The Anatomy of Web Services Hongbing Wang1,2, Yuzhong Qu2, Junyuan Xie1 1
Department of computer science and technology ，Nanjing University，210093 Nanjing，China
[email protected] 2
Department of computer science and engineering，Southeast University， 210096 Nanjing，China {hbw, yzqu}@seu.edu.cn
Abstract. With WSDL, SOAP and UDDI, Web services are becoming popular with Web application. However, current Web services architecture is confronted with a few intractable problems. We shall give an overview of these problems and current efforts. Finally, we set an eye on the amalgamation of Web services and Grid technologies.
1 Introduction Web services are a new breed of Web application. They are self-contained, selfdescribing, modular applications that can be published, located, and invoked across the Web. Web services perform functions, which can be anything from simple requests to complicated business processes. Once a Web service is deployed, other applications (and other Web services) can discover and invoke the deployed service. The key to Web services is software creation through the use of loosely coupled, reusable software components. The Web services framework [1] is divided into three areas — communication protocols, service descriptions, and service discovery — and specifications are being developed for each. However, current Web services specifications (SOAP, WSDL, and UDDI) meet only lowest requirements, to develop the complete potential of Web services, there are more issues which have to be resolved.
2 The problems and Current Efforts SOAP，WSDL, and UDDI are important steps in the direction of a Web populated by services. However, they only address part of the overall stack that needs to be This work is jointly supported by NSFC with grant no. 60173036 and JSNSF with grant no. BK2003001.
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Hongbing Wang1,2, Yuzhong Qu2, Junyuan Xie1
available in order to achieve Web services’ full Web-wide and global potential. Web services would have to address following problems: security, composition, and semantics 2.1 Security Of course though, at the most basic level the problems that are likely to affect Web services are the same as for more conventional Web-based systems. Many of these have been discussed at length in past articles. Certainly though the following depiction sums up the current situation: Security is critical to the adoption of Web services by enterprises, but, as it stands today, the Web services framework doesn’t meet basic security requirements. In the Web services context security means that the recipient of a message should be able to verify the integrity of the message to make sure that it has not been modified, receive a message confidentially so that unauthorized users can’t read it, determine the identity of the sender by authenticating them and determine whether or not the center is authorized to carry out the operation requested in the message. These are usually met by encrypting messages. On the other hand, Web services allow all your systems, both internally and externally, to communicate on HTTP ports so you gain the flexibility. Thereby you inevitably open up the application servers to “application level” attacks. There are some standards coming to alleviate the former however, such as WS Security (http://www.oasis-open.org/committees/wss/) and various other initiatives (mostly from the major vendors and PKI suppliers, see The XML Signature specification (http://www.w3.org/Signature/), W3C's XML Encryption specification (http://www.w3.org/Encryption/2001/), Security Assertion Markup Language (SAML) (http://www.oasis-open.org/committees/security/), eXtensible Access Control Markup Language (XACML) (http://www.oasisopen.org/committees/xacml/)) towards enabling digital signatures on XML messages and transactions. But the latter was concerned hardly. 2.2 Composition Early work in Web services composition included eCo (http://www.commerce.net/), WSCL (WSCL, http://www.w3.org/TR/wscl10/), XLANG (http://www.gotdotnet.com/team/xml_wsspecs/xlang-c/default.htm), and http://wwwWSFL (WSFL, 3.ibm.com/software/solutions/webservices/pdf/WSFL.pdf). The Web services workflow specifications outlined by XLANG and WSFL have recently been superseded by a new specification from IBM, Microsoft, and BEA called BPEL4WS (Business Process Execution Language for Web Services, http://www-106.ibm.com/developerworks/webservices/library/ws-bpel/). BPEL4WS is a specification that models the behavior of Web services in a business process interaction. The Web Services Choreography Interface (WSCI, http://www.sun.com/software/xml/developers/wsci/wsci-spec-10.pdf) is a
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specification from Sun, SAP, BEA, and Intalio that defines an XML-based language for Web services collaboration. The Business Process Management Language (BPML) is a meta-language for describing business processes (http://www.bpmi.org/specifications.esp). Business Process Management Initiative, an independent organization chartered by Intalio, Sterling Commerce, Sun, CSC, and others, developed the specification. Besides above efforts to constitute Web services composition standards [2], many researchers do some helpful works in Web services’ composition [3,4] 2.3 Semantics Most researchers hope now that the Semantic Web vision of a next-generation Web that computers can unambiguously interpret addresses precisely semantic problem of the Web services [5]. Why Web services framework need supports from semantic Web? We should note that current Web services standards, such as SOAP, WSDL, XLANG, WSFL, BPEL4WS, WSCI and BPML, all describe Web service content in terms of XML syntax. Unfortunately, XML alone lacks both a well-defined semantics and sufficient expressive power to realize the vision of diverse Web services having wide-scale interoperability. Seamless interoperability between services that have not been predesigned to work together requires programs to describe their own capabilities and understand other services’ capabilities. To realize this vision, we must describe Web content, particularly Web service content and capabilities, in a language that goes beyond XML. The Semantic Web vision of a next-generation Web that computers can unambiguously interpret addresses precisely this problem. The Semantic Web services vision is to describe Web services’ capabilities and content in a computer-interpretable language and improve the quality of existing tasks, including Web service discovery, invocation, composition, monitoring and recovery [6-8]. Initial attempts have already been made to apply semantic Web services technology to a few applications. A key element of semantic Web services is the creation of a description language. DAML-S, an ontology created by DAML-S Coalition (http://www.daml.org/services/members.html) with support from DARPA, is such a description language. Comprehensive discussions on DAML-S could be found at this Website (http://www.daml.org/services/pub-archive.html).
3 Amalgamations of Web Services and Grid Technologies A research topic deserving attention to Web services is Grid services. The Open Grid Services Architecture (OGSA, http://www.globus.org/ogsa/) represents an evolution towards a Grid system architecture based on Web services concepts and technologies. The OGSA integrates key Grid technologies [9] with Web services mechanisms to create a distributed system framework based on the Open Grid Services Infrastructure (OGSI, http://www-unix.globus.org/toolkit/draft-ggf-ogsi-gridservice-33_2003-06-
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Hongbing Wang1,2, Yuzhong Qu2, Junyuan Xie1
27.pdf). A Grid service instance is a service that conforms to a set of conventions, expressed as Web Service Definition Language (WSDL) interfaces, extensions, and behaviors, for such purposes as lifetime management, discovery of characteristics, and notification. Grid services provide for the controlled management of the distributed and often long-lived state that is commonly required in sophisticated distributed applications. OGSI also introduces standard factory and registration interfaces for creating and discovering Grid services. Grid service instances are made accessible to (potentially remote) client applications through the use of a Grid Service Handle and a Grid Service Reference. An important issue is how OGSI interfaces are likely to be invoked from client applications. OGSI exploits an important component of the Web services framework: the use of WSDL to describe multiple protocol bindings, encoding styles, messaging styles, and so on for a given Web service. As we known, Grid services have been concerned by many international conferences. An interesting topic is semantic grid services (http://www.semanticgrid.org). The visions of the grid and the semantic Web have much in common but can perhaps be distinguished by a difference of emphasis: the Grid is traditionally focused on computation, while the goal of the Semantic Web take it towards inference, proof and trust. We think that The Grid we are now building is heading towards Semantic Grid.
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