Kubernetes Downward API Concepts

External

• https://kubernetes.io/docs/concepts/storage/volumes/#downwardapi
• https://kubernetes.io/docs/tasks/inject-data-application/downward-api-volume-expose-pod-information/

Internal

• Kubernetes Concepts
• Kubernetes Storage Concepts

Overview

Kubernetes is exposing pod information to the containers running inside the pod through files or environment variables, which are projected in the container by a mechanism known as the Downward API. Individual pieces of information are declared as "items" under a "downwardAPI" volume declaration, and the corresponding content will be projected as individual files in the pod filesystem.

The pieces of information to expose can be read from pods or containers.

File Projection

This is how the pod name and namespace can be projected as files available to containers running inside the pod.

Declare a "downwardAPI" volume, conventionally named "podinfo" and mount it in the pod:

apiVersion: v1
kind: Pod
metadata:
  name: loop
spec:
  containers:
  - name: loop
    image: docker.io/ovidiufeodorov/loop:latest
    volumeMounts:
      - name: "podinfo"
        mountPath: "/etc/podinfo"
  volumes:
  - name: podinfo
    downwardAPI:
      items:
        - path: "metadata/name"
          fieldRef:
            fieldPath: "metadata.name"

Note that individual downwardAPI items can declare a hierarchical path - the directories will be created automatically.