---
title:  Define Health of Component and Trait
---

This documentation will explain how to achieve status write back by using CUE templates in definition objects.

## Health Check

The spec of health check is `spec.status.healthPolicy`, they are the same for both Workload Type and Trait.

If not defined, the health result will always be `true`.

The keyword in CUE is `isHealth`, the result of CUE expression must be `bool` type.
KubeVela runtime will evaluate the CUE expression periodically until it becomes healthy. Every time the controller will get all the Kubernetes resources and fill them into the context field.

So the context will contain following information:

```cue
context:{
  name: <component name>
  appName: <app name>
  output: <K8s workload resource>
  outputs: {
    <resource1>: <K8s trait resource1>
    <resource2>: <K8s trait resource2>
  }
}
```

Trait will not have the `context.output`, other fields are the same.

The example of health check likes below:

```yaml
apiVersion: core.oam.dev/v1beta1
kind: ComponentDefinition
spec:
  status:
    healthPolicy: |
      isHealth: (context.output.status.readyReplicas > 0) && (context.output.status.readyReplicas == context.output.status.replicas)
   ...
```

```yaml
apiVersion: core.oam.dev/v1beta1
kind: TraitDefinition
spec:
  status:
    healthPolicy: |
      isHealth: len(context.outputs.service.spec.clusterIP) > 0
   ...
```

> Please refer to [this doc](https://github.com/kubevela/kubevela/blob/master/docs/examples/app-with-status/template.yaml) for the complete example.

The health check result will be recorded into the `Application` resource.

```yaml
apiVersion: core.oam.dev/v1beta1
kind: Application
spec:
  components:
  - name: myweb
    type: worker    
    properties:
      cmd:
      - sleep
      - "1000"
      enemies: alien
      image: busybox
      lives: "3"
    traits:
    - type: ingress
      properties:
        domain: www.example.com
        http:
          /: 80
status:
  ...
  services:
  - healthy: true
    message: "type: busybox,\t enemies:alien"
    name: myweb
    traits:
    - healthy: true
      message: 'Visiting URL: www.example.com, IP: 47.111.233.220'
      type: ingress
  status: running
```

## Custom Status

The spec of custom status is `spec.status.customStatus`, they are the same for both Workload Type and Trait.

The keyword in CUE is `message`, the result of CUE expression must be `string` type.

The custom status has the same mechanism with health check.
Application CRD controller will evaluate the CUE expression after the health check succeed.

The context will contain following information:

```cue
context:{
  name: <component name>
  appName: <app name>
  output: <K8s workload resource>
  outputs: {
    <resource1>: <K8s trait resource1>
    <resource2>: <K8s trait resource2>
  }
}
```

Trait will not have the `context.output`, other fields are the same.


Please refer to [this doc](https://github.com/kubevela/kubevela/blob/master/docs/examples/app-with-status/template.yaml) for the complete example.

```yaml
apiVersion: core.oam.dev/v1beta1
kind: ComponentDefinition
spec:
  status:
    customStatus: |-
      message: "type: " + context.output.spec.template.spec.containers[0].image + ",\t enemies:" + context.outputs.gameconfig.data.enemies
   ...
```

```yaml
apiVersion: core.oam.dev/v1beta1
kind: TraitDefinition
spec:
  status:
    customStatus: |-
      message: "type: "+ context.outputs.service.spec.type +",\t clusterIP:"+ context.outputs.service.spec.clusterIP+",\t ports:"+ "\(context.outputs.service.spec.ports[0].port)"+",\t domain"+context.outputs.ingress.spec.rules[0].host
   ...
```