Many derived metrics are provided about the facts being published. To find out whether a fact has stopped being updated, you can use Update-Age and Change-Age. These values represent the time (in milliseconds) since the fact was last updated or last changed. The difference between them is that Update-Age is reset to 0 when any value is published, and Change-Age is only reset to 0 when the actual value is different than the previous value.
For example, testing f0:Update-Age is greater than 30000 would indicate the value had not been updated in more than 30 seconds.
However, to optimize performance, a sensor's logic is only driven when its specified facts are published. This means that if the sensor only subscribes to this one fact AND if the expert stopped publishing it, the logic to test the Update-Age would not be driven. To make sure the logic is driven periodically, the solution is to include in the expert any fact that changes regularly. One example is under System facts: the CEP System fact System/time value is updated every 30 seconds. This fact does not need to be used in the sensor logic; it must simply be selected as a Fact to Monitor.
When %f0:Update-Age is greater than 60000