Copyright © TIBCO Software Inc. All Rights Reserved
Copyright © TIBCO Software Inc. All Rights Reserved


Chapter 14 Using Multicast : Overview of Multicast

Overview of Multicast
Multicast is a messaging model that broadcasts messages to many consumers at once, as opposed to sending copies of a message to each subscribing consumer individually. TIBCO Enterprise Message Service uses Pragmatic General Multicast (PGM) to broadcast messages published to multicast-enabled topics over an existing network. Messages sent to topics that are not multicast-enabled are delivered to the message consumer using TCP.
The server sends multicast messages over a multicast channel. Each multicast-enabled topic is associated with a channel. The channel determines the multicast port and multicast group address to which the server sends messages.
The multicast message is received by a multicast daemon running on the same computer with the message consumer. When an EMS client subscribes to a multicast-enabled topic, it automatically connects to the multicast daemon. The multicast daemon begins listening on the channel associated with that topic, receives any broadcast messages, and delivers them to subscribed clients.
Figure 18 shows the communication flow between a multicast message consumer, EMS server, and multicast daemon.
Figure 18 Multicast message consumer creation
The following describes the multicast message consumer creation process:
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The client will continue to receive non-multicast messages directly from the server.
When to Use Multicast
Because multicast reduces the number of operations performed by the server and reduces the amount of bandwidth used in the publish and subscribe model, multicast is highly scalable.
Figure 19 shows how using multicast can reduce the amount of bandwidth used to send a message. Where publish and subscribe messaging creates a copy of a published message for each message consumer, multicast broadcasts the message only once. Multiple multicast daemons listening on the channel receive the same broadcast.
Figure 19 The benefits of multicast
Although multicast can reduce the network resources used by the server, it is not the best messaging model for every system. Multicast offers last-hop delivery only; it cannot be used to send messages between servers. However, messages sent to multicast-enabled topics are still delivered to other subscribed servers using the standard TCP connection.
Requirements
In order to use multicast in your EMS messaging application, the following requirements must be met:
See Configuring Multicast for more information about configuring multicast.
Backwards Compatibility
Multicast is backwards compatible, and can be used in applications where not all EMS clients are using the same version of TIBCO Enterprise Message Service. The EMS sever and any clients that wish to receive messages over multicast must be Software Release 5.0 or later.
Multicast is configured primarily in the EMS server, and is largely invisible to EMS clients. Message producers do not need to be enabled for multicast in order to send multicast messages, because topics are multicast-enabled in the server. However, clients are multicast-enabled by default.
Clients that are not multicast-enabled, either because multicast has been disabled or because the client uses a release of EMS earlier than 5.0, will receive messages from the server over the TCP connection, even if the message topic is multicast-enabled.

Copyright © TIBCO Software Inc. All Rights Reserved
Copyright © TIBCO Software Inc. All Rights Reserved