The following information has been incorporated in Alarm System
Notes on the implementation of the MIDAS alarm system.
Alarms are checked inside alarm.c::al_check(). This function is called by cm_yield() every 10 seconds and by rpc_server_thread(), also every 10 seconds.
For remote midas clients, their al_check() issues an RPC_AL_CHECK RPC call into the mserver, where rpc_server_dispatch() calls the local al_check().
As result, all alarm checks run inside a process directly attached to the local midas shared memory (inside a local client or inside an mserver process for a remote client).
Each and every midas client runs the alarm checks. To prevent race conditions between different midas clients, access to al_check() is serialized using the ALARM semaphore.
Inside al_check(), alarms are triggered using al_trigger_alarm(), which in turn calls al_trigger_class(). Inside al_trigger_class(), the alarm is recorded into an elog or into midas.log using cm_msg(MTALK).
Special note should be made of the ODB setting "/Alarm/Classes/xxx/System message interval", which has a surprising effect - after an alarm is recorded into system messages (using cm_msg(MTALK)), no record is made of any subsequent alarms until the time interval set by this variable elapses. With default value of 60 seconds, after one alarm, no more alarms are recorded for 60 seconds. Also, because all the alarms are checked at the same time, only the first triggered alarm will be recorded.
As of alarm.c rev 4683, "System message interval" set to 0 ensures that every alarm is recorded into the midas log file. (In previous revisions, this setting may still miss some alarms).
There are 3 types of alarms:
- "program not running" alarms.
These alarms are enabled in ODB by setting "/Programs/ppp/Alarm class". Each time al_check() runs, every program listed in "/Programs" is tested using "cm_exist()" and if the program is not running, the time of first failure is remembered in "/Programs/ppp/First failed".
If the program has not been running for longer than the time set in ODB "/Programs/ppp/Check interval", an alarm is triggered (if enabled by "/Programs/ppp/Alarm class" and the program is restarted (if enabled by "/Programs/ppp/Auto restart").
The "not running" condition is tested every 10 seconds (each time al_check() is called), but the frequency of "program not running" alarms can be reduced by increasing the value of "/Alarms/Alarms/ppp/Check interval" (default value 60 seconds). This can be useful if "System message interval" is set to zero.
- "evaluated" alarms
- "periodic" alarms
There is nothing surprising in these alarms. Each alarm is checked with a time period set by "/Alarm/xxx/Check interval". The value of an evaluated alarm is computed using al_evaluate_condition().
K.O. ELOG V2.9.2-2455 https://ladd00.triumf.ca/elog/Midas/695