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Describe how Kustomize plugins work with examples
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README.md

Kustomize plugins

The following examples illustrate how Kustomize plugins work.

Summary

The following examples illustrate how Kustomize plugins work--that is, illustrate Kustomize Kubernetes Resource Model (KRM) Functions Specification with the following examples:

  1. Example 1 - exec - Do nothing
  2. Example 2 - exec - Log something
  3. Example 3 - container - Do nothing
  4. Example 4 - container - Reach ✨The Internet✨

Jargon note: Kubernetes Resource Model means Kubernetes data model. For an explanation about the Kubernetes Resource Model (KRM) jargon term, see Kubernetes Resource Model (KRM) page.

Setup

The following examples depend on the kustomize standalone command. To install kustomize on macOS, run the brew install kustomize command. To follow Kustomize plugin graduation--that is, to figure out if plugins are out of Alpha status and merged into kubectl, see Kustomize Plugin Graduation #2953 issue.

Example 1 - exec - Do nothing

The following example runs a do-nothing DoNothing Kustomize exec plugin that does nothing (I guess it's not nothing, you copy stdin to stdout (it's cat)).

  1. To experiment, create to a temporary directory--good for quick experiments and aligns with how Kustomize documentation tends to illustrate examples.

    DEMO=$(mktemp -d)

  2. Create a kustomization.yaml file that reaches the https://gist.github.com/mbigras/24b9d8c97a43a81d650c67a0d08f1897 public gist--also a kustomization. Also create do-nothing DoNothing Kustomize exec plugin.

    cat <<'KUSTOMIZATION' >${DEMO}/kustomization.yaml
resources:
  - https://gist.github.com/mbigras/24b9d8c97a43a81d650c67a0d08f1897
transformers:
  - |
    kind: DoNothing
    apiVersion: maxhax.net/v1apha1
    metadata:
      name: do-nothing
      annotations:
        config.kubernetes.io/function: |
          exec:
            path: cat
KUSTOMIZATION

    Jargon note: The term kustomization means: a directory with Kubernetes YAML files and a kustomization.yaml file that bundles and manipulates those Kubernetes YAML files. There is probably a more precise definition; but if you're totally bewildered by that kustomization term, then you can probably do worse than this explanation. For more details, see kustomization Kustomize glossary definition.

  3. Run kustomize command that runs your do-nothing DoNothing Kustomize exec plugin that doesn't do anything.

    kustomize build --enable-alpha-plugins --enable-exec $DEMO

    Your output should look like the following:

    $ kustomize build --enable-alpha-plugins --enable-exec $DEMO
apiVersion: v1
data:
  HELLO: world
kind: ConfigMap
metadata:
  name: hello

    Consider the following:

    1. You just ran your first Kustomize exec plugin--congratulations!
    2. Observation: Notice that do-nothing DoNothing Kustomize exec plugin that didn't do much--just passed stdin to stdout. And your command printed the hello ConfigMap in the https://gist.github.com/mbigras/24b9d8c97a43a81d650c67a0d08f1897 public gist to stdout--expected result.
    3. Observation: Notice that to run Kustomize exec plugins, you need those --enable-alpha-plugins and --enable-exec options. Gotcha: When you run container Kustomize plugins, you don't need that --enable-exec option.

Example 2 - exec - Log something

The following example runs a log-something LogSomething Kustomize exec plugin that logs the input that your log-something plugin gets from Kustomize on stdin to stderr--usual excellent Unix convention--and also copies stdin to stdout (it's tee).

  1. Create to a temporary directory.

    DEMO=$(mktemp -d)

  2. Create a log-something.sh script that runs tee command.

    cat <<'SCRIPT' >${DEMO}/log-something.sh && chmod +x ${DEMO}/log-something.sh
#!/usr/bin/env bash
# Script log-something.sh print stdin to stderr and stdout--it's tee.
tee /dev/stderr
SCRIPT

    Gotcha: Run that log-something.sh script instead of {"path":"tee","args":["/dev/stderr"]} because you can only use that path field and you can't pass args to your exec plugin, see https://github.com/kubernetes-sigs/kustomize/blob/878cda7c55aa20903cc3c6d9f85380ccb0b66d5e/kyaml/fn/runtime/runtimeutil/functiontypes.go#L142-L144 code segment.

  3. Create a kustomization.yaml file that reaches the https://gist.github.com/mbigras/24b9d8c97a43a81d650c67a0d08f1897 public gist--also a kustomization. Also create log-something LogSomething Kustomize exec plugin.

    cat <<'KUSTOMIZATION' >${DEMO}/kustomization.yaml
resources:
  - https://gist.github.com/mbigras/24b9d8c97a43a81d650c67a0d08f1897
transformers:
  - |
    kind: LogSomething
    apiVersion: maxhax.net/v1apha1
    metadata:
      name: log-something
      annotations:
        config.kubernetes.io/function: |
          exec:
            path: ./log-something.sh
            # path: tee
            # args: ["/dev/stderr"] # Gotcha: Run that log-something.sh script instead of `{"path":"tee","args":["/dev/stderr"]}` because you can only use that path field and you can't pass args to your exec plugin, see https://github.com/kubernetes-sigs/kustomize/blob/878cda7c55aa20903cc3c6d9f85380ccb0b66d5e/kyaml/fn/runtime/runtimeutil/functiontypes.go#L142-L144 code segment.
KUSTOMIZATION

  4. Run kustomize command that runs your log-something LogSomething Kustomize exec plugin that copies stdin to stderr and stdout.

    kustomize build --enable-alpha-plugins --enable-exec $DEMO

    Your output should look like the following:

    $ kustomize build --enable-alpha-plugins --enable-exec $DEMO
apiVersion: config.kubernetes.io/v1
kind: ResourceList
items:
- apiVersion: v1
  data:
    HELLO: world
  kind: ConfigMap
  metadata:
    annotations:
      kustomize.config.k8s.io/id: |
        kind: ConfigMap
        name: hello
        version: v1
      config.kubernetes.io/index: '0'
      internal.config.kubernetes.io/index: '0'
      internal.config.kubernetes.io/annotations-migration-resource-id: '0'
      internal.config.kubernetes.io/id: '1'
      config.k8s.io/id: '1'
    name: hello
functionConfig:
  apiVersion: maxhax.net/v1apha1
  kind: DoNothing
  metadata:
    annotations:
      config.kubernetes.io/function: |
        exec:
          path: ./log-something.sh
          # path: tee
          # args: ["/dev/stderr"] # Gotcha: Run that log-something.sh script instead of `{"path":"tee","args":["/dev/stderr"]}` because you can only use that path field and you can't pass args to your exec plugin, see https://github.com/kubernetes-sigs/kustomize/blob/878cda7c55aa20903cc3c6d9f85380ccb0b66d5e/kyaml/fn/runtime/runtimeutil/functiontypes.go#L142-L144 code segment.
      config.kubernetes.io/local-config: 'true'
    name: do-nothing
apiVersion: v1
data:
  HELLO: world
kind: ConfigMap
metadata:
  name: hello

    Consider the following:

    1. You printed a concrete example of the Kustomize plugin message schema to stderr--so you can see!
    2. Observation: Notice that log-something LogSomething Kustomize exec plugin also still printed the hello ConfigMap in the https://gist.github.com/mbigras/24b9d8c97a43a81d650c67a0d08f1897 public gist to stdout--expected result.

Example 3 - container - Do nothing

The following example runs a do-nothing DoNothing Kustomize container plugin that does nothing (I guess it's not nothing, you copy stdin to stdout (it's cat)). Also runs a do-nothing Docker container from the local do-nothing:latest Docker image--it's a container Kustomize plugin.

  1. Create to a temporary directory.

    DEMO=$(mktemp -d)

  2. Create a do-nothing:latest Docker image.

    cat <<'DOCKERFILE' | docker build -t do-nothing:latest -f - $DEMO
FROM debian:stable
ENTRYPOINT ["/usr/bin/env", "cat"]
DOCKERFILE

  3. Create a kustomization.yaml file that reaches the https://gist.github.com/mbigras/24b9d8c97a43a81d650c67a0d08f1897 public gist--also a kustomization. Also create do-nothing DoNothing Kustomize container plugin.

    cat <<'KUSTOMIZATION' >${DEMO}/kustomization.yaml
resources:
  - https://gist.github.com/mbigras/24b9d8c97a43a81d650c67a0d08f1897
transformers:
  - |
    kind: DoNothing
    apiVersion: maxhax.net/v1apha1
    metadata:
      name: do-nothing
      annotations:
        config.kubernetes.io/function: |
          container:
            image: do-nothing:latest
          # exec:
          #   path: cat
KUSTOMIZATION

  4. Run kustomize command that runs your do-nothing DoNothing Kustomize container plugin.

    kustomize build --enable-alpha-plugins $DEMO

    Your output should look like the following:

    $ kustomize build --enable-alpha-plugins $DEMO
apiVersion: v1
data:
  HELLO: world
kind: ConfigMap
metadata:
  name: hello

    Consider the following:

    1. You just ran your first Kustomize container plugin--congratulations!
    2. Observation: Notice that do-nothing DoNothing Kustomize container plugin that didn't do much--just passed stdin to stdout. And your command printed the hello ConfigMap in the https://gist.github.com/mbigras/24b9d8c97a43a81d650c67a0d08f1897 public gist to stdout--expected result.
    3. Observation: Notice that to run Kustomize container plugins, you need that --enable-alpha-plugins option (but not the --enable-exec option).

Example 4 - container - Reach ✨The Internet✨

The following example runs a hello-world HelloWorld Kustomize container plugin that reaches ✨The Internet✨.

  1. Create to a temporary directory.

    DEMO=$(mktemp -d)

  2. Create a hello-world:latest Docker image--reaches ✨The Internet✨ with curl command, manipulates YAML with yq command.

    cat <<'DOCKERFILE' | docker build -t hello-world:latest -f - $DEMO
# syntax=docker/dockerfile:1.3-labs
FROM mikefarah/yq AS yq
FROM debian:stable
COPY --from=yq /usr/bin/yq /usr/local/bin/yq
RUN apt update && apt install -y curl
COPY <<'SCRIPT' /hello-world.sh
#!/usr/bin/env bash
# Script hello-world.sh reaches ✨The Internet✨ and also manipulates Kubernetes YAML--it's a Kustomize plugin.
myip=$(curl -sSL https://checkip.amazonaws.com) \
yq '.items[]
|= with(select(.kind == "ConfigMap" and .metadata.name == "hello");
.metadata.annotations.myip = strenv(myip))'
SCRIPT
RUN chmod +x /hello-world.sh
ENTRYPOINT ["/hello-world.sh"]
DOCKERFILE

  3. Create a kustomization.yaml file that reaches the https://gist.github.com/mbigras/24b9d8c97a43a81d650c67a0d08f1897 public gist--also a kustomization. Also create hello-world HelloWorld Kustomize container plugin.

    cat <<'KUSTOMIZATION' >${DEMO}/kustomization.yaml
resources:
  - https://gist.github.com/mbigras/24b9d8c97a43a81d650c67a0d08f1897
transformers:
  - |
    kind: DoNothing
    apiVersion: maxhax.net/v1apha1
    metadata:
      name: do-nothing
      annotations:
        config.kubernetes.io/function: |
          container:
            image: hello-world:latest
            network: true
          # container:
          #   image: do-nothing:latest
          # exec:
          #   path: cat
KUSTOMIZATION

    Obsevation: Notice that to reach the network, you set that "config.kubernetes.io/function".container.network = true option.

  4. Run kustomize command that runs your hello-world HelloWorld Kustomize container plugin.

    kustomize build --enable-alpha-plugins --network $DEMO

    Your output should look like the following:

    $ kustomize build --enable-alpha-plugins --network $DEMO
apiVersion: v1
data:
  HELLO: world
kind: ConfigMap
metadata:
  annotations:
    myip: 35.247.30.15
  name: hello

    Consider the following:

    1. Observation: Notice that hello-world HelloWorld Kustomize container plugin added that myip = "YOURIP" Kubernetes annotation and printed the hello ConfigMap in the https://gist.github.com/mbigras/24b9d8c97a43a81d650c67a0d08f1897 public gist to stdout--expected result.
    2. Gotcha: Notice that to reach the network, you set that "config.kubernetes.io/function".container.network = true option and you also set the --network kustomize command option.

Conclusion

You can run Kustomize exec and container plugins. Kustomize exec plugins run a host command. Kustomize container plugins run a Docker container. Experiment with examples to learn and identify gotchas. After you get the hang of how Kustomize plugins work, then consider the Kustomize Kubernetes Resource Model (KRM) Functions Specification to understand the precise Kustomize plugin input/output schema.

Raw
configmap.yaml
kind: ConfigMap
apiVersion: v1
metadata:
name: hello
data:
HELLO: world
Raw
kustomization.yaml
resources:
- configmap.yaml
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