Docker containers can be terminated any time, due to an auto-scaling policy, pod or deployment deletion or while rolling out an update. In most of such cases, you will probably want to graceful shutdown your application running inside the container.
In our case, for example, we do want to wait until all current requests (or jobs processing) have completed, but the actual reasons to graceful shutdown an application may be many, including releasing resources, distributed locks or opened connections.
How it works
When a pod should be terminated:
SIGTERMsignal is sent to the main process (PID 1) in each container, and a “grace period” countdown starts (defaults to 30 seconds - see below to change it).
- Upon the receival of the
SIGTERM, each container should start a graceful shutdown of the running application and exit.
- If a container doesn’t terminate within the grace period, a
SIGKILLsignal will be sent and the container violently terminated.
For a detailed explaination, please see:
- Kubernetes: Termination of pods
- Kubernetes: Pods lifecycle hooks and termination notice
- Kubernetes: Container lifecycle hooks
A common pitfall while handling the SIGTERM
Let’s say your
Dockerfile ends with a
CMD in the shell form:
The shell form runs the command with
/bin/sh -c myapp, so the process that will get the
SIGTERM is actually
/bin/sh and not its child
myapp. Depending on the actual shell you’re running, it could or could not pass the signal to its children.
For example, the shell shipped by default with Alpine Linux does not pass signals to children, while Bash does it. If your shell doesn’t pass signals to children, you’ve a couple of options to ensure the signal will be correctly delivered to the app.
Option #1: run the CMD in the exec form
You can obviously use the
CMD in the exec form. This will run
myapp instead of
/bin/sh -c myapp, but will not allow you to pass environment variables as arguments.
Option #2: run the command with Bash
You can ensure your container includes Bash and run your command through it, in order to support environment variables passed as arguments.
How to change the grace period
The default grace period is 30 seconds. As any default, it could fit or couldn’t fit on your specific use cases. There are two way to change it:
- In the deployment .yaml file
- On the command line, when you run
You can customize the grace period setting
terminationGracePeriodSeconds at the pod spec level. For example, the following .yaml shows a simple deployment config with a 60 seconds termination grace period.
You can also change the default grace period when you manually delete a resource with
kubectl delete command, adding the parameter
--grace-period=SECONDS. For example:
There’re some circumstances where a
SIGTERM violently kill the application, vanishing all your efforts to gracefully shutdown it. Nginx, for example, quickly exit on
SIGTERM, while you should run
/usr/sbin/nginx -s quit to gracefully terminate it.
In such cases, you can use the preStop hook. According to the Kubernetes doc, PreStop works as follow:
This hook is called immediately before a container is terminated. No parameters are passed to the handler. This event handler is blocking, and must complete before the call to delete the container is sent to the Docker daemon. The SIGTERM notification sent by Docker is also still sent. A more complete description of termination behavior can be found in Termination of Pods.
The preStop hook is configured at container level and allows you to run a custom command before the
SIGTERM will be sent (please note that the termination grace period countdown actually starts before invoking the preStop hook and not once the
SIGTERM signal will be sent).
The following example, taken from the Kubernetes doc, shows how to configure a preStop command.
This blog post has been written according to my personal knowledge. If something is incorrect, don’t esitate to drop a comment: will be my care to update the post and yet another occasion for me to learn something new. Thanks!
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|Incontro DevOps 2020||Virtual||22 October 2020|