Add storage for Prometheus using a PersistentVolumeClaim

Prior to this change, Prometheus' timeseries data would
be lost whenever the chart was upgraded, the Prometheus Pod
was rescheduled, or restarted due to a configuration change.

Tested with a blank storageClassName with KinD, and the local
path provisioner. After creating load with hey, and restarting
Prometheus, the invocation metrics remained available.

Signed-off-by: Alex Ellis (OpenFaaS Ltd) <alexellis2@gmail.com>

chart/openfaas/templates/prometheus-dep.yaml

@@ -119,8 +119,19 @@ spec:
                 path: prometheus-rules.yml
                 mode: 0644
 {{- end }}
+
+{{- if .Values.prometheus.pvc.enabled }}
         - name: prom-data
-          emptyDir: {}
+          persistentVolumeClaim:
+  {{- if .Values.prometheus.pvc.name }}
+            claimName: {{.Values.prometheus.pvc.name}}
+  {{- else }}
+            claimName: prometheus-data
+  {{- end }}
+{{- else }}
+
+{{- end }}
+
     {{- with .Values.nodeSelector }}
       nodeSelector:
 {{ toYaml . | indent 8 }}

chart/openfaas/templates/prometheus-pvc.yaml

@@ -0,0 +1,28 @@
+{{- $functionNs := default .Release.Namespace .Values.functionNamespace }}
+{{- if and .Values.prometheus.create .Values.prometheus.pvc.enabled }}
+---
+apiVersion: v1
+kind: PersistentVolumeClaim
+metadata:
+  labels:
+    app: {{ template "openfaas.name" . }}
+    chart: {{ .Chart.Name }}-{{ .Chart.Version }}
+    component: prometheus
+    heritage: {{ .Release.Service }}
+    release: {{ .Release.Name }}
+{{- if .Values.prometheus.pvc.name }}
+  name: {{.Values.prometheus.pvc.name}}
+{{- else }}
+  name: prometheus-data
+{{- end }}
+  namespace: {{ .Release.Namespace | quote }}
+spec:
+  accessModes:
+  - ReadWriteOnce
+  resources:
+    requests:
+      storage: {{ .Values.prometheus.pvc.size | quote }}
+  {{- with .Values.prometheus.pvc.storageClassName }}
+  storageClassName: {{ . | quote }}
+  {{- end }}
+{{- end }}

chart/openfaas/values.yaml

@@ -359,6 +359,11 @@ iam:
     url: https://kubernetes.default.svc.cluster.local
     tokenExpiry: 2h
 
+## Prometheus is required for metrics and autoscaling
+##
+## It is bundled into OpenFaaS to be used only as an internal component
+## if you wish to retain the metrics for a longer period, you should
+## scrape this instance from an external Prometheus server
 prometheus:
   image: prom/prometheus:v2.54.0
   create: true
@@ -371,22 +376,19 @@ prometheus:
   annotations: {}
   recordingRules: []
 
-alertmanager:
-  image: prom/alertmanager:v0.27.0
-  create: true
-  resources:
-    requests:
-      memory: "25Mi"
-      cpu: "50m"
-    limits:
-      memory: "50Mi"
-
-stan:
-  # Image used for the NATS Streaming when using the deprecated 
-  # support in the Community Edition (CE)
-  image: nats-streaming:0.25.6
-
-# NATS (required for async)
+  # Set to true to enable persistent storage for the Prometheus Pod
+  # otherwise, the data will be lost when the Pod is restarted
+  pvc:
+    enabled: false
+    # You may want to set this higher for production
+    size: 10Gi
+    # Leave the storageClassName blank for the default storage class
+    # using the string "default" does not necessarily mean the default
+    # storage class
+    storageClassName:
+
+## NATS is used for OpenFaaS Pro and is required for:
+## asynchronous invocations, billing & auditing webhooks
 nats:
   channel: "faas-request"
   # Stream replication is set to 1 by default. This is only recommended for development.
@@ -405,6 +407,22 @@ nats:
       memory: "120Mi"
       cpu: "100m"
 
+## alertmanager is only used for OpenFaaS CE
+alertmanager:
+  image: prom/alertmanager:v0.27.0
+  create: true
+  resources:
+    requests:
+      memory: "25Mi"
+      cpu: "50m"
+    limits:
+      memory: "50Mi"
+
+## stan is only used for OpenFaaS CE and will be removed in
+## a fture release, having already been deprecated by the NATS team
+stan:
+  image: nats-streaming:0.25.6
+
 # ingress configuration
 ingress:
   enabled: false