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502 DISTRIBUTED OBJECT-BASED SYSTEMS CHAP. 9
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Request type Failure semantics Description
Synchronous |
At-most-once |
Caller blocks until a response is returned or an exception is raised |
One-way |
Best effort delivery |
Caller continues immediately without waiting |
|
|
for any response from the server |
Deferred |
At-most-once |
Caller continues immediately and can later |
synchronous |
|
block until response is delivered |
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Event and Notification Services
Figure 9-4. Invocation models supported in CORBA.
Although the invocation models offered by CORBA should normally cover most of the communication requirements in an object-based distributed system, it was felt that having only method invocations was not enough. In particular, there was a need to provide a service that could simply signal the occurrence of an event. Clients amenable to that event could take appropriate action.
The result is the definition of an event service. The basic model for events in CORBA is quite simple. Each event is associated with a single data item, generally represented by means of an object reference, or otherwise an applicationspecific value. An event is produced by a supplier and received by a consumer. Events are delivered through an event channel, which is logically placed between suppliers and consumers, as shown in Fig. 9-5.
Push event to consumers
Consumer Consumer
Event channel
Supplier Supplier Supplier
Figure 9-5. The logical organization of suppliers and consumers of events, following the push-style model.
Fig. 9-5 shows what is referred to as the push model. In this model, whenever an event occurs, the supplier produces the event and pushes it through the event channel, which then passes the event on to its consumers. The push model comes closest to the asynchronous behavior that most people associate with events. In effect, consumers passively wait for event propagation, and expect to be interrupted one way or the other when an event happens.
An alternative model, also supported by CORBA, is the pull model shown in Fig. 9-6. In this model, consumers poll the event channel to check whether an event has happened. The event channel, in turn, polls the various suppliers.
Although the event service provides a simple and straightforward way for