An image contains the code used to create a deployment. Then, a deployment can be created from an image, which should then create a replica set (which is the number of pods that should be created), and then the pods should be created.

If you are not familiar with the oc command, refer to OpenShift - Getting Started with the oc command.

The oc new-app command is used to deploy an application.

There are various ways to deploy an app.

• From GitHub (https://github.com)
• From Docker Hub (https://hub.docker.com)
• From an image
• From a build
• From a JSON or YAML file (templates)

From an image

The oc get images command can be used to determine if the image has already been pulled down. Something like this should be returned. In this example, there are 4 php images (versions 5.5, 5.6, 7.0 and 7.1).

~]# oc get images
NAME                                                                      IMAGE REFERENCE
sha256:31e68b58657088d3f29c857aa257f9421ec58cb73cd225c2946cf327f0337366   registry.redhat.io/rhscl/php-70-rhel7@sha256:31e68b58657088d3f29c857aa257f9421ec58cb73cd225c2946cf327f0337366
sha256:920c2cf85b5da5d0701898f0ec9ee567473fa4b9af6f3ac5b2b3f863796bbd68   registry.redhat.io/rhscl/php-56-rhel7@sha256:920c2cf85b5da5d0701898f0ec9ee567473fa4b9af6f3ac5b2b3f863796bbd68
sha256:c82d399564d21b9737ac58c1c812c31c42b4afc94443a8e276cd63979dde2930   registry.redhat.io/openshift3/php-55-rhel7@sha256:c82d399564d21b9737ac58c1c812c31c42b4afc94443a8e276cd63979dde2930
sha256:e8d6446388de7a70b97d9883ab181858cfa13cb2b41ec216faef008caf6be615   registry.redhat.io/rhscl/php-71-rhel7@sha256:e8d6446388de7a70b97d9883ab181858cfa13cb2b41ec216faef008caf6be615

The following command would deploy the php application from the latest image (version 7.1).

oc new-app php

Or, you could specify the exact image you want to deploy.

oc new-app registry.example.com/openshift3/php-55-rhel7

The --as-deployment-config flag can be used to create the deployment as a deployment config instead of a regular "deployment".

oc new-app registry.example.com/openshift3/php-55-rhel7 --as-deployment-config

In this example, there was no image for the hello-world application, thus a call was made to Docker Hub for the image, which is like deploying the image from Docker Hub.

~]$ oc new-app hello-world
warning: Cannot find git. Ensure that it is installed and in your path. Git is required to work with git repositories.
--> Found container image feb5d9f (8 months old) from Docker Hub for "hello-world"

    * An image stream tag will be created as "hello-world:latest" that will track this image

--> Creating resources ...
    imagestream.image.openshift.io "hello-world" created
    deployment.apps "hello-world" created
--> Success
    Run 'oc status' to view your app.

On the other hand, if the image exists, something like this should be displayed.

--> Found image 28c6ff6 (16 months old) in image stream "openshift/php" under tag "7.1" for "php"

    Apache 2.4 with PHP 7.1
    ----------------------- 
    PHP 7.1 . . .

    Tags: builder, php, php71, rh-php71

    * This image will be deployed in deployment config "php"
    * Ports 8080/tcp, 8443/tcp will be load balanced by service "php"
      * Other containers can access this service through the hostname "php"

--> Creating resources ...
    imagestreamtag.image.openshift.io "php:7.1" created
    deploymentconfig.apps.openshift.io "php" created
    service "php" created
--> Success
    Application is not exposed. You can expose services to the outside world by executing one or more of the commands below:
     'oc expose svc/php' 
    Run 'oc status' to view your app.

If the --as-deployment-config flag was not used, the oc get deployments command can be used to list the deployments. 

~]$ oc get deployments
NAME      READY   UP-TO-DATE   AVAILABLE   AGE
php       1/1     1            1           38s

On the other hand, if the --as-deployment-config flag is used, the oc get deploymentconfigs command can be used to list the deployment configs.

~]$ oc get deploymentconfigs
NAME      REVISION   DESIRED   CURRENT   TRIGGERED BY
my-app    1          1         1         config,image(my-app:latest)

If the --as-deployment-config flag was used, the oc rollout history command can be used to see that the latet image was rolled out. In this example, revision 1 is the original deployment.

~]# oc rollout history deploymentconfig/my-app
deploymentconfig.apps.openshift.io/my-app
REVISION    STATUS      CAUSE
1           Complete    config change

The oc get pods can be used to determine if the pod was spun up from the deployment.

~]$ oc get pods
NAME                   READY   STATUS    RESTARTS   AGE
php-68dd9b68f4-r97m9   1/1     Running   0          109s

The oc get imagestreams command can be used to display the image stream.

~]# oc get imagestreams
NAME      DOCKER REPO                                TAGS      UPDATED
php       docker-registry.default.svc:5000/foo/php   7.1 

And the oc describe is php command can display a bit more information.

~]# oc describe is php
Name:                   php
Namespace:              foo
Created:                13 minutes ago
Labels:                 <none>
Annotations:            openshift.io/image.dockerRepositoryCheck=2020-10-02T01:40:18Z
Docker Pull Spec:       docker-registry.default.svc:5000/foo/php
Image Lookup:           local=false
Unique Images:          0
Tags:                   1