--- title: Run a Single-Instance Stateful Application content_type: tutorial weight: 20 --- This page shows you how to run a single-instance stateful application in Kubernetes using a PersistentVolume and a Deployment. The application is MySQL. ## {{% heading "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. ## {{% heading "prerequisites" %}} - {{< include "task-tutorial-prereqs.md" >}} {{< version-check >}} - {{< include "default-storage-class-prereqs.md" >}} ## 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](/docs/concepts/configuration/secret/) for a secure solution. {{% code_sample file="application/mysql/mysql-deployment.yaml" %}} {{% code_sample file="application/mysql/mysql-pv.yaml" %}} 1. Deploy the PV and PVC of the YAML file: ```shell kubectl apply -f https://k8s.io/examples/application/mysql/mysql-pv.yaml ``` 1. Deploy the contents of the YAML file: ```shell kubectl apply -f https://k8s.io/examples/application/mysql/mysql-deployment.yaml ``` 1. Display information about the Deployment: ```shell 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:9 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: 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 ``` 1. List the pods created by the Deployment: ```shell 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 ``` 1. Inspect the PersistentVolumeClaim: ```shell 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: Annotations: pv.kubernetes.io/bind-completed=yes pv.kubernetes.io/bound-by-controller=yes Capacity: 20Gi Access Modes: RWO 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: ```shell kubectl run -it --rm --image=mysql:9 --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](/docs/concepts/workloads/controllers/statefulset/). - 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: ```shell 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. ## {{% heading "whatsnext" %}} - Learn more about [Deployment objects](/docs/concepts/workloads/controllers/deployment/). - Learn more about [Deploying applications](/docs/tasks/run-application/run-stateless-application-deployment/) - [kubectl run documentation](/docs/reference/generated/kubectl/kubectl-commands/#run) - [Volumes](/docs/concepts/storage/volumes/) and [Persistent Volumes](/docs/concepts/storage/persistent-volumes/)