A publisher can receive notifications when a client queue has reached a certain size and use this information to decide whether or not to act on the situation.

For example, the publisher might want to notify the client so that it can take some action (like suspending processing). As there is little point in queuing a message to tell the client that their queue is becoming full, this is probably done using a high priority message which goes to the front of the queue.

To this end, an upper notification threshold can be set for a client's queue. This is expressed as a percentage of the maximum queue depth at which a notification is sent to any client listeners that are declared. A client listener is any object that implements the ClientListener interface and such a listener can be added from within a publisher using the Publishers.addEventListener method. Listeners are notified of the limit breach using the clientQueueThresholdReached method.

In addition a lower notification threshold can be specified. The lower threshold is a percentage of the maximum queue depth at which a notification occurs when a message is removed from the queue causing the queue size to reach the threshold if (and only if) the upper threshold has been breached.

When the clientQueueThresholdReached method is called on the client listener it indicates whether it was the upper or lower threshold that was reached.

The thresholds to use for clients can be configured for a connector in connectors.properties. If not specified, the default thresholds specified in etc/Server.xml are used.

The thresholds on a client connector can be changed dynamically at run time using System Management, but the new values only take effect for new clients.

Thresholds can also be set or changed from within the publisher for a connected client using the Client.setQueueNotificationThresholds method.