Prometheus is a system used to

• Collect metrics (e.g. memory, CPU) - This is often referred to as "scraping metrics"
• Store the metrics, for example, in object storage, such as an Amazon Web Services S3 Bucket
• Configure conditions that should create an alert (e.g. high CPU or high memory usage)

For example, Prometheus can be used to gather metrics from servers, virtual machines (VMs), databases, containers (e.g. Docker, OpenShift), messaging (e.g. IBM MQ, RabbitMQ), and the list goes on. Then the metrics could be stored in object storage, such as an Amazon Web Services (AWS) S3 Bucket.

Often, an Observability system such as Kibana is used to display the metrics with a UI that is used to display query the metrics, for example, to find systems with high CPU or memory usage.

Also, an alerting system such as Alert Manager is often used to create alerts when a certain condition is met, such as a system with high CPU or high memory usage. The alerting system would route alerts to certain targets, such as an SMTP email server or OpsGenie.

The kubectl get PrometheusRules (Kubernetes) or oc get PrometheusRules (OpenShift) command can be used to list the Prometheus Rules.

~]$ oc get PrometheusRules --namespace openshift-monitoring
NAME                                           AGE
alertmanager-main-rules                        692d
cluster-monitoring-operator-prometheus-rules   692d
kube-state-metrics-rules                       692d
kubernetes-monitoring-rules                    692d
node-exporter-rules                            692d
prometheus-k8s-prometheus-rules                692d
prometheus-k8s-thanos-sidecar-rules            692d
prometheus-operator-rules                      692d
telemetry                                      692d
thanos-querier                                 692d

For example, let's say you want to make a change to one of the Alert Manager rules. Let's redirect the alertmanager-main-rules to a YAML file.

oc get PrometheusRule alertmanager-main-rules --namespace openshift-monitoring --output yaml > alertmanager-main-rules.yaml

And then let's make some change to the YAML file. For example, perhaps updating AlertmanagerFailedToSendAlerts from 5m (five minutes) to 10m (ten minutes).

    - alert: AlertmanagerFailedToSendAlerts
      annotations:
        description: Alertmanager {{ $labels.namespace }}/{{ $labels.pod}} failed
          to send {{ $value | humanizePercentage }} of notifications to {{ $labels.integration
          }}.
        runbook_url: https://github.com/openshift/runbooks/blob/master/alerts/cluster-monitoring-operator/AlertmanagerFailedToSendAlerts.md
        summary: An Alertmanager instance failed to send notifications.
      expr: |
        (
          rate(alertmanager_notifications_failed_total{job=~"alertmanager-main|alertmanager-user-workload"}[10m])
        /
          ignoring (reason) group_left rate(alertmanager_notifications_total{job=~"alertmanager-main|alertmanager-user-workload"}[10m])
        )
        > 0.01
      for: 10m
      labels:
        severity: warning

Let's apply the updated YAML to update the alertmanager-main-rules using the updated YAML.

oc apply --filename alertmanager-main-rules.yaml

This should cause the prometheus-k8s-rulefiles-0 configmap to get updated to have 10m (ten minutes) for AlertmanagerFailedToSendAlerts.

~]$ oc get configmap prometheus-k8s-rulefiles-0 --namespace openshift-monitoring --output yaml | grep AlertmanagerFailedToSendAlerts -A 11
      - alert: AlertmanagerFailedToSendAlerts
        annotations:
          description: Alertmanager {{ $labels.namespace }}/{{ $labels.pod}} failed to
            send {{ $value | humanizePercentage }} of notifications to {{ $labels.integration
            }}.
          runbook_url: https://github.com/openshift/runbooks/blob/master/alerts/cluster-monitoring-operator/AlertmanagerFailedToSendAlerts.md
          summary: An Alertmanager instance failed to send notifications.
        expr: |
          (
            rate(alertmanager_notifications_failed_total{job=~"alertmanager-main|alertmanager-user-workload"}[10m])
          /
            ignoring (reason) group_left rate(alertmanager_notifications_total{job=~"alertmanager-main|alertmanager-user-workload"}[10m])
          )
          > 0.01
        for: 10m
        labels:
          severity: warning

The kubectl get pods (Kubernetes) or oc get pods (OpenShift) command can be used to list the Prometheus pods.

~]$ oc get pods --namespace openshift-monitoring
NAME                                                     READY   STATUS    RESTARTS        AGE
prometheus-adapter-6b98c646c7-m4g76                      1/1     Running   0               8d
prometheus-adapter-6b98c646c7-tczr2                      1/1     Running   0               8d
prometheus-k8s-0                                         6/6     Running   0               11d
prometheus-k8s-1                                         6/6     Running   0               11d
prometheus-operator-6766f68555-mkfv9                     2/2     Running   0               11d
prometheus-operator-admission-webhook-8589888cbc-mq2jx   1/1     Running   0               11d
prometheus-operator-admission-webhook-8589888cbc-t62mt   1/1     Running   0               11d

There should be a directory /etc/prometheus/rules/prometheus-k8s-rulefiles-0 in the prometheus pod.

~]$ oc exec pod/prometheus-k8s-0 --container prometheus --namespace openshift-monitoring -- ls -l /etc/prometheus/rules/
total 12
drwxrwsrwx. 3 root nobody 8192 May 12 20:30 prometheus-k8s-rulefiles-0

And there should be a YAML file in the pod that contains AlertmanagerFailedToSendAlerts.

~]$ oc exec pod/prometheus-k8s-0 --container prometheus --namespace openshift-monitoring -- cat /etc/prometheus/rules/prometheus-k8s-rulefiles-0/openshift-monitoring-alertmanager-main-rules-1b98ab31-7439-4f52-9f48-c04a696979c3.yaml
- name: alertmanager.rules
  rules:
  - alert: AlertmanagerFailedToSendAlerts
    annotations:
      description: Alertmanager {{ $labels.namespace }}/{{ $labels.pod}} failed to
        send {{ $value | humanizePercentage }} of notifications to {{ $labels.integration
        }}.
      runbook_url: https://github.com/openshift/runbooks/blob/master/alerts/cluster-monitoring-operator/AlertmanagerFailedToSendAlerts.md
      summary: An Alertmanager instance failed to send notifications.
    expr: |
      (
        rate(alertmanager_notifications_failed_total{job=~"alertmanager-main|alertmanager-user-workload"}[5m])
      /
        ignoring (reason) group_left rate(alertmanager_notifications_total{job=~"alertmanager-main|alertmanager-user-workload"}[5m])
      )
      > 0.01
    for: 5m
    labels:
      severity: warning

The kubectl exec (Kubernetes) or oc exec (OpenShift) and curl commands can be used to issue a POST request inside of your Prometheus pod to the /-/reload to reload Prometheus configurations.

oc exec prometheus-k8s-0 --container prometheus --namespace openshift-monitoring -- curl --request POST --url http://localhost:9090/-/reload