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Step-by-step instructions for performing operations with Kubernetes.

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Run a Single-Instance Stateful Application

This page shows you how to run a single-instance stateful application in Kubernetes using a PersistentVolume and a Deployment. The application is MySQL.

Objectives

Before you begin

Deploy MySQL

You can run a stateful application by creating a Kubernetes Deployment and connecting it to an existing PersistentVolume using a PersistentVolumeClaim. For example, this YAML file describes a Deployment that runs MySQL and references the PersistentVolumeClaim. The file defines a volume mount for /var/lib/mysql, and then creates a PersistentVolumeClaim that looks for a 20G volume. This claim is satisfied by any existing volume that meets the requirements, or by a dynamic provisioner.

Note: The password is defined in the config yaml, and this is insecure. See Kubernetes Secrets for a secure solution.

mysql-deployment.yaml
apiVersion: v1
kind: Service
metadata:
  name: mysql
spec:
  ports:
  - port: 3306
  selector:
    app: mysql
  clusterIP: None
---
apiVersion: v1
kind: PersistentVolumeClaim
metadata:
  name: mysql-pv-claim
spec:
  accessModes:
    - ReadWriteOnce
  resources:
    requests:
      storage: 20Gi
---
apiVersion: apps/v1beta2
kind: Deployment
metadata:
  name: mysql
spec:
  selector:
    matchLabels:
      app: mysql
  strategy:
    type: Recreate
  template:
    metadata:
      labels:
        app: mysql
    spec:
      containers:
      - image: mysql:5.6
        name: mysql
        env:
          # Use secret in real usage
        - name: MYSQL_ROOT_PASSWORD
          value: password
        ports:
        - containerPort: 3306
          name: mysql
        volumeMounts:
        - name: mysql-persistent-storage
          mountPath: /var/lib/mysql
      volumes:
      - name: mysql-persistent-storage
        persistentVolumeClaim:
          claimName: mysql-pv-claim
  1. Deploy the contents of the YAML file:

    kubectl create -f https://k8s.io/docs/tasks/run-application/mysql-deployment.yaml
    
  2. Display information about the Deployment:

    kubectl describe deployment mysql
    
     Name:                 mysql
     Namespace:            default
     CreationTimestamp:    Tue, 01 Nov 2016 11:18:45 -0700
     Labels:               app=mysql
     Annotations:          deployment.kubernetes.io/revision=1
     Selector:             app=mysql
     Replicas:             1 desired | 1 updated | 1 total | 0 available | 1 unavailable
     StrategyType:         Recreate
     MinReadySeconds:      0
     Pod Template:
       Labels:       app=mysql
       Containers:
        mysql:
         Image:      mysql:5.6
         Port:       3306/TCP
         Environment:
           MYSQL_ROOT_PASSWORD:      password
         Mounts:
           /var/lib/mysql from mysql-persistent-storage (rw)
       Volumes:
        mysql-persistent-storage:
         Type:       PersistentVolumeClaim (a reference to a PersistentVolumeClaim in the same namespace)
         ClaimName:  mysql-pv-claim
         ReadOnly:   false
     Conditions:
       Type          Status  Reason
       ----          ------  ------
       Available     False   MinimumReplicasUnavailable
       Progressing   True    ReplicaSetUpdated
     OldReplicaSets:       <none>
     NewReplicaSet:        mysql-63082529 (1/1 replicas created)
     Events:
       FirstSeen    LastSeen    Count    From                SubobjectPath    Type        Reason            Message
       ---------    --------    -----    ----                -------------    --------    ------            -------
       33s          33s         1        {deployment-controller }             Normal      ScalingReplicaSet Scaled up replica set mysql-63082529 to 1
    
  3. List the pods created by the Deployment:

    kubectl get pods -l app=mysql
    
     NAME                   READY     STATUS    RESTARTS   AGE
     mysql-63082529-2z3ki   1/1       Running   0          3m
    
  4. Inspect the PersistentVolumeClaim:

    kubectl describe pvc mysql-pv-claim
    
     Name:         mysql-pv-claim
     Namespace:    default
     StorageClass:
     Status:       Bound
     Volume:       mysql-pv
     Labels:       <none>
     Annotations:    pv.kubernetes.io/bind-completed=yes
                     pv.kubernetes.io/bound-by-controller=yes
     Capacity:     20Gi
     Access Modes: RWO
     Events:       <none>
    

Accessing the MySQL instance

The preceding YAML file creates a service that allows other Pods in the cluster to access the database. The Service option clusterIP: None lets the Service DNS name resolve directly to the Pod’s IP address. This is optimal when you have only one Pod behind a Service and you don’t intend to increase the number of Pods.

Run a MySQL client to connect to the server:

kubectl run -it --rm --image=mysql:5.6 --restart=Never mysql-client -- mysql -h mysql -ppassword

This command creates a new Pod in the cluster running a MySQL client and connects it to the server through the Service. If it connects, you know your stateful MySQL database is up and running.

Waiting for pod default/mysql-client-274442439-zyp6i to be running, status is Pending, pod ready: false
If you don't see a command prompt, try pressing enter.

mysql>

Updating

The image or any other part of the Deployment can be updated as usual with the kubectl apply command. Here are some precautions that are specific to stateful apps:

Deleting a deployment

Delete the deployed objects by name:

kubectl delete deployment,svc mysql
kubectl delete pvc mysql-pv-claim

If you manually provisioned a PersistentVolume, you also need to manually delete it, as well as release the underlying resource. If you used a dynamic provisioner, it automatically deletes the PersistentVolume when it sees that you deleted the PersistentVolumeClaim. Some dynamic provisioners (such as those for EBS and PD) also release the underlying resource upon deleting the PersistentVolume.

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