If you’ve been playing around with OpenShift and kicking the tyres a bit, you might have come across the mysterious object called the image stream.
What is it? (pokes awkwardly at object lying in the corner)
“It’s this thing which seems to make your images available in the cluster, or something?”
Yes, but what exactly are image streams? How do you use them? And what do you really need to know about them?
In this OpenShift image stream tutorial, I’m going to tell you how I understand image streams, and how I use them. Let’s go.
Firstly, the terminology sucks
The name “stream” makes it so confusing. There’s no streaming involved here.
No Netflix, no data streaming.
An image stream is just a set of references (tags) which point to Docker images.
If I could rename image streams, I’d probably call them image references.
What does an image stream do?
An image stream acts as a pointer to a set of images. It’s like a shortcut or a reference.
An image stream doesn’t contain the Docker image itself. It’s a signpost to images, which can be in any registry – either OpenShift’s internal registry, or an external one.
I regularly use image streams in builds and deployments on OpenShift.
But what’s the point? Why use them?
Why would you use them?
A single place to reference a set of related images
An image stream is a single pointer to a set of related images.
One image stream can contain many different tags (
1.0, etc), each of which points to an image in a registry.
Whenever you want to deploy an application in a project, instead of having to hardcode the registry URL and tag, you can just refer to the image stream tag.
If the source image changes location in future, you just need to update the image stream definition, instead of updating all your deployments individually!
This is how the Red Hat images work in OpenShift. The Red Hat images are installed as image streams in the
openshift project. You can reference them from any other project in the cluster.
When an image changes, trigger a new build or deployment
Image streams can also be used as triggers. This means that you can set an image stream as a source for a Build or a Deployment. When the image changes, this can trigger a new Build, or trigger a redeployment of your app.
Access control for images
You can lock away your images in a private registry and control access to them through image streams.
Because image streams are like other objects in OpenShift, they can be restricted, if you like. So you can restrict images to only be used by specific service accounts.
How do you create an image stream?
Here’s how I would create an image stream using the command line.
Create an image stream using the
oc import-image command. Give the image stream a name and then the image you want to import.
oc import-image approved-apache:2.4 \ --from=bitnami/apache:2.4 \ --confirm
This creates an image stream in your project, called approved-apache. It has one tag, 2.4, which points to the tag 2.4 on the image bitnami/apache.
But wait, you didn’t specify a registry?
That’s right. I don’t specify a registry here, because Docker Hub is configured as one of OpenShift’s default search registries. So it’ll search in the Red Hat registry and Docker Hub, which is where it will find the image.
Then the image will be pulled into OpenShift’s internal registry.
If you’re a cluster-admin, you can see the image in the OpenShift registry, by typing
oc get images:
$ oc get images | grep bitnami sha256:aa742bfa021b81f8d... bitnami/apache@sha256:aa742bfa021b81f8d...
Declaring it with YAML
Once you’ve learned how to create image streams using the
oc command, you probably want to progress on to creating objects declaratively. This is a much better way to do it, because you can store all of your configuration as files in Git, rather than a bunch of commands.
To do this, you create an
ImageStream object in YAML. The YAML for an
ImageStream looks something like this:
apiVersion: image.openshift.io/v1 kind: ImageStream metadata: name: approved-apache spec: lookupPolicy: local: false tags: - name: "2.4" from: kind: DockerImage name: bitnami/apache:2.4 referencePolicy: type: Source
Then apply this using
How do you use an image stream?
The way I use image streams is as sources for Builds and Deployments.
Deploying an image stream
Once you’ve created an image stream, you can run
oc new-app --image-stream=<name> to easily create a Deployment and a Service from it.
OpenShift will inspect the image to see which ports to expose, and then create a Deployment and a Service for your app.
Let’s import a sample image that I made:
oc import-image my-python \ --from=quay.io/tdonohue/python-hello-world:latest \ --confirm
Now I create a Deployment and a Service to deploy it in my project:
oc new-app --image-stream=my-python
Using an image stream as a source image in a build
Or, you can use an image stream as a source for a build.
Here’s an example of a BuildConfig which uses an image stream as a source:
kind: BuildConfig apiVersion: build.openshift.io/v1 metadata: name: example-app spec: source: type: Git git: uri: 'https://github.com/sclorg/nodejs-ex.git' strategy: type: Source sourceStrategy: from: kind: ImageStreamTag name: 'nodejs-8-centos7:latest' output: to: kind: ImageStreamTag name: 'example-app:latest' triggers: - type: ConfigChange - type: ImageChange
What if your image is in a private registry?
Yep, you can still use image streams if your image is in a private registry.
Firstly, if the registry is password-protected, make sure you create a Secret with your credentials for the external registry, so OpenShift can pull the image.
Here I’m connecting to the registry on the Mars rover (well, maybe one day Mars rovers will run OpenShift):
oc create secret docker-registry my-mars-secret \ --docker-server=registry.marsrover.space \ --docker-username="email@example.com" \ --docker-password=thepasswordishere
Then link the secret to your builder and default service accounts:
oc secrets link builder my-mars-secret oc secrets link default my-mars-secret --for=pull
Now we’re good to import the image, using the
oc import-image command:
oc import-image marsview \ --from=registry.marsrover.space/engine/marsview \ --confirm
Congrats! You’ve created an image stream to import an image from a private registry, with authentication.
What about triggering when the source image changes?
If your image stream points to an image in OpenShift’s internal registry (perhaps, like an image you’ve built inside OpenShift), then OpenShift gets notified automatically when your image changes.
But if you want to tell OpenShift to keep checking an external image for changes, you’ll need to use the
--scheduled tag when you use
oc import-image marsview \ --from=registry.marsrover.space/engine/marsview \ --scheduled --confirm
When you use this option, you’ll see this in the output from
oc import-image. Note how it says “updates automatically”:
Name: marsview Namespace: toms-project Created: 12 minutes ago Labels: <none> Annotations: openshift.io/image.dockerRepositoryCheck=2021-01-13T18:54:19Z Image Repository: image-registry.openshift-image-registry.svc:5000/toms-project/marsview Image Lookup: local=false Unique Images: 1 Tags: 1 latest updates automatically from registry registry.marsrover.space/engine/marsview:latest * registry.marsrover.space/engine/marsview@sha256:a7ca9b84c7a642ac14294679ba6fb9c80918ea210f3c63154d23231912d41394 About a minute ago
When does the image update?
OpenShift will keep checking the image, usually around every 15 minutes (see below for how to change this.)
Wait 15 minutes. Put the kettle on. Whatever it takes to run down the clock.
If a new image is pushed, you’ll see something like this, when you run
oc describe is/marsview:
latest updates automatically from registry registry.marsrover.space/engine/marsview:latest * registry.marsrover.space/engine/marsview@sha256:77dc2d9740a2450dec03cef69d4c42ad3ca900b6b0d93dcbf0c7f46b598f8d9c About a minute ago registry.marsrover.space/engine/marsview@sha256:a7ca9b84c7a642ac14294679ba6fb9c80918ea210f3c63154d23231912d41394 19 minutes ago
Bingo, the image updated about a minute ago!
Then, as long as your DeploymentConfig is configured with a Trigger of “ImageChange”, it will automatically trigger a redeployment.
Here’s what a DeploymentConfig for this app would look like, with a trigger:
kind: DeploymentConfig apiVersion: apps.openshift.io/v1 metadata: name: marsview spec: selector: deploymentconfig: marsview replicas: 1 template: metadata: labels: deploymentconfig: marsview spec: containers: - name: main image: marsview # OpenShift will replace this with the 'real' image URL ports: - containerPort: 8080 protocol: TCP imagePullPolicy: Always restartPolicy: Always triggers: - type: ConfigChange - type: ImageChange imageChangeParams: automatic: true # Set automatic=true to trigger a redeployment on image change containerNames: - marsview from: kind: ImageStreamTag namespace: toms-project name: 'marsview:latest'
This allows you to implement basic continuous deployment. The moment that a new image is pushed to your latest tag, OpenShift will automatically trigger an update of your app. Fab.
How often are images imported?
How often images are imported, is a cluster-wide setting. Look for these settings in
imagePolicyConfig: MaxScheduledImageImportsPerMinute: 10 ScheduledImageImportMinimumIntervalSeconds: 1800 disableScheduledImport: false maxImagesBulkImportedPerRepository: 3
Adding additional tags to the image stream
To add additional tags to an image stream, you can use
The new tag doesn’t need to be in the same registry, it could be somewhere else entirely.
Here I’m adding a develop tag to my image stream, pointing to a different registry (registry.spacedev.com):
oc tag registry.spacedev.com/engine/marsview:develop \ marsview:develop \ --scheduled
which will say this:
Tag marsview:develop set to import registry.marsrover.space/engine/marsview:develop periodically.
Job done. Now I can reference the
develop tag from my development registry.
TL;DR. Do I really need them?
No, you don’t really need them. But they are useful for:
Creating one place to pull images from external registries, so you don’t have to keep track of everywhere you’re using an image
Creating triggers for your builds and deployments
Understanding how the built-in Red Hat images are installed in the cluster (you’ll see them in the
And here are the key terms.
|Term||What it is|
|Image stream||A set of pointers (tags). Each tag points to a Docker image.|
|Image stream tag||A tag within an image stream. The tag points to an individual Docker image in a registry somewhere, either the internal registry or an external one.|
|Image stream image||A pointer from within an image stream, to a particular image ID. Basically lets you get all the details (metadata) of the actual image itself.|
Just….don’t cross the streams! (Bad joke.)
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