Overview

The downward API is a mechanism that allows containers to consume information about API objects without coupling to OpenShift Container Platform. Such information includes the pod’s name, namespace, and resource values. Containers can consume information from the downward API using environment variables or a volume plug-in.

Selecting Fields

Fields within the pod are selected using the FieldRef API type. FieldRef has two fields:

Field Description

fieldPath

The path of the field to select, relative to the pod.

apiVersion

The API version to interpret the fieldPath selector within.

Currently, the valid selectors in the v1 API include:

Selector Description

metadata.name

The pod’s name. This is supported in both environment variables and volumes.

metadata.namespace

The pod’s namespace.This is supported in both environment variables and volumes.

metadata.labels

The pod’s labels. This is only supported in volumes and not in environment variables.

metadata.annotations

The pod’s annotations. This is only supported in volumes and not in environment variables.

status.podIP

The pod’s IP. This is only supported in environment variables and not volumes.

The apiVersion field, if not specified, defaults to the API version of the enclosing pod template.

Consuming the Container Values Using the Downward API

Using Environment Variables

One mechanism for consuming the downward API is using a container’s environment variables. The EnvVar type’s valueFrom field (of type EnvVarSource) is used to specify that the variable’s value should come from a FieldRef source instead of the literal value specified by the value field. In the future, additional sources may be supported; currently the source’s fieldRef field is used to select a field from the downward API.

Only constant attributes of the pod can be consumed this way, as environment variables cannot be updated once a process is started in a way that allows the process to be notified that the value of a variable has changed. The fields supported using environment variables are:

  • Pod name

  • Pod namespace

    1. Create a pod.json file:

      apiVersion: v1
      kind: Pod
      metadata:
        name: dapi-env-test-pod
      spec:
        containers:
          - name: env-test-container
            image: gcr.io/google_containers/busybox
            command: [ "/bin/sh", "-c", "env" ]
            env:
              - name: MY_POD_NAME
                valueFrom:
                  fieldRef:
                    fieldPath: metadata.name
              - name: MY_POD_NAMESPACE
                valueFrom:
                  fieldRef:
                    fieldPath: metadata.namespace
        restartPolicy: Never
    2. Create the pod from the pod.json file:

      $ oc create -f pod.json
    3. Check the container’s logs for the MY_POD_NAME and MY_POD_NAMESPACE values:

      $ oc logs -p dapi-env-test-pod

Using the Volume Plug-in

Another mechanism for consuming the downward API is using a volume plug-in. The downward API volume plug-in creates a volume with configured fields projected into files. The metadata field of the VolumeSource API object is used to configure this volume. The plug-in supports the following fields:

  • Pod name

  • Pod namespace

  • Pod annotations

  • Pod labels

Example 1. Downward API Volume Plug-in Configuration
spec:
  volumes:
    - name: podinfo
      metadata: (1)
        items:  (2)
          - name: "labels" (3)
            fieldRef:
              fieldPath: metadata.labels  (4)
1 The metadata field of the volume source configures the downward API volume.
2 The items field holds a list of fields to project into the volume.
3 The name of the file to project the field into.
4 The selector of the field to project.

For example:

  1. Create a volume-pod.json file:

    kind: Pod
    apiVersion: v1
    metadata:
      labels:
        zone: us-east-coast
        cluster: downward-api-test-cluster1
        rack: rack-123
      name: dapi-volume-test-pod
      annotations:
        annotation1: 345
        annotation2: 456
    spec:
      containers:
        - name: volume-test-container
          image: gcr.io/google_containers/busybox
          command: ["sh", "-c", "cat /etc/labels /etc/annotations"]
          volumeMounts:
            - name: podinfo
              mountPath: /etc
              readOnly: false
      volumes:
        - name: podinfo
          metadata:
            items:
              - name: "labels"
                fieldRef:
                  fieldPath: metadata.labels
              - name: "annotations"
                fieldRef:
                  fieldPath: metadata.annotations
      restartPolicy: Never
  2. Create the pod from the volume-pod.json file:

    $ oc create -f volume-pod.json
  3. Check the container’s logs and verify the presence of the configured fields:

    $ oc logs -p dapi-volume-test-pod
    cluster=downward-api-test-cluster1
    rack=rack-123
    zone=us-east-coast
    annotation1=345
    annotation2=456
    kubernetes.io/config.source=api

Consuming Container Resources Using the Downward API

When creating pods, you can use the downward API to inject information about computing resource requests and limits so that image and application authors can correctly create an image for specific environments.

You can do this using both the environment variable and volume plug-in methods.

Using Environment Variables

  1. When creating a pod configuration, specify environment variables that correspond to the contents of the resources field in the spec.container field:

    ...
    spec:
      containers:
        - name: test-container
          image: gcr.io/google_containers/busybox:1.24
          command: [ "/bin/sh", "-c", "env" ]
          resources:
            requests:
              memory: "32Mi"
              cpu: "125m"
            limits:
              memory: "64Mi"
              cpu: "250m"
          env:
            - name: MY_CPU_REQUEST
              valueFrom:
                resourceFieldRef:
                  resource: requests.cpu
            - name: MY_CPU_LIMIT
              valueFrom:
                resourceFieldRef:
                  resource: limits.cpu
            - name: MY_MEM_REQUEST
              valueFrom:
                resourceFieldRef:
                  resource: requests.memory
            - name: MY_MEM_LIMIT
              valueFrom:
                resourceFieldRef:
                  resource: limits.memory
    ...

    If the resource limits are not included in the container configuration, the downward API defaults to the node’s CPU and memory allocatable values.

  2. Create the pod from the pod.json file:

    $ oc create -f pod.json

Using the Volume Plug-in

  1. When creating a pod configuration, use the spec.volumes.downwardAPI.items field to describe the desired resources that correspond to the spec.resources field:

    ....
    spec:
      containers:
        - name: client-container
          image: gcr.io/google_containers/busybox:1.24
          command: ["sh", "-c", "while true; do echo; if [[ -e /etc/cpu_limit ]]; then cat /etc/cpu_limit; fi; if [[ -e /etc/cpu_request ]]; then cat /etc/cpu_request; fi; if [[ -e /etc/mem_limit ]]; then cat /etc/mem_limit; fi; if [[ -e /etc/mem_request ]]; then cat /etc/mem_request; fi; sleep 5; done"]
          resources:
            requests:
              memory: "32Mi"
              cpu: "125m"
            limits:
              memory: "64Mi"
              cpu: "250m"
          volumeMounts:
            - name: podinfo
              mountPath: /etc
              readOnly: false
      volumes:
        - name: podinfo
          downwardAPI:
            items:
              - path: "cpu_limit"
                resourceFieldRef:
                  containerName: client-container
                  resource: limits.cpu
              - path: "cpu_request"
                resourceFieldRef:
                  containerName: client-container
                  resource: requests.cpu
              - path: "mem_limit"
                resourceFieldRef:
                  containerName: client-container
                  resource: limits.memory
              - path: "mem_request"
                resourceFieldRef:
                  containerName: client-container
                  resource: requests.memory
    ....

    If the resource limits are not included in the container configuration, the downward API defaults to the node’s CPU and memory allocatable values.

  2. Create the pod from the volume-pod.json file:

    $ oc create -f volume-pod.json