On-change monitors are used to periodically check the value of a BACI property for changes. Basically they are used in the following manner:

1. Before even starting ACS, the developer should change the min_timer_trig characteristic of BACI properties he or she is interested in creating on-change monitors for (found somewhere in $ACS_CDB/CDB/alma/*) to some positive non-zero value. This particular characteristic is the polling rate at which the BACI property will eventually poll its value at to determine if it has changed by some delta value since the last time period. Once the property has been created, there is no way to change this behaviour short of editing the CDB, clearing its cache, and then restarting the container.
2. The first optional step is to change the default_timer_trig characteristic of BACI properties within the CDB. This defines a span of time in which a property's value must change by some delta value for the on-change monitor to be triggered. To better explain this, an example is in order. Suppose you have an on-change monitor for a ROlong property where the min_timer_trig is two seconds, the default_timer_trig is ten seconds, and the delta value is 20. This monitor is triggered under two different conditions the first being that a time period equivalent to the default_timer_trig, ten seconds, has passed. Looking at this behaviour, it's identical to the way normal monitors work and is not too interesting. However, what's going on behind the scenes is that once every two seconds within that ten second duration the property's value is being retrieved. If the value changes by the delta value, 20, between the beginning of the ten second duration and any time before the end of the ten second duration, the monitor is triggered early. User can check if the value was sent due the value had been changed or timer had expired by examining the type and code value of associated callback value:
   • If type is ACSErrTypeMonitor (1) and code is ACSErrMonitorOnTimer (0) than value was sent because the timer had expired.
   • If type is ACSErrTypeMonitor (1) and code is ACSErrMonitorOnValue (1) than the value was sent because the value had been changed.
3. The second optional step is to modify the min_delta_trig characteristic of BACI properties within the CDB. This makes it impossible at run-time to set a delta value for an on-change monitor smaller than the value of min_delta_trig.
4. A developer creates a monitor for a specific BACI property via one of it's IDL methods (typically something like create_monitor)
5. Using the Monitorxyz interface returned by the BACI property, the developer can turn on on-change monitors by utilizing the set_value_by_trigger method. The first parameter to this method would be the delta value that was spoken of earlier and the second value is boolean true.
6. To turn off on-change monitors, the developer simply invokes set_value_by_trigger again, this time passing boolean false as the second parameter.

NOTES:

• The delta value is ignored for Pattern monitors. Any change from one min_timer_trig epoch to the next will trigger the monitor.
• The delta value is ignored for String monitors. Any change from one min_timer_trig epoch to the next will trigger the monitor.