helm/docs/chart_template_guide/subcharts_and_globals.md

# Subcharts and Global Values

To this point we have been working only with one chart. But charts can have dependencies, called _subcharts_, that also have their own values and templates. In this section we will create a subchart and see the different ways we can access values from within templates.

Before we dive into the code, there are a few important details to learn about subcharts.

1. A subchart is considered "stand-alone", which means a subchart can never explicitly depend on its parent chart.
2. For that reason, a subchart cannot access the values of its parent.
3. A parent chart can override values for subcharts.
4. Helm has a concept of _global values_ that can be accessed by all charts.

As we walk through the examples in this section, many of these concepts will become clearer.

## Creating a Subchart

For these exercises, we'll start with the `mychart/` chart we created at the beginning of this guide, and we'll add a new chart inside of it.

```console
$ cd mychart/charts
$ helm create mysubchart
Creating mysubchart
$ rm -rf mysubchart/templates/*.*
```

Notice that just as before, we deleted all of the base templates so that we can start from scratch. In this guide, we are focused on how templates work, not on managing dependencies. But the [Charts Guide](../charts.md) has more information on how subcharts work.

## Adding Values and a Template to the Subchart

Next, let's create a simple template and values file for our `mysubchart` chart. There should already be a `values.yaml` in `mychart/charts/mysubchart`. We'll set it up like this:

```yaml
dessert: cake
```

Next, we'll create a new ConfigMap template in `mychart/charts/mysubchart/templates/configmap.yaml`:

```
apiVersion: v1
kind: ConfigMap
metadata:
  name: {{ .Release.Name }}-cfgmap2
data:
  dessert: {{ .Values.dessert }}
```

Because every subchart is a _stand-alone chart_, we can test `mysubchart` on its own:

```console
$ helm install --dry-run --debug mychart/charts/mysubchart
SERVER: "localhost:44134"
CHART PATH: /Users/mattbutcher/Code/Go/src/k8s.io/helm/_scratch/mychart/charts/mysubchart
NAME:   newbie-elk
TARGET NAMESPACE:   default
CHART:  mysubchart 0.1.0
MANIFEST:
---
# Source: mysubchart/templates/configmap.yaml
apiVersion: v1
kind: ConfigMap
metadata:
  name: newbie-elk-cfgmap2
data:
  dessert: cake
```

## Overriding Values of a Child Chart

Our original chart, `mychart` is now the _parent_ chart of `mysubchart`. This relationship is based entirely on the fact that `mysubchart` is within `mychart/charts`.

Because `mychart` is a parent, we can specify configuration in `mychart` and have that configuration pushed into `mysubchart`. For example, we can modify `mychart/values.yaml` like this:

```yaml
favorite:
  drink: coffee
  food: pizza
pizzaToppings:
  - mushrooms
  - cheese
  - peppers
  - onions

mysubchart:
  dessert: ice cream
```

Note the last two lines. Any directives inside of the `mysubchart` section will be sent to the `mysubchart` chart. So if we run `helm install --dry-run --debug mychart`, one of the things we will see is the `mysubchart` ConfigMap:

```yaml
# Source: mychart/charts/mysubchart/templates/configmap.yaml
apiVersion: v1
kind: ConfigMap
metadata:
  name: unhinged-bee-cfgmap2
data:
  dessert: ice cream
```

The value at the top level has now overridden the value of the subchart.

There's an important detail to notice here. We didn't change the template of `mychart/charts/mysubchart/templates/configmap.yaml` to point to `.Values.mysubchart.dessert`. From that template's perspective, the value is still located at `.Values.dessert`. As the template engine passes values along, it sets the scope. So for the `mysubchart` templates, only values specifically for `mysubchart` will be available in `.Values`.

Sometimes, though, you do want certain values to be available to all of the templates. This is accomplished using global chart values.

## Global Chart Values

Global values are values that can be accessed from any chart or subchart by exactly the same name. Globals require explicit declaration. You can't use an existing non-global as if it were a global.

The Values data type has a reserved section called `Values.global` where global values can be set. Let's set one in our `mychart/values.yaml` file.

```yaml
favorite:
  drink: coffee
  food: pizza
pizzaToppings:
  - mushrooms
  - cheese
  - peppers
  - onions

mysubchart:
  dessert: ice cream

global:
  salad: caesar
```

Because of the way globals work, both `mychart/templates/configmap.yaml` and `mychart/charts/mysubchart/templates/configmap.yaml` should be able to access that value as `{{ .Values.global.salad}}`.

`mychart/templates/configmap.yaml`:

```yaml
apiVersion: v1
kind: ConfigMap
metadata:
  name: {{ .Release.Name }}-configmap
data:
  salad: {{ .Values.global.salad }}
```

`mychart/charts/mysubchart/templates/configmap.yaml`:

```yaml
apiVersion: v1
kind: ConfigMap
metadata:
  name: {{ .Release.Name }}-cfgmap2
data:
  dessert: {{ .Values.dessert }}
  salad: {{ .Values.global.salad }}
```

Now if we run a dry run install, we'll see the same value in both outputs:

```yaml
# Source: mychart/templates/configmap.yaml
apiVersion: v1
kind: ConfigMap
metadata:
  name: silly-snake-configmap
data:
  salad: caesar

---
# Source: mychart/charts/mysubchart/templates/configmap.yaml
apiVersion: v1
kind: ConfigMap
metadata:
  name: silly-snake-cfgmap2
data:
  dessert: ice cream
  salad: caesar
```

Globals are useful for passing information like this, though it does take some planning to make sure the right templates are configured to use globals.

## Sharing Templates with Subcharts

Parent charts and subcharts can share templates. Any defined block in any chart is
available to other charts.

For example, we can define a simple template like this:

```yaml
{{- define "labels" }}from: mychart{{ end }}
```

Recall how the labels on templates are _globally shared_. Thus, the `labels` chart
can be included from any other chart.

While chart developers have a choice between `include` and `template`, one advantage
of using `include` is that `include` can dynamically reference templates:

```yaml
{{ include $mytemplate }}
```

The above will dereference `$mytemplate`. The `template` function, in contrast,
will only accept a string literal.

## Avoid Using Blocks

The Go template language provides a `block` keyword that allows developers to provide
a default implementation which is overridden later. In Helm charts, blocks are not
the best tool for overriding because if multiple implementations of the same block
are provided, the one selected is unpredictable.

The suggestion is to instead use `include`.