Monitoring

The monitoring service complies with the JMX specification and provides a polling mechanism based on the value of MBean attributes. There are three monitor MBeans, one for counter attributes, another for gauge-like attributes, and a third for strings. These monitors send notifications when the observed attribute meets certain conditions, mainly equalling or exceeding a threshold.

Monitor MBeans observe the variation of an MBean attribute's value over time. All monitors have a configurable granularity period that determines how often the attribute is polled. Each of the monitors has specific settings for the type of the observed attribute:

• Counter monitor - Observes an attribute of integer type that is monotonically increasing. The counter monitor has a threshold value and an offset value to detect counting intervals. The counter monitor will reset the threshold if the counter rolls over.

• Gauge monitor - Observes an attribute of integer or floating point types that fluctuates within a given range. The gauge monitor has both a high and low threshold, each of which can trigger a distinct notification. The two thresholds can also be used to avoid repeated triggering when an attribute oscillates around a threshold.

• String monitor - Observes an attribute of type String. The string monitor performs a full string comparison between the observed attribute and its match string. A string monitor sends notifications both when the string matches and when it differs at the observation time. Repeated notifications are not sent, meaning that only one notification is sent the first time the string matches or differs.

Monitor notifications contain the name of the observed MBean, the name of the observed attribute, the value which triggered the event, as well as the previous value for comparison. Using this information, listeners know which MBean triggered an event, and they don't need to access the MBean before taking the appropriate action.

Monitor MBeans may also send notifications when certain error cases are encountered during an observation.