create an empty file inside a volume in Kubernetes pod - kubernetes

Question

I have a legacy app which keep checking an empty file inside a directory and perform certain action if the file timestamp is changed.
I am migrating this app to Kubernetes so I want to create an empty file inside the pod. I tried subpath like below but it doesn't create any file.
apiVersion: v1
kind: Pod
metadata:
name: demo-pod
spec:
containers:
- name: demo
image: alpine
command: ["sleep", "3600"]
volumeMounts:
- name: volume-name
mountPath: '/volume-name-path'
subPath: emptyFile
volumes:
- name: volume-name
emptyDir: {}
describe pods shows
Containers:
demo:
Container ID: containerd://0b824265e96d75c5f77918326195d6029e22d17478ac54329deb47866bf8192d
Image: alpine
Image ID: docker.io/library/alpine#sha256:08d6ca16c60fe7490c03d10dc339d9fd8ea67c6466dea8d558526b1330a85930
Port: <none>
Host Port: <none>
Command:
sleep
3600
State: Running
Started: Wed, 10 Feb 2021 12:23:43 -0800
Ready: True
Restart Count: 0
Environment: <none>
Mounts:
/var/run/secrets/kubernetes.io/serviceaccount from default-token-4gp4x (ro)
/volume-name-path from volume-name (rw,path="emptyFile")
ls on the volume also shows nothing.
k8 exec -it demo-pod -c demo ls /volume-name-path
any suggestion??
PS: I don't want to use a ConfigMap and simply wants to create an empty file.

Answer 1

If the objective is to create a empty file when the Pod starts, then the most easy way is to either use the entrypoint of the docker image or an init container.
With the initContainer, you could go with something like the following (or with a more complex init image which you build and execute a whole bash script or something similar):
apiVersion: v1
kind: Pod
metadata:
name: demo-pod
spec:
initContainers:
- name: create-empty-file
image: alpine
command: ["touch", "/path/to/the/directory/empty_file"]
volumeMounts:
- name: volume-name
mountPath: /path/to/the/directory
containers:
- name: demo
image: alpine
command: ["sleep", "3600"]
volumeMounts:
- name: volume-name
mountPath: /path/to/the/directory
volumes:
- name: volume-name
emptyDir: {}
Basically the init container gets executed first, runs its command and if it is successful, then it terminates and the main container starts running. They share the same volumes (and they can also mount them at different paths) so in the example, the init container mount the emptyDir volume, creates an empty file and then complete. When the main container starts, the file is already there.
Regarding your legacy application which is getting ported on Kubernetes:
If you have control of the Dockerfile, you could simply change it create an empty file at the path you are expecting it to be, so that when the app starts, the file is already created there, empty, from the beginning, just exactly as you add the application to the container, you can add also other files.
For more info on init container, please check the documentation (https://kubernetes.io/docs/concepts/workloads/pods/init-containers/)

Answer 2

I think you may be interested in Container Lifecycle Hooks.
In this case, the PostStart hook may help create an empty file as soon as the container is started:
This hook is executed immediately after a container is created.
In the example below, I will show you how you can use the PostStart hook to create an empty file-test file.
First I created a simple manifest file:
# demo-pod.yml
apiVersion: v1
kind: Pod
metadata:
labels:
run: demo-pod
name: demo-pod
spec:
containers:
- image: alpine
name: demo-pod
command: ["sleep", "3600"]
lifecycle:
postStart:
exec:
command: ["touch", "/mnt/file-test"]
After creating the Pod, we can check if the demo-pod container has an empty file-test file:
$ kubectl apply -f demo-pod.yml
pod/demo-pod created
$ kubectl exec -it demo-pod -- sh
/ # ls -l /mnt/file-test
-rw-r--r-- 1 root root 0 Feb 11 09:08 /mnt/file-test
/ # cat /mnt/file-test
/ #