DevOps Classroom Series – 11/Jul/2020 (Evening)

Kubernetes Contd..

Storage of Applications

• Classic Datacenter (On-Premise)
  • Generally application data is stored on Network storage or some databases
  • Storage is treated separtely in datacenters
• With Container into picture
  • With containers into play our applications will be running inside containers or pods
  • The data that is generated by application will be alive as long as pod is alive, once pod is deleted data will also be deleted.
  • Since this is not ok as we loose critical data, K8s/docker containers also need storage options, But storage options are same when it comes to vm’s or containers
  • Since Pod(s) can run literally anywhere (on-premise, hybrid, cloud) we have many storage options
  • K8s had to hide the implementation details of storage like aws ebs, azure disk, google persistent disk, nfs etc..
  • K8s gives us the following major options for storage
    • Volumes
    • Persistent Volumes
      • Persistent Volume Claims
      • Storage Classes
    • CSI Standard

k8s Volumes

• K8s Pods contain containers. If the containers are crashed, kubelet will restart container but files of the container will be lost.
• Docker already has concept of Volumes
• A kubernetes volume has an explicit lifetime same as the Pod that encloses
• K8s volume can be used so that data is not lost while Pod is running. Once the Pod is deleted, k8s volumes also will be deleted.
• K8s supports several types of volumes
  • awsElasticBlockStore
  • azureDisk
  • azurefile
  • configMap
  • csi
  • emptyDir
  • gcePersistentDisk
  • hostPath
  • nfs
  • and many more
• K8s volume can preserve the data with respect to failures in the containers inside Pod
• K8s volume is also just like any other k8s object

K8s Persistent Volumes

• k8s Persistent Volumes can create volumes outside of lifecycle of Pod. Even if the Pod is deleted data can be retained.

• When a storage needs to be provided as volume to the docker container running inside Pod the following phases happen

  • Manually create a storage (aws ebs, azure disk, local drive storage, nfs etc)
  • Make it available to k8s
  • Mounting the storage into Pods

• Static Provisioning

  • Manually create storage (administrator)
  • Create a Persistent Volume to make this storage space available to k8s cluster
  • Create a PersistentVolumeClaim from the Pod to gain access to storage

• Dynamic Provisioning

  • Dynamic Provisioning can be done by using Storage classes
  • You just need to create Persistent Volume Claim

• Storage Class Provides a way for administrators to describe the storage which has to be offered to k8s cluster

• A PersistentVolumeClaim (PVC) is a request for storage by a user. Pods can request specific levels of resources (CPU and Memory), in the same way Claims can request specific sizes and access modes

• K8s supports 3 Access modes

  • ReadWriteOnce
  • ReadOnlyMany
  • ReadWriteMany

• All plugins might not support all the access modes Refer Here

• Types of persistent Volumes Refer Here

• What happens if the Persistent Volume Claim is deleted

  • Reclaim Policy
    • Retain
    • Recycle
    • Delete

• K8s has supported Raw Block Volume. This is supported in

  • AWSElasticBlockStore
  • AzureDisk
  • Local volume
  • GCEPersistentDisk

• Raw Block will make storage available as Raw Block device

• Lets try to create a simple mysql pod with hostPath as Volume

---
apiVersion: v1
kind: Pod
metadata:
  name: mysql-pod
  labels:
    app: db
spec:
  volumes:
    - name: "mysql-data"
      hostPath:
        path: "/home/ubuntu/mysql"
  containers:
    - name: mysql
      image: mysql:5.7
      volumeMounts:
        - mountPath: /var/lib/mysql
          name: "mysql-data"
      ports:
        - containerPort: 3306
          name: dbport
          protocol: TCP
      env:
        - name: MYSQL_DATABASE
          value: 'test'
        - name: MYSQL_USER
          value: 'directdevops'
        - name: MYSQL_PASSWORD
          value: 'directdevops'
        - name: MYSQL_ROOT_PASSWORD
          value: 'password'

• This spec will help in creating a volume in the node where mysqlpod is running. SO if the node is down we might loose the storage, so we need better options than this.

• When we run k8s by installing manually using kubeadm or kops or kube-spray, we need to manage master, network addons (weavenet, loadbalancers), storage plugins (storage classes, storage plugins) and also identity.

• K8s is also offered as a service by cloud providers like aws, azure and google. If we use these services

  • master nodes is managed by cloud providers
  • networking driver, loadbalancers, ingress etc are already supported in k8s cluster
  • Storage classes and suitable plugins for storage are already made available
  • You can use existing cloud identity and cloud based IAM for k8s identity

• AWS has k8s as a service by a product line called as EKS (Elastic Kubernetes Services) and Azure has a Service called as AKS (Azure Kubernetes Service)

AKS (Azure Kuberenetes Services)

• Master node highly available & is free and is managed by Azure
• You will paying only for other nodes which your create.
• Networking plugin azure-cni
• Loadbalancer, Ingress support with Azure Loadbalancer and Azure application gateways
• Storage classes are provided by default, you can use AzureDisk or Azure file in Persistent Volumes
• Other Azure Service integrations like Azure DNS, Azure network policy, Azure DDOS protections can be added
• Lets create an AKS cluster using docs over here
• Once your cluster is up, you can use k8s from any where where azure cli/azure powershell is installed
• Once you login into cloud shell execute the following command

az aks get-credentials --resource-group <rgname> --name <k8scluster>
az aks get-credentials --resource-group k8slearning --name myAKScluster
kubectl get nodes
kubectl get sc

• Now lets create a Persistent Volume claim for a dummy pod. API Reference Click Here
• Lets create a persistent volume claim

---
apiVersion: v1
kind: PersistentVolumeClaim
metadata:
  name: azure-managed-disk
spec:
  accessModes:
    - ReadWriteOnce
  storageClassName: managed-premium
  resources:
    requests:
      storage: 1Gi

• Lets create a nginx pod with Persistent volume claim

---
apiVersion: v1
kind: Pod
metadata:
  name: nginx-pod
  labels:
    app: nginx
spec:
  volumes:
    - name: myvolume
      persistentVolumeClaim:
        claimName: azure-managed-disk
  containers:
    - name: mysql
      image: nginx
      volumeMounts:
        - mountPath: /mnt/azure
          name: myvolume
      ports:
        - containerPort: 80
          name: appport
          protocol: TCP
      

• Now apply both the yaml files
• So lets add a basic svc using Loadbalancer

---
apiVersion: v1
kind: Service
metadata:
  name: dummy-svc
spec:
  type: LoadBalancer
  ports:
    - port: 80
  selector:
    app: nginx

• Resources in azure resource groups

EKS (Elastic Kubernetes Services)

• Master node is highly available and is charage 0.2$ per hour
• You will pay for other nodes hourly
• Networking plugin aws-vpc-cni
• Loadbalancer and ingress support is available using Elastic Load Balancer and application load balancer
• Storage classes are provided by default and you can create aws elastic block storage and elastic file system in Persistent volumes
• Other AWS Service integrations are possible.
• Exercise: Create an eks cluster by referring over here