Downward API

There are two ways to expose Pod and container fields to a running container: environment variables, and as files that are populated by a special volume type. Together, these two ways of exposing Pod and container fields are called the downward API.

It is sometimes useful for a container to have information about itself, without being overly coupled to Kubernetes. The downward API allows containers to consume information about themselves or the cluster without using the Kubernetes client or API server.

An example is an existing application that assumes a particular well-known environment variable holds a unique identifier. One possibility is to wrap the application, but that is tedious and error-prone, and it violates the goal of low coupling. A better option would be to use the Pod's name as an identifier, and inject the Pod's name into the well-known environment variable.

In Kubernetes, there are two ways to expose Pod and container fields to a running container:

Together, these two ways of exposing Pod and container fields are called the downward API.

Available fields

Only some Kubernetes API fields are available through the downward API. This section lists which fields you can make available.

You can pass information from available Pod-level fields using fieldRef. At the API level, the spec for a Pod always defines at least one Container. You can pass information from available Container-level fields using resourceFieldRef.

Information available via fieldRef

For most Pod-level fields, you can provide them to a container either as an environment variable or using a downwardAPI volume. The fields available via either mechanism are:
the pod's name
the pod's namespace
the pod's unique ID
the value of the pod's annotation named <KEY> (for example, metadata.annotations['myannotation'])
the text value of the pod's label named <KEY> (for example, metadata.labels['mylabel'])
the name of the pod's service account
the name of the node where the Pod is executing
the primary IP address of the node to which the Pod is assigned
the pod's primary IP address (usually, its IPv4 address)

In addition, the following information is available through a downwardAPI volume fieldRef, but not as environment variables:

all of the pod's labels, formatted as label-key="escaped-label-value" with one label per line
all of the pod's annotations, formatted as annotation-key="escaped-annotation-value" with one annotation per line

Information available via resourceFieldRef

These container-level fields allow you to provide information about requests and limits for resources such as CPU and memory.

resource: limits.cpu
A container's CPU limit
resource: requests.cpu
A container's CPU request
resource: limits.memory
A container's memory limit
resource: requests.memory
A container's memory request
resource: limits.hugepages-*
A container's hugepages limit (provided that the DownwardAPIHugePages feature gate is enabled)
resource: requests.hugepages-*
A container's hugepages request (provided that the DownwardAPIHugePages feature gate is enabled)
resource: limits.ephemeral-storage
A container's ephemeral-storage limit
resource: requests.ephemeral-storage
A container's ephemeral-storage request

Fallback information for resource limits

If CPU and memory limits are not specified for a container, and you use the downward API to try to expose that information, then the kubelet defaults to exposing the maximum allocatable value for CPU and memory based on the node allocatable calculation.

What's next

You can read about downwardAPI volumes.

You can try using the downward API to expose container- or Pod-level information:

Last modified June 20, 2022 at 5:58 PM PST: Downward API concept made Correct (bf59d416b4)