Participants
Shared Passive Resource. A shared resource is a part of the system that might be used by several potential clients. This resource is called passive if it is unable to notify its environment (the potential clients) about changes of its state.
Clients. Clients have to have access to the shared resource to fulfill their responsibilities. They may read as well as modify the resource’s state.
Notification Server. A Notification Server is an entity attached to a shared resource. It allows clients to register to receive notifications when the state of the shared resource changes.
Structure
Figure 3 shows the basic structure of a system based on the Notification Server.
Figure 3
Dynamics
Every client can join a group of clients sharing certain resources (that is a database or a part of a database) by registering itself on the Notification Server responsible for these resources. After a client has changed a database resource, it informs the Notification Server about that change and the Notification Server in turn informs all the other registered clients (figure 4). A client that was notified about some changes may react on this notice by refetching information from the database. If a client isn’t interested in change notifications related to some resources anymore, it leaves the group by unregistering itself from the responsible Notification Server.
Figure 4
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