Difference between revisions of "Helm Templates"

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(Helm Template Function Reference)
(Template Control Structures)
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==range==
 
==range==
{{External|[https://helm.sh/docs/chart_template_guide/#looping-with-the-range-action Looping with the <tt>range</tt> Action]}}
+
{{Internal|Helm Template range|range}}
 
+
<tt>range</tt> provides a "for each" loop.
+
 
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<font color=darkgray>TODO.</font>
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==Named Template Actions==
 
==Named Template Actions==

Revision as of 23:42, 9 October 2019

External

Internal

Overview

Templates are files living under a chart's templates/ directory. They are written in YAML with Helm templates extensions. Upon processing by Helm, they become Kubernetes manifest files. Helm template extensions are written in the Help template language, which is based on Go templates.

The templates/ Directory

The 'templates' directory contains templates that, after combination with values, will the Kubernetes manifests. When Tiller evaluates a chart, it will send all of the files in the directory - with a few exceptions - through the template rendering engine, then collect the results and send them to to Kubernetes. Note that the NOTES.txt and _helpers.tpl files are also subject to template rendering, but they are not sent to Kubernetes as manifests.

Template names do not follow a rigid naming pattern. It is, however, recommended to use the suffix .yaml for YAML files and .tpl for helpers.

The files whose name begins with an underscore ('_') are assumed to not have a manifest inside, so they are not rendered into manifest definitions. However, they are available everywhere within other chart templates for use. These files are conventionally used to store partials and helpers. _helpers.tpl is the default location for template partials.

Installation and De-Installation Order

During installation, Helm collects all of the resources in a given chart and its dependences, groups them by resource type, and installs them in the order specified here https://github.com/helm/helm/blob/release-2.14/pkg/tiller/kind_sorter.go#L29-L57. Upon de-installation the order is reversed: https://github.com/helm/helm/blob/release-2.14/pkg/tiller/kind_sorter.go#L62-L90

Template Comments

 # This is a comment
{{/* Generate basic labels */}}

Multi-Line Comments

 {{- /*
 This is another 
 multi-line
 comment
 */ -}}

Template Directives

A template directive, sometimes also referred as tag, is enclosed in {{ and }} blocks, and it is recommended to pad the directive with space at its left and right. The simplest directive renders a value. A value is a namespaced object, where each dot (.) separates each namespaced element. A leading dot indicates that we start with the top-most namespace for the scope.

kind: ConfigMap
metadata:
  name: {{ .Release.Name }}-configmap

Directives may also include functions and other constructs.

With the exception of the multi-line comments, all directives must be specified on one line only. Space present inside a directive is irrelevant, the following formats are equivalent:

{{.Values.color}}
{{ .Values.color }}
{{            .Values.color               }}

The recommended format is to allow for a space after '{{' and before '}}':

{{ .Values.color }}

Directives and Whitespace Handling

Controlling Whitespace

The spaces inside a directive is irrelevant (we cannot have newlines inside a directive, all directives must be specified on a single line). The '{{' and '}}' directive delimiters, without any other modifications, leave the template whitespace surrounding them alone, and do not interfere with it in any way. A hyphen '-' placed after the '{{' delimiter or before the '}}' delimiter instructs the rendering engine to trim the whitespace preceding, respectively trailing the delimiter. Whitespace includes spaces, tabs, newline ('\n') and carriage return ('\r'). When encountering "-", the template engine will simply drop the corresponding whitespace until a non-whitespace character is found.

metadata:
  color: {{ .Values.color }}
spec:

will produce (assuming that "color" is declared to be "blue" in values.yaml:

metadata:
  color: blue
spec:

To trim preceding whitespace, use '{{-'.

metadata:
  color: {{- .Values.color }}
spec:

will produce:

metadata:
  color:blue
spec:

To trim trailing whitespace (whitespace includes newlines), use '-}}'.

metadata:
  color: {{ .Values.color -}}
spec:

will produce:

metadata:
  color: bluespec:

(which is something you most like don't want).

For details related to whitespace handling when declaring named templates, see Named Templates and Whitespace Handling, below.

Scopes

Scopes are declared with 'with'.

TODO.

Template Objects

Objects are passed into a template from the template engine. The template directives can create new objects and pass them around. There are also built-in objects, which are made available by default. Objects can be simple - have just one value -, or they can contain other objects or functions. For example the "Release" built-in object contains several other objects (like "Release.Name"). The "Files" object contains functions.

Built-in Objects

Built-in Objects
The built-in values always begin with a capital letter, based on Go's naming convention. For a fully working examples of built-in objects replacement see:
https://github.com/ovidiuf/playground/tree/master/helm/bactrosaurus

Chart

This object contains value passed into the template from the Chart.yaml file. An existing field is available as (note leading dot) .Chart.<UpperCasedFirstLetterFieldName>. It is important to capitalize the first letter of the field name, otherwise the directive evaluation fails.

Example:

{{ .Chart.Name }}
{{ .Chart.Version }}

Values

This object provides access the effective values of all configuration element, as present in the runtime configuration tree, and it essentially exposes the chart configuration to templates. The value of an existing configuration element can be access using the following syntax: (note leading dot) .Values.<fieldName>. Unlike in Chart's case, the fields are allowed to keep their original capitalization. For example, a value declared as such in values.yaml:

size: 10

can be references in a template as:

kind: ConfigMap
...
data:
  size: {{ .Values.size }}

Values may contain structured content:

characteristics:
  size: 10
  shape: "large"

can be referenced in template as:

kind: ConfigMap
 ...
data:
  size: {{ .Values.characteristics.size }}
  shape: {{ .Values.characteristics.shape }}

While structuring data this way is possible, the recommendation is to keep values trees shallow, favoring flatness.

In case the structure contains an array, individual elements can be referred from the template with the index function.

Release

This object describes the release itself.

Release.Name

Exposes the release name:

{{ .Release.Name }}

Release.Revision

Exposes the release revision:

{{ .Release.Revision }}

Release.Time

Exposes the time of the release:

{{ .Release.Time }}

Release.Namespace

Exposes the namespace to be released info, if the manifest does not override:

{{ .Release.Namespace }}

Release.IsUpgrade

This is set to true if the current operation is an upgrade or rollback.

{{ .Release.IsUpgrade }}

Release.IsInstall

This is set to true if the current operation is an install.

{{ .Release.IsInstall }}

Release.Service

Exposes the releasing service - always Tiller

Files

The object provide access to all non-special files in the chart. It cannot be used to access templates. The access is provided via several functions:

Files.Get

{{ .Files.Get <file-name> }}

Files.GetBytes

Accessing Files inside Templates

Accessing Files inside Templates

TODO.

Capabilities

Provides information about the capabilities of the Kubernetes cluster:

{{ .Capabilities.APIVersions }}
{{ .Capabilities.APIVersions.Has }}
{{ .Capabilities.KubeVersion }}
{{ .Capabilities.KubeVersion.Major|
 Minor|GitVersion|GitCommit|GitTreeState|BuildDate|GoVersion|Compiler|Platform}}
{{ .Capabilities.TillerVersion }}

Template

Contains information about the current template that is being executed:

{{ .Template.Name }}
{{ .Template.BasePath }}

Template Functions

Template Functions and Pipelines
Go Templates
sprig Template Functions

A template function modifies data provided to the template via template objects, and it is declared inside the template, in a template directive. Template functions follow the syntax:

functionName arg1 arg2 ...

Example:

{{ quote .Values.color }}

Helm Template Function Reference

Operators

Operators are Functions

Operators are implemented as functions that return a boolean value:

 {{ eq Values.color "blue" }}
 {{ ne }}
 {{ lt }}
 {{ gt }}
 {{ and }}
 {{ or }}
 {{ not }}

Accessing Array Elements

To access a specific element of an array data structure, use the index function. The index function is 0-based.

Assuming we declare a simple array:

colors:
  - 'blue'
  - 'red'
  - 'green'

then the first element of the array can be accessed with:

{{ index .Values.colors 0 }}

The directive is rendered to "blue".

When the array contains maps:

colors:
  - name: blue
    shade: dark
  - name: red
    shade: light

the fields of the maps can be accessed with - note (...):

{{ (index .Values.colors 0).name }}

This pattern can be extrapolated to more complex data structures.

Template Pipelines

Pipelines
{{ <object> | <function1> | <function2>  }}
{{ .Values.color | upper | repeat 5 }}

Template Control Structures

Flow control structures are called "actions".

if/else

if/else

with

Modifying scope using with

with specifies a scope.

TODO.

range

range

Named Template Actions

Named Templates

A named template, sometimes called a partial, a subtemplate or an embedded template, is a template define inside of a file, and given a names. There are two different ways to create named templates, with the define action and with the template action.

Template names are global. That means if two template are declared with the same name, whichever is loaded last will be the one that will be used. Moreover, the templates in subcharts, which we are not necessarily too familiar with, are compiled together with top-level templates, so it is possible to run into template name collisions we are not aware of. For this reason, it is a good practice to name your templates with unique, chart-specific names.

One popular naming convention is to prefix the name of each defined template with the name of the chart:

{{ define "mychart.labels" }}

If everyone follows the same convention, chart-specific named templates get their own namespace and that decreases the probability of conflict that may arise due to two different charts that implement templates with the same name.

define

Declaring and Using Templates with define

define declares a new named template inside of a template file, conventionally _helpers.tpl:

{{/* 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.

The body of the partial may include directives.

Partials may be used instead of variables, by declaring a template that acts like a variable:

{{/*
Compute the service name that depends on the release name. This partial should be inlined every time the service name is needed.
*/}}
{{- define "myChart.postgresqlServiceName" -}}
{{- printf "%s-postgresql" .Release.Name -}}
{{- end -}}

When we need the service name in a template, we render it as such:

 ... {{ template "myChart.postgresqlServiceName" . }} ...

For more details on rendering, see template below.

Named Templates and Whitespace Handling

Unless controlled with '{{-' and '-}}', the whitespace specified inside the partial are preserved and projected in the rendered result.

Named Template Recipes

Named Template Recipes

template

A partial defined as such can be embedded inside other template with the template action:

kind: ConfigMap
metadata:
 name: ...
 {{- template "mychart.labels" }}
...

When the template engine reads the file that contains the partial, it will store away the reference to "mychart.labels" until template "mychart.labels" is called. Then it will render that template inline. After rendering, the result will look like this:

kind: ConfigMap
metadata:
 name: ...
 labels:
   generator: helm
   date: 2019-08-29
...

Setting the Scope of a Template. The "template" action allows passing a scope. By default, no scope is passed by default, so objects like .Chart.Name are not found and rendered to blank space.

To pass the scope of the calling template, use "template" as such (note the dot):

 ...
 {{- template "mychart.labels" . }}
 ...

In the above example, we passed the top-level (".") scope. We can pass whatever scope (template object) we want:

 ...
 {{- template "mychart.labels" .Values }}
 ...

or:

 ...
 {{- template "mychart.labels" .Values.favorite }}
 ...

block

Avoid using Blocks

block declares a special kind of fillable template area.

TODO.

The include Function

The include Function

TODO.

Template Variables

Variables

TODO.

Configmap and Secrets Utility Functions

Configmap and Secrets Utility Functions

TODO.

Debugging Templates

Debugging Templates
https://github.com/databus23/schelm

TODO.

TODO