Create a prometheus-operator `PrometheusRule` (CRD) from a raw Prometheus rule file?

A Prometheus rules file like this:

groups:
- name: ./example.rules
  rules:
  - alert: ExampleAlert
    expr: vector(1)

The Kubernetes prometheus-operator requires an extra layer of yaml configuration data and looks like this:

apiVersion: monitoring.coreos.com/v1
kind: PrometheusRule
metadata:
  creationTimestamp: null
  labels:
    prometheus: example
    role: alert-rules
  name: prometheus-example-rules
spec:
  groups:
  - name: ./example.rules
    rules:
    - alert: ExampleAlert
      expr: vector(1)

With files using the former, raw Prometheus rule format, I can use Prometheus promtool to run unit tests. (see link 3 below)

With files using the latter, extended PrometheusRule format, I can use kubectl apply -f prometheus_rule_file.yaml to load the rules into my prometheus-operator installation.

I want to write and maintain rules in one of these file formats and be able to use the same rule file both with promtool's unit test system and to load the same rule into a prometheus-operator installation. I don't see an easy way to do this.

Is there a way I can have a rule using the raw Prometheus rule format and load it into a prometheus-operator PrometheusRule without maintaining a separate redundant yaml file and without writing a customized tool to convert it?

1. https://prometheus.io/docs/prometheus/latest/configuration/alerting_rules/
2. https://github.com/coreos/prometheus-operator/blob/master/Documentation/user-guides/alerting.md
3. https://prometheus.io/docs/prometheus/latest/configuration/unit_testing_rules/

The way I got this to work with helm is to create a separate chart and put rules files along with the chart files. This means you need to deploy them independently and perhaps that's what you intend.

Steps

1. Create a helm chart: helm create prometheus-rules
2. Create rules directory and put prometheus rule files in that directory.
3. Delete all unnecessary yaml files in templates/ directory
4. Create a new template file templates/prometheus-rules.yaml
5. Insert example below:

    apiVersion: v1
    kind: List
    items:
    {{- $root := .Files }}
    {{- range $path, $bytes := $root.Glob "rules/**.yaml" }}
      - apiVersion: "monitoring.coreos.com/v1"
        kind: PrometheusRule
        metadata:
          name: prometheus-rules-{{ $path | base }}
          labels:
            app: prometheus-operator
        spec:
          groups:
    {{ $root.Get $path | indent 8 }}
    {{- end}}

---

NOTE: one tricky thing is to watch out for prometheus.prometheusSpec.ruleSelectorNilUsesHelmValues value when installing the prometheus-operator chart. It will only load rules which have the same labels as were installed during prometheus-operator. You can get around that by setting it to false when installing prometheus-operator. See ruleSelector logic for more details.