# Downward API

The downward API allows containers to consume information about the system without coupling to the
kubernetes client or REST API.

### Capabilities

Containers can consume the following information via the downward API:

*   Their pod's name
*   Their pod's namespace

### Consuming information about a pod in 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.  The `valueFrom`
field of an environment variable allows you to specify an `ObjectFieldSelector` to select fields
from the pod's definition.  The `ObjectFieldSelector` has an `apiVersion` field and a `fieldPath`
field.  The `fieldPath` field is an expression designating a field on 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.

### Example: consuming the downward API

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

```json
{
  "apiVersion":"v1beta3",
  "name": "downward-api-pod",
  "kind": "Pod",
  "spec": {
    "manifest": {
      "containers": [{
        "name": "example-container",
        "image": "gcr.io/google_containers/busybox",
        "command": [ "sh", "-c", "env" ]
        "env": [{
          "name": "POD_NAMESPACE",
          "valueFrom": {
            "fieldPath": {
              "apiVersion": "v1beta3",
              "fieldPath": "metadata.namespace"
            }
          }
        },
        {
          "name": "POD_NAME",
          "valueFrom": {
            "fieldPath": {
              "apiVersion": "v1beta3",
              "fieldPath": "metadata.name"
            }
          }
        }]
      }]
    }
  }
}]
```