Allowing containers to consume API objects

The Downward API is a mechanism that allows containers to consume information about API objects without coupling to OKD. 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.

Expose Pod information to Containers using the Downward API

The Downward API contains such information as the pod’s name, project, and resource values. Containers can consume information from the downward API using environment variables or a volume plug-in.

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

FieldDescription

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:

SelectorDescription

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.

Understanding how to consume container values using the downward API

You containers can consume API values using environment variables or a volume plug-in. Depending on the method you choose, containers can consume:

  • Pod name

  • Pod project/namespace

  • Pod annotations

  • Pod labels

Annotations and labels are available using only a volume plug-in.

Consuming container values using environment variables

When using a container’s environment variables, use the EnvVar type’s valueFrom field (of type EnvVarSource) to specify that the variable’s value should come from a FieldRef source instead of the literal value specified by the value field.

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 project/namespace

Procedure

To use environment variables

  1. Create a pod.yaml file:

    1. apiVersion: v1
    2. kind: Pod
    3. metadata:
    4.   name: dapi-env-test-pod
    5. spec:
    6.   containers:
    7.     - name: env-test-container
    8.       image: gcr.io/google_containers/busybox
    9.       command: [ "/bin/sh", "-c", "env" ]
    10.       env:
    11.         - name: MY_POD_NAME
    12.           valueFrom:
    13.             fieldRef:
    14.               fieldPath: metadata.name
    15.         - name: MY_POD_NAMESPACE
    16.           valueFrom:
    17.             fieldRef:
    18.               fieldPath: metadata.namespace
    19.   restartPolicy: Never

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

    1. $ oc create -f pod.yaml

  3. Check the container’s logs for the MY_POD_NAME and MY_POD_NAMESPACE values:

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

Consuming container values using a volume plug-in

You containers can consume API values using a volume plug-in.

Containers can consume:

  • Pod name

  • Pod project/namespace

  • Pod annotations

  • Pod labels

Procedure

To use the volume plug-in:

  1. Create a volume-pod.yaml file:

    1. kind: Pod
    2. apiVersion: v1
    3. metadata:
    4.   labels:
    5.     zone: us-east-coast
    6.     cluster: downward-api-test-cluster1
    7.     rack: rack-123
    8.   name: dapi-volume-test-pod
    9.   annotations:
    10.     annotation1: "345"
    11.     annotation2: "456"
    12. spec:
    13.   containers:
    14.     - name: volume-test-container
    15.       image: gcr.io/google_containers/busybox
    16.       command: ["sh", "-c", "cat /tmp/etc/pod_labels /tmp/etc/pod_annotations"]
    17.       volumeMounts:
    18.         - name: podinfo
    19.           mountPath: /tmp/etc
    20.           readOnly: false
    21.   volumes:
    22.   - name: podinfo
    23.     downwardAPI:
    24.       defaultMode: 420
    25.       items:
    26.       - fieldRef:
    27.           fieldPath: metadata.name
    28.         path: pod_name
    29.       - fieldRef:
    30.           fieldPath: metadata.namespace
    31.         path: pod_namespace
    32.       - fieldRef:
    33.           fieldPath: metadata.labels
    34.         path: pod_labels
    35.       - fieldRef:
    36.           fieldPath: metadata.annotations
    37.         path: pod_annotations
    38.   restartPolicy: Never

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

    1. $ oc create -f volume-pod.yaml

  3. Check the container’s logs and verify the presence of the configured fields:

    1. $ oc logs -p dapi-volume-test-pod

    Example output

    1. cluster=downward-api-test-cluster1
    2. rack=rack-123
    3. zone=us-east-coast
    4. annotation1=345
    5. annotation2=456
    6. kubernetes.io/config.source=api

Understanding how to consume 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 environment variable or a volume plug-in.

Consuming container resources using environment variables

When creating pods, you can use the Downward API to inject information about computing resource requests and limits using environment variables.

Procedure

To use 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:

    1. ....
    2. spec:
    3.   containers:
    4.     - name: test-container
    5.       image: gcr.io/google_containers/busybox:1.24
    6.       command: [ "/bin/sh", "-c", "env" ]
    7.       resources:
    8.         requests:
    9.           memory: "32Mi"
    10.           cpu: "125m"
    11.         limits:
    12.           memory: "64Mi"
    13.           cpu: "250m"
    14.       env:
    15.         - name: MY_CPU_REQUEST
    16.           valueFrom:
    17.             resourceFieldRef:
    18.               resource: requests.cpu
    19.         - name: MY_CPU_LIMIT
    20.           valueFrom:
    21.             resourceFieldRef:
    22.               resource: limits.cpu
    23.         - name: MY_MEM_REQUEST
    24.           valueFrom:
    25.             resourceFieldRef:
    26.               resource: requests.memory
    27.         - name: MY_MEM_LIMIT
    28.           valueFrom:
    29.             resourceFieldRef:
    30.               resource: limits.memory
    31. ....

    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:

    1. $ oc create -f pod.yaml

Consuming container resources using a volume plug-in

When creating pods, you can use the Downward API to inject information about computing resource requests and limits using a volume plug-in.

Procedure

To use 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:

    1. ....
    2. spec:
    3.   containers:
    4.     - name: client-container
    5.       image: gcr.io/google_containers/busybox:1.24
    6.       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"]
    7.       resources:
    8.         requests:
    9.           memory: "32Mi"
    10.           cpu: "125m"
    11.         limits:
    12.           memory: "64Mi"
    13.           cpu: "250m"
    14.       volumeMounts:
    15.         - name: podinfo
    16.           mountPath: /etc
    17.           readOnly: false
    18.   volumes:
    19.     - name: podinfo
    20.       downwardAPI:
    21.         items:
    22.           - path: "cpu_limit"
    23.             resourceFieldRef:
    24.               containerName: client-container
    25.               resource: limits.cpu
    26.           - path: "cpu_request"
    27.             resourceFieldRef:
    28.               containerName: client-container
    29.               resource: requests.cpu
    30.           - path: "mem_limit"
    31.             resourceFieldRef:
    32.               containerName: client-container
    33.               resource: limits.memory
    34.           - path: "mem_request"
    35.             resourceFieldRef:
    36.               containerName: client-container
    37.               resource: requests.memory
    38. ....

    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:

    1. $ 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.

Procedure

  1. Create a secret.yaml file:

    1. apiVersion: v1
    2. kind: Secret
    3. metadata:
    4.   name: mysecret
    5. data:
    6.   password: cGFzc3dvcmQ=
    7.   username: ZGV2ZWxvcGVy
    8. type: kubernetes.io/basic-auth

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

    1. $ oc create -f secret.yaml

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

    1. apiVersion: v1
    2. kind: Pod
    3. metadata:
    4.   name: dapi-env-test-pod
    5. spec:
    6.   containers:
    7.     - name: env-test-container
    8.       image: gcr.io/google_containers/busybox
    9.       command: [ "/bin/sh", "-c", "env" ]
    10.       env:
    11.         - name: MY_SECRET_USERNAME
    12.           valueFrom:
    13.             secretKeyRef:
    14.               name: mysecret
    15.               key: username
    16.   restartPolicy: Never

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

    1. $ oc create -f pod.yaml

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

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

Consuming configuration maps using the Downward API

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

Procedure

  1. Create a ***configmap.yaml*** file:

    1. apiVersion: v1
    2. kind: ConfigMap
    3. metadata:
    4.   name: myconfigmap
    5. data:
    6.   mykey: myvalue

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

    1. $ oc create -f configmap.yaml

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

    1. apiVersion: v1
    2. kind: Pod
    3. metadata:
    4.   name: dapi-env-test-pod
    5. spec:
    6.   containers:
    7.     - name: env-test-container
    8.       image: gcr.io/google_containers/busybox
    9.       command: [ "/bin/sh", "-c", "env" ]
    10.       env:
    11.         - name: MY_CONFIGMAP_VALUE
    12.           valueFrom:
    13.             configMapKeyRef:
    14.               name: myconfigmap
    15.               key: mykey
    16.   restartPolicy: Always

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

    1. $ oc create -f pod.yaml

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

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

Referencing environment variables

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.

Procedure

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

    1. apiVersion: v1
    2. kind: Pod
    3. metadata:
    4.   name: dapi-env-test-pod
    5. spec:
    6.   containers:
    7.     - name: env-test-container
    8.       image: gcr.io/google_containers/busybox
    9.       command: [ "/bin/sh", "-c", "env" ]
    10.       env:
    11.         - name: MY_EXISTING_ENV
    12.           value: my_value
    13.         - name: MY_ENV_VAR_REF_ENV
    14.           value: $(MY_EXISTING_ENV)
    15.   restartPolicy: Never

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

    1. $ oc create -f pod.yaml

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

    1. $ 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.

Procedure

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

    1. apiVersion: v1
    2. kind: Pod
    3. metadata:
    4.   name: dapi-env-test-pod
    5. spec:
    6.   containers:
    7.     - name: env-test-container
    8.       image: gcr.io/google_containers/busybox
    9.       command: [ "/bin/sh", "-c", "env" ]
    10.       env:
    11.         - name: MY_NEW_ENV
    12.           value: $$(SOME_OTHER_ENV)
    13.   restartPolicy: Never

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

    1. $ oc create -f pod.yaml

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

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