Exec plugin on linux

Exec plugin on linux in 60 seconds

This is a (no reading allowed!) 60 second copy/paste guided example. Full plugin docs here.

This demo writes and uses a somewhat ridiculous exec plugin (written in bash) that generates a ConfigMap.

This is a guide to try it without damaging your current setup.

requirements

  • linux, git, curl, Go 1.13

Make a place to work

DEMO=$(mktemp -d)

Create a kustomization

Make a kustomization directory to hold all your config:

MYAPP=$DEMO/myapp
mkdir -p $MYAPP

Make a deployment config:

cat <<'EOF' >$MYAPP/deployment.yaml
apiVersion: apps/v1
kind: Deployment
metadata:
  name: the-deployment
spec:
  replicas: 3
  template:
    spec:
      containers:
      - name: the-container
        image: monopole/hello:1
        command: ["/hello",
                  "--port=8080",
                  "--date=$(THE_DATE)",
                  "--enableRiskyFeature=$(ENABLE_RISKY)"]
        ports:
        - containerPort: 8080
        env:
        - name: THE_DATE
          valueFrom:
            configMapKeyRef:
              name: the-map
              key: today
        - name: ALT_GREETING
          valueFrom:
            configMapKeyRef:
              name: the-map
              key: altGreeting
        - name: ENABLE_RISKY
          valueFrom:
            configMapKeyRef:
              name: the-map
              key: enableRisky
EOF

Make a service config:

cat <<EOF >$MYAPP/service.yaml
kind: Service
apiVersion: v1
metadata:
  name: the-service
spec:
  type: LoadBalancer
  ports:
  - protocol: TCP
    port: 8666
    targetPort: 8080
EOF

Now make a config file for the plugin you’re about to write.

This config file is just another k8s resource object. The values of its apiVersion and kind fields are used to find the plugin code on your filesystem (more on this later).

cat <<'EOF' >$MYAPP/cmGenerator.yaml
apiVersion: myDevOpsTeam
kind: SillyConfigMapGenerator
metadata:
  name: whatever
argsOneLiner: Bienvenue true
EOF

Finally, make a kustomization file referencing all of the above:

cat <<EOF >$MYAPP/kustomization.yaml
commonLabels:
  app: hello
resources:
- deployment.yaml
- service.yaml
generators:
- cmGenerator.yaml
EOF

Review the files

ls -C1 $MYAPP

Make a home for plugins

Plugins must live in a particular place for kustomize to find them.

This demo will use the ephemeral directory:

PLUGIN_ROOT=$DEMO/kustomize/plugin

The plugin config defined above in $MYAPP/cmGenerator.yaml specifies:

apiVersion: myDevOpsTeam
kind: SillyConfigMapGenerator

This means the plugin must live in a directory named:

MY_PLUGIN_DIR=$PLUGIN_ROOT/myDevOpsTeam/sillyconfigmapgenerator

mkdir -p $MY_PLUGIN_DIR

The directory name is the plugin config’s apiVersion followed by its lower-cased kind.

A plugin gets its own directory to hold itself, its tests and any supplemental data files it might need.

Create the plugin

There are two kinds of plugins, exec and Go.

Make an exec plugin, installing it to the correct directory and file name. The file name must match the plugin’s kind (in this case, SillyConfigMapGenerator):

cat <<'EOF' >$MY_PLUGIN_DIR/SillyConfigMapGenerator
#!/bin/bash
# Skip the config file name argument.
shift
today=`date +%F`
echo "
kind: ConfigMap
apiVersion: v1
metadata:
  name: the-map
data:
  today: $today
  altGreeting: "$1"
  enableRisky: "$2"
"
EOF

By definition, an exec plugin must be executable:

chmod a+x $MY_PLUGIN_DIR/SillyConfigMapGenerator

Install kustomize

Per the instructions:

curl -s "https://raw.githubusercontent.com/\
kubernetes-sigs/kustomize/master/hack/install_kustomize.sh"  | bash
mkdir -p $DEMO/bin
mv kustomize $DEMO/bin

Review the layout

tree $DEMO

Build your app, using the plugin

XDG_CONFIG_HOME=$DEMO $DEMO/bin/kustomize build --enable_alpha_plugins $MYAPP

Above, if you had set

PLUGIN_ROOT=$HOME/.config/kustomize/plugin

there would be no need to use XDG_CONFIG_HOME in the kustomize command above.

Last modified July 16, 2020: Docs: Auto-fix markdownlint issues (f9ee578a)