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README.md

emptyDirClone CSI plugin

This document introduces the emptyDirClone Container Storage Interface (CSI) plugin, aiming to simulate the behavior of a Kubernetes emptyDir for educational purposes.

CSI was developed as a standard for exposing arbitrary block and file storage storage systems to containerized workloads on Container Orchestration Systems (COs) like Kubernetes. With the adoption of the Container Storage Interface, the Kubernetes volume layer becomes truly extensible. The advantage of using CSI is that external storage providers can create and deploy plugins to introduce new storage systems in Kubernetes without having to modify the core Kubernetes code directly.

For more information, you can refer to the resources provided in the Required Reading and Optional Reading sections. The CSI specification itself is included in the Optional Reading section. Keep in mind that this guide is designed to be beginner-friendly and serve as a quick start for CSI plugins. The additional tutorials and resources listed in the Required Reading section are considered sufficient for understanding the topic.

We will call the plugin emptyDirClone.

Quickstart

See the instructions for running locally.

Understanding emptyDir Volumes

emptyDir volumes are temporary directories exposed to the pod. These do not persist beyond the lifetime of a pod. This is implemented by Kubernetes itself(as opposed to a separate plugin).

apiVersion: v1
kind: Pod
metadata:
  name: test-pd
spec:
  containers:
  - image: registry.k8s.io/test-webserver
    name: test-container
    volumeMounts:
    - mountPath: /cache
      name: cache-volume
  volumes:
  - name: cache-volume
    emptyDir:
      sizeLimit: 500Mi

Goals of the emptyDirClone CSI Plugin

  • Provide a temporary directory
  • Provide ReadWriteOnce capability. See Access Modes
  • Support ephemeral inline volume requests
  • It should have a invocation similar to the following 
    kind: Pod
apiVersion: v1
metadata:
  name: my-csi-app
spec:
  containers:
    - name: my-frontend
      image: busybox:1.28
      volumeMounts:
      - mountPath: "/data"                    # Mount path for the volume
        name: my-csi-inline-vol
      command: [ "sleep", "1000000" ]
  volumes:
    - name: my-csi-inline-vol
      csi:
        driver: inline.storage.kubernetes.io  # Name of the CSI driver
        volumeAttributes:
          foo: bar                            # Attributes passed to the driver to configure the volume

Implementation steps

  • Implement the CSI Identity service.

    service Identity {
  rpc GetPluginInfo(GetPluginInfoRequest)
    returns (GetPluginInfoResponse) {}

  rpc GetPluginCapabilities(GetPluginCapabilitiesRequest)
    returns (GetPluginCapabilitiesResponse) {}

  rpc Probe (ProbeRequest)
    returns (ProbeResponse) {}
}

  • Implement the CSI Node service. For ephemeral inline volume request, we only need the NodePublishVolume(read mount volume), NodeUnpublishVolume(read unmount volume)

    stateDiagram-v2
    direction TB
    [*] --> Published: NodePublishVolume
    Published --> Unpublished: NodeUnpublishVolume
    Unpublished--> [*]

    service Node {
  rpc NodeStageVolume (NodeStageVolumeRequest)
    returns (NodeStageVolumeResponse) {}

  rpc NodeUnstageVolume (NodeUnstageVolumeRequest)
    returns (NodeUnstageVolumeResponse) {}

  rpc NodePublishVolume (NodePublishVolumeRequest)
    returns (NodePublishVolumeResponse) {}

  rpc NodeUnpublishVolume (NodeUnpublishVolumeRequest)
    returns (NodeUnpublishVolumeResponse) {}

  rpc NodeGetVolumeStats (NodeGetVolumeStatsRequest)
    returns (NodeGetVolumeStatsResponse) {}


  rpc NodeExpandVolume(NodeExpandVolumeRequest)
    returns (NodeExpandVolumeResponse) {}


  rpc NodeGetCapabilities (NodeGetCapabilitiesRequest)
    returns (NodeGetCapabilitiesResponse) {}

  rpc NodeGetInfo (NodeGetInfoRequest)
    returns (NodeGetInfoResponse) {}
}

Reading/References

Required

Tools/Examples

Optional