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Persistence

All the volumes we've worked with so far have been ephemeral, meaning when the associated pod is deleted the volume is deleted as well. This is fine for some use cases, but for most CRUD apps we want to persist data even if the pod is deleted.

If you think about it, it's not even just when pods are explicitly deleted with kubectl that we need to worry about data loss. Pods can be deleted for several reasons:

The node they're running on could fail
A new version of the image was published (code was updated, etc)
A new node was added to the cluster and the pod was rescheduled

In all of these cases, we want to make sure that our data is still available. Persistent volumes allow us to do this.

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 with a few properties that can be read from and written to by multiple pods at the same time. 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

Now recreate the PVC. Dynamic provisioning is awesome! Run:

kubectl proxy

Run and submit the CLI tests.