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

  • Create a PersistentVolume referencing a disk in your environment.
  • Create a MySQL Deployment.
  • Expose MySQL to other pods in the cluster at a known DNS name.

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.

application/mysql/mysql-deployment.yaml 
apiVersion: v1
kind: Service
metadata:
  name: mysql
spec:
  ports:
  - port: 3306
  selector:
    app: mysql
  clusterIP: None
---
apiVersion: apps/v1
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
application/mysql/mysql-pv.yaml 
apiVersion: v1
kind: PersistentVolume
metadata:
  name: mysql-pv-volume
  labels:
    type: local
spec:
  storageClassName: manual
  capacity:
    storage: 20Gi
  accessModes:
    - ReadWriteOnce
  hostPath:
    path: "/mnt/data"
---
apiVersion: v1
kind: PersistentVolumeClaim
metadata:
  name: mysql-pv-claim
spec:
  storageClassName: manual
  accessModes:
    - ReadWriteOnce
  resources:
    requests:
      storage: 20Gi

  1. Deploy the PV and PVC of the YAML file:

     kubectl apply -f https://k8s.io/examples/application/mysql/mysql-pv.yaml

  2. Deploy the contents of the YAML file:

     kubectl apply -f https://k8s.io/examples/application/mysql/mysql-deployment.yaml

  3. Display information about the Deployment:

     kubectl describe deployment mysql

    The output is similar to this:

     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

  4. List the pods created by the Deployment:

     kubectl get pods -l app=mysql

    The output is similar to this:

     NAME                   READY     STATUS    RESTARTS   AGE
 mysql-63082529-2z3ki   1/1       Running   0          3m

  5. Inspect the PersistentVolumeClaim:

     kubectl describe pvc mysql-pv-claim

    The output is similar to this:

     Name:         mysql-pv-claim
 Namespace:    default
 StorageClass:
 Status:       Bound
 Volume:       mysql-pv-volume
 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:

  • Don't scale the app. This setup is for single-instance apps only. The underlying PersistentVolume can only be mounted to one Pod. For clustered stateful apps, see the StatefulSet documentation.
  • Use strategy: type: Recreate in the Deployment configuration YAML file. This instructs Kubernetes to not use rolling updates. Rolling updates will not work, as you cannot have more than one Pod running at a time. The Recreate strategy will stop the first pod before creating a new one with the updated configuration.

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-volume

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.

What's next

Last modified February 11, 2021 at 6:29 PM PST : style: separate commands from output (e0e68de17)