Web service

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This page hasn't been updated since 2010. In particular, an introduction to REST is missing ....

1 Definition

According to Wikipedia (retrieved 20:58, 23 April 2007 (MEST)), the W3C defines a Web service as a software system designed to support interoperable Machine to Machine interaction over a network. Web services are frequently just Web APIs that can be accessed over a network, such as the Internet, and executed on a remote system hosting the requested services.

A Web service is any piece of software that makes itself available over the Internet and uses a standardized XML messaging system. (Top Ten FAQs for Web Services, retrieved 20:58, 23 April 2007 (MEST)

See also: e-science, service-oriented architecture, grid computing, business process modeling, etc.

2 Web services frameworks

2.1 Traditional web services stack components

The Web service protocol stack is an evolving set of protocols used to define, discover, and implement Web services. The core protocol stack consists of four layers:

  • Transport: E.g. HTTP, SMTP, FTP, and newer protocols.
  • XML messaging: E.g. Simple Object Access Protocol (SOAP) or XML-RPC. These define messages containing a service request and a response. SOAP and XML-RPC etc. are independent of any particular transport and implementation technology.
  • Service description: E.g. Web Services Description Language (WSDL) - describes what a service does in a machine readable way
  • Service discovery: E.g. Universal Discovery, Description, Integration (UDDI) - a service to publish available services
List of standards (basic and important ones first)
  • Common Internet protocols (e.g. TCP/IP, HTTP)
  • XML, the formalism
  • Simple Object Access Protocol (SOAP) 1.1, W3C note. "SOAP is a lightweight protocol for exchange of information in a decentralized, distributed environment. It is an XML based protocol that consists of three parts: an envelope that defines a framework for describing what is in a message and how to process it, a set of encoding rules for expressing instances of application-defined datatypes, and a convention for representing remote procedure calls and responses". Note DKS: This is generally the lowest Service Layer (just on top of HTTP) and tells how to describe messages that are sent or received.
  • Web Services Description Language (WSDL), W3C Note. WSDL is an XML format for describing network services as a set of endpoints (methods/calls) operating on messages containing either document-oriented or procedure-oriented information.
  • UDDI (Universal Description, Discovery and Integration) enables organizations to (I) discover each other by a directory of service providers and services, and (II) find out what services are available
  • WSFL - Web Services Flow Language (article from IBM). WSFL is an XML language (compatible with SOAP, UDDI & WSDL) for the description of the overall process logic of Web Services. See also Cover Pages about his.
  • Apache references to the WSIF (Web Services Invocation Framework) . This is a description of APIs to invoke Web Services described in WSDL. (Can, but does not need to be built on top of SOAP).
  • WSIL (Web Services Inspection Language). The WS-Inspection specification provides an XML format for assisting in the inspection of a site for available services and a set of rules for how inspection related information should be made available for consumption (IBM/Microsoft)
  • WSRP (Web Services for Remote Portals): XML and Web services standard that will allow the plug-n-play of visual, user-facing Web services with portals or other intermediary Web applications. (Oasis led initiative)
  • ebXML Messaging Service Specification (OASIS led) provides a secure method for exchanging electronic business transactions using the Internet. (This is a rather big project with several sub-specifications). See D. Mertz "Understanding ebXML" at developerworks/IBM.
  • OASIS Web Services Resource Framework (WSRF). a generic and open framework for modeling and accessing stateful resources using Web services. This includes mechanisms to describe views on the state, to support management of the state through properties associated with the Web service, and to describe how these mechanisms are extensible to groups of Web services, e.g. used in grid computing infrastructures.
  • XPDL by the Management Coalition (WfMC) allows to exchange business process definitions between different workflow products, i.e. between different modeling tools and management suites.
  • BPEL, also known as WS-PBEL, WSBPEL, or PBEL4WS was an IBM/Microsoft/BEA initiative, standardized in 2007 as WS-PBEL 2.0 by OASIS. WS-BPEL provides a language for the specification of Executable and Abstract business processes. By doing so, it extends the Web Services interaction model and enables it to support business transactions. WS-BPEL defines an interoperable integration model that should facilitate the expansion of automated process integration in both the intra-corporate and the business-to-business spaces.
  • There is more .... some still in the specification pipeline, e.g.
    • Web Services Choreography Description Language (Candidate recommendation since 2005). The Web Services Choreography Description Language (WS-CDL) is an XML-based language that describes peer-to-peer collaborations of participants by defining, from a global viewpoint, their common and complementary observable behavior; where ordered message exchanges result in accomplishing a common business goal.

2.2 Web service frameworks

See: List of Web service Frameworks (Wikipedia)

3 Web services in education

3.1 Web services in education, Rehak et al. model

“ The era of monolithic Learning Technology Systems is over. Rather than building large, closed systems, the current focus is on flexible architectures that provide interoperability of components and learning content, and that rely on open standards for information exchange and component integration. The Learning Systems Architecture Lab has developed the concept of a Learning Services Architecture and the Learning Services Stack as a framework for developing the next generation of learning technology systems. We're in the process of developing learning services using Web services and Web services technologies.” (http://lsal.org/, retrieved 20:58, 23 April 2007 (MEST)).

The stack according to Rehak et al. has three fundamental layers:

  • User agents: Authoring, delivery, management
  • Learning services: At its heart there are knowledge and content management, user profile and tracking, sequencing. On top of this are various tools for interaction, presentation, lecturing, assessment, simulation, collaboration, etc.
  • Infrastructure: Lower level protocols (e.g. HTTP), transport (e.g. SOAP), discovery (e.g. UDDI), description (WSDL), workflow (WSFL)
The LSAL learning services stack, © Carnegie Mellon, Learning Systems Architecture Lab, reproduced without permission for the moment ...

An implemented architecture may look like this:

( HTTP -> ) SOAP -> UDDI -> WSDL | WSIF ? -> WSFL ? -> -> ebXML? | WSIL ? -> WSRP? -> Portal

This model has been further developed in the e-framework model that puts "services" on center stage.

3.2 IMS

3.3 A social software model ?

Daniel K. Schneider wonders how one could conceptualize and engineer integration of various social software (e.g. blogs, wikis, reference management, etc.) as web services into a portal.

4 Organizations

There are many players in this field, just a few examples:

Global players
  • Oasis is an other organization working on xml-based e-business standards
  • XMethods has a long list of publicly available web services , implementations, tutorials, etc.
In educational technology

5 Links

Standards and recommendations
For education

6 References

  • Papazoglou, Michael P. and Dubray, Jean-jacques (2004) A Survey of Web service technologies. Technical Report DIT-04-058, Ingegneria e Scienza dell'Informazione, University of Trento. Abstract/PDF