Kubernetes - Init Containers

Step-01: Introduction

• Init Containers run before App containers
• Init containers can contain utilities or setup scripts not present in an app image.
• We can have and run multiple Init Containers before App Container.
• Init containers are exactly like regular containers, except:
• Init containers always run to completion.
• Each init container must complete successfully before the next one starts.
• If a Pod's init container fails, Kubernetes repeatedly restarts the Pod until the init container succeeds.
• However, if the Pod has a restartPolicy of Never, Kubernetes does not restart the Pod.

Kubernetes Manifests

01

#01-storage-class.yml
apiVersion: storage.k8s.io/v1
kind: StorageClass
metadata: 
  name: ebs-sc
provisioner: ebs.csi.aws.com
volumeBindingMode: WaitForFirstConsumer 

02

#02-persistent-volume-claim.yml
apiVersion: v1
kind: PersistentVolumeClaim
metadata:
  name: ebs-mysql-pv-claim
spec: 
  accessModes:
    - ReadWriteOnce
  storageClassName: ebs-sc
  resources: 
    requests:
      storage: 4Gi

03

#03-UserManagement-ConfigMap.yml
apiVersion: v1
kind: ConfigMap
metadata:
  name: usermanagement-dbcreation-script
data: 
  mysql_usermgmt.sql: |-
    DROP DATABASE IF EXISTS usermgmt;
    CREATE DATABASE usermgmt; 

04

#04-mysql-deployment.yml
apiVersion: apps/v1
kind: Deployment
metadata:
  name: mysql
spec: 
  replicas: 1
  selector:
    matchLabels:
      app: mysql
  strategy:
    type: Recreate 
  template: 
    metadata: 
      labels: 
        app: mysql
    spec: 
      containers:
        - name: mysql
          image: mysql:5.6
          env:
            - name: MYSQL_ROOT_PASSWORD
              valueFrom:
                secretKeyRef:
                  name: mysql-db-password
                  key: db-password 
          ports:
            - containerPort: 3306
              name: mysql    
          volumeMounts:
            - name: mysql-persistent-storage
              mountPath: /var/lib/mysql    
            - name: usermanagement-dbcreation-script
              mountPath: /docker-entrypoint-initdb.d #https://hub.docker.com/_/mysql Refer Initializing a fresh instance                                            
      volumes: 
        - name: mysql-persistent-storage
          persistentVolumeClaim:
            claimName: ebs-mysql-pv-claim
        - name: usermanagement-dbcreation-script
          configMap:
            name: usermanagement-dbcreation-script

05

#05-mysql-clusterip-service.yml
apiVersion: v1
kind: Service
metadata: 
  name: mysql
spec:
  selector:
    app: mysql 
  ports: 
    - port: 3306  
  clusterIP: None # This means we are going to use Pod IP    

06

#06-UserManagementMicroservice-Deployment-Service.yml
apiVersion: apps/v1
kind: Deployment 
metadata:
  name: usermgmt-microservice
  labels:
    app: usermgmt-restapp
spec:
  replicas: 1
  selector:
    matchLabels:
      app: usermgmt-restapp
  template:  
    metadata:
      labels: 
        app: usermgmt-restapp
    spec:
      initContainers:
        - name: init-db
          image: busybox:1.31
          command: ['sh', '-c', 'echo -e "Checking for the availability of MySQL Server deployment"; while ! nc -z mysql 3306; do sleep 1; printf "-"; done; echo -e "  >> MySQL DB Server has started";']      
      containers:
        - name: usermgmt-restapp
          image: stacksimplify/kube-usermanagement-microservice:1.0.0
          ports: 
            - containerPort: 8095           
          env:
            - name: DB_HOSTNAME
              value: "mysql"            
            - name: DB_PORT
              value: "3306"            
            - name: DB_NAME
              value: "usermgmt"            
            - name: DB_USERNAME
              value: "root"            
            - name: DB_PASSWORD
              valueFrom:
                secretKeyRef:
                  name: mysql-db-password
                  key: db-password             

07

#07-UserManagement-Service.yml
apiVersion: v1
kind: Service
metadata:
  name: usermgmt-restapp-service
  labels: 
    app: usermgmt-restapp
spec:
  type: NodePort
  selector:
    app: usermgmt-restapp
  ports: 
    - port: 8095
      targetPort: 8095
      nodePort: 31231

08

#08-kubernetes-secrets.yml
apiVersion: v1
kind: Secret
metadata:
  name: mysql-db-password
type: Opaque
data: 
  db-password: ZGJwYXNzd29yZDEx

Step-02: Implement Init Containers

• Update initContainers section under Pod Template Spec which is spec.template.spec in a Deployment

    template:
      metadata:
        labels:
          app: usermgmt-restapp
      spec:
        initContainers:
          - name: init-db
            image: busybox:1.31
            command: ['sh', '-c', 'echo -e "Checking for the availability of MySQL Server deployment"; while ! nc -z mysql 3306; do sleep 1; printf "-"; done; echo -e "  >> MySQL DB Server has started";']
  

Step-03: Create & Test

# Create All Objects
kubectl apply -f kube-manifests/

# List Pods
kubectl get pods

# Watch List Pods screen
kubectl get pods -w

# Describe Pod & Discuss about init container
kubectl describe pod <usermgmt-microservice-xxxxxx>

# Access Application Health Status Page
http://<WorkerNode-Public-IP>:31231/usermgmt/health-status

AWS EKS - Elastic Kubernetes Service - Masterclass

Step-04: Clean-Up

• Delete all k8s objects created as part of this section

  # Delete All
  kubectl delete -f kube-manifests/
  
  # List Pods
  kubectl get pods
  
  # Verify sc, pvc, pv
  kubectl get sc,pvc,pv
  

References:

• https://kubernetes.io/docs/concepts/workloads/pods/init-containers/