Triggers are logical expressions that "evaluate" data gathered by items and represent the current system state.
While items are used to gather system data, it is highly impractical to follow these data all the time waiting for a condition that is alarming or deserves attention. The job of "evaluating" data can be left to trigger expressions.
Trigger expressions allow to define a threshold of what state of data is "acceptable". Therefore, should the incoming data surpass the acceptable state, a trigger is "fired" - or changes status to PROBLEM.
A trigger may have the following status:
|OK||This is a normal trigger state.|
|PROBLEM||Normally means that something happened. For example, the processor load is too high.|
Trigger status (the expression) is recalculated every time Zabbix server receives a new value that is part of the expression.
Most trigger functions are evaluated based on history data, while some trigger functions for long-term analytics, e.g. trendavg(), trendcount(), etc, use trend data.
If time-based functions (nodata(), date(), dayofmonth(), dayofweek(), time(), now()) are used in the expression, the trigger is recalculated every 30 seconds by a Zabbix history syncer process. If both time-based and non-time-based functions are used in an expression, it is recalculated when a new value is received and every 30 seconds.
You can build trigger expressions with different degrees of complexity.