Semantic Web Service Architecture —

Evolving Web Service Standards toward the Semantic Web

½ ½ ¾ ½ Tanja Sollazzo½ ,Siegfried Handschuh ,Steffen Staab ,MartinFrank ,NenadStojanovic

½ Institute AIFB, University of Karlsruhe, 76128 Karlsruhe, Germany [email protected], sha, sst, nst @aifb.uni-karlsruhe.de

¾ Information Sciences Institute of the University of Southern California, Marina del Rey, California 90292 [email protected]

Abstract and layering of languages in the Semantic Web. To the detri- ment of the latter community the adoption of their schemes Theimportance of Web services has been recognized and into industry (quasi-)standards for Web services is far from widely accepted by industry and academic research. How- trivial, because there is no coherent architecture of immedi- ever, the two worlds have proposed solutions that progress ate practical benefit. along different dimensions. Academic research has been, mostly concerned with expressiveness of service descrip- The core idea of this paper is to present an architecture tions, while industry has focused on modularization of ser- that combines the two worlds and their potential benefits. vice layers — mostly for usability in the short term. This pa- The benefits of the integration include increased visibility per is concerned with merging these two streams of progress. of Web services because open ontology frameworks allow Our point of departure is the current proposal by IBM. Its pro- for semantically expressive advertising on the Web that may posal is extended by Semantic Web technologies such that be found by Web crawlers. They include better usability asmooth evolution from Web services in the current Web because of more expressive Web service descriptions. They to Web services in theSemantic Web appears possible and include a smooth evolution from Web services for human —infact — highly desirable. As a showcase we describe users such as targeted by current industry (quasi-)standards SWOBIS,anontology-compatible registry for software tools, toward Web services for personalized machine agents that that represents a first step towards developing a search engine forWeb services based on Semantic Web technologies. assist the user. The structure of this paper is as follows. First, we sketch the general model of the Web service setting, of the archi- Introduction tecture of IBM in particular. We consider the latter the most elaborate and the best described industry quasi-standard for Webservices are “self-contained, self-describing modu- WebServices so far. Second, we analyse assumptions of lar applications” (Martin 2001). They constitute software this (and related) architecture(s), describing several parame- modules that “describe a collection of operations that are ters which may be varied to turn the “traditional” view into network-accessible through standardized XML messaging” aSemantic Web view. Third, we outline the current descrip- (Kreger 2001, p. 6). With OAP, UDDI, WSDL, and .NET in- tion of DAML-S (Ankolenkar et al. 2001). Fourth, we crit- dustry has made a bold move and started initiatives that tar- ically evaluate the achievements of DAML-S. Fifth, we de- get the potential benefits of Web services. The focus of the scribe the integrated architecture that we propose. Finally, initiatives was an evolutionary step from current Web tech- we sketch SWOBIS, an ontology-based registry for software nology toward a technology for Web services. Key concerns tools that we are currently developing into the direction of of the initiatives were, e.g., short-term applicability or scal- our proposed integrated architecture. ability. This implies that the corresponding Web service ar- chitectures build on little really new technology inside, e.g. IBM Web Service Architecture they use standardized taxonomies and vocabularies that ex- hibit little flexibility and expressiveness and that restrict the The typical procedure in a Web service setting is the fol- usability of Web services mostly to human users rather than lowing: A Web serviceprovider offers services on the Web. machine agents. For the latter one would need, e.g., Web He may choose to register her service at an online registry service description languages that support semi-structured of a service broker (Trastour & Bartolini 2001, p.2/3). The data, constraints, types and inheritance. registry publishes and locates services. To allow for service In contrast to the industry point of view, academic re- discovery, the registry also provides standardized descrip- search has investigated languages that fulfill exactly these tion facilities, e.g. taxonomies that allow the description of, needs (Horrocks et al. 2001; Ankolenkar et al. 2001; (i),the functionality of a service, (ii), its service provider, Fensel et al. 1999) offering extensible ontology frameworks (iii),how to access and interact with the service. The corresponding information about a particular service is registered

FLAIRS 2002 425 istry. It ﬁnds one by browsing or querying the registry. Then ture, because issues like security, management and quality she uses the service description to create a binding for her of service span all other layers. application to be able to invoke or interact with the Web service implementation. In the IBM Web service architecture Discussion of the IBM (Quasi-)Standard this procedure boils down to the architecture layers depicted Architecture in Figure 1 (cf. (Kreger 2001)). This description, the current industry quasi-standard, how- Network Protocols and SOAP. The lower layers are based ever, is too “ﬂat” to be comprehensive. Rather, there are a on general network protocols, like HTTP or FTP, and SOAP number of parameters that may be varied: (simple object access protocol1), which is a standardized en- veloping mechanism for communicating document-centric Table 1: Dimensions of Web service features messages and remote procedure calls using XML. DimensionChoices WSFL Service Flow “Traditional” Semantic Web Service Simple Composed Requestor Human Machine

Static à UDDI Service Discovery o Quality

Mana Provider Registration No registration Direct à UDDI Service Publication Security Broker Key Player Facilitator gement fSer Service description Taxonomy Ontology

vice Descriptive elements Closed world Open world WSDL Service Description Data exchange Syntactic-based Semantics-based SOAP XML-Based Messaging In particular, the Semantic Webwill allow richer descriptions of Web services (e.g., semi-structured data, types, in- HTTP, FTP, Network Protocols email etc. heritance, semantic constraints). The key role of the broker maydisappear, it may still be viable as a kind of search engine for Web services (with meta search engines on top), but Figure 1: IBM Web Service Architecture it will lose its central role as aregistry, because everyone WSDL. The interface of a Web service and its service in- may publish semantic descriptions and crawlers may find teractions are described in WebServices Description Lan- them. Personalized machine agents will take over the role guage (WSDL2). WSDL allows the composition of XML of a service requestor from the human user. And, they may documents to describe technical details of a Web service. also do the composition for the human user. When we distinguish between “traditional” and Semantic UDDI. The service publication is regarded as a business- Web features, we do, however, not require that only the lat- related description of a service; e.g. answering questions ter be valid for two reasons. First, we do want to allow for such as: What products are associated with this service? an evolutionary development ofaWebService architecture. Which organization is offeringthisservice? Universal De- Second, also “traditional” features offer several advantages. scription Discovery and Integration (UDDI), initiated by Mi- For instance, a service broker as *intermediary* may take crosoft, IBM, and Ariba3, does not just consist of defining a care of concerns like validation of information or even se- data structure standard for all business-related descriptions lection of service providers. of services (i.e. service publication), but it also contains all mechanisms that allow the service requestor to gain access to the service publication and service description, and thus Service Description with DAML-S is also in charge of the service discovery layer. For the lat- DAML-S is a DAML+OIL-based Web service ontology, ter purpose, a UDDI registry contains data in a combination which supplies Web service providers with a core set of of white pages, yellow pages, and green pages. The white markup language constructs for describing the properties pages include items such as a company’s name, its address, and capabilities of their Webservices in unambiguous, and other contact information. The yellow pages help to cat- computer-interpretable form. DAML-S markup of Web ser- egorize a business; they define its business type, as well as vices facilitates the automation of Web service tasks includ- theindustry in which it operates. Finally, the green pages ing automated Web service discovery, execution, composi- define what kind of services a business offers, and how to tionand interoperation. In particular, it provides language electronically communicate with these services. primitives for technical, business-related and process-based WSFL. WebServices Flow Language (WSFL) prescribes an facts about services. Thus, DAML-S can be regarded as a XML format for specifying service composition (also called semantics-based substitution of the above-mentioned Web service flow) (Leymann 2001). WSFL allows to compose service languages for service description, service publica- complex services from given simpler ones. tion, and service flow (cf. (Ankolenkar et al. 2001)):

Remainder. Several layers are orthogonal in this architec- ¯ The Service Proﬁle of DAML-S describes “what the service does” (business-related facts); that is, it provides a 1http://www.w3.org/TR/SOAP/ service-seeking agent with an abstract speciﬁcation of a 2http://www.w3.org/TR/wsdl.html service to determine whether it meets the agent’s needs 3http://www.uddi.org/pubs/Iru UDDI Technical White Paper.pdf (e.g. input and output types, pre- and postconditions).

426 FLAIRS 2002 ¯ The Service Model demonstrates “how the service works” (cf. arrow 1 in Figure 2), viz. thetechnology-based details of (process-based facts); it describes what happens when the the service description layer, the business-related details of service is performed, which may be used by a service- the service publication layer,andtheinternal process flow seeking agent in at least four different ways: to perform details of the service of the service interaction layer. This amorein-depth analysis of whether the service meets its direct invocation, however, is only possible if the user knows needs; to compose service descriptions from multiple ser- the location of the service description, which often is not vices to perform a specific task; to coordinate the activi- the case. Alternatively, the user may interact with a registry ties of different participants during the course of the ser- located at the service discovery layer which helps her finding vice enactment; to monitor the execution of the service. aservice and retrieving the descriptions about how to invoke it (arrow 2 in Figure 2). ¯ The Service Grounding specifies the details of how an agent can access a service (technical facts), e.g. a commu- Agent nication protocol such as Remote Procedure Call (RPC), uses Hyper-Text Transfer Protocol (HTTP), and Agent Com- 1 2 3 4 munication Language (ACL) or service-specific details such as port numbers used in contacting the service. UDDI Service Discovery Currently, only the first two have been described in some DAML-S Process / WSFL Service Flow Composition detail. Discussion of DAML-S DAML-S Process Service Interaction Though there are some points within DAML-S that may re- DAML-S Profile & UDDI Service Publication quire discussion and possibly revision, we do not want to DAML-S Grounding Service Description tackle them here. Rather, we want to consider the role that DAML-S plays with regard to other specifications like the IBM Web Service architecture: Communication Protocol (KQML, ACL)

¯ DAML-S’ strength is the semantically-based deﬁnition of XML Messaging Protocol concepts. The corresponding languages like WSDL and SOAP UDDI are rather weak in this respect and focus more on Network Protocols (HTTP, FTP, SMTP) technical details.

¯ DAML-S is indecisive about how it should be realized in Figure 2: Semantic Web Service Architecture aparticular architecture (though in several papers a shal- low architecture is suggested in illustrations). This may In the second scenario, agents want to find and invoke a be considered an advantage (many possibilities) or a dis- Web service in order to fulfill a task given to them by a hu- advantage (increased complexity). We consider it a dis- manuser. This adds new possibilities to the first scenario, advantage for most practical purposes (no reliability on because agents may better find services and access them di- architectural issues). rectly (arrow 4 in Figure 2) exploiting the crawling of decen- tralized machine-processable metadata on the WWW. Fur-

¯ DAML-S does not include the role of an intermediary into thermore, they may also take advantage of registries/search its schemes (though it does not prohibit one either). Nev- engines — just like a human user (arrow 3 in Figure 2). ertheless, we consider the role of registries and service brokers as very important in order to increase quality of If the human or the machine agents queries for a com- service, reliability, etc. posed service, e.g. for arrangements to visit a conference, the registry will make use of a service flow composition In the following, we describe the Semantic Web Service ar- description. The description describes services and service chitecture — a proposal that aims to combine the advantages flows (e.g., temporal sequences). With the help of this de- of the two worlds. scription, the services contained in the composed service (atomic or composed itself) may either be located directly Semantic Web Service Architecture or retrieved via the very same registry for atomic services. Ontologies provide a large extent of flexibility and expres- Communication. Like in the IBM quasi-standard, the two siveness, the ability to express semi-structured data and con- bottom layers are standard network protocols and SOAP straints, and support types and inheritance. The industry’s allowing for exchange of object descriptions by standard Web service (quasi-)standards, however, provide better man- means. In addition, machine agents need the possibility to ageability, scalability, and modularization. The benefits of communicate at an adequate level using protocols such as both technologies can be obtained by merging DAML-S and Knowledge Query and Manipulation Language (KQML) or current Web service standards to the benefits of both. Agent Communication Language (ACL) (cf., e.g. the sur- Web Services and Web Service Access. We distinguish veypaper (Labrou 2001)). These protocols are currently not two main scenarios. In the first one, a human user wants to “ontology-capable”. Thus, there is a need for extension in access the Web service. There are (mainly) two possibilities: order to better reflect ontology requirements. The user may directly access the Web service description, Further Layers. Finally, the extended layers for Quality of

FLAIRS 2002 427 Service, security, and management as depicted in the IBM rization of software tools, their features, the developers, the Webservice architecture are still needed, but are not illus- supporting organizations, etc. and relations between them. trated in the figure for ease of presentation. It is also possible to use another ontology to describe soft- The Role of DAML-S. DAML-S is used for the full service ware tools and publish the metadata created according to it description, viz. the technical, business-related, and process- in the SWOBIS list. This can be realized by publishing onto- based description of services.Fortheservice publication logical mappings. SWOBIS is capable of interpreting these layer, DAML-S should have a core ontology modelled with mappings and thus can visualize information from various the UDDI properties and should offer further functionality ontologies in one report. with more properties. At the next layer, viz. service flow Each researcher willing to provide metadata creates and composition (corresponding to theservice flow layer in the makes it available on an annotated Web page. The informa- IBM Web service architecture), a language like WSFL or the tion of the DAMLlized Web sources can be easily extracted DAML-S subontology Process should be in charge of de- and fused by the tool WebScripter4,whichproduces the self- scribing the relations between services. The service discov- updating list of software tools in form of a spreadsheet. ery in the topmost layer will be performed by an ontology- Results of SWOBIS. SWOBIS offers various benefits in compatible registry, which is capable to point to DAML-S comparison to the current simple list, which can be seen on descriptions and support their expressiveness. UDDI needs the SWOBIS Web site at http://tools.semanticweb.org, a do- to be developed into this direction. In the long run, the cur- main of the portal SemanticWeb.org: The report always con- rently mostly passive registry should be substituted by an tains up to date information. The user saves time that would active agent that functions as an intermediary. This way the have been otherwise spent retrieving information by looking update of publication information does not only depend on at the list instead of visiting all Web pages of software tools active registration, but it may also be informed by the in- that are of interest to her. The timesaving is even higher if termediary, the tasks of which may include the crawling of personalization of the report is available or agent interaction Web service descriptions. substitutes the human tasks. Due to the automatic update Web Service Binding. The Semantic Web service architec- of information, more details about the software tools can be ture supports better service invocation because the underly- provided in the list. The classification of the software tools ing ontologies are extensible. This advantage is inherited in the appropriate categories isperformed by domain experts by DAML-S, which allows for extension of the predefined in themetadata creation process and thus should be more ac- ontologies. curate. Transparency of the domain of interest attracts non- researchers, thus leads to an increase of audience and brand SWOBIS recognition of the tools. If personalization is accomplished, personalized reports will enhance user-friendliness. We implemented the Semantic Web Ontology-Based In- However, the full potential of SWOBIS relies in its use formation Service (SWOBIS), which is analogous to the in the future Semantic Web service architecture. That is, recommended UDDI registry which makes use of service SWOBIS needs to be developed further from a registry to a descriptions to enable service discovery. It is a kind of search engine, which does not provide the information about ontology-compatible registry due to the following function- software tools, but the information about Web services by ality: It offers a self-updating list of software tools for the using an appropriate DAML-S ontology. The further step Semantic Web generated as a Web-based report by utilizing will be the transition to an agent which communicates with the descriptions of software tools provided as DAML+OIL personalized agents of users who are searching a specific metadata on the WWW. In addition to showing the next Web service for their needs. step in realizing the Semantic Web Service Architecture, SWOBIS keeps the research community updated about the Related Work performance of current Semantic Web technologies, thus This section reviews the architecture of other Semantic Web supporting the vision of Semantic Web. The information services (for humans, for machines, or for both). about the latest development of software tools provides op- ITTALKS is a Web portal that lists information technol- portunities of collaboration and eliminates duplication of ogy talks, such as distinguished lectures at universities. It work. Further, it is one of the first services to show the po- internally uses DAML for knowledge representation, rea- tential of existing Semantic Web tools and thus leads to mo- soning, and communication (Cost et al. 2001). A user can tivation of both information and service providers to create fill out a standard Web form that will result in a DAML DAML+OIL metadata. The advantages of SWOBIS can be file containing data that ITTALKS can process, which the seen in comparison to the existing list of software tools on users can then host on their own Web server. Talk informa- the DAML project Web site at http://www.daml.org/tools/, tion can similarly be entered viatraditional HTML forms, or which has shortcomings, such as having to communicate one can submit the URL of a DAML file using the standard with the human maintainer to add to the list. ITTALKS content. Talks information is kept in a database Implementation of SWOBIS. In the first step, we cre- much like in any other (non-Semantic-Web) Web service. ated the Semantic Web Software Tools (SWST) ontology, ITTALKS does not provide for robust direct external access which is needed to describe software tools in a machine- to its underlying DAML content (as of the time of writing, understandable way. SWST models in a simplified way the Semantic Web software tool development activity: catego- 4http://www.isi.edu/webscripter

428 FLAIRS 2002 to the best of our knowledge). This means that it is impos- (Sollazzo 2001), others are open because of the evolving sta- sible to robustly republish, search, or annotate its content tus of the Semantic Web, e.g. the need for Semantic Web rule –negatingthekeyidea of the Semantic Web. Thus, while languages has been recognized, but not yet accounted for in ITTALKS is a novel and immediately useful “traditional” the W3C documents. Web service, its internal use of DAML seems to provide few Acknowledgements. Research for this paper was partially benefits to external users.5 financed by US Air Force in the DARPA DAML project One division of USC’s Information Sciences Institute au- “OntoAgents” (01IN901C0). We thank our colleagues, in tomatically pulls together its people page from different particular Stefan Decker, for fruitful discussions and point- DAMLized pages 6.Someinformation is maintained by in- ers to relevant work and Juan Francisco Lopez for the help dividual employees themselves (such as their research in- with the Webscripter report. terests), other information is maintained by the division di- rector (such as project assignments), and some information References is maintained at the institute level (such as office assign- Ankolenkar, A.; Burstein, M.; Hobbs, J.; Lassila, O.; Martin, D.; ments); this e.g. relieves administrative assistants from man- McIlraith, S.; Narayanan, S.; Paolucci, M.; Payne, T.; Sycara, K.; ually maintaining everyone’s interests. The page also uses and Zeng, H. 2001. DAML-S: A Semantic Markup Language WebScripter as does SWOBIS; all the underlying DAML is For Web Services. In Proceedings of SWWS’01, Stanford, USA, available externally – scroll to the end of the cited Web page August 2001, 411–430. and clickon“report”. Benjamins, V. R.; Plaza, E.; Motta, E.; Fensel, D.; Studer, The DAML Web site maintains a DAML tools7 list in R.; Wielinga, B.; Schreiber, G.; Zdrahal, Z.; and Decker, S. DAML format in the same spirit of SWOBIS; however, ex- 1998. IBROW3: An intelligent brokering service for knowledge- ternal users cannot add to the list without human interven- component reuse on the world-wide web. In The 11th Banff tion like they can do in SWOBIS. (It is a simple static Web Knowledge Acquisition for nowledge-Based System Workshop page, not a service in any fashion.) (KAW98). DAML-S is a promising add-on to the DAML+OIL lan- Cost, R. S.; Finin, T.; Joshi, A.; Peng, Y.; Nicholas, C.; Chen, H.; guage for describing services (Ankolenkar et al. 2001). In Kagal, L.; Perich, F.; Zou, Y.; and Tolia, S. 2001. ITTALKS: A Case Study in the Semantic Web and DAML. In Proceedings of fact, the need for a merger between DAML-S and current SWWS’01, Stanford, USA, August 2001, 477–494. industry quasi-standards has even been recognized by the developers of DAML-S. However, to the best of our know- Fensel, D.; Benjamins, R.; Motta, E.; and Wielinga, B. 1999. UPML: A Frameworkforknowledge system reuse. In Proceed- ledge, it has not yet been specified in any way nor has there ings of the International Joint Conference on AI (IJCAI-99), 16– been implemented Web services using DAML-S at the time 21. of writing. Therefore, we withhold judgement on its viabil- Horrocks, I.; Harmelen, F.; Patel-Schneider, P.; Berners-Lee, ity until a reference implementation exists. T.;Brickley, D.; Connolly, D.; Dean, M.; Decker, S.; Fensel, UPML, the Unified Problem-solving Method Develop- D.; Hayes, P.; Heflin, J.; Hendler, J.; Lassila, O.; McGuin- ment Language, sits on top of the DAML+OIL layer just like ness, D.; and Stein, L. 2001. DAML+OIL language release. DAML-S, and defines an architecture for describing reason- http://www.daml.org/2001/03/daml+oil-index.html. ing services on the Web (Fensel et al. 1999). Using UPML, Kreger, H. 2001. Web Services Conceptual Architecture (WSCA IBROWaims to develop intelligent brokers that are able to 1.0).http://www-4.ibm.com/software/solutions/webservices/pdf/ configure reusable components into knowledge systems via WSCA.pdf. the World-Wide Web (and aims to do so in a distributed fash- Labrou, Y. 2001. Standardizing Agent Communication. In 9th ion) (Benjamins et al. 1998). This is an ambitious project; ECCAIAdvanced Course ACAI 2001 and Agent Link’s 3rd Euro- various software pieces such as UPML editors exist, but pean Agent Systems Summer School, EASSS 2001,volume 2086 there are no implemented distributed reasoning services to of LNCS, 74–97. Prague, Czech Republic: Springer. our knowledge, thus we similarly withhold judgement. Leymann, F. 2001. Web Services Flow Language (WSFL 1.0).Technical Paper. http://www-4.ibm.com/software/ solu- Conclusion tions/webservices/pdf/WSFL.pdf. Martin, J. 2001. Web Services: The Next Big Thing. XML- We presented a Semantic Web Service architecture that inte- Journal 2. http://www.sys-con.com/xml/. grates industry quasi-standards and academic research. We argued that such an integration is necessary in order to allow Sollazzo, T. 2001. Ontology-based Services for the Semantic Web. Services in the Area of Software Tools and Supply Chain for a stepwise evolution of current standards towards the full Management. Master’s thesis, University of Karlsruhe. benefits of Semantic Web technologies. However, we left many implementation details of the future full-featured Web Trastour, D., and Bartolini, C. 2001. Approach to Service De- scription for Matchmaking and Negotiation of Services. In Pro- services architecture open. Some of them were specified in ceedings of SWWS’01, Stanford, USA, August 2001, 447–462. 5Note that SWOBIS provides direct external access to all of its underlying DAML input files – click on the “WebScripter report definition” link on any of the SWOBIS reports to retrieve the list of raw DAML input files. 6http://www.isi.edu/divisions/div2/people.html 7http://www.daml.org/tools/

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