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Step Zero: Prerequisites

Following this example, you will create a pod with a downward API volume. A downward API volume is a k8s volume plugin with the ability to save some pod information in a plain text file. The pod information can be for example some metadata or a container’s resources.

Supported metadata fields:

  1. metadata.annotations
  2. metadata.namespace
  3. metadata.name
  4. metadata.labels

Supported container’s resources:

  1. limits.cpu
  2. limits.memory
  3. requests.cpu
  4. requests.memory

Step Zero: Prerequisites

This example assumes you have a Kubernetes cluster installed and running, and the kubectl command line tool somewhere in your path. Please see the gettingstarted for installation instructions for your platform.

Step One: Create the pod

Use the dapi-volume.yaml file to create a Pod with a downward API volume which stores pod labels and pod annotations to /etc/labels and /etc/annotations respectively.

$ kubectl create -f  docs/user-guide/downward-api/volume/dapi-volume.yaml

Step Two: Examine pod/container output

The pod displays (every 5 seconds) the content of the dump files which can be executed via the usual kubectl log command

$ kubectl logs kubernetes-downwardapi-volume-example
cluster="test-cluster1"
rack="rack-22"
zone="us-est-coast"
build="two"
builder="john-doe"
kubernetes.io/config.seen="2015-08-24T13:47:23.432459138Z"
kubernetes.io/config.source="api"

Internals

In pod’s /etc directory one may find the file created by the plugin (system files elided):

$ kubectl exec kubernetes-downwardapi-volume-example -i -t -- sh
/ # ls -laR /etc
/etc:
total 4
drwxrwxrwt    3 0        0              120 Jun  1 19:55 .
drwxr-xr-x   17 0        0             4096 Jun  1 19:55 ..
drwxr-xr-x    2 0        0               80 Jun  1 19:55 ..6986_01_06_15_55_10.473583074
lrwxrwxrwx    1 0        0               31 Jun  1 19:55 ..data -> ..6986_01_06_15_55_10.473583074
lrwxrwxrwx    1 0        0               18 Jun  1 19:55 annotations -> ..data/annotations
lrwxrwxrwx    1 0        0               13 Jun  1 19:55 labels -> ..data/labels

/etc/..6986_01_06_15_55_10.473583074:
total 8
drwxr-xr-x    2 0        0               80 Jun  1 19:55 .
drwxrwxrwt    3 0        0              120 Jun  1 19:55 ..
-rw-r--r--    1 0        0              129 Jun  1 19:55 annotations
-rw-r--r--    1 0        0               59 Jun  1 19:55 labels
/ #

The file labels is stored in a temporary directory (..6986_01_06_15_55_10.473583074 in the example above) which is symlinked to by ..data. Symlinks for annotations and labels in /etc point to files containing the actual metadata through the ..data indirection.  This structure allows for dynamic atomic refresh of the metadata: updates are written to a new temporary directory, and the ..data symlink is updated atomically using rename(2).

Example of downward API volume with container resources

Use the docs/user-guide/downward-api/volume/dapi-volume-resources.yaml file to create a Pod with a downward API volume which stores its container’s limits and requests in /etc.

$ kubectl create -f  docs/user-guide/downward-api/volume/dapi-volume-resources.yaml

Examine pod/container output

In pod’s /etc directory one may find the files created by the plugin:

$ kubectl exec kubernetes-downwardapi-volume-example -i -t -- sh
/ # ls -alR /etc
/etc:
total 4
drwxrwxrwt    3 0        0              160 Jun  1 19:47 .
drwxr-xr-x   17 0        0             4096 Jun  1 19:48 ..
drwxr-xr-x    2 0        0              120 Jun  1 19:47 ..6986_01_06_15_47_23.076909525
lrwxrwxrwx    1 0        0               31 Jun  1 19:47 ..data -> ..6986_01_06_15_47_23.076909525
lrwxrwxrwx    1 0        0               16 Jun  1 19:47 cpu_limit -> ..data/cpu_limit
lrwxrwxrwx    1 0        0               18 Jun  1 19:47 cpu_request -> ..data/cpu_request
lrwxrwxrwx    1 0        0               16 Jun  1 19:47 mem_limit -> ..data/mem_limit
lrwxrwxrwx    1 0        0               18 Jun  1 19:47 mem_request -> ..data/mem_request

/etc/..6986_01_06_15_47_23.076909525:
total 16
drwxr-xr-x    2 0        0              120 Jun  1 19:47 .
drwxrwxrwt    3 0        0              160 Jun  1 19:47 ..
-rw-r--r--    1 0        0                1 Jun  1 19:47 cpu_limit
-rw-r--r--    1 0        0                1 Jun  1 19:47 cpu_request
-rw-r--r--    1 0        0                8 Jun  1 19:47 mem_limit
-rw-r--r--    1 0        0                8 Jun  1 19:47 mem_request

/ # cat /etc/cpu_limit
1
/ # cat /etc/mem_limit
67108864
/ # cat /etc/cpu_request
1
/ # cat /etc/mem_request
33554432

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