constant.md

Rule: Constant

Constants are group decorators, adding trivial values.

Constants are useful to add to metric groups, to later compare values inside the metric group with that constant.

Constants thus added are also emitted as metrics.

Syntax

constant groupname metricname value ;

• groupname The name of a group, upon which to apply the constant.
• metricname The name of a metric.
• value A value to assign to the metricname.

If the groupname exists, a metric will be added, with the given value under the name specified as metricname. If the group does not exist, no action will be taken. If the metric already exists, it will be overwritten.

Example

constant qps critical 200;
constant specific.endpoint.qps critical 20;

If the group qps exists, will add or replace a metric critical with the value 200. If the group qps doesn't exist, the constant won't be added.

Similarly for the specific.endpoint.qps group, which will have critical = 20.

If, later in the configuration file, a match statement declares an alert for both, the alert can be made to evaluate against different values:

match **.qps as qps_metric {
  alert ${qps_metric}
  if qps_metric value > qps_metric critical
  for 10m;
}

This will evaluate to:

alert qps
if qps value > qps critical  # qps critical = 200
for 10m;

alert specific.endpoint.qps
if specific.endpoint.qps value > specific.endpoint.qps critical  # = 20
for 10m;