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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
- The pod's IP
- A container's cpu limit
- A container's cpu request
- A container's memory limit
- A container's memory request
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 or using 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, and a resourceFieldRef
to select fields from one of its container'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.
The resourceFieldRef is a structure that has a containerName field, a resource
field, and a divisor field. The containerName is the name of a container,
whose resource (cpu or memory) information is to be exposed. The containerName is
optional for environment variables and defaults to the current container. The
resource field is an expression designating a resource in a container, and the divisor
field specifies an output format of the resource being exposed. If the divisor
is not specified, it defaults to "1" for cpu and memory. The table shows possible
values for cpu and memory resources for resource and divisor settings:
| Setting | Cpu | Memory |
|---|---|---|
| resource | limits.cpu, requests.cpu | limits.memory, requests.memory |
| divisor | 1(cores), 1m(millicores) | 1(bytes), 1k(kilobytes), 1M(megabytes), 1G(gigabytes), 1T(terabytes), 1P(petabytes), 1E(exabytes), 1Ki(kibibyte), 1Mi(mebibyte), 1Gi(gibibyte), 1Ti(tebibyte), 1Pi(pebibyte), 1Ei(exbibyte) |
Example
This is an example of a pod that consumes its name and namespace via the downward API:
{% include code.html language="yaml" file="dapi-pod.yaml" ghlink="/docs/user-guide/downward-api/dapi-pod.yaml" %}
This is an example of a pod that consumes its container's resources via the downward API:
{% include code.html language="yaml" file="dapi-container-resources.yaml" ghlink="/docs/user-guide/downward-api/dapi-container-resources.yaml" %}
Downward API volume
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 downwardAPI
volume type and the different items represent the files to be created. fieldPath references the field to be exposed.
For exposing a container's resources limits and requests, containerName must be specified with resourceFieldRef.
Downward API volume permits to store more complex data like metadata.labels and metadata.annotations. Currently key/value pair set fields are saved using key="value" format:
key1="value1"
key2="value2"
In future, it will be possible to specify an output format option.
Downward API volumes can expose:
- The pod's name
- The pod's namespace
- The pod's labels
- The pod's annotations
- A container's cpu limit
- A container's cpu request
- A container's memory limit
- A container's memory request
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.
Example
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 /etc/annotations, respectively:
{% include code.html language="yaml" file="volume/dapi-volume.yaml" ghlink="/docs/user-guide/downward-api/volume/dapi-volume.yaml" %}
This is an example of a pod that consumes its container's resources via the downward API volume.
{% include code.html language="yaml" file="volume/dapi-volume-resources.yaml" ghlink="/docs/user-guide/downward-api/volume/dapi-volume-resources.yaml" %}
Some more thorough examples:
Default values for container resource limits
If cpu and memory limits are not specified for a container, the downward API will default to node's cpu and memory capacities.