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How to Share Data Using a Local Kubernetes Cluster

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how to share data using a local kubernetes cluster, using a custom PersistentVolume.

I will use the clk extension to bootstrap easily a kind cluster.

clk k8s --distribution kind flow

A hardcoded behavior of clk makes the folder /kind mounted on /shared in the kind docker container.

Then, say that you are debugging a deployment that contain a PersistentVolumeClaim that is named my-claim in the default namespace.

The PersistentVolume to create is simply.

apiVersion: v1
kind: PersistentVolume
metadata:
  name: shared-volume
spec:
  accessModes:
  - ReadWriteOnce
  claimRef:
    name: my-claim
    namespace: default
  capacity:
    storage: 1Gi
  hostPath:
    path: /shared/myvolume/
    type: DirectoryOrCreate
  nodeAffinity:
    required:
      nodeSelectorTerms:
      - matchExpressions:
        - key: kubernetes.io/hostname
          operator: In
          values:
          - clk-k8s-control-plane
  persistentVolumeReclaimPolicy: Delete
  storageClassName: local-path
  volumeMode: Filesystem

The nodeAffinity is not strictly needed as in the standard k3d cluster, you have only one node, but it might help remembering that you put this persistent volume explicitly on this node.

kubectl get pvc my-claim
NAME STATUS VOLUME CAPACITY ACCESS MODES STORAGECLASS VOLUMEATTRIBUTESCLASS AGE
my-claim Bound shared-volume 1Gi RWO <unset> 56s

bonus: create the PersistentVolumeClaim

This is a simple as that.

apiVersion: v1
kind: PersistentVolumeClaim
metadata:
  name: my-claim
spec:
  storageClassName: ""
  volumeName: shared-volume
  resources:
    requests:
      storage: 100Mi
  accessModes:
    - ReadWriteOnce

Let’s schedule a pod in it.

    apiVersion: v1
    kind: Pod
    metadata:
      name: testpod
      namespace: default
      labels:
        app.kubernetes.io/name: testpod
    spec:
      volumes:
        - name: my-volume
          persistentVolumeClaim:
            claimName: my-claim
      containers:
        - image: nginx:latest
          imagePullPolicy: IfNotPresent
          name: maincontainer
          ports:
            - containerPort: 80
              name: http
          livenessProbe:

httpGet:

              path: /
              port: 80
            initialDelaySeconds: 3
            periodSeconds: 5
          readinessProbe:

httpGet:

              path: /
              port: 80
            initialDelaySeconds: 3
            periodSeconds: 5
          volumeMounts:
            - name: my-volume
              mountPath: /usr/share/nginx/html/init

And check that the pod actually sees the content available on the host.

kubectl exec pod/testpod -- touch /usr/share/nginx/html/init/foo
ssh colima ls /kind/myvolume

foo

bonus2: Using that path without using PV and PVC

I can actually access that directory directly from a pod without needing a much work.

    apiVersion: v1
    kind: Pod
    metadata:
      name: testpod
      namespace: default
      labels:
        app.kubernetes.io/name: testpod
    spec:
      volumes:
        - name: shared-volume
          hostPath:
            path: /shared/myothervolume
      containers:
        - image: nginx:latest
          imagePullPolicy: IfNotPresent
          name: maincontainer
          ports:
            - containerPort: 80
              name: http
          livenessProbe:

httpGet:

              path: /
              port: 80
            initialDelaySeconds: 3
            periodSeconds: 5
          readinessProbe:

httpGet:

              path: /
              port: 80
            initialDelaySeconds: 3
            periodSeconds: 5
          volumeMounts:
            - name: shared-volume
              mountPath: /usr/share/nginx/html/init

kubectl exec pod/testpod -- touch /usr/share/nginx/html/init/bar
ssh colima ls /kind/myothervolume

bar

Notes linking here