W3C Push Workshop, 8-9 September 1997

[Other Papers] [Briefing Package]

Overview

The Keryx Programme in Hewlett-Packard Laboratories, Bristol, has developed a real-time, notification system for distributing announcements of changes in the dynamic content of Internet-based services. The Keryx Notification System provides an architecture and supporting components that allows the rapid development, deployment and evolution of services in a distributed environment.

The Keryx system supports the real-time exchange of structured information (a notification) between service providers and service users. The communication model allows "unsolicited" (or informational) information to be passed to users based on their predefined "interests". Users define their interests by registering a filter that is applied to all notifications. Notifications satisfying the filter are sent to the user, thus services can be flexibly developed without the notification source or the user having prior knowledge of each other.

A notification is a self-contained, non-directed package of structured information reflecting the occurrence of some event. For example, a notification may be generated when Email arrives on a mail server, or when the content of a Web page is updated. Interested consumers may register an interest in such events and will be notified as the events happen, allowing the content itself to be retrieved using normal services (POP and HTTP in the above examples).

The Keryx Notification System provides a real-time, update service that saves the need for regular polling of the original content. This can support a host of Internet-based services.

System Principles

• Notifications contain structured information. This allows a consumer of notifications to provide a description of the events it is interested in by specifying constraints on the components of the notification itself. We have developed a simple form of language independent filters that allow the selection of notifications of interest.
• Notifications are extensible. We support a non-positional, structured exchange of data. This allows event consumers to only process the parts of the notification they are interested in, and allows event producers to extend the schema of their notifications in a conservative way, without affecting current consumers.
• Notifications are easy to generate and receive. To achieve this we use a text based format for transferring notifications between systems. We have interchangeable data formats that allow for the introduction of data without the need to worry about quoting mechanisms. The system may be requested to reformat the data before delivery to simplify the reconstruction of the event by the receiver.
• Event consumers and producers are loosely bound together. There is no need to have a common interface definition before generating events, or to subscribe to them. Indeed events are not segregated by interface type, rather a consumer specifies what set of events it should receive (should they be generated). This allows for independent evolution of the producing and consuming services.
• System level services and components are implemented in a modular fashion allowing lightweight clients to interact with the system in a simple fashion.

Availability

A Java based implementation of the Keryx Notification System will be available in the near future from the KeryxSoft WebSite at <URL:http://keryxsoft.hpl.hp.com/>.