add example for transformer plugin

examples/validationTransformer/README.md

@@ -0,0 +1,222 @@
+# a transformer plugin performing validation
+
+[base]: ../docs/glossary.md#bases
+[kubeval]: https://github.com/instrumenta/kubeval
+[plugin]: ../../docs/plugins.md
+
+Kustomize doesn't validate the resources that it loads
+from files or [bases], but kustomize has a [plugin]
+system that allows one to validate the
+loaded resources.
+
+One way to validate Kubernetes resources is to run
+[kubeval] against the resource file. For example,
+an invalid resource file gives similar error:
+```shell
+$ kubeval my-invalid-rc.yaml
+The document my-invalid-rc.yaml contains an invalid ReplicationController
+--> spec.replicas: Invalid type. Expected: integer, given: string
+```
+
+One can write a Kustomize transformer [plugin] to
+run [kubeval] against the resources that have been
+loaded by Kustomize.
+
+
+Make a place to work:
+
+<!-- @makeWorkplace @test -->
+```
+DEMO_HOME=$(mktemp -d)
+mkdir -p $DEMO_HOME/valid
+mkdir -p $DEMO_HOME/invalid
+PLUGINDIR=$DEMO_HOME/kustomize/plugin/someteam.example.com/v1/validator
+mkdir -p $PLUGINDIR
+```
+
+## write a transformer plugin
+
+Download the [kubeval] binary depending on the operating system
+and add it to $PATH.
+
+<!-- @downloadKubeval @test -->
+```
+OS=`uname | sed -e 's/Linux/linux/' -e 's/Darwin/darwin/'`
+wget https://github.com/instrumenta/kubeval/releases/download/0.9.2/kubeval-${OS}-amd64.tar.gz
+tar xf kubeval-${OS}-amd64.tar.gz
+export PATH=$PATH:`pwd`
+```
+
+Kustomize has the following assumption of a transformer plugin:
+- The resources are passed to the transformer plugin from stdin.
+- The configuration file for the transformer plugin is passed in
+  as the first argument.
+- The working directory of the plugin is the kustomization
+  directory where it is used as a transformer.
+- The transformed resources are written to stdout by the plugin.
+- If the return code of the transformer plugin is non zero,
+  Kustomize regards there is an error during the transformation.
+
+A transformer plugin for the validation can be written as a
+bash script, which execute the [kubeval] binary and return proper
+output and exit code.
+
+<!-- @writePlugin @test -->
+```
+cat <<'EOF' > $PLUGINDIR/Validator
+#!/bin/bash
+
+if ! [ -x "$(command -v kubeval)" ]; then
+  echo "Error: kubeval is not installed."
+  exit 1
+fi
+
+temp_file=$(mktemp)
+output_file=$(mktemp)
+cat - > $temp_file
+
+kubeval $temp_file > $output_file
+
+if [ $? -eq 0 ]; then
+    cat $temp_file
+    rm $temp_file $output_file
+    exit 0
+fi
+
+cat $output_file
+rm $temp_file $output_file
+exit 1
+
+EOF
+chmod +x $PLUGINDIR/Validator
+```
+
+## use the transformer plugin
+
+Define a kustomization containing a valid ConfigMap
+and the transformer plugin.
+
+<!-- @writeKustomization @test -->
+```
+cat <<'EOF' >$DEMO_HOME/valid/configmap.yaml
+apiVersion: v1
+kind: ConfigMap
+metadata:
+  name: cm
+data:
+  foo: bar
+EOF
+
+cat <<'EOF' >$DEMO_HOME/valid/validation.yaml
+apiVersion: someteam.example.com/v1
+kind: Validator
+metadata:
+  name: notImportantHere
+EOF
+
+cat <<'EOF' >$DEMO_HOME/valid/kustomization.yaml
+resources:
+- configmap.yaml
+
+transformers:
+- validation.yaml
+EOF
+```
+
+Define a kustomization containing an invalid ConfigMap
+and the transformer plugin.
+
+<!-- @writeKustomization @test -->
+```
+cat <<'EOF' >$DEMO_HOME/invalid/configmap.yaml
+apiVersion: v1
+kind: ConfigMap
+metadata:
+  name: cm
+data:
+- foo: bar
+EOF
+
+cat <<'EOF' >$DEMO_HOME/invalid/validation.yaml
+apiVersion: someteam.example.com/v1
+kind: Validator
+metadata:
+  name: notImportantHere
+EOF
+
+cat <<'EOF' >$DEMO_HOME/invalid/kustomization.yaml
+resources:
+- configmap.yaml
+
+transformers:
+- validation.yaml
+EOF
+```
+
+The directory structure is as the following:
+
+```
+/tmp/tmp.fAYMfLZJs4
+├── invalid
+│   ├── configmap.yaml
+│   ├── kustomization.yaml
+│   └── validation.yaml
+├── kustomize
+│   └── plugin
+│       └── someteam.example.com
+│           └── v1
+│               ├── kubeval
+│               └── Validator
+└── valid
+    ├── configmap.yaml
+    ├── kustomization.yaml
+    └── validation.yaml
+```
+
+Define a helper function to run kustomize with the
+correct environment and flags for plugins:
+
+<!-- @defineKustomizeBd @test -->
+```
+function kustomizeBd {
+  XDG_CONFIG_HOME=$DEMO_HOME \
+  kustomize build \
+    --enable_alpha_plugins \
+    $DEMO_HOME/$1
+}
+```
+
+Build the valid variant
+
+<!-- @buildValid @test -->
+```
+kustomizeBd valid
+```
+The output contains a ConfigMap as
+
+```yaml
+apiVersion: v1
+data:
+  foo: bar
+kind: ConfigMap
+metadata:
+  name: cm
+```
+
+Build the invalid variant
+
+```
+kustomizeBd invalid
+```
+
+The output is an error as
+```shell
+data: Invalid type. Expected: object, given: array
+```
+
+## cleanup
+
+<!-- @cleanup @test -->
+```shell
+rm -rf $DEMO_HOME
+```

examples/validationTransformer/invalid.yaml

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+apiVersion: v1
+kind: ConfigMap
+metadata:
+  name: cm
+data:
+- foo: bar

examples/validationTransformer/valid.yaml

@@ -0,0 +1,6 @@
+apiVersion: v1
+kind: ConfigMap
+metadata:
+  name: cm
+data:
+  foo: bar