John Mulhausen
96 lines
3.5 KiB
Markdown
96 lines
3.5 KiB
Markdown
---
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layout: docwithnav
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title: "Downward API volume plugin"
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---
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<!-- BEGIN MUNGE: UNVERSIONED_WARNING -->
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<!-- END MUNGE: UNVERSIONED_WARNING -->
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# Downward API volume plugin
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Following this example, you will create a pod with a downward API volume.
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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.html#metadata).
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Supported metadata fields:
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1. `metadata.annotations`
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2. `metadata.namespace`
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3. `metadata.name`
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4. `metadata.labels`
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### Step Zero: Prerequisites
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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.
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### Step One: Create the pod
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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.
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{% highlight sh %}
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{% raw %}
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$ kubectl create -f docs/user-guide/downward-api/volume/dapi-volume.yaml
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{% endraw %}
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{% endhighlight %}
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### Step Two: Examine pod/container output
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The pod displays (every 5 seconds) the content of the dump files which can be executed via the usual `kubectl log` command
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{% highlight sh %}
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{% raw %}
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$ kubectl logs kubernetes-downwardapi-volume-example
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cluster="test-cluster1"
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rack="rack-22"
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zone="us-est-coast"
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build="two"
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builder="john-doe"
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kubernetes.io/config.seen="2015-08-24T13:47:23.432459138Z"
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kubernetes.io/config.source="api"
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{% endraw %}
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{% endhighlight %}
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### Internals
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In pod's `/etc` directory one may find the file created by the plugin (system files elided):
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{% highlight sh %}
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{% raw %}
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$ kubectl exec kubernetes-downwardapi-volume-example -i -t -- sh
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/ # ls -laR /etc
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/etc:
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total 32
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drwxrwxrwt 3 0 0 180 Aug 24 13:03 .
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drwxr-xr-x 1 0 0 4096 Aug 24 13:05 ..
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drwx------ 2 0 0 80 Aug 24 13:03 ..2015_08_24_13_03_44259413923
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lrwxrwxrwx 1 0 0 30 Aug 24 13:03 ..downwardapi -> ..2015_08_24_13_03_44259413923
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lrwxrwxrwx 1 0 0 25 Aug 24 13:03 annotations -> ..downwardapi/annotations
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lrwxrwxrwx 1 0 0 20 Aug 24 13:03 labels -> ..downwardapi/labels
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/etc/..2015_08_24_13_03_44259413923:
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total 8
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drwx------ 2 0 0 80 Aug 24 13:03 .
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drwxrwxrwt 3 0 0 180 Aug 24 13:03 ..
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-rw-r--r-- 1 0 0 115 Aug 24 13:03 annotations
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-rw-r--r-- 1 0 0 53 Aug 24 13:03 labels
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/ #
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{% endraw %}
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{% endhighlight %}
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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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<!-- BEGIN MUNGE: IS_VERSIONED -->
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<!-- TAG IS_VERSIONED -->
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<!-- END MUNGE: IS_VERSIONED -->
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<!-- BEGIN MUNGE: GENERATED_ANALYTICS -->
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[]()
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<!-- END MUNGE: GENERATED_ANALYTICS -->
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