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downward_api.md

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Downward API

The downward API allows containers to consume information about the system without coupling to the kubernetes client or REST API.

Capabilities

Containers can consume the following information via the downward API:

  • Their pod's name
  • Their pod's namespace

Consuming information about a pod in a container

Containers consume information from the downward API using environment variables. In the future, containers will also be able to consume the downward API via a volume plugin. The valueFrom field of an environment variable allows you to specify an ObjectFieldSelector to select fields from the pod's definition. The ObjectFieldSelector has an apiVersion field and a fieldPath field. The fieldPath field is an expression designating a field on the pod. The apiVersion field is the version of the API schema that the fieldPath is written in terms of. If the apiVersion field is not specified it is defaulted to the API version of the enclosing object.

Example: consuming the downward API

This is an example of a pod that consumes its name and namespace via the downward API:

apiVersion: v1
kind: Pod
metadata:
  name: dapi-test-pod
spec:
  containers:
    - name: test-container
      image: gcr.io/google_containers/busybox
      command: [ "/bin/sh", "-c", "env" ]
      env:
        - name: POD_NAME
          valueFrom:
            fieldRef:
              fieldPath: metadata.name
        - name: POD_NAMESPACE
          valueFrom:
            fieldRef:
              fieldPath: metadata.namespace
  restartPolicy: Never

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