Write a Helm chart for an application#

Argo CD manages applications in the Rubin Science Platform through a set of Helm charts. Which Helm charts to deploy in a given environment is controlled by the values.yaml and values-environment.yaml files in /environments.

The /applications directory defines templates in its templates directory and values to resolve those templates in values.yaml and values-environment.yaml files to customize the application for each environment. For first-party charts, the templates directory is generally richly populated.

Here are instructions for writing a Helm chart for a newly-developed application. If you are using an external third-party chart to deploy part of the application, also see Adding an external Helm chart.

Start from a template#

Ensure that your local Phalanx development environment is set up following the instructions in Setting up a Phalanx development environment.

Then, create the files for the new application, including the start of a Helm chart:

phalanx application create <application>

Replace <application> with the name of your new application, which will double as the name of the Helm chart.

By default, this will create a Helm chart for a FastAPI web service. Use the --starter flag to specify a different Helm chart starter. There are two options:

web-service

Use this starter if the new Helm application is a web service, such as a new Safir FastAPI service.

empty

Use this starter for any other type of application. This will create an empty Helm chart, to which you can add resources or external charts.

You will be prompted for a short description of the application. Keep it succinct, ideally just a few words, and do not add a period at the end.

Write the Chart.yaml#

After the previous step, there will be a new Helm chart for your application in applications/application. A basic Chart.yaml was created by phalanx application create. A few additional fields need to be filled out.

Chart versioning#

For charts that deploy a Rubin-developed application, set appVersion to the application’s Docker image tag (which is typically the version tag). For charts that do not deploy an application (for example, charts that are only used to manage subcharts as described in Adding an external Helm chart), delete the appVersion field.

Note

The chart also has a version field, which will be set to 1.0.0. This field does not need to be changed. The top level of charts defined in the applications directory are used only by Argo CD and are never published as Helm charts. Their versions are therefore irrelevant, so we use 1.0.0 for all charts.

Write the Kubernetes resource templates#

Put all Kubernetes resource templates that should be created by your chart in the templates subdirectory. See the Helm chart template developer’s guide.

Two aspects of writing a Helm chart are specific to Phalanx:

  • All secrets must come from VaultSecret resources, not Kubernetes Secret resources. You should use the value of the global.vaultSecretsPath configuration option followed by a slash and the name of your application. Phalanx’s secret management requires that you use a Vault secret with exactly this name. global.vaultSecretsPath will be injected by Argo CD with the correct value for the environment in which your application is deployed. See Define the application secrets for more information about secrets.

  • Applications providing a web API should be protected by Gafaelfawr and require an appropriate scope. This normally means using a GafaelfawrIngress object rather than an Ingress object. If you use the web service starter, this is set up for you by the template using a GafaelfawrIngress resource in templates/ingress.yaml, but you will need to customize the scope required for access, and may need to add additional configuration. You will also need to customize the path under which your application should be served. See the Gafaelfawr documentation for more details.

Pull secrets#

If your application image resides at a Docker repository which requires authentication (either to pull the image at all or to raise the pull rate limit), then you must tell any pods deployed by your application to use a pull secret named pull-secret, and you must create a VaultSecret resource for that pull secret.

If your container image is built through GitHub Actions and stored at ghcr.io (the recommended approach), there is no rate limiting (as long as your container image is built from a public repository, which it should be). There is therefore no need for a pull secret.

If your container image is stored at Docker Hub, you should use a pull secret, because we have been (and will no doubt continue to be) rate-limited at Docker Hub. Strongly consider moving your container image to be hosted by GitHub instead.

If your container image is pulled from a private repository, you may need authentication and therefore a pull secret.

If you do need a pull secret, add a block like the following to the pod specification for any resource that creates pods.

imagePullSecrets:
  - name: "pull-secret"

If you are using an external chart, see its documentation for how to configure pull secrets.

Then, add the following VaultSecret to your application templates to put a copy of pull-secret in your application’s namespace:

apiVersion: ricoberger.de/v1alpha1
kind: VaultSecret
metadata:
  name: pull-secret
  labels:
    {{- include "<application>.labels" . | nindent 4 }}
spec:
  path: "{{- .Values.global.vaultSecretsPath }}/pull-secret"
  type: kubernetes.io/dockerconfigjson

Replace <application> with the name of your application.

Write the values.yaml file#

The values.yaml file contains the customizable settings for your application. As a general rule, only use values.yaml settings for things that may vary between Phalanx environments. If something is the same in every Phalanx environment, it can be hard-coded into the Kubernetes resource templates.

Injected values#

Three values will be injected by Argo CD into your application automatically as globals, and therefore do not need to be set for each environment. These are global.baseUrl, global.host, and global.vaultSecretsPath and are taken from the global settings for each environment.

These should be mentioned for documentation purposes at the bottom of your values.yaml file with empty defaults. This is done automatically for you by the chart starters.

Documentation#

Phalanx uses helm-docs to automate generating documentation for the values.yaml settings.

For this to work correctly, each setting must be immediately preceded by a comment that starts with # -- and is followed by documentation for that setting in Markdown. This documentation may be wrapped to multiple lines.

The default value is included in the documentation. The documentation of the default value can be overridden with a comment starting with # @default --. This can be helpful when the default value in values.yaml is not useful (if, for instance, it’s a placeholder). For example:

# -- Tag of Gafaelfawr image to use
# @default -- The appVersion of the chart
tag: ""

For large default values or default values containing a lot of structure, the default behavior of helm-docs is to reproduce the entire JSON-encoded default in the generated documentation. This is often not useful and can break the HTML formatting of the resulting table. Therefore, for settings with long or complex values, use the following convention in a comment immediately before the setting:

# -- Description of the field.
# @default -- See the `values.yaml` file.
setting:
  - Some long complex value

Referring to Docker images#

To allow automated dependency updates to work, ensure that any Docker image deployed by your Helm chart uses values.yaml settings for the repository and current tag. These fields must be named repository and tag, respectively, and are conventionally nested under a key named image along with any other image properties that may need to be customized (such as pullPolicy).

Using this format will allow Mend Renovate to detect newer versions and create PRs to update Phalanx.

The main deployment (or stateful set, or cron job, etc.) for a Helm chart should use the appVersion in Chart.yaml as the default value for the image tag. This is done in the Kubernetes resource template. For example:

image: "{{ .Values.image.repository }}:{{ .Values.image.tag | default .ChartAppVersion }}"

Examples#

Existing Helm charts that are good examples to read or copy are:

  • hips (fairly simple)

  • mobu (also simple)

  • gafaelfawr (complex, including CRDs and multiple pods)

Next steps#

Be aware that Phalanx tests will not pass until you have done both of these steps as well.