CUE based Addon Application

Previous tutorial introduces the basic structure of an addon and illustrate that any Kubernetes operator to be installed of an addon should be defined in a KubeVela application. Document YAML application description file explains the way of using YAML define the addon application. If you use CUE to write application description file, the addon will be able to have these abilities:

• Utilize the flexible and concise syntax of the CUE language, rich built-in functions and its parameter verification capabilities, to render and deploy the application and auxiliary resources with parameters and metadata of addon.
• An addon may contain multiple Definitions and CRD Operators. They can be selectively installed according to parameters of the addon.

This doc will introduce how to define addon application by writing CUE files.

Application description files contain two parts: application template files and resource files (they are in the resources/ folder).

Application template file (template.cue)

The most important part in the application template is output field, which must place a KubeVela application as follows:

package main

output: {
    apiVersion: "core.oam.dev/v1beta1"
    kind:       "Application"
    spec: {
        components: [
            {
                type: "k8s-objects"
                name: "example-namespace"
                properties: objects: [{
                    apiVersion: "v1"
                    kind:       "Namespace"
                    metadata: name: parameter.namespace
                }]
            },
        ]
    }
}

In this example, the name of the namespace defined in spec.components[0].properties.objects[0] of this application is determined by parameter.namespace, which means that its name will be dynamically rendered by the namespace parameter when the addon is enabled. If you want the created namespace to be my-namespace, you can run the following command:

vela addon enable <addon-name> namespace=my-namespace

After rendered, the resulting application is:

kind: Application
metadata:
spec:
  components:
    - name: namespace
      type: k8s-objects
      properties:
        objects:
          - apiVersion: v1
            kind: Namespace
            metadata:
              name: my-namespace

You can refer to the CUE basic to learn language details.

Please notice: The name of Application in template file will be replaced by the addon name in metadata.yaml. The application will always have a unified name in the format of addon-<addon_name>.

Parameter definition file (parameter.cue)

In the example above, we use the parameter namespace to set the name of Kubernetes namespace resource. Actually, we also need a parameter definition file (parameter.cue) to declare what parameters this addon has. For example,

parameter: {
  //+usage=namespace to create
  namespace: string
}

When enabling the addon, you can set the parameters declared in the parameter definition file by appending the parameters to the command, as follows:

vela addon enable <addon-Name> <parameter-name-1=value> <parameter-name-2=value>

Resource files (CUE files under resources/ folder)

KubeVela has supported CUE package in an addon, which means you can define any CUE files containing data or schema inside the resources/ folder and reference them in application CUE file as a while package. This also help you avoid defining all content in one template file.

Continuing with the example above, we split the CUE blocks that define the namesapce component under the resources/ folder, the folder structure is as follows:

├── resources/
│   └── namespace.cue
├── README.md
├── metadata.yaml
├── parameter.cue
└── template.cue

The namespace.cue file is as follows:

// resources/namespace.cue
package main

namespace: {
    type: "k8s-objects"
    name: "example-namespace"
    properties: objects: [{
        apiVersion: "v1"
        kind:       "Namespace"
        metadata: name: parameter.namespace
    }]
}

Then we can reference this CUE block in template.cue :

// template.cue

package main

output: {
    apiVersion: "core.oam.dev/v1beta1"
    kind:       "Application"
    spec: {
        // reference namespace block from resources/naemspace.cue
        components: [namespace]
    }
}

After enabled this addon with command $ vela addon enable <addon-name> namespace=my-namespace clusters=local,cluser1, the resulting application is:

apiVersion: core.oam.dev/v1beta1
kind: Application
metadata:
  name: example
  namespace: vela-system
spec:
  components:
    - name: namespace
      type: k8s-objects
      properties:
        objects:
          - apiVersion: v1
            kind: Namespace
            metadata:
              name: my-namespace

Please notice: Only those CUE files with header package main can be reference by template.cue, this can be used to help you filter CUE files that you don't want to use in the rendering context.

We just use namespace as example here, other resources of an operator can also be defined in KubeVela application in the same way. This also gives your addon re-usability and validation capability powered by the CUE.

Features

This section will introduce the way of writing application description file to implement several core features of addon.

Determine which clusters to be installed by parameters

If you want the resources in the addon to be installed not only in the control-plane, but also in managed clusters, you can use the topology policy in your application as shown below. The parameter clusters field will be filled when the addon is enabled with the clusters parameter specified.

package main

output: {
    apiVersion: "core.oam.dev/v1beta1"
    kind:       "Application"
    spec: {
        components:{...}
        policies: [{
            type: "topology"
            name: "deploy-topology"
            properties: {
                if parameter.clusters != _|_ {
                    clusters: parameter.clusters
                }
                if parameter.clusters == _|_ {
                    clusterLabelSelector: {}
                }
            }
        }]
    }}

If you execute the command to enable the addon as follows:

$ vela addon enable <addon-name> clusters=local,cluser1

The rendering result will be:

kind: Application
metadata:
  name: addon-example
  namespace: vela-system
spec:
  components: ...
  policies:
    - type: "topology"
      name: "deploy-topology"
      properties:
        clusters:
          - local
          - cluster1

After enabling the addon, the KubeVela controller will install components to the local and cluster1 clusters as defined in the application's topology policy.

If you need to enable the addon in all clusters, you can enable the addon by not setting the cluster parameter as follows:

$ vela addon enable <addon-name>

The rendering result is :

kind: Application
metadata:
  name: addon-example
  namespace: vela-system
spec:
  components: ...
  policies:
    - type: "topology"
      name: "deploy-topology"
      properties:
        clusterLabelSelector: {}

Since an empty ({}) clusterLabelSelector topology will choose all exist clusters as target, so the components in the application will be dispatched to all clusters including both the control-plane and the managed clusters.

Auxiliary resources

You can also define some auxiliary resources in the outputs field of the template.cue file. These resources will only be applied to the control plane.

package main

output: {
    apiVersion: "core.oam.dev/v1beta1"
    kind:       "Application"
    spec: {
        
    }
    ... 
}

outputs: resourceTree: {
    apiVersion: "v1"
    kind:       "ConfigMap"
    metadata: {
        name:      "resource-tree"
        namespace: "vela-system"
        labels: {
            "rules.oam.dev/resources":       "true"
            "rules.oam.dev/resource-format": "json"
        }
    }
    data: rules: json.Marshal(_rules)
}

_rules: {...}

In this example, we define a configmap resourceTree as an auxiliary resource, this configmap is actually a resource topology rule . The function of this resource is to establish the relationship of CustomResources in the cluster, so that it can be displayed in the topology graph. It only needs to be applied to control-plane.

You can also run the cue eval *.cue resources/*.cue -e output -d yaml command from local to see the result of resource rendering.

Use metadata of context to render application

In addition to dynamically rendering the application by parameters, you can also read fields defined in metadata.yaml for rendering. For example, you can define a template.cue file as follows:

output: {
    apiVersion: "core.oam.dev/v1beta1"
    kind:       "Application"
    spec: {
        components: [
            {
                type: "webservice"
                properties: {
                    image: "oamdev/vela-apiserver:" + context.metadata.version
                }
            },
        ]
    }
}

When rendering, the fields defined in metadata.yaml will be put into the CUE block of context and rendered together with other CUE files. For example, the metadata.yaml is:

...
name: velaux
version: 1.2.4
...

Resulting application is:

apiVersion: core.oam.dev/v1beta1
kind:       Application
metadata:
  name: addon-example
  namespace: "vela-system"
spec:
  components:
    - type: webservice
      properties:
        image: "oamdev/vela-apiserver:v1.2.4"

The image tag becomes the addon's version which the context.metadata.version field points to. The real example is VelaUX. Other available fields of metadata please refer to metadata.

When the addon is enabled, template.cue, parameter.cue and the resource files will be gathered with the addon metadata in metadata.yaml to render out the resources and apply them.

Binding the Definition to a component

If you want to bind a Definition to a component in the application, to dynamically enable the ability of one Definition, you can do it by setting addon.oam.dev/bind-component annotation on the Definition.

An actual example is fluxcd addon.

ComponentDefinition kustomize in this addon is:

kustomize: {
    attributes: workload: type: "autodetects.core.oam.dev"
    description: "kustomize can fetching, building, updating and applying Kustomize manifests from git repo."
    type:        "component"
    annotations: {
         "addon.oam.dev/ignore-without-component": "fluxcd-kustomize-controller"
   }
}

...

This Definition has an annotation "addon.oam.dev/bind-component": "fluxcd-kustomize-controller", which means, bind the ComponentDefinition to fluxcd-kustomize-controller component.

The template.cue of this addon is:

//...

kustomizeController: {
    type: "webService"
    Name: "fluxcd-kustomize-controller",
    //....
}

gitOpsController: [...]

if parameter.onlyHelmComponents == false {
    gitOpsController: [kustomizeController]
}

output: {
    apiVersion: "core.oam.dev/v1beta1"
    kind:       "Application"
    spec: {
        //...
        components: [
            helmController,
            sourceController,
        ] + gitOpsController
        //...
    }
}
//...

If you enable this addon by following the command:

$ vela addon enable fluxcd `onlyHelmComponents=true`

The fluxcd-kustomize-controller component won't be added to the application. The kustomize ComponentDefinitions will not be applied either.

Examples

An example is ingress-nginx addon. All files included in this addon are all CUE typed.