3 Triggers

Overview

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:

Trigger status (the expression) is recalculated every time Zabbix server receives a new value that is part of the expression.

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 timer 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.

Calculation time

A trigger is recalculated every time Zabbix server receives a new value that is part of the expression. When a new value is received, each function that is included in the expression is recalculated (not just the one that received the new value).

Additionally, a trigger is recalculated each time when a new value is received and every 30 seconds if time-based functions are used in the expression.

Time-based functions are nodata(), date(), dayofmonth(), dayofweek(), time(), now()); they are recalculated every 30 seconds by the Zabbix history syncer process.

Evaluation period

An evaluation period is used in functions referencing the item history. It allows to specify the interval we are interested in. It can be specified as time period (30s, 10m, 1h) or as a value range (#5 - for five latest values).

The evaluation period is measured up to "now" - where "now" is the latest recalculation time of the trigger (see Calculation time above); "now" is not the "now" time of the server.

The evaluation period specifies either:

• To consider all values between "now-time period" and "now" (or, with time shift, between "now-time shift-time period" and "now-time_shift")
• To consider no more than the num count of values from the past, up to "now"
  • If there are 0 available values for the time period or num count specified - then the trigger or calculated item that uses this function becomes unsupported

Note that:

• If only a single function (referencing data history) is used in the trigger, "now" is always the latest received value. For example, if the last value was received an hour ago, the evaluation period will be regarded as up to the latest value an hour ago.
• A new trigger is calculated as soon as the first value is received (history functions); it will be calculated within 30 seconds for time-based functions. Thus the trigger will be calculated even though perhaps the set evaluation period (for example, one hour) has not yet passed since the trigger was created. The trigger will also be calculated after the first value, even though the evaluation range was set, for example, to ten latest values.