How the WMB AggregateReply node builds compound messages

Compound message building on the AggregateReply node can be triggered in three cases:

1. When the AggregateControl node timeout expires. The timeout expiration triggers a known timeout thread on which all the reply messages that are available on the SYSTEM.BROKER.AGGR.REPLY queue and replies that have reached the SYSTEM.BROKER.AGGR.UNKNOWN queue are appended under the AggregateReplyBody of the compound message. If no reply message is available on the SYSTEM.BROKER.AGGR.REPLY or SYSTEM.BROKER.AGGR.UNKNOWN queues, the AggregateReply node propagates a message with an empty Properties and AggregateReplyBody, as nothing has been received with which to build a compound message.
   
   
2. When an unknown message times out, based on the Unknown Message Timeout property that is set on the AggregateReply node. This timeout collates the corresponding request messages on the SYSTEM.BROKER.AGGR.REQUEST queue and any replies that have reached the SYSTEM.BROKER.AGGR.REPLY queue to build a compound message.
   
   
3. When the last reply arrives on the AggregateReply node. The last reply is appended at the end of the AggregateReplyBody.
   
   

In each of these cases a reply is required for processing and building a compound message.