---
title: Health Check
---

This documentation will introduce how to use `health` policy to apply periodical
health checking to an application.

## Background

After an application is deployed, users usually want to monitor or observe the
health condition of the running application as well as each components.
Health policy decouples health checking procedure from application workflow
execution.

It allows to set independent health inspection cycle, such as check every 30s.
That helps users to notice as soon as applications turn out unhealthy and
follow the diagnosis message to troubleshot.

## Health Policy

```yaml
apiVersion: core.oam.dev/v1beta1
kind: Application
metadata:
  name: app-healthscope-unhealthy
spec:
  components:
    - name: my-server
      type: webservice
      properties:
        cmd:
          - node
          - server.js
        image: oamdev/testapp:v1
        port: 8080
      traits:
        - type: ingress
          properties:
            domain: test.my.domain
            http:
              "/": 8080
    - name: my-server-unhealthy
      type: webservice
      properties:
        cmd:
          - node
          - server.js
        image: oamdev/testapp:boom # make it unhealthy
        port: 8080
  policies:
    - name: health-policy-demo
      type: health
      properties:
        probeInterval: 5
        probeTimeout: 10
```

We apply the sample application including two components, `my-server` is
supposed to be healthy while `my-server-unhealthy` is supposed to be unhealthy
(because of invalid image).

As shown in the sample, a `Health` policy is specified.
`Health` policy accepts two optional properties, `probeInterval` indicating time
duration between checking (default is 30s) and `probeTimeout` indicating time
duration before checking timeout (default is 10s).

```yaml
...
  policies:
    - name: health-policy-demo
      type: health
      properties:
        probeInterval: 5
        probeTimeout: 10
...
```

To learn about defining health checking rules, please refer to **[Status Write Back](../../platform-engineers/traits/status)**.

Finally we can observe application health status from its `status.services`.
Here is a snippet of health status.

```yaml
...
  services:
    - healthy: true
      message: 'Ready:1/1 '
      name: my-server
      scopes:
      - apiVersion: core.oam.dev/v1alpha2
        kind: HealthScope
        name: health-policy-demo
        namespace: default
        uid: 1d54b5a0-d951-4f20-9541-c2d76c412a94
      traits:
      - healthy: true
        message: |
          No loadBalancer found, visiting by using 'vela port-forward app-healthscope-unhealthy'
        type: ingress
      workloadDefinition:
        apiVersion: apps/v1
        kind: Deployment
    - healthy: false
      message: 'Ready:0/1 '
      name: my-server-unhealthy
      scopes:
      - apiVersion: core.oam.dev/v1alpha2
        kind: HealthScope
        name: health-policy-demo
        namespace: default
        uid: 1d54b5a0-d951-4f20-9541-c2d76c412a94
      workloadDefinition:
        apiVersion: apps/v1
        kind: Deployment
    status: running
...
```

## Appendix: Parameter List

Name | Desc | Type | Required | Default Value
:---------- | :----------- | :----------- | :----------- | :-----------
probeInterval|time duration between checking (in units of seconds) | int |false| 30
probeTimeout|time duration before checking timeout (in units of seconds) | int |false| 10