Kubernetes Batch API Resources
Objective
This tutorial aims to guide you through the process of automating tasks in OpenShift using Kubernetes jobs and cron jobs.
Prerequisites
- Access to an OpenShift cluster or a terminal emulator connected to one.
- Familiarity with Kubernetes batch resources, including jobs and cron jobs.
- Basic knowledge of YAML syntax for creating and editing configuration files.
Introduction
In the grand kingdom of OpenShift, automation plays a crucial role in maintaining order and efficiency. Just as a medieval castle relies on automated mechanisms like clock chimes and automated gates, OpenShift uses Kubernetes batch resources—jobs and cron jobs—to manage repetitive and scheduled tasks. These tasks range from simple one-off commands to complex maintenance routines that keep the infrastructure running smoothly. Understanding how to effectively utilize these batch resources is key to ensuring the kingdom's stability and prosperity.
Kubernetes Jobs
A Kubernetes job is akin to a knight given a specific quest to complete a single task. It ensures that a specified number of pods run to completion successfully. The job resource includes a pod template that describes the task to execute.
Example:
To create a job, you can use the oc create job --dry-run=client command to generate the YAML representation, like a scribe drafting the quest details for the knight:
oc create job --dry-run=client -o yaml test \
--image=registry.access.redhat.com/ubi9/ubi:9.4 \
-- curl https://example.com
This command outlines a job that uses a pod with a container running the Universal Base Image (UBI) and executes a curl command to access a website.
Kubernetes Cron Jobs
A Kubernetes cron job is like a castle bell ringer, tasked with ringing the bell at specific times. It runs jobs on a schedule, ensuring that tasks are performed regularly.
Example:
To create a cron job, you can use the oc create cronjob --dry-run=client command to get the YAML representation:
oc create cronjob --dry-run=client -o yaml test \
--image=registry.access.redhat.com/ubi9/ubi:9.4 \
--schedule='0 0 * * *' \
-- curl https://example.com
This command creates a cron job scheduled to run at midnight every day, executing a curl command using a container running the UBI.
Automating Application Maintenance Tasks
Just as the castle has regular maintenance to keep its structures sound, applications in OpenShift often need regular maintenance tasks. Here’s an example of a cron job automating a WordPress backup:
apiVersion: batch/v1
kind: CronJob
metadata:
name: wordpress-backup
spec:
schedule: 0 2 * * 7
jobTemplate:
spec:
template:
spec:
dnsPolicy: ClusterFirst
restartPolicy: Never
containers:
- name: wp-cli
image: registry.io/wp-maintenance/wp-cli:2.7
resources: {}
command:
- bash
- -xc
args:
- >
wp maintenance-mode activate ;
wp db export | gzip > database.sql.gz ;
wp maintenance-mode deactivate ;
rclone copy database.sql.gz s3://bucket/backups/ ;
rm -v database.sql.gz ;
This cron job activates WordPress maintenance mode, exports and compresses the database, uploads it to an S3 bucket, and then cleans up.
Automating Cluster Maintenance Tasks
Just as the castle's guards perform regular patrols to ensure safety, clusters need regular maintenance to stay healthy. Here’s an example of a script for image pruning:
apiVersion: v1
kind: ConfigMap
metadata:
name: maintenance
app: crictl
data:
maintenance.sh: |
#!/bin/bash
NODES=$(oc get nodes -o=name)
for NODE in ${NODES}
do
echo ${NODE}
oc debug ${NODE} -- \
chroot /host \
/bin/bash -xc 'crictl images ; crictl rmi --prune'
echo $?
done
This script lists and prunes unused images on each node. It can be executed as a cron job:
apiVersion: batch/v1
kind: CronJob
metadata:
name: image-pruner
spec:
schedule: 0 * * * *
jobTemplate:
spec:
template:
spec:
dnsPolicy: ClusterFirst
restartPolicy: Never
containers:
- name: image-pruner
image: quay.io/openshift/origin-cli:4.12
resources: {}
command:
- /opt/scripts/maintenance.sh
volumeMounts:
- name: scripts
mountPath: /opt
volumes:
- name: scripts
configMap:
name: maintenance
defaultMode: 0555
Assigning Privileged Access
Sometimes, maintenance tasks require higher privileges, much like how certain castle tasks require special keys. You can create a service account with the required privileges and associate it with the cron job:
Then, specify the service account in the cron job:
Conclusion
By leveraging Kubernetes jobs and cron jobs, you can automate and schedule critical tasks in OpenShift, ensuring your applications and clusters are well-maintained. This automation is akin to the efficient operation of a castle, where every task is carefully scheduled and executed, ensuring the kingdom runs smoothly and securely.
