Setting Override Values in Helm-based Operators

Sometimes it is useful to pass down environment variables from the Operators Deployment all the way to the helm charts templates. This allows the Operator to be configured at a global level at runtime. This is new compared to dealing with the helm CLI as they usually don’t have access to any environment variables in the context of Tiller (helm v2) or the helm binary (helm v3) for security reasons.

With the helm Operator this becomes possible by override values. This enforces that certain template values provided by the chart’s default values.yaml or by a CR spec are always set when rendering the chart. If the value is set by a CR it gets overridden by the global override value. The override value can be static but can also refer to an environment variable and use go templates. Using override values is currently the only way to pass down environment variables to the chart.

An example use case of this is when your helm chart references container images by chart variables, which is a good practice. If your Operator is deployed in a disconnected environment (no network access to the default images location) you can use this mechanism to set them globally at the Operator level using environment variables versus individually per CR / chart release.

Note that it is strongly recommended to reference container images in your chart by helm variables and then also associate these with an environment variable of your Operator like shown below. This allows your Operator to be mirrored for offline usage when packaged for OLM.

Basic usage

To configure your operator with override values, add an overrideValues map to your watches.yaml file for the GVK and chart you need to override. For example, to change the repository used by the nginx chart, you would update your watches.yaml to the following:

# Use the 'create api' subcommand to add watches to this file.
- group: example.com
  version: v1alpha1
  kind: Nginx
  chart: helm-charts/nginx
  overrideValues:
    image.repository: quay.io/mycustomrepo/myimage

By setting image.repository to quay.io/mycustomrepo/myimage you are ensuring that quay.io/mycustomrepo/myimage will always be used instead of the chart’s default repository (nginx). If the CR attempts to set this value, it will be ignored.

Using environment variables

It is also possible to reference environment variables in the overrideValues section:

  overrideValues:
    image.repository: $IMAGE_REPOSITORY # or ${IMAGE_REPOSITORY}

By using an environment variable reference in overrideValues you enable these override values to be set at runtime by configuring the environment variable on the operator deployment. For example, in config/manager/manager.yaml you could add the following snippet to the container spec:

env:
  - name: IMAGE_REPOSITORY
    value: quay.io/mycustomrepo/myimage

If an environment variable reference is listed in overrideValues, but is not present in the environment when the operator runs, it will resolve to an empty string and override all other values. Therefore, these environment variables should always be set. It is suggested to update the Dockerfile to set these environment variables to the same defaults that are defined by the chart.

Using Go templates

Lastly, you can use Go text/template strings along with slim-sprig functions to provide even more flexibility when building override values.

For example, consider a situation where your operator has an environment variable, $IMAGE, set to quay.io/mycustomrepo/myimage:latest. You can use sprig template functions to split that environment variable into its repo and tag:

  overrideValues:
    image.repository: '{{ ("$IMAGE" | split ":")._0 }}'
    image.tag: '{{ ("$IMAGE" | split ":")._1 }}'

The resulting override values sent to the helm installation would look like:

  overrideValues:
    image.repository: quay.io/mycustomrepo/myimage
    image.tag: latest

Event generation

To warn users that their CR settings may be ignored, the Helm operator creates events on the CR that include the name and value of each overridden value. For example:

$ kubectl describe nginxes.example.com
...
Events:
  Type     Reason               Age   From              Message
  ----     ------               ----  ----              -------
  Warning  OverrideValuesInUse  1m    nginx-controller  Chart value "image.repository" overridden to "quay.io/mycustomrepo" by operator's watches.yaml