From 795a97466527a5f02e79e47b7fb316c78ffde667 Mon Sep 17 00:00:00 2001
From: Vincent Ambo <tazjin@google.com>
Date: Fri, 20 Dec 2019 22:13:07 +0000
Subject: chore(kontemplate): Prepare kontemplate for depot-merge

This merge will not yet include moving over to buildGo.nix, as support
for testing and such is not present in that library yet.
---
 ops/kontemplate/docs/templates.md | 153 ++++++++++++++++++++++++++++++++++++++
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 create mode 100644 ops/kontemplate/docs/templates.md

(limited to 'ops/kontemplate/docs/templates.md')

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+Kontemplate templates
+=====================
+
+The template file format is based on Go's [templating engine][] in combination
+with a small extension library called [sprig][] that adds additional template
+functions.
+
+Go templates can either simply display variables or build more complicated
+*pipelines* in which variables are passed to functions for further processing,
+or in which conditionals are evaluated for more complex template logic.
+
+It is recommended that you check out the Golang [documentation][] for the templating
+engine in addition to the cherry-picked features listed here.
+
+<!-- markdown-toc start - Don't edit this section. Run M-x markdown-toc-refresh-toc -->
+**Table of Contents**
+
+- [Kontemplate templates](#kontemplate-templates)
+    - [Basic variable interpolation](#basic-variable-interpolation)
+        - [Example:](#example)
+    - [Template functions](#template-functions)
+    - [Examples:](#examples)
+    - [Conditionals & ranges](#conditionals--ranges)
+    - [Caveats](#caveats)
+
+<!-- markdown-toc end -->
+
+## Basic variable interpolation
+
+The basic template format uses `{{ .variableName }}` as the interpolation format.
+
+### Example:
+
+Assuming that you include a resource set as such:
+
+```
+- name: api-gateway
+  values:
+    internalHost: http://my-internal-host/
+```
+
+And the api-gateway resource set includes a ConfigMap (some fields left out for
+the example):
+
+```
+# api-gateway/configmap.yaml:
+---
+kind: ConfigMap
+metadata:
+  name: api-gateway-config
+data:
+  internalHost: {{ .internalHost }}
+```
+
+The resulting output will be:
+
+```
+
+---
+kind: ConfigMap
+metadata:
+  name: api-gateway-config
+data:
+  internalHost: http://my-internal-host/
+```
+
+## Template functions
+
+Go templates support template functions which you can think of as a sort of
+shell-like pipeline where text flows through transformations from left to
+right.
+
+Some template functions come from Go's standard library and are listed in the
+[Go documentation][]. In addition the functions declared by [sprig][] are
+available in kontemplate, as well as five custom functions:
+
+* `json`: Encodes any supplied data structure as JSON.
+* `gitHEAD`: Retrieves the commit hash at Git `HEAD`.
+* `passLookup`: Looks up the supplied key in [pass][].
+* `insertFile`: Insert the contents of the given file in the resource
+  set folder as a string.
+* `insertTemplate`: Insert the contents of the given template in the resource
+  set folder as a string.
+
+## Examples:
+
+```
+# With the following values:
+name: Donald
+certKeyPath: my-website/cert-key
+
+# The following interpolations are possible:
+
+{{ .name | upper }}
+-> DONALD
+
+{{ .name | upper | repeat 2 }}
+-> DONALD DONALD
+
+{{ .certKeyPath | passLookup }}
+-> Returns content of 'my-website/cert-key' from pass
+
+{{ gitHEAD }}
+-> Returns the Git commit hash at HEAD.
+```
+
+## Conditionals & ranges
+
+Some logic is supported in Golang templates and can be used in Kontemplate, too.
+
+With the following values:
+
+```
+useKube2IAM: true
+servicePorts:
+  - 8080
+  - 9090
+```
+
+The following interpolations are possible:
+
+```
+# Conditionally insert something in the template:
+metadata:
+  annotations:
+    foo: bar
+    {{ if .useKube2IAM -}} iam.amazonaws.com/role: my-api {{- end }}
+```
+
+```
+# Iterate over a list of values
+ports:
+  {{ range .servicePorts }}
+  - port: {{ . }}
+  {{ end }}
+```
+
+Check out the Golang documentation (linked above) for more information about template logic.
+
+## Caveats
+
+Kontemplate does not by itself parse any of the content of the templates, which
+means that it does not validate whether the resources you supply are valid YAML
+or JSON.
+
+You can perform some validation by using `kontemplate apply --dry-run` which
+will make use of the Dry-Run functionality in `kubectl`.
+
+[templating engine]: https://golang.org/pkg/text/template/
+[documentation]: https://golang.org/pkg/text/template/
+[sprig]: http://masterminds.github.io/sprig/
+[Go documentation]: https://golang.org/pkg/text/template/#hdr-Functions
+[pass]: https://www.passwordstore.org/
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