Policy Definition

In this section we will introduce how to define a custom policy with CUE. Make sure you've learned the basic knowledge about Definition Concept and how to manage definition.

Generate Resources by Policy

Generate resources in policy is similar to trait, policy can be used to define things across components.

Let's use vela def init to create a basic policy scaffold:

vela def init my-plc -t policy --desc "My ingress route policy." > myroute.cue

The content of the scaffold expected to be:

$ cat myroute.cue
"my-plc": {
    annotations: {}
    attributes: {}
    description: "My ingress route policy."
    labels: {}
    type: "policy"
}

template: {
}

The rule is align with component definition, you must specify output, while you can use outputs for more objects, the format as below:

output: {
    <full template data>
}
outputs: <unique-name>: 
  <full template data>

Below is an example that we create a traffic split service mesh object in policy.

"my-plc": {
    description: "My service mesh policy."
    type:        "policy"
}

template: {
    #ServerWeight: {
        service: string
        weight:  int
    }
    parameter: {
        weights: [...#ServerWeight]
    }

    output: {
        apiVersion: "split.smi-spec.io/v1alpha3"
        kind:       "TrafficSplit"
        metadata: name: context.name
        spec: {
            service:  context.name
            backends: parameter.weights
        }
    }
}

Apply to our control plane to make this trait work:

vela def apply myroute.cue

Then our end users can discover it immediately and use it in Application.

After using vela up by the following command:

cat <<EOF | vela up -f -
apiVersion: core.oam.dev/v1beta1
kind: Application
metadata:
  name: my-test-2
spec:
  components:
    - name: server-v1
      type: webservice
      properties:
        image: oamdev/hello-world:v1
    - name: server-v2
      type: webservice
      properties:
        image: oamdev/hello-world:v2
  policies:
    - type: my-plc
      name: unified
      properties:
        weights:
          - service: server-v1
            weight: 80
          - service: server-v2
            weight: 20
EOF

The policy will generate Kubernetes resources by KubeVela like below:

apiVersion: split.smi-spec.io/v1alpha3
kind: TrafficSplit
metadata:
  name: unified
  namespace: default
spec:
  backends:
  - service: server-v1
    weight: 80
  - service: server-v2
    weight: 20
  service: unified

You can define any Kubernetes API objects in policies if you want.

Special Policy

Not all policies generate resources, there're several built-in policies which are used to control the whole delivery precess and workflows. These special polices are usually coded in the application controller.

What's Next

• Learn how to define health check and custom status of Trait.
• Learn how to define workflow step in CUE.