apiVersion: v1
kind: PersistentVolumeClaim
metadata:
  name: mysql-pv-claim
spec: 
  accessModes:
    - ReadWriteOnce
  storageClassName: standard-rwo
  resources: 
    requests:
      storage: 4Gi

# NEED FOR PVC
# 1. Dynamic volume provisioning allows storage volumes to be created 
# on-demand. 

# 2. Without dynamic provisioning, cluster administrators have to manually 
# make calls to their cloud or storage provider to create new storage 
# volumes, and then create PersistentVolume objects to represent them in k8s

# 3. The dynamic provisioning feature eliminates the need for cluster 
# administrators to pre-provision storage. Instead, it automatically 
# provisions storage when it is requested by users.

# 4. PVC: Users request dynamically provisioned storage by including 
# a storage class in their PersistentVolumeClaim

apiVersion: v1
kind: ConfigMap
metadata:
  name: usermanagement-dbcreation-script
data: 
  mysql_usermgmt.sql: |-
    DROP DATABASE IF EXISTS webappdb;
    CREATE DATABASE webappdb; 

apiVersion: apps/v1
kind: Deployment
metadata:
  name: mysql
spec: 
  replicas: 1
  selector:
    matchLabels:
      app: mysql
  strategy:
    type: Recreate # terminates all the pods and replaces them with the new version.
  template: 
    metadata: 
      labels: 
        app: mysql
    spec: 
      containers:
        - name: mysql
          image: mysql:8.0
          env:
            - name: MYSQL_ROOT_PASSWORD
              value: dbpassword11
          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: mysql-pv-claim
        - name: usermanagement-dbcreation-script
          configMap:
            name: usermanagement-dbcreation-script


# VERY IMPORTANT POINTS ABOUT CONTAINERS AND POD VOLUMES: 
## 1. On-disk files in a container are ephemeral
## 2. One problem is the loss of files when a container crashes. 
## 3. Kubernetes Volumes solves above two as these volumes are configured to POD and not container. 
## Only they can be mounted in Container
## 4. Using Compute Enginer Persistent Disk CSI Driver is a super generalized approach 
## for having Persistent Volumes for workloads in Kubernetes


## ENVIRONMENT VARIABLES
# 1. When you create a Pod, you can set environment variables for the 
# containers that run in the Pod. 
# 2. To set environment variables, include the env or envFrom field in 
# the configuration file.


## DEPLOYMENT STRATEGIES
# 1. Rolling deployment: This strategy  replaces pods running the old version of the application with the new version, one by one, without downtime to the cluster.
# 2. Recreate: This strategy terminates all the pods and replaces them with the new version.
# 3. Ramped slow rollout: This strategy  rolls out replicas of the new version, while in parallel, shutting down old replicas. 
# 4. Best-effort controlled rollout: This strategy  specifies a “max unavailable” parameter which indicates what percentage of existing pods can be unavailable during the upgrade, enabling the rollout to happen much more quickly.
# 5. Canary Deployment: This strategy  uses a progressive delivery approach, with one version of the application serving maximum users, and another, newer version serving a small set of test users. The test deployment is rolled out to more users if it is successful.

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    

apiVersion: apps/v1
kind: Deployment 
metadata:
  name: usermgmt-webapp
  labels:
    app: usermgmt-webapp
spec:
  replicas: 1
  selector:
    matchLabels:
      app: usermgmt-webapp
  template:  
    metadata:
      labels: 
        app: usermgmt-webapp
    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-webapp
          image: stacksimplify/kube-usermgmt-webapp:1.0.0-MySQLDB
          imagePullPolicy: Always
          ports: 
            - containerPort: 8080           
          env:
            - name: DB_HOSTNAME
              value: "mysql"            
            - name: DB_PORT
              value: "3306"            
            - name: DB_NAME
              value: "webappdb"            
            - name: DB_USERNAME
              value: "root"            
            - name: DB_PASSWORD
              value: "dbpassword11"            

apiVersion: v1
kind: Service
metadata:
  name: usermgmt-webapp-lb-service
  labels: 
    app: usermgmt-webapp
spec: 
  type: LoadBalancer
  selector: 
    app: usermgmt-webapp
  ports: 
    - port: 80 # Service Port
      targetPort: 8080 # Container Port