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Team Comment on Web Services Eventing (WS-Eventing) Submission

W3C is pleased to receive the Web Services Eventing (WS-Eventing) Submission from BEA Systems, Computer Associates, IBM, Microsoft, and Tibco Software.

WS-Eventing is a SOAP-based protocol to subscribe to notifications. The subscriber receives event messages from an event source, and can manage its subscription with a subscription manager.

This protocol is built using the SOAP messaging framework, both version 1.1 which is a W3C Member Submission and version 1.2 which is a W3C Recommendation. It does not define, however, a URI to identify the SOAP module defined as per the SOAP 1.2 specification.

The protocol also uses the WS-Addressing Member Submission, on which the Web Services Addressing Working Group has based WS-Addressing 1.0. In particular, the concept of endpoint references (EPR) defined is WS-Addressing is used to communicate the addresses of the subscriber, event source, and subscription manager.

A subscription corresponds to a subscription manager EPR, i.e. WS-Eventing does not define as part of its protocol the concept of subscription identifier in its protocol. Should a Web service act as a subscription manager for more than one subscriptions, the specification defines a wse:Identifier element of type xs:anyURI to be used as a reference parameter in the subscription manager EPR. It might have been useful to have this concept part of the protocol to allow identification of subscriptions in other contexts.

The delivery mode of the notification messages is open. The specification defines a Push mode, which is delivery mechanism where the source sends event messages to the sink as individual, unsolicited, asynchronous SOAP messages. The XML Protocol Working Group is working on a one-way underlying protocol binding for SOAP 1.2 which will allow such delivery over HTTP.

A subscriber can request the subscription to a specific subset of event messages from the event source by using filters. The dialect used for this filtering is open as an extensibility point. The specification defines how to use XPath 1.0 for this filtering.

Finally, the specification defines a @wse:EventSource that can be used in WSDL service descriptions to indicate that an Web service endpoint is an event source and supports WS-Eventing subscription requests, which is only defined for the WSDL 1.1 Member Submission. This WSDL extension could also be used in WSDL 2.0, once a mapping to the WSDL 2.0 component model is defined.

Related work

The OASIS Web Services Notification (WSN) Technical Committee works on standardizing a set of specifications closely related to the specification submitted:

The purpose of the Web Services Notification (WSN) TC is to define a set of specifications that standardise the way Web services interact using the Notification pattern.

In the Notification pattern a Web service, or other entity, disseminates information to a set of other Web services, without having to have prior knowledge of these other Web Services. Characteristics of this pattern include:

  1. The Web services that wish to consume information are registered with the Web service that is capable of distributing it. As part of this registration process they may provide some indication of the nature of the information that they wish to receive.
  2. The distributing Web service disseminates information by sending one-way messages to the Web services that are registered to receive it. It is possible that more than one Web service is registered to consume the same information. In such cases, each Web service that is registered receives a separate copy of the information.
  3. The distributing Web service may send any number of messages to each registered Web service; it is not limited to sending just a single message.

As noted by the Technical Committee, WS-Eventing and WS-BaseNotification address a very similar problem area and architecturally are similarly structured.

Next steps

The W3C Team will notify the Web Services Coordination Group of this Member Submission.

Authors: Hugo Haas