# kustomize plugins

Kustomize offers a plugin framework allowing
people to write their own resource _generators_
and _transformers_.

[generator options]: ../examples/generatorOptions.md
[transformer configs]: ../examples/transformerconfigs

Write a plugin when changing [generator options]
or [transformer configs] doesn't meet your needs.

[12-factor]: https://12factor.net

 * A _generator_ plugin could be a helm chart
   inflator, or a plugin that emits all the
   components (deployment, service, scaler,
   ingress, etc.) needed by someone's [12-factor]
   application, based on a smaller number of free
   variables.

 * A _transformer_ plugin might perform special
   container command line edits, or any other
   transformation that exceeds the power of the
   builtin transformations (`namePrefix`,
   `commonLabels`, etc.).

## Specification in `kustomization.yaml`

Start by adding a `generators` and/or `transformers`
field to your kustomization.

Each field accepts a string list:

> ```
> generators:
> - relative/path/to/some/file.yaml
> - relative/path/to/some/kustomization
> - /absolute/path/to/some/kustomization
> - https://github.com/org/repo/some/kustomization
>
> transformers:
> - {as above}
> ```

This is exactly like the syntax of the `resources`
field.

The value of each entry in a `resources`,
`generators` or `transformers` list must be a
relative path to a YAML file, or a path or URL
to a [kustomization].

[kustomization]: glossary.md#kustomization

In the former case the YAML is read from disk directly,
and in the latter case a kustomization is performed,
and its YAML output is merged with the YAML read
directly from files.  The net result in all three cases
is a set of YAML objects.

Each object resulting from a `generators` or
`transformers` field is now further interpreted by
kustomize as a _plugin configuration_ object.

## Configuration

A kustomization file could have the following lines:

```
generators:
- chartInflator.yaml
```

Given this, the kustomization process would expect to
find a file called `chartInflator.yaml` in the
kustomization [root](glossary.md#kustomization-root).

This is the _plugin's configuration file_.

The file `chartInflator.yaml` could contain:

```
apiVersion: someteam.example.com/v1
kind: ChartInflator
metadata:
  name: notImportantHere
chartName: minecraft
```

__The `apiVersion` and `kind` fields are
used to locate the plugin.__

[k8s object]: glossary.md#kubernetes-style-object

> Thus, these fields are required.  They are also
> required because a kustomize plugin
> configuration object is also a [k8s object].

To get the plugin ready to generator or transform,
it is given the entire contents of the
configuration file.

[NameTransformer]: ../plugin/builtin/nametransformer/NameTransformer_test.go
[ChartInflator]: ../plugin/someteam.example.com/v1/chartinflator/ChartInflator_test.go
[plugins]: ../plugin/builtin

For more examples of plugin configuration YAML,
browse the unit tests below the [plugins] root,
e.g. the tests for [ChartInflator] or
[NameTransformer].


## Placement

Each plugin gets its own dedicated directory named

```
$XDG_CONFIG_HOME/kustomize/plugin
    /${apiVersion}/LOWERCASE(${kind})
```

The default value of `XDG_CONFIG_HOME` is
`$HOME/.config`.

The one-plugin-per-directory requirement eases
creation of a plugin tarball (source, test, plugin
data files, etc.) for sharing.

In the case of a [Go plugin](#go-plugins), it also
allows one to provide a `go.mod` file for the
single plugin, easing resolution of package
version dependency skew.

When loading, kustomize will first look for an
_executable_ file called

```
$XDG_CONFIG_HOME/kustomize/plugin
    /${apiVersion}/LOWERCASE(${kind})/${kind}
```

If this file is not found or is not executable,
kustomize will look for a file called `${kind}.so`
in the same directory and attempt to load it as a
[Go plugin](#go-plugins).

If both checks fail, the plugin load fails the overall
`kustomize build`.

## Execution

Plugins are only used during a run of the
`kustomize build` command.

Generator plugins are run after processing the
`resources` field (which itself is in some sense a
generator in that it emits resources for further
processing).

The full set of resources is then passed into the
transformation pipeline, wherein builtin
transformations like `namePrefix` and
`commonLabel` are applied (if they were specified
in the kustomization file), followed by the
user-specified transformers in the `transformers`
field.

The specified order of transformers in the
`transformers` field should be respected, as
transformers cannot be expected to be commutative.

A `kustomize build` that tries to use plugins but
omits the flag

> `--enable_alpha_plugins`

will fail with a warning about plugin use.

Flag use is an opt-in acknowledging the absence of
plugin provenance.  It's meant to give pause to
someone who blindly downloads a kustomization from
the internet and attempts to run it, without
realizing that it might attempt to run 3rd party
code in plugin form.  The plugin would have to be
installed already, but nevertheless the flag is a
reminder.


## Writing plugins

### Exec plugins

A _exec plugin_ is any executable that accepts a
single argument on its command line - the name of
a YAML file containing its configuration (the file name
provided in the kustomization file).

> TODO: more restrictions on plugin to allow the same exec
> plugin to be specified in a config under both the
> `generators` and `transformers` fields.
> - first arg could be the fixed string
>   `generate` or `transform`,
>   (the name of the configuration file moves to
>   the 2nd arg), or
> - by default an exec plugin behaves as a tranformer
>   unless a flag `-g` is provided, switching the
>   exec plugin to behave as a generator.

[helm chart inflator]: ../plugin/someteam.example.com/v1/chartinflator
[bashed config map]: ../plugin/someteam.example.com/v1/bashedconfigmap
[sed transformer]: ../plugin/someteam.example.com/v1/sedtransformer

#### Examples

 * [helm chart inflator] - A generator that inflates a helm chart.
 * [bashed config map] -  Super simple configMap generation from bash.
 * [sed transformer] - Define your unstructured edits using a
   plugin like this one.


A generator plugin accepts nothing on `stdin`, but emits
generated resources to `stdout`.

A transformer plugin accepts resource YAML on `stdin`,
and emits those resources, presumably transformed, to
`stdout`.

kustomize uses an exec plugin adapter to provide
marshalled resources on `stdin` and capture
`stdout` for further processing.

### Go plugins

[Go plugin]: https://golang.org/pkg/plugin/

A [Go plugin] for kustomize looks like this:

> ```
> package main
>
> import (
>	"sigs.k8s.io/kustomize/pkg/ifc"
>	"sigs.k8s.io/kustomize/pkg/resmap"
>   ...
> )
>
> type plugin struct {...}
>
> var KustomizePlugin plugin
>
> func (p *plugin) Config(
>    ldr ifc.Loader,
>    rf *resmap.Factory,
>    c []byte) error {...}
>
> func (p *plugin) Generate() (resmap.ResMap, error) {...}
>
> func (p *plugin) Transform(m resmap.ResMap) error {...}
> ```

Use of the identifiers `plugin`, `KustomizePlugin`
and implementation of the method signature
`Config` is required.

Implementing the `Generator` or `Transformer`
method allows (respectively) the plugin's config
file to be added to the `generators` or
`transformers` field in the kustomization file.
Do one or the other or both as desired.

[secret generator]: ../plugin/someteam.example.com/v1/secretsfromdatabase
[service generator]: ../plugin/someteam.example.com/v1/someservicegenerator
[string prefixer]: ../plugin/someteam.example.com/v1/stringprefixer
[date prefixer]: ../plugin/someteam.example.com/v1/dateprefixer


#### Examples

 * [secret generator] - Generate secrets from a database.
 * [service generator] - Generate a service from a name and port argument.
 * [string prefixer] - uses the value in `metadata/name` as the prefix.
   This particular example exists to show how a plugin can
   transform the behavior of a plugin.  See the
   `TestTransformedTransformers` test in the `target` package.
 * [date prefixer] - prefix the current date to resource names, a simple
   example used to modify the string prefixer plugin just mentioned.
 * All the builtin plugins [here](../plugin/builtin).
   User authored plugins are
   on the same footing as builtin operations.

A plugin can be both a generator and a
transformer.  The `Generate` method will run along
with all the other generators before the
`Transform` method runs.

Here's a build command that sensibly assumes the
plugin source code sits in the directory where
kustomize expects to find `.so` files:

```
d=$XDG_CONFIG_HOME/kustomize/plugin\
/${apiVersion}/LOWERCASE(${kind})

go build -buildmode plugin \
   -o $d/${kind}.so $d/${kind}.go
```

#### Caveats

Go plugins allow kustomize extensions that run
without the cost marshalling/unmarshalling all
resource data to/from a subprocess for each plugin
run.

[ELF]: https://en.wikipedia.org/wiki/Executable_and_Linkable_Format

Go plugins work as [defined][Go plugin], but fall
short of common notions associated with the word
_plugin_.  Go plugin compilation creates an [ELF]
formatted `.so` file, which by definition has no
information about the provenance of the object
code.  Skew between the compilation conditions
(versions of package dependencies, `GOOS`,
`GOARCH`) of the main program ELF and the plugin
ELF will cause plugin load failure.

Exec plugins also lack provenance, but won't
complain about compilation skew.

In either case, a sensible way to share a plugin
is as a tar file of source code, tests and
associated data, unpackable under
`$XDG_CONFIG_HOME/kustomize/plugin` (exactly where
one would develop a plugin).

[Go modules]: https://github.com/golang/go/wiki/Modules

In the case of a Go plugin, an end user accepting
a shared plugin must compile both kustomize and
the plugin.  Tooling could be built to make Go
_plugin sharing_ easier, but this requires some
critical mass of _plugin authoring_, which in turn
is hampered by confusion around sharing.
[Go modules], once they are more widely adopted,
will solve one of the biggest plugin sharing
difficulties - ambiguous plugin vs host
dependencies.