How to Install and Use an Azure Connected Registry

In 2021, Microsoft released a feature of Azure Container Registry called 'connected registry' in public preview. A connected registry allows you to install a container registry on-prem, which synchronizes or mirrors an Azure Container Registry in the cloud. This allows you to have your container images nearby, which is beneficial in scenarios where you have an occasional or limited connection with the cloud. At the moment that you need to pull a container image, the image doesn't need to be pulled from Azure but is already present where you need it. In this blog post, I'll dive a bit deeper into the technicalities of how to deploy a connected registry.

Frederik Gheysels
28 July 2022

In another blog post, I explained how we can use a connected registry to bring containers onboard vessels.

In this article, I’ll dive a bit deeper and show you how you can deploy a connected Azure Container Registry, and how it must be configured so that images can be pulled.  Scripting is great, as it allows you to automate your deployments.

Let’s dive right into this!

Installing a connected registry

Before we can pull images from a connected registry, we need to install one. This consists of 2 parts:

  • Create a connected registry instance in Azure.
  • Deploy the required connected registry components on the machine that will actually host the connected registry.

Define a connected registry instance in Azure

When you want to make use of an on-prem connected registry, you first need to define a ‘connected registry’ resource in Azure.

This can be done by using the az acr connected-registry command. When creating a connected registry, you need to specify which repositories from the parent Azure Container Registry must be synced or mirrored.  This is explained in great detail here.

Specify the repositories that must be synced

If you don’t have a fixed list of repositories that must be synced with your connected registries, but want to sync all repositories that exist in a certain namespace, it can be quite cumbersome to manually list them all. Instead, you can list all the repositories that you want to sync via scripting. For instance:

Create the Connected Registry Azure resource

Once we have all the required information, the connected registry resource in Azure can be created.

With the above command, we specify that the connected registry must sync all repositories that are listed in the $repositoriesToSyncString.
The connected registry can only read from the parent ACR since the mode is set to ReadOnly.
Syncing happens every hour at 30 minutes and the sync window is set to 1 hour.

Deploy the connected registry on-prem components

Now that we have created the necessary Azure resources for the connected registry, it is time to deploy the on-prem components.
In this article, we’ll focus on deploying a connected registry in a Kubernetes cluster.  If you want to deploy a connected registry in IoT Edge, please see this article.

Get the connection-string of the connected registry

Before we can deploy the connected registry components, we need to get hold of the connection-string of the connected registry resource that exists in Azure.
We will need this information when deploying the on-prem components, so let’s just quickly retrieve the required information.

With the above command, we generate a password and retrieve the credentials with one single command.
The $credentials object contains a connection-string that we’ll use in the next step.

Deploying connected registry service

When you want to pull container images from a connected registry in a Kubernetes cluster, you will need to deploy the connected registry components inside your Kubernetes container. This can easily be done by installing a Helm chart provided by Microsoft.
I like to deploy these components in a dedicated Kubernetes namespace.  Execute the command below to create a Kubernetes namespace in an idempotent way.  This means that, when the namespace already exists, no error will be given:

Create the Connected Registry Azure resource

Once we have all the required information, the connected registry resource in Azure can be created.

With the above command, we specify that the connected registry must sync all repositories that are listed in the $repositoriesToSyncString.
The connected registry can only read from the parent ACR since the mode is set to ReadOnly.
Syncing happens every hour at minute 30 and the sync window is set to 1 hour.

Deploy the connected registry on-prem components

Now that we have created the necessary Azure resources for the connected registry, it is time to deploy the on-prem components.
In this article, we’ll focus on deploying a connected registry in a Kubernetes cluster.  If you want to deploy a connected registry in IoT Edge, please see this article.

Get the connection-string of the connected registry

Before we can deploy the connected registry components, we need to get hold of the connection-string of the connected registry resource that exists in Azure.
We will need this information when deploying the on-prem components, so let’s just quickly retrieve the required information.

With the above command, we generate a password and retrieve the credentials with one single command.
The $credentials object contains a connection-string that we’ll use in the next step.

Deploying connected registry service

When you want to pull container images from a connected registry in a Kubernetes cluster, you will need to deploy the connected registry components inside your Kubernetes container. This can easily be done by installing a Helm chart provided by Microsoft.
I like to deploy these components in a dedicated Kubernetes namespace.  Execute the command below to create a Kubernetes namespace in an idempotent way.  This means that, when the namespace already exists, no error will be given:

Getting the Helm chart that is provided by Microsoft is done by executing these commands:

Verify if everything is running correctly by executing kubectl get pods -n connected-registry.

As you can see, with the above command we specify the connection-string to the connected-registry resource that is defined in Azure.
We also specify that we want to be able to pull images from a private registry using http instead of being required to pull images using https (httpEnabled=true).

With the sync.chunkSizeInMb parameter you can specify the size of the chunks in which container layers must be downloaded. If you’re running on a slow or unstable connection, it is advised to set this parameter quite low.

The pvc.storageClassName specifies that we want to store the container images on local storage of the node. The values that you can specify for this parameter vary per Kubernetes flavor. The local-path value is a setting that is specific for Rancher’s K3s.

Configure pulling from a private registry

When you want to pull from a private registry using http instead of https, you need to configure that the connected-registry is a private registry.
How this is done again depends on your Kubernetes version.  For K3s, this is done by editing the /etc/rancher/k3s/registries.yaml file.  More information on this can be found here.
To configure this, you’ll need the IP address at which the connected registry is reachable. The command below can be used to retrieve the IP address of the connected registry service that is running in Kubernetes:

Modify the /etc/rancher/k3s/registries.yaml file so that it looks like the sample below. Of course, you need to use the IP address that you’ve acquired ($connectedRegistryIp) instead of the sample address 10.1.1.1

Modifying this file can also be automated using your favorite scripting language. Using Powershell, you can for instance use the powershell-yaml module to do that. You can find this module here.

Here is the code to do that:

After the registries.yaml file has been modified, K3s must be restarted for the changes to have effect:

The connected registry is now in place and will start pulling container images from its’ parent Azure Container Registry.

Pulling images from a connected registry

Now that the connected registry components are up and running, we need to configure it so that images can be pulled from the connected registry.

Configure Client Token

Before images can be pulled from the connected registry, a client token must be configured.  As explained here, the client token specifies from which repository images can be pulled.
A token is created using a scope map.  In the scope map, we specify the repositories from which images can be pulled. We can use the $existingRepositories that we have declared and initialized earlier for this. We can extract the repositories that the client is allowed to pull. Again, let us specify that images can be pulled if they belong to a repository that is in a certain namespace:

As can be read from the documentation of the az acr scope-map create command, each repository must be prefixed with --repository and must be suffixed with the actions that are allowed for that repository.

The following command does that:

Now we have everything to create scope map.  Again, we use the Invoke-Expression command to avoid problems with the quotes in the $allowedRepositoriesString variable:

Now, we can use the scope map to generate a client access token and add that token to our connected registry:

Note that it is a good idea to often roll the client access token. This can be done by simply executing the above commands again.

Create a Pull Secret for the Connected Registry

When you want to pull images in Kubernetes, you need a pull secret. The $token that we have just generated can be used to create a pull secret for the connected registry:

Now you can use this pull secret in your Kubernetes deployments to pull container images from your Connected Registry! Just don’t forget to specify the pull secret in your Kubernetes deployment.yaml manifest.

I hope this article helps you in setting up an Azure Connected Registry!

Frederik

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