The JGroups framework provides services to enable peer-to-peer communications between nodes in a cluster. It is built on top a stack of network communication protocols that provide transport, discovery, reliability and failure detection, and cluster membership management services. Figure 2.1, “Protocol stack in JGroups” shows the protocol stack in JGroups.

JGroups configurations often appear as a nested attribute in cluster related MBean services, such as the PartitionConfig attribute in the ClusterPartition MBean or the ClusterConfig attribute in the TreeCache MBean. You can configure the behavior and properties of each protocol in JGroups via those MBean attributes. Below is an example JGroups configuration in the ClusterPartition MBean.

<mbean code="org.jboss.ha.framework.server.ClusterPartition"
    name="jboss:service=DefaultPartition">

    ... ...
    
    <attribute name="PartitionConfig">
        <Config>
            <UDP mcast_addr="228.1.2.3" mcast_port="45566"
               ip_ttl="8" ip_mcast="true"
               mcast_send_buf_size="800000" mcast_recv_buf_size="150000"
               ucast_send_buf_size="800000" ucast_recv_buf_size="150000"
               loopback="false"/>
            <PING timeout="2000" num_initial_members="3"
               up_thread="true" down_thread="true"/>
            <MERGE2 min_interval="10000" max_interval="20000"/>
            <FD shun="true" up_thread="true" down_thread="true"
               timeout="2500" max_tries="5"/>
            <VERIFY_SUSPECT timeout="3000" num_msgs="3"
               up_thread="true" down_thread="true"/>
            <pbcast.NAKACK gc_lag="50"
               retransmit_timeout="300,600,1200,2400,4800"
               max_xmit_size="8192"
               up_thread="true" down_thread="true"/>
            <UNICAST timeout="300,600,1200,2400,4800" 
               window_size="100" min_threshold="10"
               down_thread="true"/>
            <pbcast.STABLE desired_avg_gossip="20000"
               up_thread="true" down_thread="true"/>
            <FRAG frag_size="8192"
               down_thread="true" up_thread="true"/>
            <pbcast.GMS join_timeout="5000" join_retry_timeout="2000"
               shun="true" print_local_addr="true"/>
            <pbcast.STATE_TRANSFER up_thread="true" down_thread="true"/>
        </Config>
    </attribute>
</mbean>
        

All the JGroups configuration data is contained in the <Config> element under the JGroups config MBean attribute. In the next several sections, we will dig into the options in the <Config> element and explain exactly what they mean.