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In this lesson, you will learn how to add new alerting rules using the Prometheus operator. Similar to service monitors used for target registration, the Prometheus operator provides a Custom Resource Definition (CRD) called PrometheusRule. This CRD enables you to register alerting rules with a Prometheus instance. The configuration is nearly identical to that of a service monitor, with the main differences being the resource type and the specifics of the rule specification.

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Defining a PrometheusRule

Below is an example of a PrometheusRule definition: 
apiVersion: monitoring.coreos.com/v1
kind: PrometheusRule
metadata:
  labels:
    release: prometheus
  name: api-rules
spec:
  groups:
    - name: api
      rules:
        - alert: down
          expr: up == 0
          for: 0m
          labels:
            severity: critical
          annotations:
            summary: Prometheus target missing {{$labels.instance}}
This configuration is similar to the alerting rules you would normally define in the traditional prometheus.yaml file, but now it is encapsulated as a Kubernetes resource.

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Understanding the Rule Selector

When you run the command below: 
kubectl get prometheuses.monitoring.coreos.com -o yaml
you might see an output like: 
ruleSelector:
  matchLabels:
    release: prometheus
The ruleSelector property works similarly to a service discovery selector. It tells Prometheus which PrometheusRule resources to load dynamically by matching labels. Therefore, ensure that any PrometheusRule you create includes the label release: prometheus (or the corresponding label used in your configuration).
Ensure that your PrometheusRule metadata labels match the rule selector specified in your Prometheus configuration.

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Creating and Applying New Rules

To add new rules to your deployment, follow these steps:
  1. Create a file (e.g., rules.yaml) and paste your PrometheusRule configuration into it.
  2. Apply the configuration with the following command: 
    kubectl apply -f rules.yaml
If you encounter an error indicating that no objects were provided for the apply command, verify that your file is saved and contains valid YAML content.
After applying the configuration, check the creation of your PrometheusRule by running:
kubectl get prometheusrule
You should see output similar to the example below, where your API rule appears among the pre-existing rules: 
kubectl apply -f rules.yaml
prometheusrule.monitoring.coreos.com/api-rules created
kubectl get prometheusrule
NAME
api-rules
prometheus-kube-prometheus-alertmanager.rules     16h
prometheus-kube-prometheus-config-loaders          16h
prometheus-kube-prometheus-etc                      16h
prometheus-kube-prometheus-general.rules           16h
prometheus-kube-prometheus-k8s.rules               16h
prometheus-kube-prometheus-kube-apiserver-availability.rules 16h
prometheus-kube-prometheus-burnrate.rules         16h
prometheus-kube-prometheus-history.rules          16h
prometheus-kube-prometheus-general.rules          16h
prometheus-kube-prometheus-node-recording.rules   16h
prometheus-kube-prometheus-kube-state-metrics     16h
prometheus-kube-prometheus-kubernetes-apps       52s

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Verifying in Prometheus UI

After applying the rule, you can verify its registration in the Prometheus UI:
  1. Navigate to the “Status” > “Rules” tab.
  2. Locate the “api” group with the registered alert.
This confirms that the rule was successfully detected and loaded by Prometheus.

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Advanced Alerting Rules

Below is an example of a more advanced alerting configuration that includes additional rules beyond the basic API alert: 
api:
  alert: down
  expr: up == 0
  labels:
    severity: critical
  annotations:
    summary: Prometheus target missing {{ $labels.instance }}

alertmanager.rules:
  - alert: AlertmanagerFailedReload
    expr: max_over_time(alertmanager.config_last_reload_successful{job="prometheus-kube-prometheus-alertmanager", namespace="default"}[5m]) == 0
    for: 10m
    labels:
      severity: critical
    annotations:
      description: Configuration has failed to load for {{ $labels.namespace }} {{ $labels.pod }}.
      runbook_url: https://runbooks.prometheus-operator.dev/runbooks/alertmanager/alertmanagerfailedreload
      summary: Reloading an Alertmanager configuration has failed

  - alert: AlertmanagerMembersInconsistent
    expr: max_over_time(alertmanager.cluster_members{job="prometheus-kube-prometheus-alertmanager", namespace="default"}[5m]) < on (namespace, service) group_left () count by (namespace, service) (max_over_time(alertmanager.cluster_members{job="prometheus-kube-prometheus-alertmanager", namespace="default"}[5m]))
    for: 15m

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Summary

To register new alerting rules with Prometheus:
  1. Create a PrometheusRule resource with the desired groups and rules.
  2. Ensure the metadata labels match the rule selector used by your Prometheus instance.
  3. Apply the configuration using kubectl apply -f rules.yaml.
  4. Verify the rule registration via the Prometheus UI or with kubectl get prometheusrule.
By following these steps, you can efficiently add and manage alerting rules using the Prometheus operator.

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