I have a webapp running in a Docker-container in a Kubernetes cluster. The app has an endpoint I want to be called periodically. The app runs at multiple nodes/pods, and it is important that only one node performs the task initiated by the endpoint. I have looked at Kubernetes Cron Jobs, but have not found any documentation on calling endpoints from a Kubernetes Cron Job. Does anybody have any proposal for a solution of this problem? How do you handle scheduling in a cluster where it is crucial that only one node performs the task?
1 Answers
CronJobs are a good choice. Here's a quick layout that runs 3 nginx pods accepting all traffic. Every minute, a Job curls 1 of the 3 pods (always the same pod).
apiVersion: apps/v1beta1
kind: Deployment
metadata:
name: main
labels:
app: nginx
spec:
replicas: 2
selector:
matchLabels:
app: nginx
template:
metadata:
labels:
app: nginx
spec:
containers:
- name: nginx
image: nginx:1.7.9
ports:
- containerPort: 80
---
apiVersion: apps/v1beta1
kind: Deployment
metadata:
name: singleton
labels:
app: nginx
special: singleton
spec:
replicas: 1
selector:
matchLabels:
app: nginx
special: singleton
template:
metadata:
labels:
app: nginx
special: singleton
spec:
containers:
- name: nginx
image: nginx:1.7.9
ports:
- containerPort: 80
---
kind: Service
apiVersion: v1
metadata:
name: allpods
spec:
selector:
app: nginx
ports:
- protocol: TCP
port: 80
targetPort: 80
---
kind: Service
apiVersion: v1
metadata:
name: singleton
spec:
selector:
special: singleton
ports:
- protocol: TCP
port: 80
targetPort: 80
---
apiVersion: batch/v1beta1
kind: CronJob
metadata:
name: callout
spec:
schedule: "*/1 * * * *"
concurrencyPolicy: Forbid
successfulJobsHistoryLimit: 1
failedJobsHistoryLimit: 1
jobTemplate:
spec:
template:
spec:
containers:
- name: callout
image: buildpack-deps:curl
args:
- /bin/sh
- -ec
- curl http://singleton
restartPolicy: Never
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