Kubernetes-4-Storage.md

Kubernetes 101 - Storage

1 - Volumes

Volumes & PVC (official docs)

Kubernetes supports multiple storage types. The most common storage type is a volume. Volumes depend on external storage addons and are provisioned via a Persistent Volume Claim (PVC)

apiVersion: v1
kind: PersistentVolumeClaim
metadata:
  name: task-pv-claim
spec:
  storageClassName: cinder-csi
  accessModes:
    - ReadWriteOnce
  resources:
    requests:
      storage: 1Gi

You can use the PVC like this:

apiVersion: apps/v1
kind: Deployment
metadata:
  name: ubuntu-deployment
  labels:
    app: ubuntu
spec:
  replicas: 1
  selector:
    matchLabels:
      app: ubuntu
  template:
    metadata:
      labels:
        app: ubuntu
    spec:
      containers:
      - name: ubuntu
        image: debian
        command: [ "sleep" ]
        args: [ "infinity" ]
        volumeMounts:
        - mountPath: /mnt
          name: storage
      volumes:
      - name: storage
        persistentVolumeClaim:
          claimName: task-pv-claim

The infinity sleep is needed to keep the container alive (similar to docker). A succeeding pod in a deployment is considered a failure and results in rescheduling and crashloops.

2 - Secrets / Configmaps

Configmaps (official docs) / Secrets (official docs)

Example for a secret:

apiVersion: v1
kind: Secret
metadata:
  name: mysecret
type: Opaque
data:
  username: YWRtaW4=
  password: MWYyZDFlMmU2N2Rm

Example for a configmap:

apiVersion: v1
kind: ConfigMap
metadata:
  name: game-demo
data:
  # property-like keys; each key maps to a simple value
  player_initial_lives: "3"
  ui_properties_file_name: "user-interface.properties"

  # file-like keys
  game.properties: |
    enemy.types=aliens,monsters
    player.maximum-lives=5    
  user-interface.properties: |
    color.good=purple
    color.bad=yellow
    allow.textmode=true

3 - Secrets / Configmaps as ENV-vars

Official docs section

Secrets and configmaps can be used to directly change the environment variables of a container. This is very useful when configuring containers, almost all configurable containers have the option to be configured via environment variables.

metadata:
  name: nginx-deployment
  labels:
    app: nginx
spec:
  replicas: 3
  selector:
    matchLabels:
      app: nginx
  template:
    metadata:
      labels:
        app: nginx
    spec:
      containers:
      - name: nginx
        image: nginx:1.7.9
        ports:
        - containerPort: 80
        env:
          - name: SomeName
            value: SomeValue
          - name: SECRET_USERNAME
            valueFrom:
              secretKeyRef:
                name: mysecret
                key: username
          - name: SECRET_PASSWORD
            valueFrom:
              secretKeyRef:
                name: mysecret
                key: password

Create a pod with the above YAML, connect to it interactively and check that the environment variables SomeName, SECRET_USERNAME and SECRET_PASSWORD are set.

4 - Secrets / Configmaps as volumes/files

Official docs section

Configmaps and secrets can not only be used to populate environment variables, they can also be used as volume mounts directly. This can be used to overwrite existing files inside a container which is especially useful for small configuration files.

Caveat: Please be aware that secrets and configmaps (and in fact all kubernetes resources) are stored in etcd which imposes a size limit of 1MB that cannot be exceeded.

A simple example for a configmap based volume-mount can be found here:

apiVersion: v1
kind: Pod
metadata:
  name: dapi-test-pod
spec:
  containers:
    - name: test-container
      image: busybox
      command: [ "/bin/sh", "-c", "ls /etc/config/" ]
      volumeMounts:
      - name: config-volume
        mountPath: /etc/config
  volumes:
    - name: config-volume
      configMap:
        # Provide the name of the ConfigMap containing the files you want
        # to add to the container
        name: special-config
  restartPolicy: Never