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.
The following information is available to a
Pod through the downward API:
More information will be exposed through this same API over time.
Containers consume information from the downward API using environment variables or using a volume plugin.
Most environment variables in the Kubernetes API use the
value field to carry
simple values. However, the alternate
valueFrom field allows you to specify
fieldRef to select fields from the pod’s definition. The
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
written in terms of. If the
apiVersion field is not specified it is
defaulted to the API version of the enclosing object.
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.
This is an example of a pod that consumes its name and namespace via the downward API:
Using a similar syntax it’s possible to expose pod information to containers using plain text files.
Downward API are dumped to a mounted volume. This is achieved using a
volume type and the different items represent the files to be created.
fieldPath references the field to be exposed.
In future, it will be possible to specify an output format option.
Downward API volumes can expose:
The downward API volume refreshes its data in step with the kubelet refresh loop. When labels will be modifiable on the fly without respawning the pod containers will be able to detect changes through mechanisms such as inotify.
In future, it will be possible to specify a specific annotation or label.
This is an example of a pod that consumes its labels and annotations via the downward API volume, labels and annotations are dumped in
/etc/labels and in