Persistent Volumes (PV)#

Instead of simply adding a volume to a deployment, a persistent volume is a cluster-level resource that is created separately from the pod and then attached to the pod. It’s similar to a ConfigMap in that way.

PVs can be created statically or dynamically.

• Static PVs are created manually by a cluster admin
• Dynamic PVs are created automatically when a pod requests a volume that doesn’t exist yet

Generally speaking, and especially in the cloud-native world, we want to use dynamic PVs. It’s less work and more flexible.

Persistent Volume Claims (PVC)#

A persistent volume claim is a request for a persistent volume. When using dynamic provisioning, a PVC will automatically create a PV if one doesn’t exist that matches the claim.

The PVC is then attached to a pod, just like a volume would be.

Assignment#

Create a new file called api-pvc.yaml and add the following:

apiVersion: v1
kind: PersistentVolumeClaim
metadata:
  name: xxx
spec:
  accessModes:
    - xxx
  resources:
    requests:
      storage: xxx

Add the following properties:

• metadata/name: synergychat-api-pvc
• spec/accessModes: An array with one entry:
  • ReadWriteOnce
• spec/resources/requests/storage: 1Gi

This creates a new PVC called synergychat-api-pvc. ReadWriteOnce means the volume is mounted read-write by a single node at a time - multiple pods on that same node can share it, but pods on other nodes cannot. (The full access-mode story - ReadOnlyMany, ReadWriteMany - lives in storage-classes.) It also requests 1GB of storage.

Apply the PVC.

Run both of these commands:

kubectl get pvc
kubectl get pv

You should see that a new PV was created automatically!

Now delete the PVC:

kubectl delete pvc <pvc-name>

Make sure both the PVC and PV are gone:

kubectl get pvc
kubectl get pv

Attach Persistence#

So far all we’ve done is create an empty persistent volume. Let’s get the api application to use it.

Assignment#

Use your crawler deployment and the docs as a reference. Create a new volume in the api-deployment referencing your pvc:

volumes:
  - name: synergychat-api-volume
    persistentVolumeClaim:
      claimName: synergychat-api-pvc

Then mount it in the container under the /persist directory:

volumeMounts:
  - name: synergychat-api-volume
    mountPath: /persist

Update the API_DB_FILEPATH environment variable you added earlier to instead use the new mount path: /persist/db.json

Apply the changes, then check to make sure all your pods are healthy:

kubectl get pods

With your tunnel running (minikube tunnel -c), open http://synchat.internal/ in your browser.

1. Send some messages.
2. Delete the api pod.
3. Once the new pod is running, refresh the page and make sure your messages are still there. If they are, your persistent volume is working!

api-deployment.yaml

---
apiVersion: apps/v1
kind: Deployment
metadata:
  name: synergychat-api
  labels:
    app: synergychat-api
spec:
  replicas: 1
  selector:
    matchLabels:
      app: synergychat-api
  template:
    metadata:
      labels:
        app: synergychat-api
    spec:
      containers:
        - name: synergychat-api
          image: bootdotdev/synergychat-api:latest
          volumeMounts:
            - name: synergychat-api-volume
              mountPath: /persist
          env:
            - name: API_PORT
              valueFrom:
                configMapKeyRef:
                  name: synergychat-api-configmap
                  key: API_PORT
            - name: API_DB_FILEPATH
              valueFrom:
                configMapKeyRef:
                  name: synergychat-api-configmap
                  key: API_DB_FILEPATH
      volumes:
        - name: synergychat-api-volume
          persistentVolumeClaim:
            claimName: synergychat-api-pvc
---

api-configmap.yaml

---
apiVersion: v1
kind: ConfigMap
metadata:
  name: synergychat-api-configmap
data:
  API_PORT: "8080"
  API_DB_FILEPATH: /persist/db.json
---

for more storage-classes