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Chapter 5. Configuring OSGi applications > The configuration problem - Pg. 132

132 C HAPTER 5 Configuring OSGi applications Criteria Publisher Event Broker Event Subscriber Figure 5.1 The notification publisher dispatches events to the broker, which dispatches them to the subscriber if criteria are met. 5.1 The configuration problem Before we look into how to configure a service in OSG i, you should understand why and when configuration is needed. To do this, we'll use an example--the Notification Broker service. 5.1.1 The Notification Broker service A notification system allows an application to send events of interest to another appli- cation. Such a system typically consists of three main components or services: a pub- lisher, a broker, and a subscriber. The notification subscriber registers filters in the notification broker; these filters are sometimes called event criteria. The notification publisher sends events to the notification broker, which dispatches those events that match the criteria to the registered subscribers, as shown in figure 5.1. In summary, this is similar to any other messaging infrastructure, such as the popu-