2.5.3 Monitoring

The monitoring service complies with the JMX specification and provides a polling mechanism based on the value of MBean attributes. The monitoring service contains three monitor MBeans, one MBean for counter attributes, another MBean for gauge-like attributes, and a third MBean for strings. These monitors send notifications when the observed attribute meets certain conditions, mainly equaling 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 monitor has specific settings for the type of the observed attribute, as follows.

• Counter monitor, which observes an attribute of the integer type (byte, integer, short or long) that is monotonically increasing. The counter monitor has a threshold value and an offset value to detect counting intervals. The counter monitor resets the threshold if the counter rolls over.

• Gauge monitor, which observes an attribute of integer (byte, integer, short, or long) or floating-point (float or double) 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, and the value that triggered the event, as well as the previous value for comparison. This information allows listeners to know which MBean triggered an event. The listeners do not need to access the MBean before taking the appropriate action.

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