See this lecture for a summary of Web services and the Semantic Web. The lecture includes the following two definitions of a Web service.
"A Web service is a software system designed to support interoperable
machine-to-machine interaction over a network. It has an interface
described in a machine-processable format (specifically WSDL). Other
systems interact with the Web service in a manner prescribed by its
description using SOAP messages, typically conveyed using HTTP with an XML
serialization in conjunction with other Web-related standards."
-- W3C Web Services Architecture Note (WSAN)
"Web services, in the general meaning of the term, are services offered by
one application to other applications via the World Wide Web. Clients of
these services can aggregate them to form an end-user application, enable
business transactions, or create new Web services. In a typical Web
services scenario, a business application sends a request to a service at a
given URL using the SOAP protocol over HTTP. The service receives the
request, processes it, and returns a response."
-- Java Web Services Tutorial (Introduction to Web Services)
Required standards in the W3C model of a Web service are SOAP (syntax for messages and RPCs), WSDL (for describing the syntactic interface to Web services) and UDDI (for listing available Web services).
The lecture omits reference to emerging Web service coreography languages such as:
A good, informal description of how loosely-coupled, long-lived, distributed, asynchronous, Web service transactions differ from traditional centralised database transactions, and require reliable messaging and compensation to be implemented correctly. Provides several motivating examples, including (cancelled) flight reservations, (interrupted) theatre bookings, etc. Based on "Loosely Coupled - The Missing Pieces of Web Services" (2003).
Abstract Abstract: Composition of Web services has received much interest to support business-to-business or enterprise application integration. On the one side, the business world has developed a number of XML-based standards to formalize the specification of Web services, their flow composition and execution. This approach is primarily syntactical: Web service interfaces are like remote procedure call and the interaction protocols are manually written. On the other side, the Semantic Web community focuses on reasoning about web resources by explicitly declaring their preconditions and effects with terms precisely defined in ontologies. For the composition of Web services, they draw on the goal-oriented inferencing from planning. So far, both approaches have been developed rather independently from each other. We compare these approaches and discuss their solutions to the problems of modeling, composing, executing, and verifying Web services. We discuss what makes the Web service composition so special and derive challenges for the AI planning community.
This paper provides a more detailed but still informal description of the role of transactions in Web service composition. It distinguishes between static workflows and more dynamic workflows that require an element of real-time (AI) planning.
Abstract Web service composition lets developers create applications on top of service-oriented computing's native description, discovery, and communication capabilities. Such applications are rapidly deployable and offer developers reuse possibilities and users seamless access to a variety of complex services. There are many existing approaches to service composition, ranging from abstract methods to those aiming to be industry standards. The authors describe four key issues for Web service composition.
Introduces a timed extension of the \pi-calculus and timed bisimilarity to model Web transactions (from a Web Services viewpoint).
Introduces the Colombo framework in which Web Services are characterized. Deveplops (i) a technique for handling the data in a finite symbolic way, and (ii) a technique to automatically synthesize composite web services, based on propositional dynamic logic. Potentially useful bibliography.
A good paper discussing the decidability and complexity of proving properties of fairly traditional database-backed Web sites.