4.9.5. Queue Threshold Alerts (Edge-triggered)

In MRG 3, Queue Threshold alerts are edge-triggered. Thresholds are configured per-queue and are:
  • qpid.alert_count_up - upper threshold (messages)
  • qpid.alert_size_up - upper threshold (bytes)
  • qpid.alert_count_down - lower threshold (messages)
  • qpid.alert_size_down - lower threshold (bytes)
By default, the upward threshold is set to the global threshold ratio multiplied by the maximum size/count of the queue. The global threshold ratio can be specified using the --default-event-threshold-ratio command line option, otherwise it defaults to 80%.
By default, the downward threshold is set to one half of the default upward threshold.
Note: The upper and lower thresholds should have a gap between them to limit event rates.
Events

There are two different events:

Threshold crossed increasing
The increasing event is raised when the queue depth goes from (upper-threshold - 1) to upper-threshold and the increasing event flag is not already set. When an increasing event occurs the increasing event flag is set. The increasing event flag must be cleared (by a decreasing event) before further increasing events will be raised. This prevents multiple retriggering of this event by fluctuation of queue depth around the upper-threshold.
Threshold crossed decreasing
The decreasing event is raised when the increasing event flag is set and the queue depth goes from (lower-threshold + 1) to lower-threshold. The decreasing event clears the increasing event flag, allowing further increasing events to be triggered and preventing multiple retriggering of this event by fluctuation of queue depth around the lower-threshold.
The events are sent via the QMF framework. You can subscribe to the event messages by listening to the addresses:
  • qmf.default.topic/agent.ind.event.org_apache_qpid_broker.queueThresholdCrossedUpward.#
  • qmf.default.topic/agent.ind.event.org_apache_qpid_broker.queueThresholdCrossedDownward.#
Events are sent as map messages:
qmf::org::apache::qpid::broker::EventQueueThresholdCrossedUpward(name, count, size)
qmf::org::apache::qpid::broker::EventQueueThresholdCrossedDownward(name, count, size)
The following code demonstrates subscribing to and consuming threshold event messages:

Window One

Python
import sys
from qpid.messaging import *
conn = Connection.establish("localhost:5672")
session = conn.session()
rcv = session.receiver("qmf.default.topic/agent.ind.event.org_apache_qpid_broker.queueThresholdCrossedUpward.#")
while True:
   event = rcv.fetch()
   print "Threshold exceeded on queue %s" % event.content[0]["_values"]["qName"]
   print "at a depth of %s messages, %s bytes" % (event.content[0]["_values"]["msgDepth"], event.content[0]["_values"]["byteDepth"])
   session.acknowledge()

Window Two

Python
import sys
from qpid.messaging import *
connection = Connection.establish("localhost:5672")
session = connection.session()
rcv = session.receiver("threshold-queue; {create:always, node:{x-declare:{auto-delete:True, arguments:{'qpid.alert_count_down':1,'qpid.alert_count_up':3}}}}")
snd = session.sender("threshold-queue")

snd.send("Message1")
snd.send("Message2")
snd.send("Message3")
rcv.fetch()
rcv.fetch()
rcv.fetch()
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