Reliability And Flow Control

Because “payload” messages sent and received by clients and control messages (such as broker acknowledgments) used by Message Queue itself pass over the same client-broker connection, excessive levels of payload traffic can interfere with the delivery of control messages. To help alleviate this problem, the connection factory attributes listed in Table 18–5 allow you to manage the relative flow of the two types of message. These attributes fall into four categories:

• Acknowledgment timeout specifies the maximum time (imqAckTimeout) to wait for a broker acknowledgment before throwing an exception.

• Connection flow metering limits the transmission of payload messages to batches of a specified size (imqConnectionFlowCount), ensuring periodic opportunities to deliver any accumulated control messages.

• Connection flow control limits the number of payload messages (imqConnectionFlowLimit) that can be held pending on a connection, waiting to be consumed. When the limit is reached, delivery of payload messages to the connection is suspended until the number of messages awaiting consumption falls below the limit. Use of this feature is controlled by a boolean flag (imqConnectionFlowLimitEnabled).

• Consumer flow control limits the number of payload messages (imqConsumerFlowLimit) that can be held pending for any single consumer, waiting to be consumed. (This limit can also be specified as a property of a specific queue destination, consumerFlowLimit.) When the limit is reached, delivery of payload messages to the consumer is suspended until the number of messages awaiting consumption, as a percentage of imqConsumerFlowLimit, falls below the limit specified by the imqConsumerFlowThreshold attribute. This helps improve load balancing among multiple consumers by preventing any one consumer from starving others on the same connection.

The use of any of these flow control techniques entails a trade-off between reliability and throughput; see Client Runtime Message Flow Adjustments for further discussion.