It defaults to Allow but can be changed to Forbid (prevent new jobs from starting while one already exists) or Replace (terminate an existing job as soon as a new one starts). You can customize the ConcurrencyPolicy to control whether Kubernetes allows your jobs to overlap. The frequency is given as a regular cron definition to the schedule key in the resource's spec. Be aware that CronJob is a beta resource which may change in future Kubernetes releases.Ĭommand: Īpply the above manifest to your cluster to create a new cron job that will run /my-cron-script.sh within your container every five minutes. You don't need to install cron in your image or setup specialized containers if you're using Kubernetes. Kubernetes comes with its own CronJob resource which you can use in your manifests. As long as the image still has cron installed and your crontab configured, you can use docker-compose up to bring up your application.įinally, let's look at a simple example of running scheduled tasks within Kubernetes. Make sure this does not start the cron daemon! The second container overrides the image's entrypoint to run cron. Using the above example, one container serves our application using the default entrypoint in the image. To show each scenario, we’ll use a sample Dockerfile. We’ll examine those and fix the problem based on the root cause in each case. A Docker container can become stopped for several reasons. Using docker-compose also simplifies attaching the container to any shared volumes and networks it requires. Overview In this tutorial, we’ll learn how to restart a terminated or exited container. You could use your application's base image, overriding the entrypoint command to start the cron daemon. You'd define the cron container as an extra service. One way to get setup with a separate cron container would be to use docker-compose. In that case, the cron container may use a minimal base image and do away with connections to unnecessary peripheral resources. This is not a hard-and-fast rule - in some projects, your scheduled tasks might be trivial scripts which operate independently of your codebase. The service would be started within the ephemeral container used to build the layer, not the final containers running the completed image. You can't achieve this with a RUN stage in your Dockerfile because these are transient steps which don't persist beyond the image's build phase. To complete this setup, you'll need to amend your image's command or entrypoint to start the cron daemon when containers begin to run. Finally, use the crontab command to make the file known to the cron daemon. Next, we need to amend the permissions on our crontab to make sure it's accessible to cron. We install cron and copy our codebase's crontab into the /etc/cron.d directory. RUN chmod 0644 /etc/cron.d/example-crontab & RUN apt-get update & apt-get install -y cronĬOPY example-crontab /etc/cron.d/example-crontab Next, amend your Dockerfile to install cron and register your crontab - here's how you can do that with a Debian-based image: */5 * * * * /usr/bin/sh /example-scheduled-task.sh You can install it within your Dockerfile and then register your application's crontab.įirst, create a new crontab file within your codebase: Most popular Docker base images do not include the cron daemon by default.
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