Example using kubernetes StatefulSet to target a CronJob to a specific pod

README.md

Example showing how to use a StatefulSet in coordination with a CronJob to run a scheduled command on a specific pod.

Download k8s.yaml locally and then run:

kubectl apply -f k8s.yaml && \
  kubectl rollout status statefulset/web && \
  sleep 65 && \
  for podNum in 0 1 2; do echo -n "Pod $podNum: "; kubectl exec pod/web-$podNum -- ls -1 / | grep cron || echo "<notfound>"; done

Expected output:

statefulset.apps/web created
cronjob.batch/crontest created
Waiting for 3 pods to be ready...
Waiting for 2 pods to be ready...
Waiting for 1 pods to be ready...
partitioned roll out complete: 3 new pods have been updated...
Pod 0: <notfound>
Pod 1: ran-cron          // <-- cronjob runs against pod 1 only
Pod 2: <notfound>

k8s.yaml

# Slightly modified from the StatefulSet example at 
# https://kubernetes.io/docs/concepts/workloads/controllers/statefulset/
apiVersion: apps/v1
kind: StatefulSet
metadata:
  name: web
spec:
  selector:
    matchLabels:
      app: nginx # has to match .spec.template.metadata.labels
  serviceName: "nginx"
  replicas: 3 # by default is 1
  template:
    metadata:
      labels:
        app: nginx # has to match .spec.selector.matchLabels
    spec:
      containers:
      - name: nginx
        image: alpine:latest
        command: [ "/bin/sh", "-c", "--" ]
        args: [ "while true; do sleep 30; done;" ]
---
apiVersion: batch/v1beta1
kind: CronJob
metadata:
  name: crontest
spec:
  schedule: "* * * * *"
  jobTemplate:
    spec:
      template:
        spec:
          containers:
          - name: crontest
            image: alpine/k8s:1.20.7
            imagePullPolicy: IfNotPresent
            command:
            - /bin/sh
            - -c
            - kubectl exec pod/web-1 -- touch ran-cron
          restartPolicy: OnFailure