PRINCIPLES AND APPLICATIONS OF WEB SERVICES

Principles and Applications of Web Services

Principles and Applications of Web Services

In an event-driven approach, services interact by exchanging events which flow dynamically between service providers and service consumers. Each event causes other services to respond and generate events in turn, resulting in a global event cloud which in theory makes service behaviour observable and controllable. Most event-driven approaches provide stateless and asynchronous message passing. This requires an application programmer to decouple the act of generating an event from the act of reacting to one, even if the event generation should cause a near-direct reply. Adopting an asynchronous approach facilitates loose couplings, and is often seen as a solution to achieve reliability and performance. Event-driven interactions are characterized by a continuous flow of messages representing “things that happen.” Events are commonly represented by structured messages which are exchanged between the event generator and any number of event receivers, and carry contextual information in which the event is described, plus the circumstances in which it occurred. Such contextual information may be an explicit part of the event representation, or can be implicitly deduced from said circumstances. There are distinct differences between event-oriented message exchanges versus request-response message exchange patterns when considering the routing and delivery of messages. A service which operates according to the request response pattern will actively go out and attempt to discover other services to interact with. In an event-driven approach, selection of services to provide certain operations does not principally lay with the generator of the event. Instead, service providers express their interest in handling certain events. Based on those preferences, either an event broker or the requesting service will then decide which service will handle the event. While the concept of a global event cloud is a useful metaphor to visualize a turbulent flow of events, it is important to realize that the “global” nature of that event cloud has only theoretical value in service-oriented computing. This is caused by the fact that each service operates as an autonomous and separate entity and is not subject to a form of centralized monitoring and/or control. For this reason, we partition the global event cloud into smaller event spaces. Each event space represents a group of collaborating services which interact by exchanging events, and changes over time as services join or leave the collaboration. Event-driven interactions are only now gaining more interest of the WS-* community. Initiatives, such as WS-Eventing and WS-Notification are emerging and are starting to establish a firm foothold. Both initiatives will be briefly discussed in the following two sections. WS-Eventing provides a number of basic mechanisms that can be used to manage the subscriptions of services to specific messages, such as subscribe, renew, unsubscribe, etc. The WS-Eventing specification recommends using standards such as WSsecurity to achieve message-level security. While the issue of access control is touched upon, no recommendations are made, and no mechanism to achieve a system for access control is suggested (Papazoglou, 2003, ...