202 lines
6.7 KiB
Markdown
202 lines
6.7 KiB
Markdown
---
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title: Run a Single-Instance Stateful Application
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content_type: tutorial
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weight: 20
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---
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<!-- overview -->
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This page shows you how to run a single-instance stateful application
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in Kubernetes using a PersistentVolume and a Deployment. The
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application is MySQL.
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## {{% heading "objectives" %}}
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* Create a PersistentVolume referencing a disk in your environment.
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* Create a MySQL Deployment.
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* Expose MySQL to other pods in the cluster at a known DNS name.
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## {{% heading "prerequisites" %}}
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* {{< include "task-tutorial-prereqs.md" >}} {{< version-check >}}
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* {{< include "default-storage-class-prereqs.md" >}}
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<!-- lessoncontent -->
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## Deploy MySQL
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You can run a stateful application by creating a Kubernetes Deployment
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and connecting it to an existing PersistentVolume using a
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PersistentVolumeClaim. For example, this YAML file describes a
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Deployment that runs MySQL and references the PersistentVolumeClaim. The file
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defines a volume mount for /var/lib/mysql, and then creates a
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PersistentVolumeClaim that looks for a 20G volume. This claim is
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satisfied by any existing volume that meets the requirements,
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or by a dynamic provisioner.
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Note: The password is defined in the config yaml, and this is insecure. See
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[Kubernetes Secrets](/docs/concepts/configuration/secret/)
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for a secure solution.
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{{< codenew file="application/mysql/mysql-deployment.yaml" >}}
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{{< codenew file="application/mysql/mysql-pv.yaml" >}}
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1. Deploy the PV and PVC of the YAML file:
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kubectl apply -f https://k8s.io/examples/application/mysql/mysql-pv.yaml
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1. Deploy the contents of the YAML file:
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kubectl apply -f https://k8s.io/examples/application/mysql/mysql-deployment.yaml
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1. Display information about the Deployment:
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kubectl describe deployment mysql
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Name: mysql
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Namespace: default
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CreationTimestamp: Tue, 01 Nov 2016 11:18:45 -0700
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Labels: app=mysql
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Annotations: deployment.kubernetes.io/revision=1
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Selector: app=mysql
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Replicas: 1 desired | 1 updated | 1 total | 0 available | 1 unavailable
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StrategyType: Recreate
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MinReadySeconds: 0
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Pod Template:
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Labels: app=mysql
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Containers:
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mysql:
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Image: mysql:5.6
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Port: 3306/TCP
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Environment:
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MYSQL_ROOT_PASSWORD: password
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Mounts:
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/var/lib/mysql from mysql-persistent-storage (rw)
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Volumes:
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mysql-persistent-storage:
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Type: PersistentVolumeClaim (a reference to a PersistentVolumeClaim in the same namespace)
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ClaimName: mysql-pv-claim
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ReadOnly: false
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Conditions:
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Type Status Reason
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---- ------ ------
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Available False MinimumReplicasUnavailable
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Progressing True ReplicaSetUpdated
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OldReplicaSets: <none>
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NewReplicaSet: mysql-63082529 (1/1 replicas created)
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Events:
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FirstSeen LastSeen Count From SubobjectPath Type Reason Message
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--------- -------- ----- ---- ------------- -------- ------ -------
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33s 33s 1 {deployment-controller } Normal ScalingReplicaSet Scaled up replica set mysql-63082529 to 1
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1. List the pods created by the Deployment:
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kubectl get pods -l app=mysql
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NAME READY STATUS RESTARTS AGE
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mysql-63082529-2z3ki 1/1 Running 0 3m
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1. Inspect the PersistentVolumeClaim:
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kubectl describe pvc mysql-pv-claim
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Name: mysql-pv-claim
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Namespace: default
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StorageClass:
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Status: Bound
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Volume: mysql-pv-volume
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Labels: <none>
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Annotations: pv.kubernetes.io/bind-completed=yes
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pv.kubernetes.io/bound-by-controller=yes
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Capacity: 20Gi
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Access Modes: RWO
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Events: <none>
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## Accessing the MySQL instance
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The preceding YAML file creates a service that
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allows other Pods in the cluster to access the database. The Service option
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`clusterIP: None` lets the Service DNS name resolve directly to the
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Pod's IP address. This is optimal when you have only one Pod
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behind a Service and you don't intend to increase the number of Pods.
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Run a MySQL client to connect to the server:
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```
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kubectl run -it --rm --image=mysql:5.6 --restart=Never mysql-client -- mysql -h mysql -ppassword
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```
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This command creates a new Pod in the cluster running a MySQL client
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and connects it to the server through the Service. If it connects, you
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know your stateful MySQL database is up and running.
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```
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Waiting for pod default/mysql-client-274442439-zyp6i to be running, status is Pending, pod ready: false
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If you don't see a command prompt, try pressing enter.
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mysql>
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```
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## Updating
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The image or any other part of the Deployment can be updated as usual
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with the `kubectl apply` command. Here are some precautions that are
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specific to stateful apps:
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* Don't scale the app. This setup is for single-instance apps
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only. The underlying PersistentVolume can only be mounted to one
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Pod. For clustered stateful apps, see the
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[StatefulSet documentation](/docs/concepts/workloads/controllers/statefulset/).
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* Use `strategy:` `type: Recreate` in the Deployment configuration
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YAML file. This instructs Kubernetes to _not_ use rolling
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updates. Rolling updates will not work, as you cannot have more than
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one Pod running at a time. The `Recreate` strategy will stop the
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first pod before creating a new one with the updated configuration.
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## Deleting a deployment
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Delete the deployed objects by name:
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```
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kubectl delete deployment,svc mysql
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kubectl delete pvc mysql-pv-claim
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kubectl delete pv mysql-pv-volume
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```
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If you manually provisioned a PersistentVolume, you also need to manually
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delete it, as well as release the underlying resource.
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If you used a dynamic provisioner, it automatically deletes the
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PersistentVolume when it sees that you deleted the PersistentVolumeClaim.
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Some dynamic provisioners (such as those for EBS and PD) also release the
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underlying resource upon deleting the PersistentVolume.
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## {{% heading "whatsnext" %}}
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* Learn more about [Deployment objects](/docs/concepts/workloads/controllers/deployment/).
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* Learn more about [Deploying applications](/docs/tasks/run-application/run-stateless-application-deployment/)
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* [kubectl run documentation](/docs/reference/generated/kubectl/kubectl-commands/#run)
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* [Volumes](/docs/concepts/storage/volumes/) and [Persistent Volumes](/docs/concepts/storage/persistent-volumes/)
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