Avesh Agarwal
119 lines
4.8 KiB
Markdown
119 lines
4.8 KiB
Markdown
---
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---
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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](https://github.com/kubernetes/kubernetes/tree/{{page.githubbranch}}/docs/devel/api-conventions.md#metadata) or a container's [resources](/docs/user-guide/compute-resources).
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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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Supported container's resources:
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1. `limits.cpu`
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2. `limits.memory`
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3. `requests.cpu`
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4. `requests.memory`
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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 [dapi-volume.yaml](/docs/user-guide/downward-api/volume/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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```shell
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$ kubectl create -f docs/user-guide/downward-api/volume/dapi-volume.yaml
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```
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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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```shell
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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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```
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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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```shell
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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 4
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drwxrwxrwt 3 0 0 120 Jun 1 19:55 .
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drwxr-xr-x 17 0 0 4096 Jun 1 19:55 ..
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drwxr-xr-x 2 0 0 80 Jun 1 19:55 ..6986_01_06_15_55_10.473583074
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lrwxrwxrwx 1 0 0 31 Jun 1 19:55 ..data -> ..6986_01_06_15_55_10.473583074
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lrwxrwxrwx 1 0 0 18 Jun 1 19:55 annotations -> ..data/annotations
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lrwxrwxrwx 1 0 0 13 Jun 1 19:55 labels -> ..data/labels
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/etc/..6986_01_06_15_55_10.473583074:
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total 8
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drwxr-xr-x 2 0 0 80 Jun 1 19:55 .
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drwxrwxrwt 3 0 0 120 Jun 1 19:55 ..
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-rw-r--r-- 1 0 0 129 Jun 1 19:55 annotations
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-rw-r--r-- 1 0 0 59 Jun 1 19:55 labels
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/ #
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```
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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)`.
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## Example of downward API volume with container resources
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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.
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```shell
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$ kubectl create -f docs/user-guide/downward-api/volume/dapi-volume-resources.yaml
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```
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### Examine pod/container output
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In pod's `/etc` directory one may find the files created by the plugin:
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```shell
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$ kubectl exec kubernetes-downwardapi-volume-example -i -t -- sh
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/ # ls -alR /etc
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/etc:
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total 4
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drwxrwxrwt 3 0 0 160 Jun 1 19:47 .
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drwxr-xr-x 17 0 0 4096 Jun 1 19:48 ..
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drwxr-xr-x 2 0 0 120 Jun 1 19:47 ..6986_01_06_15_47_23.076909525
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lrwxrwxrwx 1 0 0 31 Jun 1 19:47 ..data -> ..6986_01_06_15_47_23.076909525
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lrwxrwxrwx 1 0 0 16 Jun 1 19:47 cpu_limit -> ..data/cpu_limit
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lrwxrwxrwx 1 0 0 18 Jun 1 19:47 cpu_request -> ..data/cpu_request
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lrwxrwxrwx 1 0 0 16 Jun 1 19:47 mem_limit -> ..data/mem_limit
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lrwxrwxrwx 1 0 0 18 Jun 1 19:47 mem_request -> ..data/mem_request
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/etc/..6986_01_06_15_47_23.076909525:
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total 16
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drwxr-xr-x 2 0 0 120 Jun 1 19:47 .
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drwxrwxrwt 3 0 0 160 Jun 1 19:47 ..
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-rw-r--r-- 1 0 0 1 Jun 1 19:47 cpu_limit
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-rw-r--r-- 1 0 0 1 Jun 1 19:47 cpu_request
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-rw-r--r-- 1 0 0 8 Jun 1 19:47 mem_limit
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-rw-r--r-- 1 0 0 8 Jun 1 19:47 mem_request
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/ # cat /etc/cpu_limit
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1
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/ # cat /etc/mem_limit
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67108864
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/ # cat /etc/cpu_request
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1
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/ # cat /etc/mem_request
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33554432
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```
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