--- title: "Kubernetes: Cron Jobs" source: "https://wordpress-1075849-4005834.cloudwaysapps.com/blog/kubernetes-cron-jobs/" --- [Technical Blog](https://wordpress-1075849-4005834.cloudwaysapps.com/category/technical-blog/) # Kubernetes: Cron Jobs By [Chris Short](https://wordpress-1075849-4005834.cloudwaysapps.com/blog/author/chrisshort/) on Jan 01, 2019 • 7 min read One of the reasons I stood up a [Kubernetes cluster on Raspberry Pis](http://chrisshort.net/my-raspberry-pi-kubernetes-cluster/) in my house was because of the savings I wanted to gain by not running high-available, redundant infrastructure in the cloud. Kubernetes provides high-availability by design. It’s pretty awesome the possibilities that exist given this capability. Need a web server to constantly run? Build a container and throw it in the Kubernetes cluster. Need a service available all the time? Package it and ship it to the Kubernetes cluster. ## Legacy Systems I have four old Raspberry Pi boxes doing various things here in the office for me. They do a single task fine, but I have over-extended one of the first-generation Raspberry Pis. I’d like to deprecate the older Raspberry Pis as they are no longer effective boxes. To do that, I need to move the workloads (like a cloud migration). One thing I have no shortage of running on these Raspberry Pi boxes are cron jobs. I have a cron for almost everything I do. Monitoring, updating web apps, detecting changes in domain name configurations, etc. ## Kubernetes Jobs and Cron Jobs Kubernetes has the concept of *Jobs*. To quote the official [Jobs documentation](http://kubernetes.io/docs/concepts/workloads/controllers/jobs-run-to-completion/#what-is-a-job), “A job creates one or more pods and ensures that a specified number of them successfully terminate.” If you have a pod that needs to run until completion no matter what, a Kubernetes Job is for you. Think of Jobs as a batch processor. [Kubernetes *Cron Jobs*](http://kubernetes.io/docs/concepts/workloads/controllers/cron-jobs/) are a relatively new thing. But, I am ecstatic that this is a standard feature in modern Kubernetes clusters. This means that I can tell the cluster one time that I want a job to run at certain times. Since Cron Jobs build on top of the existing Job functionality, I know that the job will be run to completion. The job will run on one of the six nodes I have in my Kubernetes cluster. Even if a pod is destroyed mid-job, it will spin up on another node and run there. High-available cron jobs have been a beast I’ve tried to slay many times. This is now a solved problem and all I have to do is implement it. The implementation of Kubernetes *Cron Jobs* is like many other things with Kubernetes: YAML. There are a few projects to help with wrangling your YAML ([ksonnet](http://ksonnet.io/) for example) but, that is a discussion for another article. For now, let’s get a Dockerfile and Kubernetes configuration file put together. ## Use Case I moved my newsletter, [DevOps’ish](http://devopsish.com/), off of Medium and on to [Netlify](http://www.netlify.com/) with [Hugo](http://gohugo.io/) as a static site generator. This makes for a very fast and easy to manage website. But, the one piece of functionality lost in the move is the ability to schedule posts. Netlify provides a build hook that will trigger builds when called. I can write the newsletter and set it to a date in the future. Hugo, by default, will not publish articles unless a build is completed after the specified date. Calling the build hook URL via `curl` with a cron job is a way to implement scheduled posts with Hugo on Netlify. ## Dockerfile The Dockerfile is pretty simple. Pull from `alpine:latest`, install `curl`, and run a `curl`command. But, I don’t want the build hook URL exposed in the Dockerfile. Loading the URL as a variable via a [Kubernetes Secret](http://kubernetes.io/docs/concepts/configuration/secret/) is advisable. Do this so that the artifacts have no sensitive data and so that they can be shared publicly. Here is the Dockerfile: ``` `FROM alpine:latest LABEL maintainer="Chris Short " RUN set -x && apk update && apk upgrade && apk add --no-cache curl ENTRYPOINT [ "/bin/sh", "-c" ] CMD [ "/usr/bin/curl -vvv -X POST -d '' ${URL}" ]` ```   ## Docker Build As my Kubernetes cluster runs on Raspberry Pi, I make sure to pull this Dockerfile down to a Raspberry Pi dev box I have and build it there: ``` `docker build -t devopsish-netlify-cron .` ``` The name is whatever you want it to be. I will likely rename this to netlify-curl or some other more appropriate name when I’m ready. ## Docker Registry The next step is to add the image to a Docker registry. I thought about running a Docker registry in the Kubernetes cluster itself, but then I realized [Google Container Registry](http://cloud.google.com/container-registry/)(GCR) is a thing. Since I have a fair amount of stuff in Google Cloud, I decided to use GCR for simplicity and availability (also that whole “state inside Kubernetes” thing). Heptio has a great guide titled [*Google Cloud Registry (GCR) with external Kubernetes*](http://docs.heptio.com/content/private-registries/pr-gcr.html). If you are going to use GCR with an external Kubernetes cluster, I highly recommend reading this first. Once GCR is configured, your Kubernetes cluster is configured to use GCR, and the container is built, you have to tag it for GCR: ``` `docker tag devopsish-netlify-cron gcr.io/chrisshort-net/devopsish-netlify-cron` ``` Then push the newly tagged container image to GCR: ``` `gcloud docker -- push gcr.io/chrisshort-net/devopsish-netlify-cron:latest` ``` ## Kubernetes Secret As previously mentioned, the next piece will be the Kubernetes Secret. There are a lot of ways to skin the k8s secret cat. Secrets can be loaded one time via command line or by applying a configuration file. I chose the configuration file method because I will save them in 1Password then delete them. The secret file will look something like this: ``` `apiVersion: v1 kind: Secret metadata: name: devopish-build-hook type: Opaque data: url: [REDACTED]` ``` The redacted url string will be the Netlify build hook URL. As this is an Opaque secret, the string will need to be base64 encoded: ``` `echo -n "" | base64` ``` Once the base64 string is added to the file, apply it: ``` `kubectl apply -f secret.yml` ``` ## Cron Job Configuration Piecing together the [Kubernetes Cron Job](http://kubernetes.io/docs/concepts/workloads/controllers/cron-jobs) configuration file is relatively easy. Schedule is a required field, and if you’re familiar with cron, it will look identical to the Cron format string. ``` `apiVersion: batch/v1beta1 kind: CronJob metadata: name: devopsish-netlify-cronjob spec: schedule: "1 2-14 * * 0-1,5-6" jobTemplate: spec: template: spec: imagePullSecrets: - name: gcr-secret containers: - name: devopsish-netlify-cronjob image: gcr.io/chrisshort-net/devopsish-netlify-cron:latest env: - name: URL valueFrom: secretKeyRef: name: devopish-build-hook key: url restartPolicy: OnFailure` ``` One pitfall I experienced is Kubernetes uses UTC exclusively. Make sure you take that into account when you’re creating your schedule. Here is what the Kubernetes configuration file is specifying: - Create a CronJob named devopsish-netlify-cronjob - Schedule it to run the first minute of every hour from 0200 to 1400 UTC on Sunday, Monday, Friday, and Saturday - Pull the image from gcr.io/chrisshort-net/devopsish-netlify-cron:latest using the provided secrets for gcr - Set an environment based off the url key in the secret named devopish-build-hook - Run container on CronJob schedule Apply the configuration file and you’re off to the races: ``` `kubectl apply -f devopsish-netlify-cronjob.yml` ``` ## Conclusion And voilà! You have built a Docker container, deployed the image to Google Container Registry, configured the Kubernetes cluster to pull images from GCR, created a secret to store the build hook, and created the CronJob. If everything works okay, the following command should show an active cron job: ``` `cshort@michiganjfrog ~> kubectl get cronjob NAME SCHEDULE SUSPEND ACTIVE LAST SCHEDULE AGE devopsish-netlify-cronjob 1 2-14 * * 0-1,5-6 False 0 8h 2d` ``` Now go celebrate your high-availability, damn near guaranteed to run every time Kubernetes Cron Job! Congratulations! This article is originated from[http://chrisshort.net/kubernetes-cron-jobs/](http://chrisshort.net/kubernetes-cron-jobs/) [Chris Short](http://www.linkedin.com/in/thechrisshort/) is a Tigera guest blogger. 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