How to cp data from one container to another using kubernetes - kubernetes

Question

Say we have a simple deployment.yml file:
apiVersion: apps/v1
kind: Deployment
metadata:
namespace: ikg-api-demo
name: ikg-api-demo
spec:
selector:
matchLabels:
app: ikg-api-demo
replicas: 3
template:
metadata:
labels:
app: ikg-api-demo
spec:
containers:
- name: ikg-api-demo
imagePullPolicy: Always
image: example.com/main_api:private_key
ports:
- containerPort: 80
the problem is that this image/container depends on another image/container - it needs to cp data from the other image, or use some shared volume.
How can I tell kubernetes to download another image, run it as a container, and then copy data from it to the container declared in the above file?
It looks like this article explains how.
but it's not 100% clear how it works. It looks like you create some shared volume, launch the two containers, using that shared volume?
so I according to that link, I added this to my deployment.yml:
spec:
volumes:
- name: shared-data
emptyDir: {}
containers:
- name: ikg-api-demo
imagePullPolicy: Always
volumeMounts:
- name: shared-data
mountPath: /nltk_data
image: example.com/nltk_data:latest
- name: ikg-api-demo
imagePullPolicy: Always
volumeMounts:
- name: shared-data
mountPath: /nltk_data
image: example.com/main_api:private_key
ports:
- containerPort: 80
my primary hesitation is that mounting /nltk_data as a shared volume will overwrite what might be there already.
So I assume what I need to do is mount it at some other location, and then make the ENTRYPOINT for the source data container:
ENTRYPOINT ['cp', '-r', '/nltk_data_source', '/nltk_data']
so that will write it to the shared volume, once the container is launched.
So I have two questions:
How to run one container and finish a job, before another container starts using kubernetes?
How to write to a shared volume without having that shared volume overwrite what's in your image? In other words, if I have /xyz in the image/container, I don't want to have to copy /xyz to /shared_volume_mount_location if I don't have to.

Answer 1

How to run one container and finish a job, before another container starts using kubernetes?
Use initContainers - updated your deployment.yml, assuming example.com/nltk_data:latest is your data image
How to write to a shared volume without having that shared volume overwrite?
As you know what is there in your image, you need to select an appropriate mount path. I would use /mnt/nltk_data
Updated deployment.yml with init containers
spec:
volumes:
- name: shared-data
emptyDir: {}
initContainers:
- name: init-ikg-api-demo
imagePullPolicy: Always
# You can use command, if you don't want to change the ENTRYPOINT
command: ['sh', '-c', 'cp -r /nltk_data_source /mnt/nltk_data']
volumeMounts:
- name: shared-data
mountPath: /mnt/nltk_data
image: example.com/nltk_data:latest
containers:
- name: ikg-api-demo
imagePullPolicy: Always
volumeMounts:
- name: shared-data
mountPath: /nltk_data
image: example.com/main_api:private_key
ports:
- containerPort: 80