching. The relationships between the concepts in this mapping ontology are shown pictorially in Figure 1 below.

Below, we list the three ontologies that are used in matching the checkAvailabilityRequest() request WSDL with the checkInventoryService() advertisement WSDL. The first ontology shown in listing 3.2-3 is the one used by the requesting organization (referred to using the name space http://org1.example.com/ontologies/SampleOntologyOrg1) that defined the checkAvailabilityRequest() request WSDL. The second ontology shown in listing 3.2-4 is the one used by the service advertising organization (referred to using the namespace ) that defined the checkInventoryService() advertisement WSDL. The third one shown in listing 3.2-5 is the mapping ontology defined by a third organization (could possibly be defined by either of the first two organizations as well) that relates the concepts from the two ontologies defined by org1 and org2.

@prefix rdfs: <http://www.w3.org/2000/01/rdf-schema#> .
@prefix rdf: <http://www.w3.org/1999/02/22-rdf-syntax-ns#> .
@prefix: <http://org1.example.com/ontologies/SampleOntologyOrg1#> .
@prefix owl: <http://www.w3.org/2002/07/owl#> .

  <http://org1.example.com/ontologies/SampleOntologyOrg1#> rdf:type owl:Ontology  .
  
  :PartNumber rdf:type owl:Class .    
  :DueDate rdf:type owl:Class . 
  :Quantity rdf:type owl:Class . 
  :AvailabilityConfirmation rdf:type owl:Class .

Listing 3.2-3:An RDF Turtle excerpt of an Ontology used by the organization that defined checkAvailabilityRequest() request WSDL.

@prefix rdfs: <http://www.w3.org/2000/01/rdf-schema#> .
@prefix rdf: <http://www.w3.org/1999/02/22-rdf-syntax-ns#> .
@prefix: <http://org2.example.com/ontologies/SampleOntologyOrg2#> .
@prefix owl: <http://www.w3.org/2002/07/owl#> .

  <http://org2.example.com/ontologies/SampleOntologyOrg2#> rdf:type owl:Ontology  .
 
  :SKU rdf:type owl:Class .
  :DeliveryDate rdf:type owl:Class .
  :NumBundles rdf:type owl:Class .
  :AvailabilityConfirmation rdf:type owl:Class .
  

Listing 3.2-4: An RDF Turtle excerpt of Ontology used by the organization that defined checkInventoryService() advertisement WSDL

@prefix rdfs: <http://www.w3.org/2000/01/rdf-schema#> .
@prefix rdf: <http://www.w3.org/1999/02/22-rdf-syntax-ns#> .
@prefix: <http://org3.example.com/ontologies/MappingOntology#> .
@prefix Org1Ont: <http://org1.example.com/ontologies/SampleOntologyOrg1#> .
@prefix Org2Ont: <http://org2.example.com/ontologies/SampleOntologyOrg2#> .
@prefix owl: <http://www.w3.org/2002/07/owl#> .

  <http://org3.example.com/ontologies/MappingOntology#> rdf:type owl:Ontology  .
  
  Org2Ont:SKU rdfs:subClassOf Org1Ont:PartNumber .
  Org2Ont:NumBundles owl:equivalentClass Org1Ont:Quantity .
  Org2Ont:DeliveryDate owl:equivalentClass Org1Ont:DueDate .
  Org2Ont:AvailabilityConfirmation owl:equivalentClass Org1Ont:AvailabilityConfirmation .

Listing 3.2-5: An RDF Turtle excerpt showing the mapping ontology mapping concepts from ontologies in listings 3.2-3 and 3.2-4. This mapping ontology is used to match the request WSDL in listing 3.2-1 with the advertisement WSDL in listing 3.2-2.

It is to be noted that SAWSDL specifies mechanisms to annotate WSDLs but does not say anything about how these annotations are generated. These annotations can either be created manually or by tools that can infer relationships between the terms used in the WSDLs and those that are given in a semantic model. Also, SAWSDL does not say anything about how the semantic models themselves are created. Just like the creation of annotations, these semantic models can either be created manually by a human expert or generated automatically using ontology learning tools or by a combination of both.

In the next section, we extend the above item availability check example to show how semantic annotations added to a WSDL can be used to compose Web services.

3.3 Composing Web Services - A Simple Example

This section illustrates how semantic annotations can be used to compose Web services. The request and advertisement WSDLs to be considered in this scenario are the same as the ones shown in listings 3.2-1 and 3.2-2 respectively. However, we introduce a variation to the mapping ontology from section 3.2. Suppose that in the item availability checking scenario that we have been discussing so far, the subClassOf relationship between the concepts PartNumber and SKU is unspecified in the ontology. In this case, the request checkAvailabilityRequest() would not match with the service checkInventoryService() because there is no relationship between PartNumber and SKU. Also, suppose that there is another Web service called PartNumber2SKULookup ( ) that can take a PartNumber as input and produce SKU as output (Listing 3.3-1). Say that a matching engine would be able to find this service in the appropriate category in a registry - in this case it would be in 'Utilities/LookupServices' category (because the modelReference on the operation of this service contains the categorization information. So, presumably a publishing agent/program would have placed this service in the appropriate category that is referred to by the modelReference URI). Say, also, that a mapping ontology such as the one shown in listing 3.3-2 is available for the matching engine. Now, the service PartNumber2SKULookup() can be composed with checkInventoryService() to match the request checkAvailabilityRequest() as shown in Figure 2. This can be achieved by simply extracting the semantic annotations from the request, and all the advertisement WSDLs and using the relationships in the mapping ontology to match the corresponding concepts. A simple Web service composition using the semantics annotations on the request and advertisement services is illustrated in Figure 2.

<wsdl:description
  targetNamespace="http://org3.example.com/wsdl/PartNumber2SKULookupService/"
  xmlns="http://org3.example.com/wsdl/PartNumber2SKULookupService/"
  xmlns:wsdl="http://schemas.xmlsoap.org/wsdl/"
  xmlns:xs="http://www.w3.org/2001/XMLSchema#"
  xmlns:sawsdl="http://www.w3.org/2002/sw/sawsdl/spec/sawsdl# 
  xmlns:Org3Ont="http://org3.example.com/ontologies/SampleOntologyOrg3#">

  <wsdl:types>
    <xs:schema targetNamespace="http://org3.example.com/wsdl/PartNumber2SKULookupService>
      <xs:element name="PartNumber2SKULookupServiceRequest" type="partNum"/>
      <xs:simpleType name="partNum">
        sawsdl:modelReference="http://org3.example.com/ontologies/SampleOntologyOrg3#PartNumber">
        <xs:restriction base="xs:string"/>
      </xs:simpleType>

      <xs:element name="PartNumber2SKULookupServiceResponse" type="SKU"/>
      <xs:simpleType name="SKU">
        sawsdl:modelReference="http://org3.example.com/ontologies/SampleOntologyOrg3#SKU">
        <xs:restriction base="xs:string"/>
      </xs:simpleType>
    </xs:schema>
  </wsdl:types>

  <wsdl:interface name="PartNumber2SKULookupService">
    <wsdl:operation name="PartNumber2SKULookupService" pattern="http://www.w3.org/2006/01/wsdl/in-out"  
        sawsdl:modelReference="http://org3.example.com/taxonomies/Utilities#LookupServices" >
        <wsdl:input element="PartNumber2SKULookupServiceRequest"/>
        <wsdl:output element="PartNumber2SKULookupServiceResponse"/>
    </wsdl:operation>
  </wsdl:interface>
</wsdl:description>

@prefix rdfs: <http://www.w3.org/2000/01/rdf-schema#> .
@prefix rdf: <http://www.w3.org/1999/02/22-rdf-syntax-ns#> .
@prefix: <http://org3.example.com/ontologies/MappingOntology#> .
@prefix Org1Ont: <http://org1.example.com/ontologies/SampleOntologyOrg1#> .
@prefix Org2Ont: <http://org2.example.com/ontologies/SampleOntologyOrg2#> .
@prefix Org3Ont: <http://org2.example.com/ontologies/SampleOntologyOrg3#> .
@prefix owl: <http://www.w3.org/2002/07/owl#> .

  <http://org3.example.com/ontologies/MappingOntology#> rdf:type owl:Ontology  .
  
  Org2Ont:NumBundles rdf:type owl:Class;
     owl:equivalentClass Org1Ont:Quantity .
  Org2Ont:DeliveryDate rdf:type owl:Class;
     owl:equivalentClass Org1Ont:DueDate .
  Org2Ont:AvailabilityConfirmation rdf:type owl:Class;
     owl:equivalentClass Org1Ont:AvailabilityConfirmation .
 
  Org3Ont:PartNumber rdf:type owl:Class;
     owl:equivalentClass Org1Ont:PartNumber .    
  Org3Ont:SKU rdf:type owl:Class;
     owl:equivalentClass Org2Ont:SKU . 
 

3.4 Composing Web Services using Ontology Reasoning

We introduce three variations to our item availability checking scenario in this section to illustrate how one can use semantic annotations to compose Web services with ontology reasoning. First, suppose that we have a checkInventoryService() that takes an UPC as input inplace of SKU ((as introduced in Listing 1.2-2 but with semantic annotations as in Listing 3.4-1 below). Then, this service will not match the request checkAvailabilityRequest() (unchanged from Listing 3.2-1) because there is no relationship between the concepts PartNumber and UPC in the mapping ontology. So, even semantic annotations will not be able to help in matching the request checkAvailabilityRequest() and the advertisement checkInventoryService(). Second, say that there is another Web service called SKU2UPCLookup( ) that can take an SKU as input and produce UPC (Universal Product Code) as output (Listing 3.4-2). Third, the original mapping ontology introduced in Listing 3.2-3 has an addition of UPC concept. But it is to be noted that the ontology does not specify any relationship between SKU and UPC codes. Figure 3 summarizes the relationships between the concepts PartNumber, SKU, and UPC in SampleOntology. Now with these three variations, we have enough information to see how a semantic engine can compose Web services to match the request checkAvailabilityRequest() with checkInventoryService().

<wsdl:description
  targetNamespace="http://org2.example.com/wsdl/CheckInventoryService/"
  ....
  xmlns:Org2Ont="http://org2.example.com/ontologies/SampleOntologyOrg2#>

  <wsdl:types>
    <xs:schema targetNamespace="http://org2.example.com/wsdl/CheckInventoryService">
    <xs:element name="CheckInventoryServiceRequest">
      <xs:complexType>
        <xs:sequence>
          <xs:element name="UPC" type="xsd:string"
            sawsdl:modelReference="http://org2.example.com/ontologies/SampleOntologyOrg2#UPC"/>
          <wsdl:element name="deliveryDate" type="xs:string"
            sawsdl:modelReference="http://org2.example.com/ontologies/SampleOntologyOrg2#DeliveryDate"/>
          <wsdl:element name="numBundles" type="xs:float"
            sawsdl:modelReference="http://org2.example.com/ontologies/SampleOntologyOrg2#NumBundles/>
        </xs:sequence>
      </xs:complexType>
    </xs:element>
    <xs:element name="CheckInventoryServiceResponse">
      <xs:complexType>
        <xs:sequence>
          <xs:element name="conf" type="xsd:boolean"
            sawsdl:modelReference="http://org2.example.com/ontologies/SampleOntologyOrg2#AvailabilityConfirmation"/>
          <xs:element name="numBundles_available" type="xsd:string"
            sawsdl:modelReference="http://org2.example.com/ontologies/SampleOntologyOrg2#NumBundles"/>
        </xs:sequence>
      </xs:complexType>
    </xs:element>
    </xs:schema>
  </wsdl:types>

  <wsdl:interface name="CheckInventoryService">
     <wsdl:operation name="checkInventoryOperation" pattern="http://www.w3.org/2006/01/wsdl/in-out">
       <wsdl:input element="CheckInventoryServiceRequest"/>
       <wsdl:output element="CheckInventoryServiceResponse"/>
    </wsdl:operation>
  </wsdl:interface>
</wsdl:description>

Listing 3.4-1: CheckInventoryService() WSDL excerpt with a variation where the input SKU is replaced by UPC for the purpose of illustrating Web service composition with ontology reasoning.

A WSDL excerpt of SKU2UPCLookupService() is shown below.

<wsdl:description
  targetNamespace="http://org3.example.com/wsdl/SKU2UPCLookupService/"
  .....
  xmlns:Org3Ont="http://org3.example.com/ontologies/SampleOntologyOrg3#>

  <wsdl:types>
    <xs:schema targetNamespace="http://org3.example.com/wsdl/SKU2UPCLookupService>
      <xs:element name="SKU2UPCLookupServiceRequest" type="SKU"/>
      <xs:simpleType name="SKU">
        sawsdl:modelReference="http://org3.example.com/ontologies/SampleOntologyOrg3#SKU">
        <xs:restriction base="xs:string"/>
      </xs:simpleType>

      <xs:element name="SKU2UPCLookupServiceResponse" type="UPC"/>
      <xs:simpleType name="UPC">
        sawsdl:modelReference="http://org3.example.com/ontologies/SampleOntologyOrg3#UPC">
        <xs:restriction base="xs:string"/>
      </xs:simpleType>
    </xs:schema>
  </wsdl:types>

  <wsdl:interface name="SKU2UPCLookupService">
    <wsdl:operation name="SKU2UPCLookupService" pattern="http://www.w3.org/2006/01/wsdl/in-out"  
        sawsdl:modelReference="http://org3.example.com/taxonomies/Utilities#LookupServices">
        <wsdl:input element="SKU2UPCLookupServiceRequest"/>
        <wsdl:output element="SKU2UPCLookupServiceResponse"/>
    </wsdl:operation>
  </wsdl:interface>
</wsdl:description>

Listing 3.4-2: A WSDL excerpt representing SKU2UPCLookup service.

@prefix rdfs: <http://www.w3.org/2000/01/rdf-schema#> .
@prefix rdf: <http://www.w3.org/1999/02/22-rdf-syntax-ns#> .
@prefix: <http://org1.example.com/ontologies/SampleOntologyOrg1#> .
@prefix owl: <http://www.w3.org/2002/07/owl#> .

  <http://org1.example.com/ontologies/SampleOntologyOrg1#> rdf:type owl:Ontology  .
  
  :PartNumber rdf:type owl:Class .
  :SKU rdf:type owl:Class;
    rdfs:subClassOf :PartNumber .
  :DueDate rdf:type owl:Class . 
  :Quantity rdf:type owl:Class . 
  :AvailabilityConfirmation rdf:type owl:Class .

@prefix rdfs: <http://www.w3.org/2000/01/rdf-schema#> .
@prefix rdf: <http://www.w3.org/1999/02/22-rdf-syntax-ns#> .
@prefix: <http://org2.example.com/ontologies/SampleOntologyOrg2#> .
@prefix owl: <http://www.w3.org/2002/07/owl#> .

  <http://org2.example.com/ontologies/SampleOntologyOrg2#> rdf:type owl:Ontology  .
 
  :UPC rdf:type owl:Class;
  :DeliveryDate rdf:type owl:Class .
  :NumBundles rdf:type owl:Class .
  :AvailabilityConfirmation rdf:type owl:Class .
  

Listing 3.4-5 represents the updated sample ontology that enables composition in this example. It relates the concepts in the ontologies shown in listings 3.4-3 and 3.4-4. Note that the ontology as such does not specify any relationship between SKU and UPC. Given an SKU, its corresponding UPC code is retrieved by the Web service SKU2UPCLookup(). Here also, we assume that the organization that is providing the SKU2UPCLookupService() is providing the mapping ontology as well. As mentioned earlier in section 3.3, this does not always have to be the case.

@prefix rdfs: <http://www.w3.org/2000/01/rdf-schema#> .
@prefix rdf: <http://www.w3.org/1999/02/22-rdf-syntax-ns#> .
@prefix: <http://org3.example.com/ontologies/MappingOntology#> .
@prefix Org1Ont: <http://org1.example.com/ontologies/SampleOntologyOrg1#> .
@prefix Org2Ont: <http://org2.example.com/ontologies/SampleOntologyOrg2#> .
@prefix Org3Ont: <http://org2.example.com/ontologies/SampleOntologyOrg3#> .
@prefix owl: <http://www.w3.org/2002/07/owl#> .

  <http://org3.example.com/ontologies/MappingOntology#> rdf:type owl:Ontology  .
  
  Org2Ont:NumBundles owl:equivalentClass Org1Ont:Quantity .
  Org2Ont:DeliveryDate owl:equivalentClass Org1Ont:DueDate .
  Org2Ont:AvailabilityConfirmation owl:equivalentClass Org1Ont:AvailabilityConfirmation .
 
  Org3Ont:SKU rdf:type owl:Class;
     owl:equivalentClass Org1Ont:SKU .    
  Org3Ont:UPC rdf:type owl:Class;
     owl:equivalentClass Org2Ont:UPC . 

Listing 3.4-5: Revised Mapping Ontology that contains relationships between concepts PartNumber, SKU, and UPC among other things that is used in this section to illustrate Web service composition using ontology reasoning.

Figure 3 represents an altered version of the ontology that enables composition in this example.

Figure 4 shows how Web service composition can be achieved by using the semantic annotations in the WSDL files discussed in this example. As can be noted, in this scenario, a semantic engine retrieves the semantic annotations from checkAvailabilityRequest() for element itemCode namely PartNumber and for the element SKU in SKU2UPCLookup() service namely SKU. The annotation PartNumber doesnot match lexically with the concept SKU. So, the semantic engine could then consult the MappingOntology to see if there is any relationship between PartNumber and SKU. The ontology reasoner can return the existance of 'subClassOf' relationship between SKU and PartNumber. This information can then be used in composing SKU2UPCLookup() with CheckInventoryService() as shown in Figure 4 to match the request CheckAvailabilityRequest(). Again, as mentioned earlier in Section 3.3, a matching engine would be able to find a utility service such as SKU2UPCLookup() in the appropriate category in a registry - in this case it would be in 'Utilities/LookupServices' category because the modelReference on the operation of this service contains the categorization information. So, presumably a publishing agent/program would have placed this service in the appropriate category that is referred to by the modelReference URI.

The example presented in this section was a variation of the one discussed in section 3.4. It illustrated how Web service composition can be achieved usng the relationships between concepts in an ontology even when the semantic annotations on the services don't match directly.

3.5 Matching Using Multiple Annotations

SAWSDL allows multiple annotations to be associated with those WSDL elements that can have a modelReference extension. These annotations could be pointing to different concepts from the same semantic model or from different models altogether. For example, a WSDL element with the name itemCode can be associated with SampleOntology#PartNumber and SampleOntology#SKU concepts. SAWSDL does not specify any relationship between these multiple annotations other than saying that they all apply. It is upto the consumers of these annotated WSDLs to use the ones that are relevant to them or to figure out the relationship between the concepts, if they so choose, by consulting the ontology that defines them. The following example in Listing 3.5-1 shows the annotation of itemCode with multiple annotations. If a requesting WSDL is annotated with multiple concepts like shown in listing 3.5-1, this increases the likelihood of matching this request with other advertisement/service WSDLs. Similarly, if a service WSDL is annotated with multiple concepts, possibly more request WSDLs will match it.

  ...
  <xs:simpleType name="itemCode"
    sawsdl:modelReference="http://org3.example.com/ontologies/SampleOntologyOrg3#PartNumber http://org3.example.com/ontologies/SampleOntologyOrg3#SKU"/>
    <xs:restriction base="xs:string"/>
  </xs:simpleType>
  ...

Listing 3.5-1: A WSDL excerpt showing multiple annotations to a simple type

The example given in this section so far illustrated multiple annotations that come from the same ontology. It is possible for elements to be annotated with concepts from different ontologies as well. The mechanism for doing so is exactly the same as the one employed in Listing 3.5-1 except in the latter case, the namespaces for the concepts coming from different ontolgies would be different.

3.6 Matching Using Complex Type Annotations

In the examples that we discussed so far, all annotations are added at the member element levels of a complex type. SAWSDL allows annotations to be added either at the complex type level (top level) or at the member element level (bottom level) or both. In cases where both complex type and the member elements have annotations, SAWSDL does not specify any relationship between the modelReferences on a complex type and those on the elements contained within a complex type. The listing below (Listing 3.6-1) illustrates the usage of modelReference for annotating the complex type 'ItemRequest' as well as the member elements.

<wsdl:types>
  ...
  <xs:element name="ItemRequest" sawsdl:modelReference="http://org3.example.com/ontologies/SampleOntologyOrg3#OrderItem">
    <xs:complexType>
      <xs:sequence>
        <xs:element name="itemCode" type="xs:string"/>
        <xs:element name="date" type="xs:string"/>
        <xs:element name="qty" type="xs:string"/>
       </xs:sequence>
    </xs:complexType>
  </xs:element>
  ...
</wsdl:types>

Listing 3.6-1: WSDL excerpt from CheckAvailabilityRequest() showing the annotation of a complexType.

Here is one possible way in which the annotation on a complex type can be used. The annotation on the complex type 'itemRequest' such as the one in 3.6-1 which is part of checkAvailabilityRequest() can be used to match another complex type such as the one in checkInventoryService() shown in Listing 3.6-2 below. The annotation on a complex type can be used a first level of matching in structure matching i.e., a semantic matching engine could use these annotations to see if two complex types have any relationship at all. If they do, perhaps spending more time on matching the member elements of complex types may be worth the time. If not, perhaps there is no need to do detailed matching of complex structures. Therefore, the annotations on complex types can be used for first level of structure matching.

 <wsdl:types>
  ...

    <xs:element name="Item" sawsdl:modelReference="http://org3.example.com/ontologies/SampleOntologyOrg3#OrderItem">
      <xs:complexType>
        <xs:sequence>
          <xs:element name="SKU" type="xsd:string"/>
          <xs:element name="deliveryDate" type="xs:string"/>
          <xs:element name="numBundles" type="xs:float"/>
        </xs:sequence>
      </xs:complexType>
    </xs:element>
   ...
 </wsdl:types>

Listing 3.6-2: WSDL excerpt from CheckInventoryService() showing the annotation of a compex type.

So far, we have discussed how to annotate WSDL documents with semantic concepts for use in Web service discovery, matching and composition.These annotations that can be added to a WSDL using modelReferences are meant to add semantic clarity to WSDL elements by pointing to concepts in a semantic model that describes the larger context. They help understand whether a service matches clients' requests at a semantic level. However, there may still be mismatches at the data level that would need to be addressed and captured to enable the invocation of a Web service. In the following section, we discuss the mechanisms defined in SAWSDL to capture such data transformation maps (knowns as schema mappings) to enable Web service invocation.

4. Defining Schema Mappings to Enable Web Service Invocation

Consider the example where a client/requester may have a 'first name' and 'last name' among its data as shown in listing 4-1 and the advertisement service requires a 'full name' as shown in listing 4-2. In this case, when the client invokes the Web service of the service provider, the data values of firstName and lastName need to be concatenated in the message to the advertisement Web service to pass the correct values for the fullName element.

..
  <xs:element name="firstName" type=xs:string/>
  <xs:element name="lastName" type=xs:string/>
..

Listing 4-1: An XML excerpt showing two elements firstName and lastName.

..
  <xs:element name="fullName" type=xs:string/>
 ..

Listing 4-2: An XML excerpt showing two elements fullName.

To facilitate the association of such types of data transformations with Web services, SAWSDL provides a mechanism called schemaMapping. A schemaMapping concept is provided to allow the specification of transformation functions on the WSDL elements to map instance data defined by that XML schema document to the semantic data of the concepts in a semantic model. Conversely, it allows the specification of transformation functions that map the semantic data of ontological concepts to the instance data values that adheres to the XML schema document that is being annotated. In the former case, the transformation functions are referred to using the extensibility feature liftingSchemaMapping and in the latter case it is called loweringSchemaMapping. These kinds of mappings are useful in general when the structure of the instance data does not correspond directly to the organization of the semantic data. Also, these types of mappings can be used to generate mediation code to support invocation of a Web service.

Consider the scenario where a client/requester is able to supply the parameters 'firstName', 'lastName' and 'middleName'. Say that the semantic model that the that the client would like to use to annotate it's parameters contains 'fullName' and 'middleName' concepts. Then the client can specify a lifting schema mapping to map it's 'firstName' and 'lastName' concepts with 'fullName' via concatenation. 'middleName' concepts would be mapped directly in this case since no transformation is required. Now, let's consider that a service provider has an advertisement service that expects a parameter 'completeName'. This 'completeName' is meant to be a string consisting of 'firstName lastName middleInitial'. Say that the servide provider is using the same semantic model as the one that the client is using for simplifying the discussion. Then, the service provider can provide a lowering schema mapping from the semantic data of the semantic model to create an XML instance for 'completeName'. This lowering schemaMapping would essentially concatenate the 'fullName' concept with the first letter of the concept 'middleName' to create an instance of 'completeName'. In this case, together the lifting and lowering schema mapping transformation functions can enable the client to invoke the service provider's service.

Editor's note: TODO: The remaining part of this section and the following SPARQL and XSLT examples will be revised to make it complete. 
In their current form, they do not work.

The following example in Listing 4-2 illustrates lifting schema mapping notion with XSLT.

<wsdl:description>
  ...
  <wsdl:types>
  <xs:schema
    targetNamespace="http://org1.example.com/wsdl/order#" elementFormDefault="qualified">

    <xs:element name="OrderRequest">
      <xs:complexType sawsdl:liftingSchemaMapping="http://org1.example.com/mapping/OrderRequest.xslt">
        <xs:sequence>
          <xs:element name="firstName" type="xs:integer" />
          <xs:element name="lastName" type="xs:integer" />
          <xs:element name="item" type="item" minOccurs="1" maxOccurs="unbounded" />
        </xs:sequence>
      </xs:complexType>
    </xs:element>

    <xs:complexType name="item">
      <xs:all>
        <xs:element name="itemCode" type="xs:string" />
        <xs:element name="quantity" type="xs:float"/>
        <xs:element name="dueDate" type="xs:string"/>
        <xs:element name="billingInfo" type="xsd1:POBilling"/>
      </xs:all>
    </xs:complexType>

    <xs:element name="OrderResponse" type="Confirmation"/>
    <xs:simpleType name="Confirmation">
      <xs:restriction base="xs:string">
        <xs:enumeration value="Confirmed"/>
        <xs:enumeration value="Pending"/>
        <xs:enumeration value="Rejected"/>
      </xs:restriction>
    </xs:simpleType>
  </xs:schema>
  </wsdl:types>

  <wsdl:interface name="Order">
    <wsdl:operation name="order" pattern="http://www.w3.org/2006/01/wsdl/in-out">
      <wsdl:input element="OrderRequest"/>
      <wsdl:output element="OrderResponse"/>
    </wsdl:operation>
  </wsdl:interface>
</wsdl:description>
  ...

Listing 4-2: A WSDL excerpt showing liftingSchemaMapping notion.

Listing 4-2 refers to OrderRequest.xslt. This XSLT maps the elements of OrderRequest complex type with the corresponding ones in the ontology. The part where an XSLT would be of most value in this example is where the values of firstName and lastName are concatenated to match with the semantic data of the concept 'FullName' in the SampleOntology. An excerpt of such a transformation in XSLT [XSLT] is shown Listing 4-3 below.

<xsl:transform version="2.0"
  xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
  xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
  xmlns:po="http://org1.example.com/wsdl/order#"
  xmlns:POOntology="http://org1.example.com/ontology/purchaseorder#">
  <xsl:output method="xml" version="1.0" encoding="iso-8859-1" indent="yes" />
  <xsl:variable name="firstName" select="wsdl:description/xs:element[@name='OrderRequest']/xs:complexType/xs:sequence/xs:element[@name='firstName']"/>
  <xsl:variable name="lastName" select="wsdl:description/xs:element[@name='OrderRequest']/xs:complexType/xs:sequence/xs:element[@name='lastName']"/>
  <xsl:variable name="fullName" select="concat($firstName,' ', $lastName)"/>
  <xsl:template match="/">
    <xsl:apply-templates/>
  <xsl:template>
</xsl:transform> 

Listing 4-3: An XSLT excerpt that concatenates the values of firstName and lastName elements to form the semantic data of the concept FullName in the ontology.

When specifying a liftingSchemaMapping, an XSLT transform can create a specific XML that can be treated as the representation of semantic data for an RDF/OWL ontology. While specifying loweringSchemaMapping, on the other hand, it may not be clear what format the semantic data of the ontology is expressed in (since there are many variations of RDF semantic data representation format as defined by XML schema). To overcome this, SAWSDL proposes the use of SPARQL language to query the semantic data of the ontology to obtain a specific XML format for the semantic data of the ontology so that it can then be used to formulate an XSLT transform to obtain an XML that could correspond with the instance of a Web service. A sample semantic data is shown in Listing 4-4 and a sample SPARQL query that could be used in loweringSchemaMapping is shown in Listing 4-5 below.

<!DOCTYPE rdf:RDF[
    <!ENTITY xsd  "http://www.w3.org/2001/XMLSchema#" >
]> <rdf:RDF
  xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
  xmlns:owl="http://www.w3.org/2002/07/owl#"
  xmlns="http://org1.example.com/ontology/purchaseorder#"
  xml:base="http://org1.example.com/ontology/purchaseorder#">

  <owl:Ontology />
  <OrderRequest>
    <hasLineItems>
      <LineItem>
        <hasPart>
          <part>
            <hasPartCode>
              <partNum>
                <hasLexicalRespresentation>348290187318</hasLexicalRespresentation>
              </partNum>
            </hasPartCode>
          </part>
        </hasPart>

        <hasDueDate>
          <date>
            <dueDate>
              <hasLexicalRespresentation>20060906</hasLexicalRespresentation>
            </dueDate>
          </date>
        </hasDueDate>
        <hasQuantity rdf:datatype="&xsd;float">12</hasQuantity>
        <hasQuantity rdf:billingInfo="&xsd1;POBilling">23 XYZ Road</hasQuantity>
      </LineItem>
    </hasLineItems>
    <hasCustomer>
      <Customer>
        <hasCustomerName>
          <Name>
            <hasLexicalRespresentation>ABC DEF</hasLexicalRespresentation>
          </Name>
        </hasCustomerName>
      </Customer>
    </hasCustomer>
  </OrderRequest>
</rdf:RDF> 

Listing 4-4: An excerpt of semantic data that adheres to the variation on SampleOntology.

<lowering>
  <sparqlQuery>
    PREFIX po: <http://org1.example.com/ontology/purchaseorder#>
    SELECT ?name ?partNum ?quantity ?dueDate ?billingInfo
    WHERE {
      ?order po:hasCustomer ?customer .
        ?customer po:hasCustomerName ?Name .
          ?Name po:hasLexicalRespresentation ?name .
      ?order po:hasLineItems ?item .
        ?item po:hasQuantity ?quantity .
        ?item po:hasBillingInfo ?billingInfo .
        ?item po:hasDueDate ?Date .
          ?date po:hasLexicalRepresentation ?dueDate .
        ?item po:hasPart ?part .
          ?part po:hasPartCode ?code .
            ?code po:hasLexicalRespresentation ?partNum }
  </sparqlQuery> 

Listing 4-5: A SPARQL query on the semantic data (of an ontology) to obtain an XML that could be used to specify XSLT transform in loweringSchemaMapping

When SPARQL [SPARQL] is applied to the RDF graph in Listing 4-4, the following XML will be generated as query answer.

<sparql xmlns="http://www.w3.org/2005/sparql-results#">
  <head>
    <variable name="quantity" />
    <variable name="partNum" />
    <variable name="name" />
    <variable name="dueDate" />
    <variable name="billingInfo" />
  </head>

  <results>
    <result>
      <binding name="quantity">
        <literal>12</literal>
      </binding>

      <binding name="partNum">
        <literal>348290187318</literal>
      </binding>

      <binding name="name">
        <literal>ABC DEF</literal>
      </binding>

      <binding name="billingInfo">
        <literal>23 XYZ Road</literal>
      </binding>

      <binding name="dueDate">
        <literal>20060925</literal>
      </binding>

    </result>
    </results>
</sparql>

Listing 4-6: Result of applying SPARQL from Listing 4-5 to RDF in Listing 4-4.

Once an XML is obtained from SPARQL query an XSLT transform can be created. Listing 4.7 shows such an XSLT transform.

 <xsl:transform version="2.0"
    xmlns:po="http://org1.example.com/wsdl/order#"
    xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
    xmlns:sp="http://www.w3.org/2005/sparql-results#">

    <xsl:output method="xml" version="1.0" encoding="iso-8859-1" indent="yes" />
    <xsl:template match="/sp:sparql">
      <po:OrderRequest>
        <po:Name>
          <xsl:value-of
            select="sp:results/sp:result[position()=1]/sp:binding[@name='name']/sp:literal" />
        </po:Name>

        <xsl:apply-templates select="sp:results/sp:result" />
      </po:OrderRequest>
    </xsl:template>
    <xsl:template match="sp:result">
      <po:orderItem>
        <xsl:attribute name="quantity">
          <xsl:value-of select="sp:binding[@name='quantity']/sp:literal" />
        </xsl:attribute>
        <po:partNum>
          <xsl:value-of select="sp:binding[@name='partNum']/sp:literal" />
        </po:partNum>
        <po:dueDate>
          <xsl:value-of select="sp:binding[@name='dueDate']/sp:literal" />
        </po:dueDate>
        <po:billingInfo>
          <xsl:value-of select="sp:binding[@name='billingInfo']/sp:literal" />
        </po:billingInfo>
      </po:orderItem>

    </xsl:template>
  </xsl:transform>
</lowering> 

Listing 4-7: An XSLT excerpt that shows loweringSchemaMapping notion.

 <ex:OrderRequest
  xmlns:POOntology="http://org1.example.org/ontologies/purchaseorder#"
  xmlns:sp="http://www.w3.org/2005/sparql-results#"
  xmlns:po="http://org1.example.com/wsdl/order#"
  xmlns:ex="http://org1.example.com/wsdl/order#"
  xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
  xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
  
  <ex:name>ABC DEF</ex:name>
  <ex:orderItem quantity="12">
    <ex:partNum>348290187318</ex:partNum>
    <ex:dueDate>20060925</ex:dueDate>
    <ex:billingInfo>23 XYZ Road</ex:dueDate>
  </ex:orderItem>
</ex:OrderRequest> 

Listing 4-8: Result of applying XSLT from Listing 4-7 to SPARQL result of Listing 4-6.

In summary, the lifting and lower schema mapping notions together enable the transformation of data between Web services which can be an important part of enabling the invocation of Web services.

5. Conclusions

The SAWSDL Usage Guide is an accompanying document to SAWSDL specification. This guide presented examples to illustrate how to associate semantic annotations to a Web service (represented in SAWSDL format) and how to use these annotations for classifying, discovering, matching, composing, and invoking Web services. Arguably, this usage guide presents only some of the ways in which the annotations that can be associated with a WSDL document can be used.

6. References

[NAICS]

North American Industry Classification System. Available at http://www.census.gov/epcd/www/naics.html.

[OWL]

OWL Web Ontology Language Reference, Mike Dean and Guus Schreiber, Editors. W3C Recommendation, 10 February 2004,http://www.w3.org/TR/2004/REC-owl-ref-20040210/.
Latest version available at http://www.w3.org/TR/owl-ref/.

[RDF]

Resource Description Framework, Dave Beckett, Editor. World Wide Web Consortium, 10th February 2004. This version is http://www.w3.org/TR/rdf-syntax-grammar/

[RDF Turtle]

Terse RDF Triple Language, Dave Beckett, Editor. World Wide Web Consortium, 4th April 2004. This version is http://www.dajobe.org/2004/01/turtle/

[SAWSDL]

Semantic Annotations for WSDL, Joel Farrell and Holger Lauson, Editors. World Wide Web Consortium, 28th Sept 2006. This version is http://www.w3.org/TR/2006/WD-sawsdl-20060928/.

[SPARQL]

SPARQL Query Language for RDF, Eric Prud'hommeaux and Andy Seaborne, Editors. World Wide Web Consortium, 6 April 2006. This version is http://www.w3.org/TR/2006/CR-rdf-sparql-query-20060406/. The latest version is available at http://www.w3.org/TR/rdf-sparql-query/.

[UDDI]

UDDI Version 3.0.2, Luc Clement, Andrew Hately, Claus von Riegen, and Tony Rogers, Editors. Organization for the Advancement of Structured Information Standards (OASIS). Available at http://uddi.org/pubs/uddi_v3.htm.

[WSDL 2.0]

Web Services Description Language (WSDL) Version 2.0 Part 1: Core Language, R. Chinnici, J-J. Moreau, A. Ryman, S. Weerawarana, Editors. World Wide Web Consortium, 27 March 2006. This version of the "Web Services Description Language (WSDL) Version 2.0 Part 1: Core Language" Specification is available is available at http://www.w3.org/TR/2006/CR-wsdl20-20060327. The latest version of "Web Services Description Language (WSDL) Version 2.0 Part 1: Core Language" is available at http://www.w3.org/TR/wsdl20.

[WSDL-S]

W3C Member Submission, Web Service Semantics - WSDL-S, Rama Akkiraju, Joel Farrell, John Miller, Meenakshi Nagarajan, Marc-Thomas Schmidt, Amit Sheth, and Kunal Verma, Authors. World Wide Web Consortium, 7 November 2005. Available at http://www.w3.org/Submission/WSDL-S/.

[XSLT]

XSL Transformations (XSLT) Version 2.0, Michael Kay, Editor. World Wide Web Consortium, 3 Nov 2005. This version is http://www.w3.org/TR/2005/CR-xslt20-20051103/. The latest version is available at http://www.w3.org/TR/xslt20/.

A. WSDL Files For the Examples

A version of the WSDL files and the ontology used in this document are given below. Please note that these WSDLs and the OWL files do not contain all the variations that have been discussed in the examples.

Editor's note: TODO: These examples will be made more complete.

Check Availability Request: Please see http://www.w3.org/TR/sawsdl-guide/wsdl/CheckAvailability-req-sem.wsdl
Check Inventory Service: Please see http://www.w3.org/TR/sawsdl-guide/wsdl/CheckInventory-sem.wsdl,
Sample Ontology: Please see http://www.w3.org/TR/sawsdl-guide/wsdl/ElectronicsOntology.owl

B. Acknowledgements

This document is the work of the W3C Semantic Annotations for Web Service Description Language Working Group.

Members of the Working Group are (at the time of writing, and by alphabetical order): Rama Akkiraju (IBM Corporation), Carine Bournez (W3C), J.B. Domingue (The Open University), Joel Farrell (IBM Corporation), Laura Ferrari (Telecom Italia SpA), Laurent Henocque (ILOG, S.A.), Mathias Kleiner (ILOG, S.A.), Jacek Kopecký (DERI Innsbruck at the Leopold-Franzens-Universität Innsbruck, Austria), Holger Lausen (DERI Innsbruck at the Leopold-Franzens-Universität Innsbruck, Austria), John Miller (University of Georgia Research Foundation, Inc (UGARF)) Carlos Pedrinaci (The Open University), Eric Prud'hommeaux (W3C), Brahmananda Sapkota (DERI Galway at the National University of Ireland, Galway, Ireland), Amit Sheth (Fortent), Claudio Venezia (Telecom Italia SpA), Tomas Vitvar (DERI Galway at the National University of Ireland, Galway, Ireland).

C. Change Log

Table B-1. Summary of Changes Related to Semantic Annotations for WSDL Specification — Use Cases and Examples

Date	Author	Description
20060508	BNS	Initial skeleton created from Rama's document
20060515	BNS	WSDL-S files attached to Sections 4.1, 5.1 and 5.2 sent by Rama Section 6 added from Rama's document
20060614	BNS, RA	Best Practices Section 7 added.
20060814	RA	Completely revamped and rewrote the document. The focus of the document is now on how to associate semantic annotations with a WSDL document for use in , composition and invocation NOT on how to do matching, composition and invocation using semantic annotations. Still some sections need some more work.
20060815	BNS	Some clean ups.
20060816	RA	Many more cleanups. Writeups for some more sections. Examples are much cleaner now and there is really only one running example that we keep building throughout the document. More Todos to be finished up still.
20060819	BNS	Added schemaMapping examples.
20060821	BNS	Added PartNumber2EANLookup SAWSDL examples.
20060823	RA	Many more cleanups and additions to the sections. Created more images and text throughout the document to explain things better. Added a better example in the schemaMapping section. This should be more amenable to creating schema mappings (in XSLT or RDF). The placement of images need some cleanup. Some how things are getting justified to be in the middle. Even if I change it to left justification, it doesn't seem to work. The formatting needs cleanup as well.
20060824	BNS	Clean up, and some additions in section 4.
20060831	RA	Some clean up. implemented comments from 2nd F2F meeting throughout the document.
20060909	BNS	Some clean up. implemented comments from 2nd F2F meeting, in section 3.
20060910	BNS	Converted OWL examples to RDF - Turtle format.
20060918	BNS	Implemented comments from Laura Added Section on Mapping Choices from main spec.
20060922	RA	Many more cleanups. Added new sections 1.2, 2.1, 3.1 and many more updates throughout the document .
20060923	BNS	Addition to Appendix, XSLT and SPARQL examples added.
20060924	RA	Major Revisions to sections 3 and 4. Added References.
20060925	RA	Updated SampleOntology. Commented out the Mapping Choices section. I don't think it belongs here in this document. It is in WSDL-S and we have a pointer to WSDL-S from here. That should suffice.
20060926	EGP	fixed the formatting of listings 3.2-3 and 4-3 per Jacek's comment # 6
20060926	RA	Addressed many of the issues in Jacek's feedback note
20060927	RA	Made significant revisions to section 3 and 4. Addressed more the remaining issues from Jacek's feedback note. Also addressed Joel's comments sent in a mail on Sept 25th 2006.

D. Editors' Todo List

Table C-1. Summary of Editors' Semantic Annotations for WSDL Specification — Use Cases and Examples Work Items

Item	Description
Item 3	To update SampleOntology so that it contains all the concepts and their relationships
Item 5	Need to clean up SPARQL and XSLT examples