Podman Hello-World container to Kubernetes Hello-World Pod

In this blog, we will perform the following activities:

• Create a HelloWorld Containerfile
• Build an image from the Containerfile using Podman (helloworldimage)
• Now create a container using Podman and the image you built. (hellocontainer)
• Now use the ‘podman kube generate’ command to create a yaml file (hellopod.yaml) from hellocontainer.
• Create a pod using kubectl and hellopod.yaml file.

Let's first create a Dockerfile or Containerfile that pulls an OracleLinux 8 image and then echos “Hello_World”

Step 1:

Create a Containerfile:

vi Containerfile

FROM os/oraclelinux:8
MAINTAINER Puneeth
CMD [“echo”, “Hello World”]

OR

FROM alpine
MAINTAINER Puneeth
CMD [“echo”, “Hello World”]

Step 2:

Build an image:

If your Dockerfile/Containerfile is in the current working directory:

podman build -t helloworldimage:v1 .

If you have created a Containerfile/Dockerfile with a different name say, HelloWorldfile, and if it is on a different location, then use the following command:

podman build -t helloworldimage:v1 -f /home/opc/lab/Hello-World/HelloWorldfile

Step 3:

Check if the image is built successfully:

podman images

Step 4:

Let's start a container using this image:

podman run -d --name hellocontainer localhost/helloworldimage:v1

Step 5:

Use Podman command to generate a kubernetes yaml file that can be used to create a pod:

podman generate kube <container-id> -f=hellopod.yaml

Step 6:

Let’s create a pod using kubectl and the yaml file generated by podman:

kubectl create -f hellopod.yaml

Known Issue 1:

If you are using an older version of podman, say:

[opc@control Hello-World]$ podman version
Client: Podman Engine
Version: 4.0.2
API Version: 4.0.2
Go Version: go1.17.7

Then the yaml file generated will have an “_” for Pod name, which is not allowed according to RFC 1123.

The Pod “hellocontainer_pod” is invalid: metadata.name: Invalid value: “hellocontainer_pod”: a lowercase RFC 1123 subdomain must consist of lower case alphanumeric characters, ‘-’ or ‘.’, and must start and end with an alphanumeric character (e.g. ‘example.com’, regex used for validation is ‘[a-z0–9]([-a-z0–9]*[a-z0–9])?(\.[a-z0–9]([-a-z0–9]*[a-z0–9])?)*’)

You can manually edit the yaml file generated and change the pod name from say “hellocontainer_pod” to “hellocontainer-pod” and then create the pod using kubectl command.

OR

• Upgrade podman to the latest version:

[opc@control Hello-World]$ sudo yum update podman

[opc@control Hello-World]$ podman version
Client: Podman Engine
Version: 4.6.1
API Version: 4.6.1
Go Version: go1.20.10
Built: Wed Dec 13 02:31:03 2023
OS/Arch: linux/amd64

Now generate the yaml file and you should have a valid pod name in the yaml file.

Known Issue 2:

You may see an issue while trying to create a pod using the image you build locally using podman:

[opc@control Hello-World]$ kubectl get po
NAME READY STATUS RESTARTS AGE

[opc@control Hello-World]$ 
[opc@control Hello-World]$ kubectl create -f hellopod.yaml 
pod/hellocontainer-pod created

[opc@control Hello-World]$ 
[opc@control Hello-World]$ kubectl get po
NAME READY STATUS RESTARTS AGE
hellocontainer-pod 0/1 ErrImagePull 0 118s

[opc@control Hello-World]$ kubectl describe po hellocontainer-pod
...
Events:
  Type     Reason     Age                 From               Message
  ----     ------     ----                ----               -------
  Normal   Scheduled  2m9s                default-scheduler  Successfully assigned default/hellocontainer-pod to worker1.us.oracle.com
  Normal   Pulling    35s (x4 over 2m8s)  kubelet            Pulling image "localhost/helloworldimage:v1"
  Warning  Failed     35s (x4 over 2m8s)  kubelet            Failed to pull image "localhost/helloworldimage:v1": rpc error: code = Unknown desc = pinging container registry localhost: Get "http://localhost/v2/": dial tcp [::1]:80: connect: connection refused
  Warning  Failed     35s (x4 over 2m8s)  kubelet            Error: ErrImagePull
  Warning  Failed     24s (x6 over 2m8s)  kubelet            Error: ImagePullBackOff
  Normal   BackOff    11s (x7 over 2m8s)  kubelet            Back-off pulling image "localhost/helloworldimage:v1"

podman is rootless and is not tied to a registry.

K8s cluster cannot talk to local machine. To workaround this issue you can tag and push the image to a registry where k8s cluster can pull from.

podman tag 00512f95bb0f ghcr.io/thelearnloop/exploringkubernetes/helloworldimage

podman push ghcr.io/thelearnloop/exploringkubernetes/helloworldimage

Now edit the hellopod.yaml file to use this new image name :

Then create the pod using kubectl command.