Clustered grouping follows a different approach relative to normal message grouping. In a cluster, message groups with specific group ids can arrive on any of the nodes. It is important for a node to determine which group ids are bound to which consumer on which node. Each node is responsible for routing message groups correctly to the node which has the consumer processing those group ids irrespective of where the message groups arrive by default.

This situation is addressed by a grouping handler. Each node has a grouping handler and this grouping handler (along with other handlers) is responsible for routing the message groups to the correct node. There are two types of grouping handlers namely local and remote.

The local handler is responsible for deciding the route which a message group should take. The remote handlers communicate with the local handler and work accordingly. Each cluster should choose a specific node to have a local grouping handler and all the other nodes should have remote handlers.

You can configure "local" and "remote" grouping handlers in server configuration files (standalone.xml and domain.xml) as follows:

<grouping-handler name="my-grouping-handler">
   <type>LOCAL</type>
   <address>jms</address>
   <timeout>5000</timeout>
</grouping-handler>

<grouping-handler name="my-grouping-handler">
   <type>REMOTE</type>
   <address>jms</address>
   <timeout>5000</timeout>
</grouping-handler>

The "timeout" attribute ensures that a routing decision is made quickly within the specified time. If a decision is not made within this time an exception is thrown.

The node which initially receives a message group takes the routing decision based on regular cluster routing conditions (round-robin queue availability). The node proposes this decision to the respective grouping handler which then routes the messages to the proposed queue if it accepts the proposal.

If the grouping handler rejects the proposal, it proposes some other route and the routing takes place accordingly. The other nodes follow suite and forward the message groups to the chosen queue. After a message arrives on a queue it is pinned to a customer on that queue.