Use template to define sub-chart values with Helm

Question

Use case

"I want to deploy elasticsearch + kibana, by using elastic official helm charts. I want to create a helm chart called data-viz with these charts as dependency."

Running helm install data-viz --set cluster=toto must create an elasticsearch cluster "toto", and a kibana configured with elasticsearchHosts = toto.

Problem

I see here https://github.com/helm/helm/blob/master/docs/chart_template_guide/subcharts_and_globals.md it's possible to configure sub-chart from main-chart values.yml. But I would like templatize sub-chart values.yml from main-chart values.yaml, is it possible?

I was thinking to something simple as:

.
├── Chart.yaml
├── charts
│   ├── elasticsearch
│   │   ├── Chart.yaml
│   │   └── templates
│   │       └── values.yaml
│   ├── elasticsearch-7.4.0.tgz
│   └── kibana-7.4.0.tgz
├── requirements.lock
├── requirements.yaml
└── values.yaml

Hack solution

A small Python script that creates the values.yaml file from sub-chart/values-template.yaml + data.

Answer 1

The PR 6876 "feat(helm): Adding values templates in order to customize values with go-template, for the chart and its dependencies" could be of interest:

There have been many requests for a way to use templates for chart values.yaml (#2492, #2133, ...).
The main reason being that it's currently impossible to derive the values of the subcharts from templates.

However, having templates in values.yaml make them unparsable and create a "chicken or the egg" problem when rendering them.

This merge request creates an intuitive solution without such a problem: an optional values/ directory which templates are rendered using values.yaml, and then merge them with it.
Those values/ templates are only required for specific cases, work the same way as templates/ templates, and values.yaml will remain the main parsable source of value.

The rendering order has also been changed to allow those new values to enable or disable dependencies, and to avoid rendering values templates of disabled dependencies.

New possibilities:

Can now customize dependencies values, which was previously totally impossible

values/subchart.yaml

subchart:
    fullnameOverride: subchart-{{ .Relese.Name }}
    debug: {{ default "false" .Values.debug }}

Can derive dependencies conditions from values

Chart.yaml

dependencies:
- name: subchart
  condition: subchart.enabled

values/subchart.yaml

subchart:
{{- if eq .Values.environment "production" -}}
  enabled: true
{{- else -}}
  enabled: false
{{- end -}}

---

Similarly, PR 8580 "Added support to pass values to sub-charts via map.yaml" is of interest

This PR allows developers to declare a map.yaml file on their chart, which can be used to map values in values.yaml to derived values that are used in templating, including sub-charts (see #8576 for the long explanation).

This allows developers to write e.g.

apiVersion: v1
description: Chart with map and subcharts
name: chart-with-map
version: 0.0.1
dependencies:
  - name: backend
    version: 0.0.1
  - name: frontend
    version: 0.0.1

values.yaml

domain: example.com

map.yaml

backend:
  uri: {{ printf "https://api.%s" .Values.domain }}

frontend:
  uri: {{ printf "https://app.%s" .Values.domain }}

other_uri: {{ printf "https://blabla.%s" .Values.domain }}

thereby not having to expose backend: uri, frontend: uri in values.yaml (for the subchart), nor ask chart users to have to pass the same value in multiple keys for consistency (or use global names that lead to naming collisions).

I.e. it allows subcharts to be populated with derived (or maped) values without exposing these values to values.yaml (the public interface of the chart).

---

This is being implemented/evaluated in:

• PR 8677: "fix: limit the usage of chartutil.Values to avoid conversion bugs"
• PR 8679: "fix: do not merge and import values from disabled dependencies"
• PR 8690: "feat: add values templates to customize values with go-template"

But that will likely require an HIP (Helm Improvement Proposal).

---

Update Feb. 2021: PR 6876 confirms in this comment a formal proposal is needed.