Chapter 7. Porting containers to OpenShift using Podman

This chapter describes how to generate portable descriptions of containers and pods using the YAML ("YAML Ain’t Markup Language") format. The YAML is a text format used to describe the configuration data.

The YAML files are:

  • Readable.
  • Easy to generate.
  • Portable between environments (for example between RHEL and OpenShift).
  • Portable between programming languages.
  • Convenient to use (no need to add all the parameters to the command line).

Reasons to use YAML files:

  1. You can re-run a local orchestrated set of containers and pods with minimal input required which can be useful for iterative development.
  2. You can run the same containers and pods on another machine. For example, to run an application in an OpenShift environment and to ensure that the application is working correctly. You can use podman generate kube command to generate a Kubernetes YAML file. Then, you can use podman play command to test the creation of pods and containers on your local system before you transfer the generated YAML files to the Kubernetes or OpenShift environment. Using the podman play command, you can also recreate pods and containers originally created in OpenShift or Kubernetes environments.

7.1. Generating a Kubernetes YAML file using Podman

This procedure describes how to create a pod with one container and generate the Kubernetes YAML file using the podman generate kube command.

Prerequisites

Procedure

  1. List all pods and containers associated with them: 

    $ podman ps -a --pod
CONTAINER ID  IMAGE                                       COMMAND    CREATED                 STATUS                     PORTS  NAMES               POD
5df5c48fea87  registry.access.redhat.com/ubi8/ubi:latest  /bin/bash  Less than a second ago  Up Less than a second ago         myubi               223df6b390b4
3afdcd93de3e  k8s.gcr.io/pause:3.1                                   Less than a second ago  Up Less than a second ago         223df6b390b4-infra  223df6b390b4

  2. Use the pod name or ID to generate the Kubernetes YAML file: 

    $ podman generate kube mypod > mypod.yaml

    Note that the podman generate command does not reflect any Logical Volume Manager (LVM) logical volumes or physical volumes that might be attached to the container.

  3. Display the mypod.yaml file: 

    $ cat mypod.yaml
# Generation of Kubernetes YAML is still under development!
#
# Save the output of this file and use kubectl create -f to import
# it into Kubernetes.
#
# Created with podman-1.6.4
apiVersion: v1
kind: Pod
metadata:
  creationTimestamp: "2020-06-09T10:31:56Z"
  labels:
app: mypod
  name: mypod
spec:
  containers:
  - command:
        - /bin/bash
        env:
        - name: PATH
              value: /usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin
        - name: TERM
              value: xterm
        - name: HOSTNAME
        - name: container
              value: oci
        image: registry.access.redhat.com/ubi8/ubi:latest
        name: myubi
        resources: {}
        securityContext:
              allowPrivilegeEscalation: true
              capabilities: {}
              privileged: false
              readOnlyRootFilesystem: false
        tty: true
        workingDir: /
status: {}

  4. To stop the pod mypod: 

    $ podman pod stop mypod

  5. List all pods and containers associated with them: 

    $ podman ps -a --pod
CONTAINER ID  IMAGE                               COMMAND    CREATED             STATUS                    PORTS   NAMES               POD ID        PODNAME
12fc1ada3d4f  registry.redhat.io/ubi8/ubi:latest  /bin/bash  About a minute ago  Exited (0) 7 seconds ago          myubi               8a4e6527ac9d  mypod

8bb1daaf81fe  k8s.gcr.io/pause:3.2                           About a minute ago  Exited (0) 7 seconds ago          8a4e6527ac9d-infra  8a4e6527ac9d  mypod

    Here, the pod mypod and container myubi are in "Exited" status.

  6. To remove the pod mypod: 

    $ podman pod rm mypod
8a4e6527ac9d2276e8a6b9c2670608866dbcb5da3efbd06f70ec2ecc88e247eb

    Note that removing the pod automatically removes all containers inside it.

  7. To check that all containers and pods were removed: 

    $ podman ps
$ podman pod ps

Additional resources

7.2. Generating a Kubernetes YAML file in OpenShift environment

In the OpenShift environment, use the oc create command to generate the YAML files describing your application.

Procedure

  • Generate the YAML file for your myapp application: 

    $ oc create myapp --image=me/myapp:v1 -o yaml --dry-run > myapp.yaml

    The oc create command creates and run the myapp image. The object is printed using the --dry-run option and redirected into the myapp.yaml output file.

Note

In the Kubernetes environment, you can use the kubectl create command with the same flags.

7.3. Starting containers and pods with Podman

With the generated YAML files, you can automatically start containers and pods in any environment. Note that the YAML files must not be generated by the Podman. The podman play kube command allows you to recreate pods and containers based on the YAML input file.

Procedure

  1. Create the pod and the container from the mypod.yaml file: 

    $ podman play kube mypod.yaml
Pod:
b8c5b99ba846ccff76c3ef257e5761c2d8a5ca4d7ffa3880531aec79c0dacb22
Container:
848179395ebd33dd91d14ffbde7ae273158d9695a081468f487af4e356888ece

  2. List all pods: 

    $ podman pod ps
POD ID         NAME    STATUS    CREATED          # OF CONTAINERS   INFRA ID
b8c5b99ba846   mypod   Running   19 seconds ago   2                 aa4220eaf4bb

  3. List all pods and containers associated with them: 

    $ podman ps -a --pod
CONTAINER ID  IMAGE                                       COMMAND    CREATED             STATUS                 PORTS  NAMES               POD
848179395ebd  registry.access.redhat.com/ubi8/ubi:latest  /bin/bash  About a minute ago  Up About a minute ago         myubi               b8c5b99ba846
aa4220eaf4bb  k8s.gcr.io/pause:3.1                                   About a minute ago  Up About a minute ago         b8c5b99ba846-infra  b8c5b99ba846

    The pod IDs from podman ps command matches the pod ID from the podman pod ps command.

Additional resources

7.4. Starting containers and pods in OpenShift environment

You can use the oc create command to create pods and containers in the OpenShift environment.

Procedure

  • Create a pod from the YAML file in the OpenShift environment: 

    $ oc create -f mypod.yaml
Note

In the Kubernetes environment, you can use the kubectl create command with the same flags.