PLEASE NOTE: This document applies to the HEAD of the source tree
If you are using a released version of Kubernetes, you should
refer to the docs that go with that version.
The latest 1.0.x release of this document can be found
[here](http://releases.k8s.io/release-1.0/docs/user-guide/downward-api/volume/README.md).
Documentation for other releases can be found at
[releases.k8s.io](http://releases.k8s.io).
--
# Downward API volume plugin
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](../../../../docs/devel/api-conventions.md#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](../../../../docs/getting-started-guides/) 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.
```shell
$ 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
```shell
$ 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):
```shell
$ 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)`.
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