concept downwardAPI volume in category kubernetes

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Both these problems are solved by the Kubernetes Downward API. It allows you to pass metadata about the pod and its environment through environment variables or files (in a downwardAPI volume). Don’t be confused by the name. The Downward API isn’t like a REST endpoint that your app needs to hit so it can get the data. It’s a way of having environment variables or files populated with values from the pod’s specification or status, as shown in figure 8.1.

8.1.3. Passing metadata through files in a downwardAPI volume

If you prefer to expose the metadata through files instead of environment variables, you can define a downwardAPI volume and mount it into your container. You must use a downwardAPI volume for exposing the pod’s labels or its annotations, because neither can be exposed through environment variables. We’ll discuss why later.

Listing 8.3. Pod with a downwardAPI volume: downward-api-volume.yaml

apiVersion: v1
kind: Pod
metadata:
  name: downward
  labels:                                     #1
    foo: bar                                  #1
  annotations:                                #1
    key1: value1                              #1
    key2: |                                   #1
      multi                                   #1
      line                                    #1
      value                                   #1
spec:
  containers:
  - name: main
    image: busybox
    command: ["sleep", "9999999"]
    resources:
      requests:
        cpu: 15m
        memory: 100Ki
      limits:
        cpu: 100m
        memory: 4Mi
    volumeMounts:                             #2
    - name: downward                          #2
      mountPath: /etc/downward                #2
  volumes:
  - name: downward                            #3
    downwardAPI:                              #3
      items:
      - path: "podName"                       #4
        fieldRef:                             #4
          fieldPath: metadata.name            #4
      - path: "podNamespace"
        fieldRef:
          fieldPath: metadata.namespace
      - path: "labels"                            #5

        fieldRef:                                 #5

          fieldPath: metadata.labels              #5

      - path: "annotations"                       #6
        fieldRef:                                 #6
          fieldPath: metadata.annotations         #6
      - path: "containerCpuRequestMilliCores"
        resourceFieldRef:
          containerName: main
          resource: requests.cpu
          divisor: 1m
      - path: "containerMemoryLimitBytes"
        resourceFieldRef:
          containerName: main
          resource: limits.memory
          divisor: 1

Figure 8.3. Using a downwardAPI volume to pass metadata to the container