Kubernetes Downward API

Disclaimer

Opinions expressed here are solely my own and do not express the views or opinions of JPMorgan Chase.
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Introduction

1.1 Kubernetes Downward API
1.2 Exposing metadata through environment variables
1.3 Exposing metadata though volumes

1. Introduction

1.1 Kubernetes Downward API

The Downward API enables you to expose the pod’s own metadata to the processes running inside that pod.
Currently, it allows you to pass the following information to your containers:
- The pod’s name
- The pod’s IP address
- The namespace the pod belongs to
- The name of the node the pod is running on
- The name of the service account the pod is running under
- The CPU and memory requests for each container
- The CPU and memory limits for each container
- The pod’s labels
- The pod’s annotations

1.2 Exposing metadata through environment variables

Sample YAML to showcase exposing pod metadata via env: downward-api-env.yaml

To test this grab the file here: wget https://raw.githubusercontent.com/jamesbuckett/kubernetes-downward-api/master/downward-api-env.yaml

apiVersion: v1
kind: Pod
metadata:
  name: downward-env
spec:
  containers:
  - name: main
    image: busybox
    command: ["sleep", "9999999"]
    resources:
      requests:
        cpu: 15m
        memory: 128M
      limits:
        cpu: 100m
        memory: 256M
    env:
    - name: POD_NAME
      valueFrom:
        fieldRef:
          fieldPath: metadata.name
    - name: POD_NAMESPACE
      valueFrom:
        fieldRef:
          fieldPath: metadata.namespace
    - name: POD_IP
      valueFrom:
        fieldRef:
          fieldPath: status.podIP
    - name: NODE_NAME
      valueFrom:
        fieldRef:
          fieldPath: spec.nodeName
    - name: SERVICE_ACCOUNT
      valueFrom:
        fieldRef:
          fieldPath: spec.serviceAccountName
    - name: CONTAINER_CPU_REQUEST_MILLICORES
      valueFrom:
        resourceFieldRef:
          resource: requests.cpu
          divisor: 1m
    - name: CONTAINER_MEMORY_LIMIT_KIBIBYTES
      valueFrom:
        resourceFieldRef:
          resource: limits.memory
          divisor: 1Ki

Sample output after running downward-api-env.yaml:

`kubectl exec pod/downward-env env`

root@digital-ocean-droplet:~# kubectl exec -it pod/downward-env env
kubectl exec [POD] [COMMAND] is DEPRECATED and will be removed in a future version. Use kubectl exec [POD] -- [COMMAND] instead.
PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin
HOSTNAME=downward-env
NODE_NAME=digital-ocean-pool-xxxxx - The name of the node the pod is running on
SERVICE_ACCOUNT=default - The name of the service account
CONTAINER_CPU_REQUEST_MILLICORES=15 - The CPU and memory requests for each container
CONTAINER_MEMORY_LIMIT_KIBIBYTES=250000 - The CPU and memory limits for each container
POD_NAME=downward-env - The pod’s name
POD_NAMESPACE=ckad - The namespace the pod belongs to
POD_IP=10.244.0.1 - The pod’s IP address
KUBERNETES_PORT_443_TCP_PROTO=tcp
KUBERNETES_PORT_443_TCP_PORT=443
KUBERNETES_PORT_443_TCP_ADDR=10.245.0.1
KUBERNETES_SERVICE_HOST=10.245.0.1
KUBERNETES_SERVICE_PORT=443
KUBERNETES_SERVICE_PORT_HTTPS=443
KUBERNETES_PORT=tcp://10.245.0.1:443
KUBERNETES_PORT_443_TCP=tcp://10.245.0.1:443
TERM=xterm
HOME=/root

1.3 Exposing metadata through volumes

Sample YAML to showcase exposing pod metadata via env: downward-api-vol.yaml

apiVersion: v1
kind: Pod
metadata:
  name: downward-vol
  labels:
    foo: bar
  annotations:
    key1: value1
    key2: |
      multi
      line
      value
spec:
  containers:
    - name: main
      image: busybox
      command: ["sleep", "9999999"]
      resources:
        requests:
          cpu: 15m
          memory: 16M
        limits:
          cpu: 100m
          memory: 128M
      volumeMounts:
        - name: downward
          mountPath: /etc/downward
  volumes:
    - name: downward
      downwardAPI:
        items:
          - path: "podName"
            fieldRef:
              fieldPath: metadata.name
          - path: "podNamespace"
            fieldRef:
              fieldPath: metadata.namespace
          - path: "labels"
            fieldRef:
              fieldPath: metadata.labels
          - path: "annotations"
            fieldRef:
              fieldPath: metadata.annotations
          - path: "containerCpuRequestMilliCores"
            resourceFieldRef:
              containerName: main
              resource: requests.cpu
              divisor: 1m
          - path: "containerMemoryLimitBytes"
            resourceFieldRef:
              containerName: main
              resource: limits.memory
              divisor: 1