W3C W3C Member Submission

WSMO-Lite: Lightweight Semantic Descriptions for Services on the Web

W3C Member Submission 23 August 2010

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Latest version:
Dieter Fensel, University of Innsbruck
Florian Fischer, University of Innsbruck
Jacek Kopecký, The Open University (formerly University of Innsbruck)
Reto Krummenacher, University of Innsbruck
Dave Lambert, The Open University
Tomas Vitvar, University of Innsbruck
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Copyright © 2010 by the Submitting Members. This document is available under the W3C Document License. See the W3C Intellectual Rights Notice and Legal Disclaimers for additional information.


In this document we define a lightweight set of semantic service descriptions in RDFS that can be used for annotations of various WSDL elements using the SAWSDL annotation mechanism. These annotations cover functional, behavioral, nonfunctional and information semantics of Web services, and are intended to support tasks such as (semi-)automatic discovery, negotiation, composition and invocation of services. We exploit RDF and RDFS as well as their various extensions such as OWL and RIF for semantic service descriptions.

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Table of Contents

1. Introduction

Existing service description languages, especially WSDL, allow service providers to describe their service offerings so that a client can make an up-front decision on whether and how to consume the service's functionality. Their uptake will lead to environments where thousands of services will have to be searched, integrated and mediated, and where automation will be the key enabler of service provisioning to end-users. In order to fulfill these challenges, existing service specifications need to be augmented with semantic descriptions [Studer et al., 2007]

In 2007, the W3C finished its work on Semantic Annotations for WSDL and XML Schema (SAWSDL). SAWSDL defines simple extensions for WSDL and XML Schema to link WSDL components with arbitrary semantic descriptions. It thus provides the building blocks for a bottom-up approach to semantic service modeling: it supports the piecemeal addition of semantics on top of WSDL, without prescribing a semantic framework, thereby allowing selected results from various existing approaches to be adopted in an incremental fashion. As the basis for bottom-up modeling, SAWSDL is independent of any particular semantic technology, i.e., it does not define any types, forms or languages for semantic descriptions.

In this document, we specify the WSMO-Lite service ontology as the next evolutionary step after SAWSDL, filling the SAWSDL annotations with concrete semantic service descriptions. With the ultimate goal to support real-world challenges in intelligent service integration, WSMO-Lite addresses the following requirements:

Even though we adopt the base Web service model from WSDL and SAWSDL, WSMO-Lite is inspired by the WSMO ontology, focusing on a small subset of it to define a limited extension of SAWSDL.

2. Web Service Descriptions

Our conceptual model for Web Service Descriptions adopts the following general types of service contracts (adapted from [Sheth, 2003]):

In the following sections, we show how the above general description types for service contracts are represented at the syntactic and the semantic level.

2.1 Syntactic Level

In regard to SOA technology developments today, the Semantic Service Stack represents service contracts at the non-semantic level using the existing standards: WSDL, SAWSDL, and related WS-* specifications. They all use XML as a common flexible data exchange format. Service contracts are represented as follows:

In addition, while SAWSDL does not fall into any of the service contract descriptions categories listed above, it is an essential part of the non-semantic level of the stack, providing the groundwork for the semantic layer. SAWSDL defines a simple extension layer that allows WSDL components to be annotated with semantics, using three extension attributes:

2.2 Semantic Level

With the WSMO-Lite service ontology we represent service contracts at the semantic level as follows (see Section 3 for a detailed description of WSMO-Lite):

In order to create or reuse domain-specific service ontologies, a service engineer can use any RDF-compliant language. This preserves the choice of language expressivity according to domain-specific requirements. Such languages may include RDF Schema (RDFS), Web Ontology Language (OWL), Rule Interchange Format (RIF) or the Web Service Modeling Language (WSML).

RDFS: On top of RDF, RDF Schema (RDFS) defines constructs that allow the expression of some semantics for the RDF model: RDFS allows the definition of classes describing the terminology of the domain of discourse, properties of those classes as well as class and property hierarchies (i.e. subClassOf and subPropertyOf). Thus, RDFS provides a set of constructs that allow the specification of lightweight ontologies.

OWL, WSML, RIF (on top of RDFS): Where the expressivity of RDFS is not sufficient for modeling of the required knowledge, various specializations of RDFS can be used. Such specializations are being developed both inside and outside of W3C along the lines of knowledge representation paradigms of Description Logic (DL) and Logic Programming (LP).

The Web Ontology Language (OWL) provides a vocabulary along with a formalism based on Description Logics. The Web Service Modeling Language (WSML) defines several variants allowing for both paradigms of Description Logics (WSML-DL) and Logic Programming (WSML-Flight, WSML-Rule). All WSML variants can be represented using RDF syntax and they are layered on top of RDFS. While WSML-DL has a direct mapping to OWL, WSML-Rule is the basis of the Web Rule Language (WRL) specification which served as one input to the W3C's Rule Interchange Format recommendations.

SKOS: as a lightweight ontology for simple knowledge organization systems, SKOS is well suited for capturing classifications of service and operation functionalities. As presented in this submission (in the section below), WSMO-Lite does not directly support SKOS functional classifications, but it would be straightforward to add such support, and the exact specification in the context of WSMO-Lite is part of our planned future work.

3. WSMO-Lite Service Ontology

Listing 1 shows the WSMO-Lite service ontology in RDFS, serialized in Notation 3. Below, we explain the semantics of the WSMO-Lite elements.

Listing 1. WSMO-Lite Service Ontology

1  @prefix rdfs: <http://www.w3.org/2000/01/rdf-schema#> .
2  @prefix rdf:  <http://www.w3.org/1999/02/22-rdf-syntax-ns#> .
3  @prefix wsl:  <http://www.wsmo.org/ns/wsmo-lite#> .
5  wsl:FunctionalClassificationRoot rdfs:subClassOf rdfs:Class .
6  wsl:NonfunctionalParameter  a  rdfs:Class .
7  wsl:Condition  a  rdfs:Class .
8  wsl:Effect     a  rdfs:Class .

WSMO-Lite defines behavioral descriptions through functional annotations of operations. While it does not have a special construct for choreography descriptions, the behavioral aspects can be inferred from functional (capability) annotations of individual service operations. Such annotations can be transformed into a WSMO Choreography, using the algorithm described in [Vitvar et al., 2008].

4. WSMO-Lite Annotations for WSDL

In this section, we define the particular types of annotations supported by WSMO-Lite. For this purpose we define two types of annotations, namely reference annotations and transformation annotations. A reference annotation points from a WSDL component (XML Schema element declaration or type definition, WSDL interface, operation, service) to a WSMO-Lite semantic concept. SAWSDL represents this type of annotation using the modelReference extension attribute. A transformation annotation specifies a data transformation called lifting from a component of XML schema to an element of ontology, and a reverse transformation (from ontology to XML) called lowering. SAWSDL represents this annotation using extension attributes liftingSchemaMapping and loweringSchemaMapping respectively. The actual expression of concrete lifting or lowering transformations is out of scope of this specification. Non-normatively, we expect WSMO-Lite to work well with transformation technologies such as XSLT, XQuery, and especially XSPARQL.

Listing 2 depicts an example ontology which we use to show various annotations.

Listing 2. Example of domain-specific service ontology

 1  # namespaces and prefixes
 2  @prefix rdfs: <http://www.w3.org/2000/01/rdf-schema#> .
 3  @prefix rdf:  <http://www.w3.org/1999/02/22-rdf-syntax-ns#> .
 4  @prefix wsl:  <http://www.wsmo.org/ns/wsmo-lite#> .
 5  @prefix ex:   <http://example.org/onto#> .
 6  @prefix xs:   <http://www.w3.org/2001/XMLSchema#> .
 8  # domain ontology
 9  ex:Customer  a  rdfs:Class .
10  ex:hasService  a  rdf:Property ;
11      rdfs:domain ex:Customer ; rdfs:range ex:Service .
12  ex:Service  a  rdfs:Class .
13  ex:hasConnection  a  rdf:Property ;
14      rdfs:domain ex:Customer ; rdfs:range ex:NetworkConnection .
15  ex:NetworkConnection  a  rdfs:Class .
16  ex:providesBandwidth  a  rdf:Property ;
17      rdfs:domain ex:NetworkConnection ; rdfs:range xs:integer .
18  ex:VideoOnDemandService rdfs:subClassOf ex:Service .
20  # capability description example
21  ex:VideoOnDemandSubscriptionPrecondition  a  wsl:Condition ;
22      rdf:value
23          "Prefix (ex <http://example.org/onto#>)
24           Prefix (pred <http://www.w3.org/2007/rif-builtin-predicate#>)
25           And (?Customer#ex:Customer
26                ?Customer[ex:hasConnection->?Connection]
27                ?Connection#ex:NetworkConnection
28                ?Connection[ex:providesBandwith->?Y]
29                External(pred:numeric-greater-than(?Y 1000)))" .
30  ex:VideoOnDemandSubscriptionEffect  a  wsl:Effect ;
31      rdf:value
32          "Prefix (ex <http://example.org/onto#>)
33           Prefix (pred <http://www.w3.org/2007/rif-builtin-predicate#>)
34           And (?Customer#ex:Customer
35                ?Customer[ex:hasService->?Service]
36                ?Service#ex:VideoOnDemandSubscription)" .
37  # nonfunctional property example
38  ex:PriceSpecification rdfs:subClassOf wsl:NonfunctionalParameter .
39  ex:VideoOnDemandPrice  a  ex:PriceSpecification ;
40      ex:pricePerChange "30"^^ex:euroAmount ;
41      ex:installationPrice "49"^^ex:euroAmount .
43  # classification example
44  ex:SubscriptionService  a  wsl:FunctionalClassificationRoot .
45  ex:VideoSubscriptionService rdfs:subClassOf ex:SubscriptionService .
46  ex:NewsSubscriptionService rdfs:subClassOf ex:SubscriptionService .

The listing defines a simple ontology for a telecommunication service (lines 9-18); the capability for a concrete Video-on-Demand subscription service (lines 21-36) for which the condition says that the customer must have a network connection with some minimal bandwidth and the effect says that the customer will be subscribed to the service; a nonfunctional property describing the pricing (lines 38-41); and a simple functionality classification with three categories (lines 44-46). In this example, the condition and effect are specified in the W3C's Rule Interchange Format. For clarity and brevity, the example uses RIF's presentation syntax, but in practice RIF rules would be given in the XML serialization syntax.

Figure 1. Illustration of WSMO-Lite Annotations
Illustration of
WSMO-Lite Annotations

Figure 1 illustrates the positioning of WSMO-Lite annotations (marked A1...A5), which are defined as follows:

Annotation A1: Ontological annotations of XML Schema. The schema used in WSDL to describe messages, i.e., the element declarations and type definitions (and in WSDL 1.1, messages and their parts), can carry reference annotations linking to classes from the service information model ontology.

Annotation A2: Transformation annotations of XML Schema. To be able to communicate with a service, the client needs to transform data between its semantic model and the service-specific XML message structures. The schema may contain transformation annotations (lifting or lowering) which specify the appropriate mappings.

The following listing shows an example of annotations A1 and A2 (the content of the lowering transformation is omitted for brevity).

Listing 3. Example of annotations A1 and A2

 1  <xs:element name="NetworkConnection" type="NetworkConnectionType"
 2     sawsdl:modelReference="http://example.org/onto#NetworkConnection"
 3     sawsdl:loweringSchemaMapping="http://example.org/NetCn.xslt"/>

Annotation A3: Functional annotations of WSDL Interface and Service. Functional descriptions (both capabilities and categories) apply both to concrete Web services and to the reusable and abstract interfaces. A reference annotation points from a service or an interface to its appropriate functional description.

The following listing shows an example of multiple A3 annotations.

Listing 4. Example of annotation A3

 1  <wsdl:interface name="NetworkSubscription"
 2     sawsdl:modelReference="http://example.org/onto#VideoSubscriptionService
 3                            http://example.org/onto#VideoOnDemandSubscriptionPrecondition
 4                            http://example.org/onto#VideoOnDemandSubscriptionEffect">
 5      <wsdl:operation name="CheckNetworkConnection" ... />
 6  </wsdl:interface>

Note that a WSDL interface may be shared by multiple services, therefore the functional description of the interface should be general. A concrete functional description attached to the service then refines the functional description of the interface. Additionally, aggregate interfaces or services (i.e., those that combine multiple potentially independent functionalities) may be annotated with multiple functional descriptions.

Annotation A4: Functional annotations of WSDL Interface operations. Functional descriptions (both capabilities and categories) apply also to interface operations, to indicate their particular functionalities. A reference annotation points from an operation to its appropriate functional description.

Functional annotation of interface operations can be used for services whose functionality has different, separate sub-parts. For example, a network subscription service may offer operations for subscription to a bundle, cancelation of a subscription, or price inquiry. A client will generally only want to use one or two of these operations, not all three. Service invocation will therefore need to select only the operations applicable to the current goal, and invoke them in the correct order, which is also partially implied by the functional operation annotations.

Please note that annotations A3 and A4 apply to both types of functional descriptions, i.e., a capability made up of logical conditions, or a category from some functional classification. It is even possible to combine capabilities and classification together. However, WSMO-Lite does not define any formal relationship between capabilities and classifications on the same component, or between annotations A3 on a service or its interface, and A4 on the operations of that interface.

Annotation A5: Nonfunctional annotations of WSDL Service. Nonfunctional descriptions apply to a concrete instance of a Web service, that is, a WSDL Service. A reference annotation can point from a service component to a nonfunctional property. The following listing (Listing 5) shows an example of annotation A5, attaching pricing information to a Web service on line 3.

Listing 5. Example of annotation A5

 1  <wsdl:service name="ExampleCommLtd"
 2      interface="NetworkSubscription"
 3      sawsdl:modelReference="http://example.org/onto#VideoOnDemandPrice">
 4    <wsdl:endpoint ...  />
 5  </wsdl:service>

Nonfunctional descriptions are always specific to a concrete service, therefore nonfunctional properties should not be expressed on a WSDL interface or on interface operations. In case nonfunctional properties need to be specified on the operations of a service, WSDL binding operation components (which mirror the operations of some interface) may be used to capture these properties.

5. Notes On Related Work

The major stream of related work is in the frameworks for Semantic Web services, including WSMO, Semantic Markup for Web Services (OWL-S) and Web Service Semantics (WSDL-S). Recently there has been increased attention given to RESTful services and Web APIs where, in particular, the work on hRESTS and MicroWSMO also connects to WSMO-Lite.

WSMO is a top-down conceptual model for SWS that defines four top-level components: ontologies, mediators, goals and web services. As we already mentioned, WSMO was the major inspiration for WSMO-Lite. WSMO-Lite was created to address the need for a lightweight service ontology that directly builds on the newest W3C standards and allows bottom-up modeling of services. On the other hand, WSMO is an established framework for Semantic Web Services representing a top-down model identifying semantics, useful in a semantics-first environment. WSMO-Lite adapts the WSMO model and makes its semantics lighter in the following major aspects:

Earlier, OWL-S was the first major ontology for SWS; it defined three interlinked ontologies: Service Profile (for the functional and nonfunctional descriptions), Service Model (for the behavioral descriptions), and Service Grounding (for physical Web service access). There was also some work published on OWL-S grounding that uses SAWSDL [Martin et al., 2007] [Paolucci et al., 2007]. In comparison with that work, WSMO-Lite takes the additional step of simplifying the annotations into a lightweight ontology.

WSDL-S was created in the METEOR-S project as a specification of how WSDL can be annotated with semantic information. WSDL-S itself does not provide a concrete model for SWS, instead it assumes the concrete model will be expressible as annotations in WSDL and XML Schema documents. The core parts of WSDL-S were taken as the basis for SAWSDL. WSDL-S also included WSDL extensions for attaching preconditions, effects, and categories, but these were out of scope for SAWSDL, and can be moved out to ontologies and attached through model references, as we do in WSMO-Lite.

Finally, hRESTS and MicroWSMO aim to enrich the informal descriptions of RESTful services, usually available in HTML, with microformat or RDFa annotations [Kopecký et al., 2008]. In effect, hRESTS forms an analogue of WSDL for RESTful services, and MicroWSMO is analogous to SAWSDL. The WSMO-Lite service semantics ontology is directly applicable in MicroWSMO and hRESTS annotations.


The work on WSMO-Lite has been mainly supported by the European Union project SOA4All. The editors would like to thank to all the members of the Conceptual Models for Services Working Group (CMS WG) for their advice and input to this document.


[Martin et al., 2007] D. Martin, M. Paolucci, and M. Wagner: Bringing Semantic Annotations to Web Services: OWL-S from the SAWSDL Perspective, in Proc. of 6th International Semantic Web Conference (ISWC/ASWC), pp. 340-352, 2007.

[Kopecký et al., 2008] J. Kopecký, K. Gomadam, and T. Vitvar: hRESTS: An HTML Microformat for Describing RESTful Web Services, in Proc. of IEEE/WIC/ACM International Conference on Web Intelligence and Intelligent Agent Technology (WI-IAT), pp. 619-625, 2008.

[Paolucci et al., 2007] M. Paolucci, M. Wagner, and D. Martin: Grounding OWL-S in SAWSDL, in Proc. of 5th International Conference on Service-Oriented Computing (ICSOC), pp. 416-421, 2007.

[Sheth, 2003] A.P. Sheth: Semantic Web Process Lifecycle: Role of Semantics in Annotation, Discovery, Composition and Orchestration, Invited Talk at Workshop on E-Services and the Semantic Web, at WWW 2003 (Presentation).

[Studer et al., 2007] R. Studer, S. Grimm, and A. Abecker: Semantic Web Services: Concepts, Technologies, and Applications, Springer-Verlag New York, Inc. Secaucus, NJ, USA. 2007.

[Vitvar et al., 2008] T. Vitvar, J. Kopecký, J. Viskova, and D. Fensel, WSMO-Lite Annotations for Web Services, in Proc. of 5th European Semantic Web Conference (ESWC), pp. 674-689, 2008.