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Downward API

It is sometimes useful for a container to have information about itself, but we want to be careful not to over-couple containers to Kubernetes. The downward API allows containers to consume information about themselves or the system and expose that information how they want it, without necessarily coupling to the Kubernetes client or REST API.

An example of this is a "legacy" app that is already written assuming that a particular environment variable will hold a unique identifier. While it is often possible to "wrap" such applications, this is tedious and error prone, and violates the goal of low coupling. Instead, the user should be able to use the Pod's name, for example, and inject it into this well-known variable.

Capabilities

The following information is available to a Pod through the downward API:

  • The pod's name
  • The pod's namespace

More information will be exposed through this same API over time.

Exposing pod information into a container

Containers consume information from the downward API using environment variables. In the future, containers will also be able to consume the downward API via a volume plugin.

Environment variables

Most environment variables in the Kubernetes API use the value field to carry simple values. However, the alternate valueFrom field allows you to specify a fieldRef to select fields from the pod's definition. The fieldRef field is a structure that has an apiVersion field and a fieldPath field. The fieldPath field is an expression designating a field of the pod. The apiVersion field is the version of the API schema that the fieldPath is written in terms of. If the apiVersion field is not specified it is defaulted to the API version of the enclosing object.

The fieldRef is evaluated and the resulting value is used as the value for the environment variable. This allows users to publish their pod's name in any environment variable they want.

Example

This is an example of a pod that consumes its name and namespace via the downward API:

apiVersion: v1
kind: Pod
metadata:
  name: dapi-test-pod
spec:
  containers:
    - name: 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

Some more thorough examples:

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