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120 lines
4.8 KiB
Markdown
120 lines
4.8 KiB
Markdown
---
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title: Alerting rules
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sort_rank: 3
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---
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# Alerting rules
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Alerting rules allow you to define alert conditions based on Prometheus
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expression language expressions and to send notifications about firing alerts
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to an external service. Whenever the alert expression results in one or more
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vector elements at a given point in time, the alert counts as active for these
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elements' label sets.
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### Defining alerting rules
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Alerting rules are configured in Prometheus in the same way as [recording
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rules](recording_rules.md).
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An example rules file with an alert would be:
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```yaml
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groups:
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- name: example
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labels:
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team: myteam
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rules:
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- alert: HighRequestLatency
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expr: job:request_latency_seconds:mean5m{job="myjob"} > 0.5
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for: 10m
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keep_firing_for: 5m
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labels:
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severity: page
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annotations:
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summary: High request latency
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```
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The optional `for` clause causes Prometheus to wait for a certain duration
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between first encountering a new expression output vector element and counting
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an alert as firing for this element. In this case, Prometheus will check that
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the alert continues to be active during each evaluation for 10 minutes before
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firing the alert. Elements that are active, but not firing yet, are in the pending state.
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Alerting rules without the `for` clause will become active on the first evaluation.
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There is also an optional `keep_firing_for` clause that tells Prometheus to keep
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this alert firing for the specified duration after the firing condition was last met.
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This can be used to prevent situations such as flapping alerts, false resolutions
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due to lack of data loss, etc. Alerting rules without the `keep_firing_for` clause
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will deactivate on the first evaluation where the condition is not met (assuming
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any optional `for` duration desribed above has been satisfied).
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The `labels` clause allows specifying a set of additional labels to be attached
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to the alert. Any existing conflicting labels will be overwritten. The label
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values can be templated.
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The `annotations` clause specifies a set of informational labels that can be used to store longer additional information such as alert descriptions or runbook links. The annotation values can be templated.
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#### Templating
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Label and annotation values can be templated using [console
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templates](https://prometheus.io/docs/visualization/consoles). The `$labels`
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variable holds the label key/value pairs of an alert instance. The configured
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external labels can be accessed via the `$externalLabels` variable. The
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`$value` variable holds the evaluated value of an alert instance.
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# To insert a firing element's label values:
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{{ $labels.<labelname> }}
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# To insert the numeric expression value of the firing element:
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{{ $value }}
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Examples:
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```yaml
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groups:
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- name: example
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rules:
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# Alert for any instance that is unreachable for >5 minutes.
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- alert: InstanceDown
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expr: up == 0
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for: 5m
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labels:
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severity: page
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annotations:
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summary: "Instance {{ $labels.instance }} down"
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description: "{{ $labels.instance }} of job {{ $labels.job }} has been down for more than 5 minutes."
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# Alert for any instance that has a median request latency >1s.
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- alert: APIHighRequestLatency
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expr: api_http_request_latencies_second{quantile="0.5"} > 1
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for: 10m
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annotations:
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summary: "High request latency on {{ $labels.instance }}"
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description: "{{ $labels.instance }} has a median request latency above 1s (current value: {{ $value }}s)"
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```
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### Inspecting alerts during runtime
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To manually inspect which alerts are active (pending or firing), navigate to
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the "Alerts" tab of your Prometheus instance. This will show you the exact
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label sets for which each defined alert is currently active.
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For pending and firing alerts, Prometheus also stores synthetic time series of
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the form `ALERTS{alertname="<alert name>", alertstate="<pending or firing>", <additional alert labels>}`.
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The sample value is set to `1` as long as the alert is in the indicated active
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(pending or firing) state, and the series is marked stale when this is no
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longer the case.
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### Sending alert notifications
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Prometheus's alerting rules are good at figuring what is broken *right now*, but
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they are not a fully-fledged notification solution. Another layer is needed to
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add summarization, notification rate limiting, silencing and alert dependencies
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on top of the simple alert definitions. In Prometheus's ecosystem, the
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[Alertmanager](https://prometheus.io/docs/alerting/alertmanager/) takes on this
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role. Thus, Prometheus may be configured to periodically send information about
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alert states to an Alertmanager instance, which then takes care of dispatching
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the right notifications.
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Prometheus can be [configured](configuration.md) to automatically discover available
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Alertmanager instances through its service discovery integrations.
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