kubectl/docs/book/pages/app_customization/config_reflection.md

{% panel style="success", title="Providing Feedback" %} Provide feedback at the survey {% endpanel %}

{% panel style="info", title="TL;DR" %}

• Inject the values of other Resource Config fields into Pod Env Vars and Command Args with vars. {% endpanel %}

Config Reflection

Motivation

Applications running in Pods may need to know about Application context or configuration. For example, a Pod may take the name of Service defined in the Project as a command argument. Instead of hard coding the value of the Service directly into the PodSpec, users can reference the Service value using a vars entry. If the value is updated or transformed by the kustomization.yaml file (e.g. by setting a namePrefix), the value will be propagated to where it is referenced in the PodSpec.

{% panel style="info", title="Reference" %}

• vars {% endpanel %}

Vars

The vars section contains variable references to Resource Config fields within the project. They require the following to be defined:

• Resource Kind
• Resource Version
• Resource name
• Field path

{% method %}

Example: Set the Pod command argument to the value of a Service name.

Apply will resolve $(BACKEND_SERVICE_NAME) to a value using the object reference specified in vars.

{% sample lang="yaml" %} Input: The kustomization.yaml, deployment.yaml and service.yaml files

# kustomization.yaml
namePrefix: "test-"
vars:
  # Name of the variable so it can be referenced
- name: BACKEND_SERVICE_NAME
  # GVK of the object with the field
  objref:
    kind: Service
    name: backend-service
    apiVersion: v1
  # Path to the field
  fieldref:
    fieldpath: metadata.name
resources:
- deployment.yaml
- service.yaml

# service.yaml
kind: Service
apiVersion: v1
metadata:
  # Value of the variable.  This will be customized with
  # a namePrefix, and change the Variable value.
  name: backend-service
spec:
  selector:
    app: backend
  ports:
  - protocol: TCP
    port: 80
    targetPort: 9376

# deployment.yaml
apiVersion: apps/v1
kind: Deployment
metadata:
  name: curl-deployment
  labels:
    app: curl
spec:
  selector:
    matchLabels:
      app: curl
  template:
    metadata:
      labels:
        app: curl
    spec:
      containers:
      - name: curl
        image: ubuntu
        # Reference the Service name field value as a variable
        command: ["curl", "$(BACKEND_SERVICE_NAME)"]

Applied: The Resources that are Applied to the cluster

apiVersion: v1
kind: Service
metadata:
  name: test-backend-service
spec:
  ports:
  - port: 80
    protocol: TCP
    targetPort: 9376
  selector:
    app: backend
---
apiVersion: apps/v1
kind: Deployment
metadata:
  name: test-curl-deployment
  labels:
    app: curl
spec:
  selector:
    matchLabels:
      app: curl
  template:
    metadata:
      labels:
        app: curl
    spec:
      containers:
      - command:
        - curl
        # $(BACKEND_SERVICE_NAME) has been resolved to
        # test-backend-service
        - test-backend-service
        image: ubuntu
        name: curl

{% endmethod %}

{% panel style="warning", title="Referencing Variables" %} Variables are intended only to inject Resource Config into Pods. They are not intended as a general templating mechanism. Overriding values should be done with patches instead of variables. See Bases and Variations. {% endpanel %}