flavianmissi/imagestreams

This repo reflects my path into understanding OpenShift's image streams.

Repository Contents

app-sample is a Go application that serves a "Hello, World!" under the / endpoint.
appsample-manifest.yaml contains the Deployment, Service, and Route needed to deploy app-sample
imagestreamimport.yaml is an ImageStreamImport that will create an image stream for the app-sample container image (the image needs to be built and pushed for it to work, more details below)

How-to

This section will go through the steps involved in importing an image stream (without copying it locally to OpenShift), and triggering a new deployment when the source tag changes.

We'll deploy an example application. The source code is included in this repository, under app-sample.

Requirements

You can probably run the below steps in the OpenShift sandbox, but I'll be using a cluster created on AWS with openshift-install.

We will use an already built version of the app-sample: quay.io/fmissi/sample. You won't have access to write in this repository, and we'll need to do that eventually to trigger a deployment, so you might want to build your own version of the app-sample and push it to a registry namespace you can access, then change imagestreamimport.yaml to reference your version instead of mine.

We'll be using our own project, which you can create with:

oc new-project myapp

If you're on OpenShift sandbox you won't be able to do this. In this case, you'll need to change the namespace references in the yaml files in this repository to use the namespace you've been provided with.

Importing the image

Before we can reference an image stream from our app Deployment we need to create it:

oc create -f imagestreamimport.yaml

this will create the app image stream:

oc get is
NAME   IMAGE REPOSITORY                                             TAGS     UPDATED
app    image-registry.openshift-image-registry.svc:5000/myapp/app   latest   3 hours ago

oc get istag

We set spec.images[0].referencePolicy.type to Source so that OpenShift's own image registry doesn't end up pulling this locally. Instead, the image is used directly from its source (quay.io/fmissi/sample in this case). More on that later.

Enabling deployments to reference image streams

It's not possible to reference deployment from image streams out of the box. We'll enable this directly in the image stream, but it could be done to the deployment as well.

oc set image-lookup app
oc set image-lookup imagestream --list

This will change the image stream yaml for us, enabling it to be referenced by other objects.

Deploy the sample app

oc apply -f appsample-manifests.yaml

You can see it working by making a request to it:

curl -Lk "$(oc get route app-sample -ojson|jq -r '.spec.host')"
Hello, World!

Let's have a look at what image the app pods are using:

oc get pod -l=app=app-sample -ojson|jq '.items[0].spec.containers[0].image'
"quay.io/fmissi/sample@sha256:05d9c9067053d05ad54a8b383238b59be2c5192fb0ea49708c982e9133b98977"

This manifest digest belongs to the v0.1.0 tag of quay.io/fmissi/sample. The latest tag currently points to that same manifest.

Enabling the deployment trigger

We want OpenShift to trigger a deployment whenever the manifest under the latest tag changes, so that when a new version of quay.io/fmissi/sample is imported into the image stream, OpenShift can immediately deploy it for us.

oc set triggers deploy/app-sample --from-image=app:latest -c app

Trigering a deployment

For OpenShift to trigger a deployment two things need to happen:

the source image (quay.io/fmissi/sample:latest) needs to change
the image stream needs to import the new source

1. Changing the source image

This is where you'll need access to the repository where the sample app image is stored. You'll need to adapt the commands below so they work for you.

podman pull quay.io/fmissi/sample:v0.2.0 # you can keep this line intact
podman tag quay.io/fmissi/sample:v0.2.0 quay.io/fmissi/sample:latest
podman push quay.io/fmissi/sample:latest

Version v0.2.0 of the sample app prints a "Hello, World! 123" instead of "Hello, World!".

2. Importing the new source image to the image stream

This step is the very same one we started with:

oc create -f imagestreamimport.yaml

This will update the image stream with the new reference, and that will in turn trigger a deployment to our app-sample.

curl -Lk "$(oc get route app-sample -ojson|jq -r '.spec.host')"
Hello, World! 123

oc get pod -l=app=app-sample -ojson|jq '.items[0].spec.containers[0].image'
"quay.io/fmissi/sample@sha256:62b5cf5989265441a1507dc4e10f2bdb982f0ba2707487120612f055b7f2bdb8"

Conclusion

We have:

created an image stream via ImageStreamImport
enabled it to be used by kubernetes objects (such as Deployment)
deployed a sample app
setup a deployment trigger for our app
seen the deployment trigger at work

Hope this helps someone else wrap their head around OpenShift image streams and deployment triggers. Writing it definitely helped me.

flavianmissi / imagestreams