Shared directory for a kubernetes Deployment between it's replicas

First of all, you need to decide what type of a Persistent Volume to use. Here are several examples of an on-premise cluster:

• HostPath - local Path on a Node. Therefore, if the first Pod is located on Node1 and the second is on Node2, storages will be different. To resolve this problem, you can use one of the following options. Example of a HostPath:

  kind: PersistentVolume
  apiVersion: v1
  metadata:
    name: example-pv
    labels:
      type: local
  spec:
    storageClassName: manual
    capacity:
      storage: 3Gi
    accessModes:
      - ReadWriteOnce
    hostPath:
      path: "/mnt/data"
  

• NFS - PersistentVolume of that type uses Network File System. NFS is a distributed file system protocol that allows you to mount remote directories on your servers. You need to install NFS server before using the NFS in Kubernetes; here is the example How To Set Up an NFS Mount on Ubuntu. Example in Kubernetes:

  apiVersion: v1
  kind: PersistentVolume
  metadata:
    name: example-pv
  spec:
    capacity:
      storage: 3Gi
    volumeMode: Filesystem
    accessModes:
      - ReadWriteOnce
    persistentVolumeReclaimPolicy: Recycle
    storageClassName: slow
    mountOptions:
      - hard
      - nfsvers=4.1
    nfs:
      path: /tmp
      server: 172.17.0.2
  

• GlusterFS - GlusterFS is a scalable, distributed file system that aggregates disk storage resources from multiple servers into a single global namespace. As for the NFS, you need to install GlusterFS before using it in Kubernetes; here is the link with instructions, and one more with the sample. Example in Kubernetes:

  apiVersion: v1
  kind: PersistentVolume
  metadata:
    name: example-pv 
    annotations:
      pv.beta.kubernetes.io/gid: "590" 
  spec:
    capacity:
      storage: 3Gi 
    accessModes: 
      - ReadWriteMany
    glusterfs:
      endpoints: glusterfs-cluster 
      path: myVol1 
      readOnly: false
    persistentVolumeReclaimPolicy: Retain
  ---
  apiVersion: v1
  kind: Service
  metadata:
    name: glusterfs-cluster 
  spec:
    ports:
    - port: 1
  ---
  apiVersion: v1
  kind: Endpoints
  metadata:
    name: glusterfs-cluster 
  subsets:
    - addresses:
        - ip: 192.168.122.221 
      ports:
        - port: 1 
    - addresses:
        - ip: 192.168.122.222 
      ports:
        - port: 1 
    - addresses:
        - ip: 192.168.122.223 
      ports:
        - port: 1 
  

After creating a PersistentVolume, you need to create a PersistaentVolumeClaim. A PersistaentVolumeClaim is a resource used by Pods to request volumes from the storage. After you create the PersistentVolumeClaim, the Kubernetes control plane looks for a PersistentVolume that satisfies the claim’s requirements. Example:

kind: PersistentVolumeClaim
apiVersion: v1
metadata:
  name: example-pv-claim
spec:
  storageClassName: manual
  accessModes:
    - ReadWriteOnce
  resources:
    requests:
      storage: 3Gi

And the last step, you need to configure a Pod to use the PersistentVolumeClaim. Here is the example:

apiVersion: apps/v1
kind: Deployment
metadata:
  name: 'test-tomcat'
  labels:
    app: test-tomcat
spec:
  selector:
    matchLabels:
      app: test-tomcat
  replicas: 3
  template:
    metadata:
      name: 'test-tomcat'
      labels:
        app: test-tomcat
    spec:
      volumes:
      - name: 'data'
        persistentVolumeClaim:
          claimName: example-pv-claim #name of the claim should be the same as defined before
      containers:
      - image: 'tomcat:9-alpine'
        volumeMounts:
        - name: 'data'
          mountPath: '/app/data'
        imagePullPolicy: Always
        name: 'tomcat'
        command: ['bin/catalina.sh', 'jpda', 'run']