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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

Set up a disk in your environment

You can use any type of persistent volume for your stateful app. See Types of Persistent Volumes for a list of supported environment disks. For Google Compute Engine, run:

gcloud compute disks create --size=20GB mysql-disk

Next create a PersistentVolume that points to the mysql-disk disk just created. Here is a configuration file for a PersistentVolume that points to the Compute Engine disk above:

gce-volume.yaml
 
apiVersion: v1
kind: PersistentVolume
metadata:
  name: mysql-pv
spec:
  capacity:
    storage: 20Gi
  accessModes:
    - ReadWriteOnce
  gcePersistentDisk:
    pdName: mysql-disk
    fsType: ext4

Notice that the pdName: mysql-disk line matches the name of the disk in the Compute Engine environment. See the Persistent Volumes for details on writing a PersistentVolume configuration file for other environments.

Create the persistent volume:

kubectl create -f http://k8s.io/docs/tutorials/stateful-application/gce-volume.yaml

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 volume that meets the requirements, in this case, the volume created above.

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: extensions/v1beta1
kind: Deployment
metadata:
  name: mysql
spec:
  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 http://k8s.io/docs/tutorials/stateful-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
 Selector:		app=mysql
 Replicas:		1 updated | 1 total | 0 available | 1 unavailable
 StrategyType:		Recreate
 MinReadySeconds:	0
 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 Persistent Volume:

     kubectl describe pv mysql-pv

 Name:		mysql-pv
 Labels:		<none>
 Status:		Bound
 Claim:		default/mysql-pv-claim
 Reclaim Policy:	Retain
 Access Modes:	RWO
 Capacity:	20Gi
 Message:	
 Source:
     Type:	GCEPersistentDisk (a Persistent Disk resource in Google Compute Engine)
     PDName:	mysql-disk
     FSType:	ext4
     Partition:	0
     ReadOnly:	false
 No events.

  5. Inspect the PersistentVolumeClaim:

     kubectl describe pvc mysql-pv-claim

 Name:		mysql-pv-claim
 Namespace:	default
 Status:		Bound
 Volume:		mysql-pv
 Labels:		<none>
 Capacity:	20Gi
 Access Modes:	RWO
 No events.

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 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
kubectl delete pv mysql-pv

Also, if you are using Compute Engine disks:

gcloud compute disks delete mysql-disk

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