Named Templates
It is time to move beyond one template, and begin to create others. In this section, we will see how to define named templates in one file, and then use them elsewhere. A named template (sometimes called a partial or a subtemplate) is simply a template defined inside of a file, and given a name. We’ll see two ways to create them, and a few different ways to use them.
In the
Flow Control section we introduced three actions
for declaring and managing templates: define
, template
, and block
. In this
section, we’ll cover those three actions, and also introduce a special-purpose
include
function that works similarly to the template
action.
An important detail to keep in mind when naming templates: template names are global. If you declare two templates with the same name, whichever one is loaded last will be the one used. Because templates in subcharts are compiled together with top-level templates, you should be careful to name your templates with chart-specific names.
One popular naming convention is to prefix each defined template with the name
of the chart: {{ define "mychart.labels" }}
. By using the specific chart name
as a prefix we can avoid any conflicts that may arise due to two different
charts that implement templates of the same name.
Partials and _
files
So far, we’ve used one file, and that one file has contained a single template. But Helm’s template language allows you to create named embedded templates, that can be accessed by name elsewhere.
Before we get to the nuts-and-bolts of writing those templates, there is file naming convention that deserves mention:
- Most files in
templates/
are treated as if they contain Kubernetes manifests - The
NOTES.txt
is one exception - But files whose name begins with an underscore (
_
) are assumed to not have a manifest inside. These files are not rendered to Kubernetes object definitions, but are available everywhere within other chart templates for use.
These files are used to store partials and helpers. In fact, when we first
created mychart
, we saw a file called _helpers.tpl
. That file is the default
location for template partials.
Declaring and using templates with define
and template
The define
action allows us to create a named template inside of a template
file. Its syntax goes like this:
{{ define "MY.NAME" }}
# body of template here
{{ end }}
For example, we can define a template to encapsulate a Kubernetes block of labels:
{{- define "mychart.labels" }}
labels:
generator: helm
date: {{ now | htmlDate }}
{{- end }}
Now we can embed this template inside of our existing ConfigMap, and then
include it with the template
action:
{{- define "mychart.labels" }}
labels:
generator: helm
date: {{ now | htmlDate }}
{{- end }}
apiVersion: v1
kind: ConfigMap
metadata:
name: {{ .Release.Name }}-configmap
{{- template "mychart.labels" }}
data:
myvalue: "Hello World"
{{- range $key, $val := .Values.favorite }}
{{ $key }}: {{ $val | quote }}
{{- end }}
When the template engine reads this file, it will store away the reference to
mychart.labels
until template "mychart.labels"
is called. Then it will
render that template inline. So the result will look like this:
# Source: mychart/templates/configmap.yaml
apiVersion: v1
kind: ConfigMap
metadata:
name: running-panda-configmap
labels:
generator: helm
date: 2016-11-02
data:
myvalue: "Hello World"
drink: "coffee"
food: "pizza"
Conventionally, Helm charts put these templates inside of a partials file,
usually _helpers.tpl
. Let’s move this function there:
{{/* Generate basic labels */}}
{{- define "mychart.labels" }}
labels:
generator: helm
date: {{ now | htmlDate }}
{{- end }}
By convention, define
functions should have a simple documentation block
({{/* ... */}}
) describing what they do.
Even though this definition is in _helpers.tpl
, it can still be accessed in
configmap.yaml
:
apiVersion: v1
kind: ConfigMap
metadata:
name: {{ .Release.Name }}-configmap
{{- template "mychart.labels" }}
data:
myvalue: "Hello World"
{{- range $key, $val := .Values.favorite }}
{{ $key }}: {{ $val | quote }}
{{- end }}
As mentioned above, template names are global. As a result of this, if two
templates are declared with the same name the last occurrence will be the one
that is used. Since templates in subcharts are compiled together with top-level
templates, it is best to name your templates with chart specific names. A
popular naming convention is to prefix each defined template with the name of
the chart: {{ define "mychart.labels" }}
.
Setting the scope of a template
In the template we defined above, we did not use any objects. We just used functions. Let’s modify our defined template to include the chart name and chart version:
{{/* Generate basic labels */}}
{{- define "mychart.labels" }}
labels:
generator: helm
date: {{ now | htmlDate }}
chart: {{ .Chart.Name }}
version: {{ .Chart.Version }}
{{- end }}
If we render this, the result will not be what we expect:
# Source: mychart/templates/configmap.yaml
apiVersion: v1
kind: ConfigMap
metadata:
name: moldy-jaguar-configmap
labels:
generator: helm
date: 2016-11-02
chart:
version:
What happened to the name and version? They weren’t in the scope for our defined
template. When a named template (created with define
) is rendered, it will
receive the scope passed in by the template
call. In our example, we included
the template like this:
{{- template "mychart.labels" }}
No scope was passed in, so within the template we cannot access anything in .
.
This is easy enough to fix, though. We simply pass a scope to the template:
apiVersion: v1
kind: ConfigMap
metadata:
name: {{ .Release.Name }}-configmap
{{- template "mychart.labels" . }}
Note that we pass .
at the end of the template
call. We could just as easily
pass .Values
or .Values.favorite
or whatever scope we want. But what we want
is the top-level scope.
Now when we execute this template with helm install --dry-run --debug plinking-anaco ./mychart
, we get this:
# Source: mychart/templates/configmap.yaml
apiVersion: v1
kind: ConfigMap
metadata:
name: plinking-anaco-configmap
labels:
generator: helm
date: 2016-11-02
chart: mychart
version: 0.1.0
Now {{ .Chart.Name }}
resolves to mychart
, and {{ .Chart.Version }}
resolves to 0.1.0
.
The include
function
Say we’ve defined a simple template that looks like this:
{{- define "mychart.app" -}}
app_name: {{ .Chart.Name }}
app_version: "{{ .Chart.Version }}"
{{- end -}}
Now say I want to insert this both into the labels:
section of my template,
and also the data:
section:
apiVersion: v1
kind: ConfigMap
metadata:
name: {{ .Release.Name }}-configmap
labels:
{{ template "mychart.app" . }}
data:
myvalue: "Hello World"
{{- range $key, $val := .Values.favorite }}
{{ $key }}: {{ $val | quote }}
{{- end }}
{{ template "mychart.app" . }}
The output will not be what we expect:
# Source: mychart/templates/configmap.yaml
apiVersion: v1
kind: ConfigMap
metadata:
name: measly-whippet-configmap
labels:
app_name: mychart
app_version: "0.1.0+1478129847"
data:
myvalue: "Hello World"
drink: "coffee"
food: "pizza"
app_name: mychart
app_version: "0.1.0+1478129847"
Note that the indentation on app_version
is wrong in both places. Why? Because
the template that is substituted in has the text aligned to the right. Because
template
is an action, and not a function, there is no way to pass the output
of a template
call to other functions; the data is simply inserted inline.
To work around this case, Helm provides an alternative to template
that will
import the contents of a template into the present pipeline where it can be
passed along to other functions in the pipeline.
Here’s the example above, corrected to use indent
to indent the mychart_app
template correctly:
apiVersion: v1
kind: ConfigMap
metadata:
name: {{ .Release.Name }}-configmap
labels:
{{ include "mychart.app" . | indent 4 }}
data:
myvalue: "Hello World"
{{- range $key, $val := .Values.favorite }}
{{ $key }}: {{ $val | quote }}
{{- end }}
{{ include "mychart.app" . | indent 2 }}
Now the produced YAML is correctly indented for each section:
# Source: mychart/templates/configmap.yaml
apiVersion: v1
kind: ConfigMap
metadata:
name: edgy-mole-configmap
labels:
app_name: mychart
app_version: "0.1.0+1478129987"
data:
myvalue: "Hello World"
drink: "coffee"
food: "pizza"
app_name: mychart
app_version: "0.1.0+1478129987"
It is considered preferable to use
include
overtemplate
in Helm templates simply so that the output formatting can be handled better for YAML documents.
Sometimes we want to import content, but not as templates. That is, we want to
import files verbatim. We can achieve this by accessing files through the
.Files
object described in the next section.