16.3. HAPartition

HAPartition is a general purpose service used for a variety of tasks in AS clustering. At its core, it is an abstraction built on top of a JGroups Channel that provides support for making/receiving RPC invocations on/from one or more cluster members. HAPartition also supports a distributed registry of which clustering services are running on which cluster members. It provides notifications to interested listeners when the cluster membership changes or the clustered service registry changes. HAPartition forms the core of many of the clustering services we'll be discussing in the rest of this guide, including smart client-side clustered proxies, EJB 2 SFSB replication and entity cache management, farming, HA-JNDI and HA singletons.

The following example shows the HAPartition MBean definition packaged with the standard JBoss AS distribution. So, if you simply start JBoss servers with their default clustering settings on a local network, you would get a default cluster named DefaultPartition that includes all server instances as its nodes.

<mbean code="org.jboss.ha.framework.server.ClusterPartition"
    name="jboss:service=DefaultPartition">
         
    <! -- Name of the partition being built -->
    <attribute name="PartitionName">
        ${jboss.partition.name:DefaultPartition}
    </attribute>

    <! -- The address used to determine the node name -->
    <attribute name="NodeAddress">${jboss.bind.address}</attribute>

    <! -- Determine if deadlock detection is enabled -->
    <attribute name="DeadlockDetection">False</attribute>
     
    <! -- Max time (in ms) to wait for state transfer to complete. 
        Increase for large states -->
    <attribute name="StateTransferTimeout">30000</attribute>

    <! -- The JGroups protocol configuration -->
    <attribute name="PartitionConfig">
        ... ...
    </attribute>
</mbean>

Here, we omitted the detailed JGroups protocol configuration for this channel. JGroups handles the underlying peer-to-peer communication between nodes, and its configuration is discussed in Section 22.1, “JGroups Configuration”. The following list shows the available configuration attributes in the HAPartition MBean.

PartitionName is an optional attribute to specify the name of the cluster. Its default value is DefaultPartition. Use the -g (a.k.a. --partition) command line switch to set this value at JBoss startup.
NodeAddress is an optional attribute used to help generate a unique name for this node.
DeadlockDetection is an optional boolean attribute that tells JGroups to run message deadlock detection algorithms with every request. Its default value is false.
StateTransferTimeout is an optional attribute to specify the timeout for state replication across the cluster (in milliseconds). State replication refers to the process of obtaining initial application state from other already-running cluster members at service startup. Its default value is 30000.
PartitionConfig is an element to specify JGroup configuration options for this cluster (see Section 22.1, “JGroups Configuration”).

In order for nodes to form a cluster, they must have the exact same PartitionName and the ParitionConfig elements. Changes in either element on some but not all nodes would cause the cluster to split.

You can view the current cluster information by pointing your browser to the JMX console of any JBoss instance in the cluster (i.e., http://hostname:8080/jmx-console/) and then clicking on the jboss:service=DefaultPartition MBean (change the MBean name to reflect your partitionr name if you use the -g startup switch). A list of IP addresses for the current cluster members is shown in the CurrentView field.

Note

While it is technically possible to put a JBoss server instance into multiple HAPartitions at the same time, this practice is generally not recommended, as it increases management complexity.