Product: OpenFusion Notification Service

Version: ALL

Description:

What is the difference between the Notification Service and the Event Service?

Resolution:

The OMG Notification Service is a powerful mechanism for sending event notifications between applications in a distributed and heterogeneous environment.

The Notification Service is an extension and replacement of the original Event Service. It provides backward compatibility with the Event Service and maintains an architecture that allows easy partitioning of an event system into logical groups that communicate using federated event channels.

The Notification Service overcomes a number of limitations with the Event Service. An important extension is the support for event filtering where both structured events and any type events can be filtered using a powerful generic constraint language. Although filters can apply to any part of an event, consumers will typically filter on the filterable body of structured events, which contains a sequence of name/value pairs. In order to constrain the implementations of the Notification Service specification, a large number of configurable QoS properties are supported. These can be divided into reliability, queue management, event management and event batch management.

The Notification Service supports coarse or fine-grained QoS settings as different QoS properties can be applied all the way from the channel level down to individual events.

The Notification Service also informs event suppliers and consumers about changes to the event types required or offered by a channel. The channel obtains information about the event types implicitly via filter objects or explicitly from the connected consumers and suppliers. Intelligent supplier and consumer can use this offer and subscription information to limit the amount of events.

An optional part of the Notification Service specification is the event type repository. The repository contains event types and all the filterable properties required for each event type. The repository supports inheritance and import relationships between event types and all information in the repository can be queried and modified at run-time. The event type repository serves two distinct and important purposes. First of all, it is a powerful way of maintaining an evolving set of event types while a complex distributed system is being constructed. More importantly, for systems that evolve during their lifetime, the event repository is a mechanism for new event suppliers and consumers to add additional event types or obtain information about the properties of existing types.

In conclusion, the Notification Service is a powerful event mechanism for distributed systems that directly supports important properties such as flexibility, scalability, and extensibility. With support for different communication models, event filtering, a wide range of QoS properties and the event type repository, the Notification Service covers a very wide range of distributed application requirements - and makes it an obvious candidate for the event notification component of distributed systems.