View Documentation Try Kubernetes

Edit This Page

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.

Supported metadata fields:

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

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 docs/user-guide/downward-api/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 32
drwxrwxrwt    3 0        0              180 Aug 24 13:03 .
drwxr-xr-x    1 0        0             4096 Aug 24 13:05 ..
drwx------    2 0        0               80 Aug 24 13:03 ..2015_08_24_13_03_44259413923
lrwxrwxrwx    1 0        0               30 Aug 24 13:03 ..downwardapi -> ..2015_08_24_13_03_44259413923
lrwxrwxrwx    1 0        0               25 Aug 24 13:03 annotations -> ..downwardapi/annotations
lrwxrwxrwx    1 0        0               20 Aug 24 13:03 labels -> ..downwardapi/labels

/etc/..2015_08_24_13_03_44259413923:
total 8
drwx------    2 0        0               80 Aug 24 13:03 .
drwxrwxrwt    3 0        0              180 Aug 24 13:03 ..
-rw-r--r--    1 0        0              115 Aug 24 13:03 annotations
-rw-r--r--    1 0        0               53 Aug 24 13:03 labels
/ #

The file labels is stored in a temporary directory (..2015_08_24_13_03_44259413923 in the example above) which is symlinked to by ..downwardapi. Symlinks for annotations and labels in /etc point to files containing the actual metadata through the ..downwardapi indirection.  This structure allows for dynamic atomic refresh of the metadata: updates are written to a new temporary directory, and the ..downwardapi symlink is updated atomically using rename(2).

Analytics