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Go Template

Introduction

ApplicationSet is able to use Go Text Template. To activate this feature, add goTemplate: true to your ApplicationSet manifest.

The Sprig function library (except for env, expandenv and getHostByName) is available in addition to the default Go Text Template functions.

An additional normalize function makes any string parameter usable as a valid DNS name by replacing invalid characters with hyphens and truncating at 253 characters. This is useful when making parameters safe for things like Application names.

Motivation

Go Template is the Go Standard for string templating. It is also more powerful than fasttemplate (the default templating engine) as it allows doing complex templating logic.

Limitations

Go templates are applied on a per-field basis, and only on string fields. Here are some examples of what is not possible with Go text templates:

  • Templating a boolean field.

    apiVersion: argoproj.io/v1alpha1
    kind: ApplicationSet
    spec:
      goTemplate: true
      template:
        spec:
          source:
            helm:
              useCredentials: "{{.useCredentials}}"  # This field may NOT be templated, because it is a boolean field.
    
  • Templating an object field:

    apiVersion: argoproj.io/v1alpha1
    kind: ApplicationSet
    spec:
      goTemplate: true
      template:
        spec:
          syncPolicy: "{{.syncPolicy}}"  # This field may NOT be templated, because it is an object field.
    
  • Using control keywords across fields:

    apiVersion: argoproj.io/v1alpha1
    kind: ApplicationSet
    spec:
      goTemplate: true
      template:
        spec:
          source:
            helm:
              parameters:
              # Each of these fields is evaluated as an independent template, so the first one will fail with an error.
              - name: "{{range .parameters}}"
              - name: "{{.name}}"
                value: "{{.value}}"
              - name: throw-away
                value: "{{end}}"
    

Migration guide

Globals

All your templates must replace parameters with GoTemplate Syntax:

Example: {{ some.value }} becomes {{ .some.value }}

Cluster Generators

By activating Go Templating, {{ .metadata }} becomes an object.

  • {{ metadata.labels.my-label }} becomes {{ index .metadata.labels "my-label" }}
  • {{ metadata.annotations.my/annotation }} becomes {{ index .metadata.annotations "my/annotation" }}

Git Generators

By activating Go Templating, {{ .path }} becomes an object. Therefore, some changes must be made to the Git generators' templating:

  • {{ path }} becomes {{ .path.path }}
  • {{ path[n] }} becomes {{ .path.segments[n] }}

Here is an example:

apiVersion: argoproj.io/v1alpha1
kind: ApplicationSet
metadata:
  name: cluster-addons
spec:
  generators:
  - git:
      repoURL: https://github.com/argoproj/argo-cd.git
      revision: HEAD
      directories:
      - path: applicationset/examples/git-generator-directory/cluster-addons/*
  template:
    metadata:
      name: '{{path.basename}}'
    spec:
      project: default
      source:
        repoURL: https://github.com/argoproj/argo-cd.git
        targetRevision: HEAD
        path: '{{path}}'
      destination:
        server: https://kubernetes.default.svc
        namespace: '{{path.basename}}'

becomes

apiVersion: argoproj.io/v1alpha1
kind: ApplicationSet
metadata:
  name: cluster-addons
spec:
  goTemplate: true
  generators:
  - git:
      repoURL: https://github.com/argoproj/argo-cd.git
      revision: HEAD
      directories:
      - path: applicationset/examples/git-generator-directory/cluster-addons/*
  template:
    metadata:
      name: '{{.path.basename}}'
    spec:
      project: default
      source:
        repoURL: https://github.com/argoproj/argo-cd.git
        targetRevision: HEAD
        path: '{{.path.path}}'
      destination:
        server: https://kubernetes.default.svc
        namespace: '{{.path.basename}}'

It is also possible to use Sprig functions to construct the path variables manually:

with goTemplate: false with goTemplate: true with goTemplate: true + Sprig
{{path}} {{.path.path}} {{.path.path}}
{{path.basename}} {{.path.basename}} {{base .path.path}}
{{path.filename}} {{.path.filename}} {{.path.filename}}
{{path.basenameNormalized}} {{.path.basenameNormalized}} {{normalize .path.path}}
{{path.filenameNormalized}} {{.path.filenameNormalized}} {{normalize .path.filename}}
{{path[N]}} {{.path.segments[N]}} {{index (splitList "/" .path.path) N}}

Examples

Basic Go template usage

This example shows basic string parameter substitution.

apiVersion: argoproj.io/v1alpha1
kind: ApplicationSet
metadata:
  name: guestbook
spec:
  goTemplate: true
  generators:
  - list:
      elements:
      - cluster: engineering-dev
        url: https://1.2.3.4
      - cluster: engineering-prod
        url: https://2.4.6.8
      - cluster: finance-preprod
        url: https://9.8.7.6
  template:
    metadata:
      name: '{{.cluster}}-guestbook'
    spec:
      project: my-project
      source:
        repoURL: https://github.com/infra-team/cluster-deployments.git
        targetRevision: HEAD
        path: guestbook/{{.cluster}}
      destination:
        server: '{{.url}}'
        namespace: guestbook

Fallbacks for unset parameters

For some generators, a parameter of a certain name might not always be populated (for example, with the values generator or the git files generator). In these cases, you can use a Go template to provide a fallback value.

apiVersion: argoproj.io/v1alpha1
kind: ApplicationSet
metadata:
  name: guestbook
spec:
  goTemplate: true
  generators:
  - list:
      elements:
      - cluster: engineering-dev
        url: https://kubernetes.default.svc
      - cluster: engineering-prod
        url: https://kubernetes.default.svc
        nameSuffix: -my-name-suffix
  template:
    metadata:
      name: '{{.cluster}}{{default "" .nameSuffix}}'
    spec:
      project: default
      source:
        repoURL: https://github.com/argoproj/argo-cd.git
        targetRevision: HEAD
        path: applicationset/examples/list-generator/guestbook/{{.cluster}}
      destination:
        server: '{{.url}}'
        namespace: guestbook

This ApplicationSet will produce an Application called engineering-dev and another called engineering-prod-my-name-suffix.