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Update documentation for VolumeSubpathEnvExpansion #11843

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Feb 20, 2019
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Update documentation for VolumeSubpathEnvExpansion #11843

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3239ed3
Update documentation for VolumeSubpathEnvExpansion
Dec 21, 2018
3e427dd
Address comments - improve descriptions
Dec 21, 2018
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9 changes: 5 additions & 4 deletions content/en/docs/concepts/storage/volumes.md
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Expand Up @@ -1071,13 +1071,14 @@ spec:

### Using subPath with expanded environment variables

{{< feature-state for_k8s_version="v1.11" state="alpha" >}}
{{< feature-state for_k8s_version="v1.14" state="alpha" >}}


`subPath` directory names can also be constructed from Downward API environment variables.
`subPath` directory names can also be constructed from Downward API environment variables by using the `subPathExpr` field.
Before you use this feature, you must enable the `VolumeSubpathEnvExpansion` feature gate.
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I could use a bit more clarification on why & how to use the subPathExpr:

  • In the example, you show the value of subPathExpr equal to an environment variable. Is this always the case?
  • What is the relationship between constructing subPath directory names from Downward API environment variables (which fields or variables?) and using volumeMounts.subPathExpr?
  • Is it downward API or Downward API?
  • Why is the feature flag VolumeSubpathEnvExpansion using lowercase p?

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@kevtaylor kevtaylor Dec 21, 2018 •
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  • It can be any combination of values, but has been designed primarily to give environment variables which are dynamic and available from the Downward API - so it can be $(POD_NAME)/kev
  • You can use any fields from the Downward API - any environment variables that are currently expanded in the container args can be used
  • It is referred to in the documentation as Downward https://kubernetes.io/docs/tasks/inject-data-application/downward-api-volume-expose-pod-information/#the-downward-api
  • The feature gate follows the same naming convention as VolumeSubpath in pkg/features/kube_features.go

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Incidentally, this feature gate and all the documentation with it has already been approved at 1.11 - this is to switch the code structure to a new api field to differentiate it from the subPath which was the original alpha implementation

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thanks for clarifying, @kevtaylor. Looks good. I think re-using the sub header, Using subPath with expanded environment variables was confusing.
nit, possible change first sentence:
Use the subPathExpr field to construct subPath directory names from Downward API environment variables.
nit, possible change: The subPath and subPathExpr properties are mutually exclusive.

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Agreed - I think those suggestions sound better. Pushed change

`subPath` and `subPathExpr` are mutually exclusive.

In this example, a Pod uses `subPath` to create a directory `pod1` within the hostPath volume `/var/log/pods`, using the pod name from the Downward API. The host directory `/var/log/pods/pod1` is mounted at `/logs` in the container.
In this example, a Pod uses `subPathExpr` to create a directory `pod1` within the hostPath volume `/var/log/pods`, using the pod name from the Downward API. The host directory `/var/log/pods/pod1` is mounted at `/logs` in the container.

```yaml
apiVersion: v1
Expand All @@ -1098,7 +1099,7 @@ spec:
volumeMounts:
- name: workdir1
mountPath: /logs
subPath: $(POD_NAME)
subPathExpr: $(POD_NAME)
restartPolicy: Never
volumes:
- name: workdir1
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3 changes: 2 additions & 1 deletion content/en/docs/reference/command-line-tools-reference/feature-gates.md
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Expand Up @@ -133,7 +133,7 @@ different Kubernetes components.
| `VolumeScheduling` | `false` | Alpha | 1.9 | 1.9 |
| `VolumeScheduling` | `true` | Beta | 1.10 | 1.12 |
| `VolumeScheduling` | `true` | GA | 1.13 | |
| `VolumeSubpathEnvExpansion` | `false` | Alpha | 1.11 | |
| `VolumeSubpathEnvExpansion` | `false` | Alpha | 1.14 | |
| `VolumeSnapshotDataSource` | `false` | Alpha | 1.12 | - |
| `ScheduleDaemonSetPods` | `false` | Alpha | 1.11 | 1.11 |
| `ScheduleDaemonSetPods` | `true` | Beta | 1.12 | |
Expand Down Expand Up @@ -298,5 +298,6 @@ Each feature gate is designed for enabling/disabling a specific feature:
enables the usage of [`local`](/docs/concepts/storage/volumes/#local) volume
type when used together with the `PersistentLocalVolumes` feature gate.
- `VolumeSnapshotDataSource`: Enable volume snapshot data source support.
- `VolumeSubpathEnvExpansion`: Enable `subPathExpr` field for expanding environment variables into a `subPath`.

{{% /capture %}}