Kubernetes Podspec to download only container image but it should not install - kubernetes

Question

I want to download the container image, but dont want to deploy/install the image.
How can i deploy podspec to download only images but it should not create container.
Any podspec snapshot for this?

Answer 1

As far as I know there is no direct Kubernetes resources to only download an image of your choosing. To have the images of your applications on your Nodes you can consider using following solutions/workarounds:
Use a Daemonset with an initContainer('s)
Use tools like Ansible to pull the images with a playbook
Use a Daemonset with InitContainers
Assuming the following situation:
You've created 2 images that you want to have on all of the Nodes.
You can use a Daemonset (spawn a Pod on each Node) with initContainers (with images as source) that will run on all Nodes and ensure that the images will be present on the machine.
An example of such setup could be following:
apiVersion: apps/v1
kind: DaemonSet
metadata:
name: pull-images
labels:
k8s-app: pull-images
spec:
# AS THIS DAEMONSET IS NOT SUPPOSED TO SERVE TRAFFIC I WOULD CONSIDER USING THIS UPDATE STRATEGY FOR SPEEDING UP THE DOWNLOAD PROCESS
updateStrategy:
type: RollingUpdate
rollingUpdate:
maxUnavailable: 100
selector:
matchLabels:
name: pull-images
template:
metadata:
labels:
name: pull-images
spec:
initContainers:
# PUT HERE IMAGES THAT YOU WANT TO PULL AND OVERRIDE THEIR ENTRYPOINT
- name: ubuntu
image: ubuntu:20.04 # <-- IMAGE #1
imagePullPolicy: Always # SPECIFY THE POLICY FOR SPECIFIC IMAGE
command: ["/bin/sh", "-c", "exit 0"]
- name: nginx
image: nginx:1.19.10 # <-- IMAGE #2
imagePullPolicy: IfNotPresent # SPECIFY THE POLICY FOR SPECIFIC IMAGE
command: ["/bin/sh", "-c", "exit 0"]
containers:
# MAIN CONTAINER WITH AS SMALL AS POSSIBLE IMAGE SLEEPING
- name: alpine
image: alpine
command: [sleep]
args:
- "infinity"
Kubernetes Daemonset controller will ensure that the Pod will run on each Node. Before the image is run, the initContainers will act as a placeholders for the images. The images that you want to have on the Nodes will be pulled. The ENTRYPOINT will be overridden to not run the image continuously. After that the main container (alpine) will be run with a sleep infinity command.
This setup will also work when the new Nodes are added.
Following on that topic I would also consider checking following documentation on imagePullPolicy:
Kubernetes.io: Docs: Concepts: Containers: Images: Updating images
A side note!
I've set the imagePullPolicy for the images in initContainers differently to show you that you can specify the imagePullPolicy independently for each container. Please use the policy that suits your use case the most.
Use tools like Ansible to pull the images with a playbook
Assuming that you have SSH access to the Nodes you can consider using Ansible with it's community module (assuming that you are using Docker):
community.docker.docker_image
Citing the documentation for this module:
This plugin is part of the community.docker collection (version 1.3.0).
To install it use: ansible-galaxy collection install community.docker.
Synopsis
Build, load or pull an image, making the image available for creating containers. Also supports tagging an image into a repository and archiving an image to a .tar file.
-- Docs.ansible.com: Ansible: Collections: Community: Docker: Docker image module
You can use it with a following example:
hosts.yaml
all:
hosts:
node-1:
ansible_port: 22
ansible_host: X.Y.Z.Q
node-2:
ansible_port: 22
ansible_host: A.B.C.D
playbook.yaml
- name: Playbook to download images
hosts: all
user: ENTER_USER
tasks:
- name: Pull an image
community.docker.docker_image:
name: "{{ item }}"
source: pull
with_items:
- "nginx"
- "ubuntu"
A side note!
In the ansible way, I needed to install docker python package:
$ pip3 install docker
Additional resources:
Kubernetes.io: Docs: Concepts: Workloads: Controllers: Daemonset
Kubernetes.io: Docs: Concepts: Workloads: Pods: initContainers