4. Notifications and events

The preceding Web services specifications cover a mode of communication in which a client application or service initiates a dialogue, sometimes reduced to a simple request-response round-trip, with a remote Web service. The other important mode in SOA is that in which communication is initiated by the service rather than the client. This type of communication, of which we have just seen an important example in WS-Discovery, is generally summarized by the notions of notification and event.

OASIS has published WS-Notification, an XML-based mechanism for peer-to-peer information exchange between Web services and applications. The specification is subdivided into three chapters covering so-called basic notifications (WS-BaseNotification), notifications via intermediaries (WS-BrokeredNotification) and the object of the notifications themselves (WS-Topics).