website/content/en/docs/tasks/run-application/run-single-instance-stateful-application.md

---
title: Run a Single-Instance Stateful Application
content_type: tutorial
weight: 20
---

<!-- overview -->

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" >}}




<!-- lessoncontent -->

## 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.

{{< codenew file="application/mysql/mysql-deployment.yaml" >}}
{{< codenew file="application/mysql/mysql-pv.yaml" >}}

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

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

1. Deploy the contents of the YAML file:

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

1. 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

1. 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

1. Inspect the PersistentVolumeClaim:

        kubectl describe pvc mysql-pv-claim

        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](/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:

```
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/)