Add device plugin doc under concepts/cluster-administration. (#5261)

* Add device plugin doc under concepts/cluster-administration.

* Update device-plugins.md

* Update device-plugins.md

Add meta description. Fix typo. Change bare metal deployment to manual deployment.

* Update device-plugins.md

Fix typo again.

_data/concepts.yml

@@ -93,6 +93,7 @@ toc:
   - docs/concepts/cluster-administration/master-node-communication.md
   - docs/concepts/cluster-administration/proxies.md
   - docs/concepts/cluster-administration/controller-metrics.md
+  - docs/concepts/cluster-administration/device-plugins.md
   - title: Policies
     section:
     - docs/concepts/policy/resource-quotas.md

docs/concepts/cluster-administration/device-plugins.md

@@ -0,0 +1,114 @@
+---
+approvers:
+title: Device Plugins
+description: Use the Kubernetes device plugin framework to implement plugins for GPUs, NICs, FPGAs, InfiniBand, and similar resources that require vendor-specific setup.
+---
+
+{% include feature-state-alpha.md %}
+
+{% capture overview %}
+Starting in version 1.8, Kubernetes provides a [device plugin framework](https://github.com/kubernetes/community/blob/master/contributors/design-proposals/device-plugin.md)
+for vendors to advertise their resources to the kubelet without changing Kubernetes core code.
+Instead of writing custom Kubernetes code, vendors can implement a device plugin that can
+be deployed manually or as a DaemonSet. The targeted devices include GPUs,
+High-performance NICs, FPGAs, InfiniBand, and other similar computing resources
+that may require vendor specific initialization and setup.
+{% endcapture %}
+
+{% capture body %}
+
+## Device plugin registration
+
+The device plugins feature is gated by the `DevicePlugins` feature gate and is disabled by default.
+When the device plugins feature is enabled, the kubelet exports a `Registration` gRPC service:
+
+```gRPC
+service Registration {
+	rpc Register(RegisterRequest) returns (Empty) {}
+}
+```
+A device plugin can register itself with the kubelet through this gRPC service.
+During the registration, the device plugin needs to send:
+
+  * The name of its Unix socket.
+  * The API version against which it was built.
+  * The `ResourceName` it wants to advertise. Here `ResourceName` needs to follow the
+    [extended resource naming scheme](https://github.com/kubernetes/kubernetes/pull/48922)
+    as `vendor-domain/resource`.
+    For example, an Nvidia GPU is advertised as `nvidia.com/gpu`.
+
+Following a successful registration, the device plugin sends the kubelet the
+list of devices it manages, and the kubelet is then in charge of advertising those
+resources to the API server as part of the kubelet node status update.
+For example, after a device plugin registers `vendor-domain/foo` with the kubelet
+and reports two healthy devices on a node, the node status is updated
+to advertise 2 `vendor-domain/foo`.
+
+Then, developers can request devices in a
+[Container](/docs/api-reference/{{page.version}}/#container-v1-core)
+specification by using the same process that is used for
+[opaque integer resources](/docs/tasks/configure-pod-container/opaque-integer-resource/).
+In version 1.8, extended resources are spported only as integer resources and must have
+`limit` equal to `request` in the Container specification.
+
+## Device plugin implementation
+
+The general workflow of a device plugin includes the following steps:
+
+* Initialization. During this phase, the device plugin performs vendor specific
+  initialization and setup to make sure the devices are in a ready state.
+  
+* The plugin starts a gRPC service, with a Unix socket under host path
+  `/var/lib/kubelet/device-plugins/`, that implements the following interfaces:
+
+  ```gRPC
+  service DevicePlugin {
+        // ListAndWatch returns a stream of List of Devices
+        // Whenever a Device state change or a Device disapears, ListAndWatch
+        // returns the new list
+        rpc ListAndWatch(Empty) returns (stream ListAndWatchResponse) {}
+
+        // Allocate is called during container creation so that the Device
+        // Plugin can run device specific operations and instruct Kubelet
+        // of the steps to make the Device available in the container
+        rpc Allocate(AllocateRequest) returns (AllocateResponse) {}
+  }
+  ```
+
+* The plugin registers itself with the kubelet through the Unix socket at host
+  path `/var/lib/kubelet/device-plugins/kubelet.sock`.
+
+* After successfully registering itself, the device plugin runs in serving mode, during which it keeps
+monitoring device health and reports back to the kubelet upon any device state changes.
+It is also responsible for serving `Allocate` gRPC requests. During `Allocate`, the device plugin may
+do device-specific preparation; for example, GPU cleanup or QRNG initialization.
+If the operations succeed, the device plugin returns an `AllocateResponse` that contains container
+runtime configurations for accessing the allocated devices. The kubelet passes this information
+to the container runtime.
+
+A device plugin is expected to detect kubelet restarts and re-register itself with the new
+kubelet instance. In version 1.8, a new kubelet instance cleans up all the existing Unix sockets
+under `/var/lib/kubelet/device-plugins` when it starts. A device plugin can monitor the deletion
+of its Unix socket and re-register itself upon such an event.
+
+## Device plugin deployment
+
+A device plugin can be deployed manually or as a DaemonSet. Being deployed as a DaemonSet has
+the benefit that Kubernetes can restart the device plugin if it fails.
+Otherwise, an extra mechanism is needed to recover from device plugin failures.
+The canonical directory `/var/lib/kubelet/device-plugins` requires privileged access,
+so a device plugin must run in a privileged security context. 
+If a device plugin is running as a DaemonSet, `/var/lib/kubelet/device-plugins`
+must be mounted as a
+[Volume](/docs/api-reference/{{page.version}}/#volume-v1-core)
+in the plugin's
+[PodSpec](/docs/api-reference/{{paage.version}}/#podspec-v1-core).
+
+## Examples
+
+For an example device plugin implementation, see
+[nvidia GPU device plugin for COS base OS](https://github.com/GoogleCloudPlatform/container-engine-accelerators/tree/master/nvidia_gpu).
+
+{% endcapture %}
+
+{% include templates/concept.md %}