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Downward API

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 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.yaml 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.yaml file:

      $ oc create -f pod.yaml

    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.yaml 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:
   - downwardAPI:
     name: podinfo
       defaultMode: 420
       items:
      - fieldRef:
          fieldPath: metadata.name
        path: pod_name
      - fieldRef:
          fieldPath: metadata.namespace
        path: pod_namespace
      - fieldRef:
          fieldPath: metadata.labels
        path: pod_labels
      - fieldRef:
          fieldPath: metadata.annotations
        path: pod_annotations
  restartPolicy: Never

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

    $ oc create -f volume-pod.yaml

  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.yaml file:

    $ oc create -f pod.yaml

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.yaml file:

    $ oc create -f volume-pod.yaml

Consuming Secrets Using the Downward API

When creating pods, you can use the downward API to inject Secrets so image and application authors can create an image for specific environments.

Using Environment Variables

  1. Create a secret.yaml file:

    apiVersion: v1
kind: Secret
metadata:
  name: mysecret
data:
  password: cGFzc3dvcmQ=
  username: ZGV2ZWxvcGVy
type: kubernetes.io/basic-auth

  2. Create a Secret from the secret.yaml file:

    oc create -f secret.yaml

  3. Create a pod.yaml file that references the username field from the above Secret:

    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_SECRET_USERNAME
          valueFrom:
            secretKeyRef:
              name: mysecret
              key: username
  restartPolicy: Never

  4. Create the pod from the pod.yaml file:

    $ oc create -f pod.yaml

  5. Check the container’s logs for the MY_SECRET_USERNAME value:

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

Consuming ConfigMaps Using the Downward API

When creating pods, you can use the downward API to inject ConfigMap values so image and application authors can create an image for specific environments.

Using Environment Variables

  1. Create a configmap.yaml file:

    apiVersion: v1
kind: ConfigMap
metadata:
  name: myconfigmap
data:
  mykey: myvalue

  2. Create a ConfigMap from the configmap.yaml file:

    oc create -f configmap.yaml

  3. Create a pod.yaml file that references the above ConfigMap:

    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_CONFIGMAP_VALUE
          valueFrom:
            configMapKeyRef:
              name: myconfigmap
              key: mykey
  restartPolicy: Never

  4. Create the pod from the pod.yaml file:

    $ oc create -f pod.yaml

  5. Check the container’s logs for the MY_CONFIGMAP_VALUE value:

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

Environment Variable References

When creating pods, you can reference the value of a previously defined environment variable by using the $() syntax. If the environment variable reference can not be resolved, the value will be left as the provided string.

Using Environment Variable References

  1. Create a pod.yaml file that references an existing environment variable:

    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_EXISTING_ENV
          value: my_value
        - name: MY_ENV_VAR_REF_ENV
          value: $(MY_EXISTING_ENV)
  restartPolicy: Never

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

    $ oc create -f pod.yaml

  3. Check the container’s logs for the MY_ENV_VAR_REF_ENV value:

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

Escaping Environment Variable References

When creating a pod, you can escape an environment variable reference by using a double dollar sign. The value will then be set to a single dollar sign version of the provided value.

  1. Create a pod.yaml file that references an existing environment variable:

    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_NEW_ENV
          value: $$(SOME_OTHER_ENV)
  restartPolicy: Never

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

    $ oc create -f pod.yaml

  3. Check the container’s logs for the MY_NEW_ENV value:

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