JBoss.org Community Documentation

19.7.10. Changing the multicast address and port

The -u (a.k.a. --udp) command line switch may be used to control the multicast address used by the JGroups channels opened by all standard AS services.

/run.sh -u -g QAPartition -b -c all

This switch sets the jboss.partition.udpGroup system property, which you can see referenced in all of the standard protocol stack configs in JBoss AS:

<UDP mcast_addr="${jboss.partition.udpGroup:}"

Unfortunately, setting the multicast ports is not so simple. As described above, by default there are four separate JGroups channels in the standard JBoss AS all configuration, and each should be given a unique port. There are no command line switches to set these, but the standard configuration files do use system properties to set them. So, they can be configured from the command line by using -D. For example,

	/run.sh -u -g QAPartition -Djboss.hapartition.mcast_port=12345 -Djboss.webpartition.mcast_port=23456 -Djboss.ejb3entitypartition.mcast_port=34567 -Djboss.ejb3sfsbpartition.mcast_port=45678 -b -c all

Why isn't it sufficient to change the group name?

If channels with different group names share the same multicast address and port, the lower level JGroups protocols in each channel will see, process and eventually discard messages intended for the other group. This will at a minimum hurt performance and can lead to anomalous behavior.

Why do I need to change the multicast port if I change the address?

It should be sufficient to just change the address, but there is a problem on several operating systems whereby packets addressed to a particular multicast port are delivered to all listeners on that port, regardless of the multicast address they are listening on. So the recommendation is to change both the address and the port.