Lab 5 - Persistent Storage¶

30 min Intermediate

Problem¶

The Death Star plans can't be lost no matter what happens, so we need to store them in a database whose data survives Pod restarts.

A container's filesystem is thrown away when its Pod is deleted. To keep data, the Pod needs a PersistentVolumeClaim (PVC). On OpenShift you normally don't create PersistentVolumes by hand. The cluster's default StorageClass provisions one automatically when a PVC asks for storage.

Create a PersistentVolumeClaim¶

• The PVC is named postgresql-pvc.
• It requests 1Gi of storage.
• It uses the access mode ReadWriteOnce.
• It doesn't set a storageClassName, so the cluster's default StorageClass is used.

Create a PostgreSQL Pod that uses the claim¶

• The Pod is named postgresql-pod and has the label app: postgresql.
• It uses the image quay.io/sclorg/postgresql-16-c9s. This is the upstream build of Red Hat's PostgreSQL image, and it runs as any non-root user.
• It exposes containerPort 5432.

• It sets these environment variables:

  Name	Value
  POSTGRESQL_USER	rebel
  POSTGRESQL_PASSWORD	password
  POSTGRESQL_DATABASE	deathstar

• It mounts the PVC at /var/lib/pgsql/data.

Setup¶

oc new-project lab5
oc get storageclass

One StorageClass should be marked (default).

Verification¶

1. The PVC is Bound and the Pod is Running:

   oc get pvc,pod
   

2. Save the plans to the database:

   oc exec postgresql-pod -- psql -d deathstar -c "CREATE TABLE plans (weakness text); INSERT INTO plans VALUES ('thermal exhaust port');"
   

3. Delete the Pod, then recreate it from the same manifest:

   oc delete pod postgresql-pod
   oc apply -f postgresql-pod.yaml
   oc wait --for=condition=Ready pod/postgresql-pod --timeout=120s
   

4. The plans survived:

   oc exec postgresql-pod -- psql -d deathstar -c "SELECT * FROM plans;"
   

         weakness
   ----------------------
    thermal exhaust port
   (1 row)
   

Going further¶

• Look at the PersistentVolume that was created for you with oc get pv. You may need cluster-reader access. What is its RECLAIM POLICY, and what happens to the data when you delete the PVC?
• A bare Pod is fine for learning, but real databases run as a StatefulSet with volumeClaimTemplates. See StatefulSets.

Cleanup¶

oc delete project lab5