Templates

This part of the Best Practices Guide focuses on templates.

Structure of templates/

The templates/ directory should be structured as follows:

  • Template files should have the extension .yaml if they produce YAML output. The extension .tpl may be used for template files that produce no formatted content.
  • Template file names should use dashed notation (my-example-configmap.yaml), not camelcase.
  • Each resource definition should be in its own template file.
  • Template file names should reflect the resource kind in the name. e.g. foo-pod.yaml, bar-svc.yaml

Names of Defined Templates

Defined templates (templates created inside a {{ define }} directive) are globally accessible. That means that a chart and all of its subcharts will have access to all of the templates created with {{ define }}.

For that reason, all defined template names should be namespaced.

Correct:

  1. {{- define "nginx.fullname" }}
  2. {{/* ... */}}
  3. {{ end -}}

Incorrect:

  1. {{- define "fullname" -}}
  2. {{/* ... */}}
  3. {{ end -}}

It is highly recommended that new charts are created via helm create command as the template names are automatically defined as per this best practice.

Formatting Templates

Templates should be indented using two spaces (never tabs).

Template directives should have whitespace after the opening braces and before the closing braces:

Correct:

  1. {{ .foo }}
  2. {{ print "foo" }}
  3. {{- print "bar" -}}

Incorrect:

  1. {{.foo}}
  2. {{print "foo"}}
  3. {{-print "bar"-}}

Templates should chomp whitespace where possible:

  1. foo:
  2.   {{- range .Values.items }}
  3.   {{ . }}
  4.   {{ end -}}

Blocks (such as control structures) may be indented to indicate flow of the template code.

  1. {{ if $foo -}}
  2.   {{- with .Bar }}Hello{{ end -}}
  3. {{- end -}}

However, since YAML is a whitespace-oriented language, it is often not possible for code indentation to follow that convention.

Whitespace in Generated Templates

It is preferable to keep the amount of whitespace in generated templates to a minimum. In particular, numerous blank lines should not appear adjacent to each other. But occasional empty lines (particularly between logical sections) is fine.

This is best:

  1. apiVersion: batch/v1
  2. kind: Job
  3. metadata:
  4.   name: example
  5.   labels:
  6.     first: first
  7.     second: second

This is okay:

  1. apiVersion: batch/v1
  2. kind: Job
  4. metadata:
  5.   name: example
  7.   labels:
  8.     first: first
  9.     second: second

But this should be avoided:

  1. apiVersion: batch/v1
  2. kind: Job
  4. metadata:
  5.   name: example
  11.   labels:
  12.     first: first
  14.     second: second

Comments (YAML Comments vs. Template Comments)

Both YAML and Helm Templates have comment markers.

YAML comments:

  1. # This is a comment
  2. type: sprocket

Template Comments:

  1. {{- /*
  2. This is a comment.
  3. */}}
  4. type: frobnitz

Template comments should be used when documenting features of a template, such as explaining a defined template:

  1. {{- /*
  2. mychart.shortname provides a 6 char truncated version of the release name.
  3. */}}
  4. {{ define "mychart.shortname" -}}
  5. {{ .Release.Name | trunc 6 }}
  6. {{- end -}}

Inside of templates, YAML comments may be used when it is useful for Helm users to (possibly) see the comments during debugging.

  1. # This may cause problems if the value is more than 100Gi
  2. memory: {{ .Values.maxMem | quote }}

The comment above is visible when the user runs helm install --debug, while comments specified in {{- /* */}} sections are not.

Beware of adding # YAML comments on template sections containing Helm values that may be required by certain template functions.

For example, if required function is introduced to the above example, and maxMem is unset, then a # YAML comment will introduce a rendering error.

Correct: helm template does not render this block

  1. {{- /*
  2. # This may cause problems if the value is more than 100Gi
  3. memory: {{ required "maxMem must be set" .Values.maxMem | quote }}
  4. */ -}}

Incorrect: helm template returns Error: execution error at (templates/test.yaml:2:13): maxMem must be set

  1. # This may cause problems if the value is more than 100Gi
  2. # memory: {{ required .Values.maxMem "maxMem must be set" | quote }}

Review Debugging Templates for another example of this behavior of how YAML comments are left intact.

Use of JSON in Templates and Template Output

YAML is a superset of JSON. In some cases, using a JSON syntax can be more readable than other YAML representations.

For example, this YAML is closer to the normal YAML method of expressing lists:

  1. arguments:
  2.   - "--dirname"
  3.   - "/foo"

But it is easier to read when collapsed into a JSON list style:

  1. arguments: ["--dirname", "/foo"]

Using JSON for increased legibility is good. However, JSON syntax should not be used for representing more complex constructs.

When dealing with pure JSON embedded inside of YAML (such as init container configuration), it is of course appropriate to use the JSON format.