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About the PolicyGenerator plugin

See the PolicyGenerator reference YAML to see the full structure of the PolicyGenerator manifest.

The Kustomize exec plugin explained

The policy generator plugin is a binary written in Go. The binary accepts an input manifest of the PolicyGenerator kind in the policy.open-cluster-management.io/v1 API. When Kustomize finds a manifest of that kind under the generators: array in kustomization.yaml, it looks in its directories for a binary of the same name ("PolicyGenerator") in the policy.open-cluster-management.io/vi/policygenerator directory and calls the binary as <path>/PolicyGenerator <cached-manifest> to run the generator as its plugin. The flag --enable-alpha-plugins must be specified in the Kustomize command in order for the external generator plugin to be enabled.

Placement

By default, a Placement and PlacementBinding are created for each policy with the policy name as the suffix. To signal that you'd like to consolidate policies that use the same Placement under a single PlacementBinding, specify placement.placementPath to an existing Placement manifest or set placement.name along with placement.labelSelector. When the PlacementBinding is consolidated in this way, placementBindingDefaults.name must be specified so that the generator can create unique names for the bindings.

Policy expanders

Policy expanders provide logic to create additional policies based on a given kind to give a complete picture of violations or status using Open Cluster Management policies. Generally, the expanders point to kinds provided by policy engines such as Kyverno and Gatekeeper. These expanders are enabled by default but can be disabled individually by setting the flag associated with the expander in the PolicyGenerator manifest either in policyDefaults or for a particular manifest in the policies array.

Contributing a policy expander

To contribute a policy expander, you'll need to:

  1. Add your expander to the getExpanders() method in expanders.go and familiarize yourself with the Interface there that you'll be implementing.
  2. Add your expander file to the internal/expanders/ directory. You can follow the other files there as an example.
  3. Choose a name for your boolean expander setting. (Existing names have followed the pattern Inform<engine-name>Policies.)
  4. Add your expander setting to both the PolicyDefaults and the PolicyConfig structs in types.go
  5. Add your expander setting to the applyDefaults() method in plugin.go to set defaults for both PolicyDefaults and Policies.
  6. Update the policygenerator-reference.yaml with your expander setting.
  7. Add tests for your expander to the internal/expanders/ directory.

PolicyGenerator code structure

DIRECTORY TREE              PACKAGE                 DESCRIPTION
================================================================================================
.
├── cmd
│   └── main.go             main                    Parent binary (imports the internal package)
└── internal
    ├── expanders
    │   ├── expanders.go    expanders               Policy expander interface
    ├── types
    │   └── types.go        types                   Generator structs
    ├── patches.go          internal                Code to patch input manifests
    ├── plugin.go           internal                Primary generator methods
    ├── typohelper.go       internal                Helpers for identifying manifest typos
    ├── utils.go            internal                Helper/utility functions

OpenAPI schema support

The Policy Generator supports OpenAPI schemas through Kustomize (see the openapi Kustomize documentation). The goal of this feature is to support patching non-Kubernetes custom resource objects that contain list of objects. The OpenAPI object in this project has the same format of the OpenAPI object in the Kustomize project. The path indicates the relative path of the schema JSON file relative to the kustomization.yaml file:

openapi:
  path: schema.json

The OpenAPI object is included with the manifest object in the plugin file:

apiVersion: policy.open-cluster-management.io/v1
kind: PolicyGenerator
. . .
policies:
  - name: myapp
    manifests:
      - path: input-kustomize/
        patches: []
        openapi:
          path: schema.json

How to create a Kustomize schema manually

Ideally the OpenAPI schema is provided by the developper of the Custom Resource (CR). To retrieve a schema from a running Kubernetes cluster manually, do the following:

kustomize openapi fetch

Then cut and paste the subset containing the resources that need to be patched. Next, identify the list objects in the schema and select a key from the fields of the object that would be use to index the list, for instance a name. After the definition of the list, add the following text:

"x-kubernetes-patch-merge-key": "name",
"x-kubernetes-patch-strategy": "merge"

x-kubernetes-patch-merge-key indicates the field in the object that is used to uniquely identify it in the list in this case the name field. x-kubernetes-patch-strategy indicates the patch strategy. Merge would merge fields, replace would replace the object identified by the key with patch content.

Note: The "key" selected in this step is used in patches to uniquely identify a list object. An example of schema is shown at openapi-schema.json