W3C W3C Member Submission

Semantic Web Services Framework (SWSF) Overview

W3C Member Submission 9 September 2005

This version:
http://www.w3.org/submissions/2005/SUBM-SWSF-20050909/
Latest version:
http://www.w3.org/submissions/SWSF/
Authors:
Steve Battle (Hewlett Packard)
Abraham Bernstein (University of Zurich)
Harold Boley (National Research Council of Canada)
Benjamin Grosof (Massachusetts Institute of Technology)
Michael Gruninger (NIST)
Richard Hull (Bell Labs Research, Lucent Technologies)
Michael Kifer (State University of New York at Stony Brook)
David Martin (SRI International)
Sheila McIlraith (University of Toronto)
Deborah McGuinness (Stanford University)
Jianwen Su (University of California, Santa Barbara)
Said Tabet (The RuleML Initiative)

Abstract

This document provides an overview of the Semantic Web Services Framework (SWSF), which includes the Semantic Web Services Language (SWSL) and the Semantic Web Services Ontology (SWSO).

Status of this document

This document is part of a member submission, offered by National Institute of Standards and Technology (NIST), National Research Council of Canada, SRI International, Stanford University, Toshiba Corporation, and University of Southampton on behalf of themselves and the authors.

This is one of four documents that make up the submission. These documents define the Semantic Web Services Framework (SWSF). This submission has been prepared by the Semantic Web Services Language Committee of the Semantic Web Services Initiative.

The W3C Team Comment discusses this submission in the context of W3C activities. Public comment on this document is invited on the mailing list public-sws-ig@w3.org (public archive). Announcements and current information may also be available on the SWSL Committee Web site.

By publishing this document, W3C acknowledges that National Institute of Standards and Technology (NIST), National Research Council of Canada, SRI International, Stanford University, Toshiba Corporation, and University of Southampton have made a formal submission to W3C for discussion. Publication of this document by W3C indicates no endorsement of its content by W3C, nor that W3C has, is, or will be allocating any resources to the issues addressed by it. This document is not the product of a chartered W3C group, but is published as potential input to the W3C Process. Publication of acknowledged Member Submissions at the W3C site is one of the benefits of W3C Membership. Please consult the requirements associated with Member Submissions of section 3.3 of the W3C Patent Policy. Please consult the complete list of acknowledged W3C Member Submissions.


Table of contents

1 Introduction to SWSL and SWSO
2 Document Roadmap
3 Acknowledgements
4 Glossary
5 References


1 Introduction to SWSL and SWSO

The promise of Web services and the need for widely accepted standards enabling them are widely recognized, and considerable efforts are underway to define and evolve such standards in the commercial realm. In particular, the Web Services Description Language (WSDL) [WSDL 1.1] is already well established as an essential building block in the evolving stack of Web service technologies, and is being standardized in the W3C's Web Services Description Working Group. WSDL, in essence, allows for the specification of the syntax of the input and output messages of a basic service, as well as other details needed for the invocation of the service. WSDL does not, however, support the specification of workflows composed of basic services. In this area, the Business Process Execution Language for Web Services (BPEL4WS) [BPEL 1.1], under development at OASIS, has the most prominent status. The Choreography Description Language under development by W3C's Web Services Choreography Working Group, serves to "define from a global viewpoint ... the information exchanges that occur and the jointly agreed ordering rules that need to be satisfied" in carrying out a Web service-based transaction [WS-Choreography]. With respect to registering Web services for purposes of advertising and discovery, Universal Description, Discovery and Integration (UDDI) [UDDI v3.02] has received the most attention to date. Standards are also being developed in connection with various other aspects of Web service provisioning, such as reliable messaging, security, and resource management.

At the same time, recognition is growing of the need for richer semantic specifications of Web services, based on a compressive representational framework that spans the full range of service-related concepts. Such a framework will enable fuller, more flexible automation of service provision and use, support the construction of more powerful tools and methodologies, and promote the use of semantically well-founded reasoning about services. Because an expressive representation framework permits the specification of many different aspects of services, it can provide a foundation for a broad range of activities, across the Web service lifecycle. For example, richer semantics can support greater automation of service selection and invocation; automated translation of message content between heterogeneous interoperating services; automated or semi-automated approaches to service composition; more comprehensive approaches to service monitoring and recovery from failure; and fuller automation of verification, simulation, configuration, supply chain management, contracting, and negotiation for services. The technologies presented in this document are designed to realize this Semantic Web Service vision [McIlraith01].

This report presents two major components:

More specifically, SWSL is a general-purpose logical language, with certain features to make it usable with the basic languages and infrastructure of the Web. These features include URIs, integration of XML built-in types, and XML-compatible namespace and import mechanisms. SWSL includes two layers of expressiveness: SWSL-FOL and SWSL-Rules. SWSL-FOL is a first-order logic, extended with features from HiLog [Chen93] and the frame syntax of F-logic [Kifer95]. SWSL-Rules is a full-featured logic programming (LP) language, which includes a novel combination of features from Courteous logic programs [Grosof99a], HiLog, and F-logic.

Nearly all the elements of the syntax are common to both SWSL-FOL and SWSL-Rules, so as to promote the ability of developers to easily work with both layers, and to facilitate various kinds of interchange and interoperation between the layers. The presentation syntax is designed for readability, and incorporates a number of convenience features, such as an object-oriented style of presentation, which can be used to improve code organization and comprehensibility, but without changing the expressiveness and tractability of the underlying logical systems. An XML-based serialization syntax, based on RuleML, is also specified.

FLOWS is an axiomatized ontology of service concepts, which provides the conceptual framework for describing and reasoning about services. FLOWS draws many of its intuitions and lessons-learned from OWL-S, the OWL ontology for Web services [OWL-S 1.1]. A key contribution of the FLOWS ontology is the development of a rich behavioural process model, based on ISO 18629 Process Specification Language (PSL) [Gruninger03a], [Gruninger03b]. Originally designed to support interoperability among process modeling languages, PSL provides the ideal foundation for interoperability among emerging Web service process models, while supporting the realization of automation task associated with the Semantic Web Service vision. FLOWS goes beyond PSL in modeling many Web-service specific process concepts including messages, channels, inputs and outputs.

FLOWS is designed modularly. It comprises an ontology for service descriptors, somewhat akin to a domain-independent yellow-pages or OWL-S service profile, an extensive process model ontology, and a grounding that relates the process model message types to WSDL messages. The process model ontology is in turn comprised of a core ontology and a number of extensions.

The statements above about FLOWS are also true of ROWS, which is derived from FLOWS.

In addition to presenting SWSL and SWSO, this document also provides some guidance as to how SWSL-FOL and SWSL-Rules can be used together, examples of applications of SWSL and SWSO, and a discussion of how FLOWS-based service descriptions can be grounded in the concrete descriptions of messages and protocols provided by WSDL.

This proposal has been prepared by the Semantic Web Services Language Committee of the Semantic Web Services Initiative, a collaborative international research effort. In addition to providing further evolution of the SWSL language and ontology, SWSI will also be a forum for working towards convergence of SWSL with the products of the WSMO research effort [Bruijn05].

The overall design of the SWSL Language and Ontology, and of the application scenarios using those, has been motivated by an overall longer-term vision and set of objectives for Semantic Web services. These are summarized in a requirements document, which is available on the SWSL Committee's Web site [SWSL Requirements]. That document expressed our thinking as we set out to design the SWSF Languge and Ontology. The current design of SWSF addresses, directly or indirectly, most (but not all) of the points in the requirements document.

OWL-S: OWL-S [OWL-S 1.1] is an ontology of service concepts expressed in OWL-DL, a decidable description logic language. In contrast, FLOWS is an ontology of service concepts expressed in first-order logic. Since OWL-DL trades off expressiveness for decidability, there were aspects of the OWL-S process model ontology whose semantics could not be defined in the OWL-DL language. This is not a problem with the FLOWS ontology. FLOWS provides a more comprehensive ontology, building on the conceptual model and lessons-learned from OWL-S. As such it captures all concepts in OWL-S. In terms of coverage it is distinguished by its axiomatization of messages, something that was not addressed in OWL-S. An important final distinction between OWL-S and FLOWS is with respect to the role it plays. Whereas both endeavours attempt to provide an ontology for Web services, FLOWS had the additional objective of acting as a focal point for interoperability, enabling other business process modeling languages to be expressed or related to FLOWS.

Process Specification Language (PSL) and related specifications: There are a number of efforts whose aim is to develop a process specification ontology. These efforts include PSL [Gruninger03a], BPMI [BPML 1.0], and others. The process ontology of SWSL is based on PSL. From the perspective of PSL, FLOWS may be viewed as a collection of extensions that situate the description of processes within a larger context of message-based communications across networks.

BPEL4WS: There is much to be said about the relationship between this effort and BPEL4WS [BPEL 1.1]. BPEL4WS provides an executable business process modeling language that enables the specification (orchestration) of executable business processes as well as the description of non-executable processes. The most obvious point of comparison between BPEL4WS and this effort is with respect to the FLOWS ontology. The FLOWS ontology is a great deal more than process modeling, supporting the axiomatization of non-functional service descriptions (for service discovery) as well as the rich mapping of message types to WSDL. We believe that the FLOWS process model subsumes the BPEL4WS process model, enabling the encoding of both executable and abstract BPEL process models in FLOWS. (Note that we have not yet confirmed this through a systematic translation.) Whereas BPEL takes a message-centric view of services, FLOWS provides for a process-centric view of services, where message exchanges are conceived as additional processes that may be inferred automatically. A notable difference between the two formalisms is that FLOWS supports the encoding of service side-effects, i.e., the effects of services on the world, which enables automated composition of Web services, in additon to the manual composition supported by BPEL. Finally, BPEL is indeed designed for Web service orchestration, whereas FLOWS may also be used for choreography of services.

Web Services Modeling Ontology (WSMO): WSMO [Bruijn05] is a parallel effort to define an ontology and a language for Semantic Web services. Like SWSL-Rules, WSMO's rule language, WSML is largely based on F-logic and these languages share much of the logical expression syntax. Nevertheless, the two groups have pursued complimentary goals. WSMO has focused heavily on the language effort, and in particular on end user issues, associated with the language. In particular, they have developed a "conceptual syntax" for top-level descriptions of services, which might make the specifications easier to read for the end user. WSMO has also paid special attention to the issue of OWL compatibility. To this end, it defined WSML-Core as a subset of both OWL and WSML, which serves as a common ground for ontology interoperability. In contrast, SWSL's focus was on extending the functionality of the rule language. In particular, SWSL-Rules supports meta-reasoning with its HiLog and reification extensions. It also supports prioritized defaults and classical negation by incorporating Courteous Logic Programming.

A major distinction between the WSMO effort and the effort presented here is with respect to the ontology. The two efforts are divergent, but complementary. WSMO has focussed on describing Web service choreography through guarded transition rules, Event-Condition-Action (ECA) rules which are viewed as abstract state machines. In contrast, FLOWS provides an extensive first-order process ontology, which enables description of process orchestration as well as message exchange among processes. The FLOWS ontology also provides the foundation for enabling automated simulation, verification and composition of Web services, something that cannot be done with the guarded transition rules, without further definition of the semantics.


2 Document Roadmap

This report comprises four top-level documents:

  1. Semantic Web Services Framework Overview, this document, includes introductory material, comments about selected other work that is most directly related to this work, and acknowledgements.
  2. The Semantic Web Services Language (SWSL) describes the syntactic elements of SWSL and, informally, its semantic underpinnings. It also explains how SWSL-FOL and SWSL-Rules can be used together, and presents an XML serialization syntax for SWSL, based on RuleML.
  3. The Semantic Web Services Ontology (SWSO) presents FLOWS, a first-order ontology for Web services, expressed in SWSL-FOL, and its partial translation into ROWS, expressed in SWSL-Rules. SWSO also includes material about grounding FLOWS/ROWS process models with WSDL.

    The SWSO document includes four appendices:

    A PSL in SWSL-FOL and SWSL-Rules
    B Axiomatization of the FLOWS Process Model
    C Axiomatization of the Process Model in SWSL-Rules
    D Reference Grammars

  4. SWSF Application Scenarios gives examples that illustrate possible uses of the ontology and the language.

So as to be more-or-less self-contained, each of these four documents includes a glossary and references. For simplicity, these are the same in each document.

For pedagogical reasons, it makes sense to introduce SWSL before SWSO, since SWSL is used in expressing SWSO. However, the SWSO document relies primarily on simple uses of SWSL-FOL. Hence, those readers who are already familiar with first-order logic may want to begin with the SWSO document, which contains the Web services-specific aspects of this work. A suggested order of reading for such readers is: the Overview document (this document); the Ontology document; the first two sections of the Applications document; the Language document; the three remaining sections of the Applications document.


3 Acknowledgements

We are grateful for the vision and support of Murray Burke and Dieter Fensel, who first envisioned the collaboration that became SWSI, and were essential in getting it started. Katia Sycara, in her role as SWSI co-executive (with Dieter) has been active and supportive throughout the efforts of the committee, and has given valuable guidance in many areas.

The participation of some members of SWSL was funded by the DARPA DAML Program. We very much appreciate the ongoing support, interest, and patience of DAML program managers Jim Hendler, Murray Burke, and Mark Greaves.

We are also grateful to Carine Bournez and Sandro Hawke, of the World Wide Web Consortium, for guidance in the preparation of this submission. We thank the W3C for permitting and supporting the use of the public-sws-ig mailing list for discussion of SWSL issues, and Carine Bournez for helping to make this possible.

We would like to give special thanks to the following individuals who have made significant contributions to this work:

We are also greatly appreciative of the efforts of the following individuals, who, as reviewers, provided valuable feedback on short notice:

This document has benefited from input from members of the Semantic Web Services Architecture (SWSA) committee.


4 Glossary

Activity
Activity. In the formal PSL ontology, the notion of activity is a basic construct, which corresponds intuitively to a kind of (manufacturing or processing) activity. In PSL, an activity may have associated occurrences, which correspond intuitively to individual instances or executions of the activity. (We note that in PSL an activity is not a class or type with occurrences as members; rather, an activity is an object, and occurrences are related to this object by the binary predicate occurrence_of.) The occurrences of an activity may impact fluents (which provide an abstract representation of the "real world"). In FLOWS, with each service there is an associated activity (called the "service activity" of that service). The service activity may specify aspects of the internal process flow of the service, and also aspects of the messaging interface of that service to other services.
Channel
Channel. In FLOWS, a channel is a formal conceptual object, which corresponds intuitively to a repository and conduit for messages. The FLOWS notion of channel is quite primitive, and under various restrictions can be used to model the form of channel or message-passing as found in web services standards, including WSDL, BPEL, WS-Choreography, WSMO, and also as found in several research investigations, including process algrebras.
FLOWS
First-order Logic Ontology for Web Services. FLOWS, also known as SWSO-FOL, is the first-order logic version of the Semantic Web Services Ontology. FLOWS is an extension of the PSL-OuterCore ontology, to incorporate the fundamental aspects of (web and other electronic) services, including service descriptors, the service activity, and the service grounding.
Fluent
Fluent. In FLOWS, following PSL and the situation calculii, a fluent is a first-order logic term or predicate whose value may vary over time. In a first-order model of a FLOWS theory, this being a model of PSL-OuterCore, time is represented as a discrete linear sequence of timees, and fluents has a value for each time in this sequence.
Grounding
Grounding. The SWSO concepts for describing service activities, and the instantiations of these concepts that describe a particular service activity, are abstract specifications, in the sense that they do not specify the details of particular message formats, transport protocols, and network addresses by which a Web service is accessed. The role of the grounding is to provide these more concrete details. A substantial portion of the grounding can be acheived by mapping SWSO concepts into corresponding WSDL constructs. (Additional grounding, e.g., of some process-related aspects of SWSO, might be acheived using other standards, such as BPEL.)
Message
Message. In FLOWS, a message is a formal conceptual object, which corresponds intuitively to a single message that is created by a service occurrence, and read by zero or more service occurrences. The FLOWS notion of message is quite primitive, and under various restrictions can be used to model the form of messages as found in web services standards, including WSDL (1.0 and 2.0), BPEL, WS-Choreography, WSMO, and also as found in several research investigations. A message has a payload, which corresponds intuitively to the body or contents of the message. In FLOWS emphasis is placed on the knowledge that is gained by a service occurrence when reading a message with a given payload (and the knowledge needed to create that message.
Occurrence
Occurence (of a service). In FLOWS, a service S has an associated FLOWS activity A (which generalizes the notion of PSL activity). An occurrence of S is formally a PSL occurrence of the activity A. Intuitively, this occurrence corresponds to an instance or execution (from start to finish) of the activity A, i.e., of the process associated with service S. As in PSL, an occurrence has a starting time time and an ending time.
PSL
Process Specification Language. The Process Specification Language (PSL) is a formally axiomatized ontology [Gruninger03a, Gruninger03b] that has been standardized as ISO 18629. PSL provides a layered, extensible ontology for specifying properties of processes. The most basic PSL constructs are embodied in PSL-Core; and PSL-OuterCore incorporates several extensions of PSL-Core that includes several useful constructs. (An overview of concepts in PSL that are relevant to FLOWS is given in Section 6 of the Semantic Web Services Ontology document.)
QName
Qualified name. A pair (URI, local-name). The URI represents a namespace and local-name represents a name used in an XML document, such as a tag name or an attribute name. In XML, QNames are syntactically represented as prefix:local-name, where prefix is a macro that expands into a concrete URI. See Namespaces in XML for more details.
ROWS
Rules Ontology for Web Services. ROWS, also known as SWSO-Rules, is the rules-based version of the Semantic Web Services Ontology. ROWS is created by a relatively straight-forward, almost faithful, transformation of FLOWS, the First-order Logic Ontology for Web Services. As with FLOWS, ROWS incorporates fundamental aspects of (web and other electronic) services, including service descriptors, the service activity, and the service grounding. ROWS enables a rules-based specification of a family of services, including both the underlying ontology and the domain-specific aspects.
Service
(Formal) Service. In FLOWS, a service is a conceptual object, that corresponds intuitively to a web service (or other electronically accessible service). Through binary predicates a service is associated with various service descriptors (a.k.a. non-functional properties) such as Service Name, Service Author, Service URL, etc.; an activity (in the sense of PSL) which specifies intuitively the process model associated with the service; and a grounding.
Service contract
Describes an agreement between the service requester and service provider, detailing requirements on a service occurrence or family of service occurrences.
Service descriptor
Service Descriptor. This is one of several non-functional properties associated with services. The Service Descriptors include Service Name, Service Author, Service Contract Information, Service Contributor, Service Description, Service URL, Service Identifier, Service Version, Service Release Date, Service Language, Service Trust, Service Subject, Service Reliability, and Service Cost.
Service offer description
Describes an abstract service (i.e. not a concrete instance of the service) provided by a service provider agent.
Service requirement description
Describes an abstract service required by a service requester agent, in the context of service discovery, service brokering, or negotiation.
sQName
Serialized QName. A serialized QName is a shorthand representation of a URI. It is a macro that expands into a full-blown URI. sQNames are not QNames: the former are URIs, while the latter are pairs (URI, local-name). Serialized QNames were originally introduced in RDF as a notation for shortening URI representation. Unfortunately, RDF introduced confusion by adopting the term QName for something that is different from QNames used in XML. To add to the confusion, RDF uses the syntax for sQNames that is identical to XML's syntax for QNames. SWSL distinguishes between QNames and sQNames, and uses the syntax prefix#local-name for the latter. Such an sQName expands into a full URI by concatenating the value of prefix with local-name.
URI
Universal Resource Identifier. A symbol used to locate resources on the Web. URIs are defined by IETF. See Uniform Resource Identifiers (URI): Generic Syntax for more details. Within the IETF standards the notion of URI is an extension and refinement of the notions of Uniform Resource Locator (URL) and Relative Uniform Resource Locators.

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