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Alerting rules

Alerting rules allow you to define alert conditions based on Prometheus expression language expressions and to send notifications about firing alerts to an external service. Whenever the alert expression results in one or more vector elements at a given point in time, the alert counts as active for these elements' label sets.

Alerting rules are configured in Prometheus in the same way as recording rules.

Defining alerting rules

Alerting rules are defined in the following syntax:

ALERT <alert name>
  IF <expression>
  [ FOR <duration> ]
  [ LABELS <label set> ]
  [ ANNOTATIONS <label set> ]

The alert name must be a valid metric name.

The optional FOR clause causes Prometheus to wait for a certain duration between first encountering a new expression output vector element (like an instance with a high HTTP error rate) and counting an alert as firing for this element. Elements that are active, but not firing yet, are in pending state.

The LABELS clause allows specifying a set of additional labels to be attached to the alert. Any existing conflicting labels will be overwritten. The label values can be templated.

The ANNOTATIONS clause specifies another set of labels that are not identifying for an alert instance. They are used to store longer additional information such as alert descriptions or runbook links. The annotation values can be templated.

Templating

Label and annotation values can be templated using console templates. The $labels variable holds the label key/value pairs of an alert instance and $value holds the evaluated value of an alert instance.

# To insert a firing element's label values:
{{ $labels.<labelname> }}
# To insert the numeric expression value of the firing element:
{{ $value }}

Examples:

# Alert for any instance that is unreachable for >5 minutes.
ALERT InstanceDown
  IF up == 0
  FOR 5m
  LABELS { severity = "page" }
  ANNOTATIONS {
    summary = "Instance {{ $labels.instance }} down",
    description = "{{ $labels.instance }} of job {{ $labels.job }} has been down for more than 5 minutes.",
  }

# Alert for any instance that have a median request latency >1s.
ALERT APIHighRequestLatency
  IF api_http_request_latencies_second{quantile="0.5"} > 1
  FOR 1m
  ANNOTATIONS {
    summary = "High request latency on {{ $labels.instance }}",
    description = "{{ $labels.instance }} has a median request latency above 1s (current value: {{ $value }}s)",
  }

Inspecting alerts during runtime

To manually inspect which alerts are active (pending or firing), navigate to the "Alerts" tab of your Prometheus instance. This will show you the exact label sets for which each defined alert is currently active.

For pending and firing alerts, Prometheus also stores synthetic time series of the form ALERTS{alertname="<alert name>", alertstate="pending|firing", <additional alert labels>}. The sample value is set to 1 as long as the alert is in the indicated active (pending or firing) state, and a single 0 value gets written out when an alert transitions from active to inactive state. Once inactive, the time series does not get further updates.

Sending alert notifications

Prometheus's alerting rules are good at figuring what is broken right now, but they are not a fully-fledged notification solution. Another layer is needed to add summarization, notification rate limiting, silencing and alert dependencies on top of the simple alert definitions. In Prometheus's ecosystem, the Alertmanager takes on this role. Thus, Prometheus may be configured to periodically send information about alert states to an Alertmanager instance, which then takes care of dispatching the right notifications. The Alertmanager instance may be configured via the -alertmanager.url command line flag.