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
- Before you begin
- Deploy MySQL
- Accessing the MySQL instance
- Updating
- Deleting a deployment
- What's next
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
You need to have a Kubernetes cluster, and the kubectl command-line tool must be configured to communicate with your cluster. If you do not already have a cluster, you can create one by using Minikube, or you can use one of these Kubernetes playgrounds:
To check the version, enter kubectl version
.
You need to either have a dynamic PersistentVolume provisioner with a default StorageClass, or statically provision PersistentVolumes yourself to satisfy the PersistentVolumeClaims used here.
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
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application/mysql/mysql-pv.yaml
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Deploy the PV and PVC of the YAML file:
kubectl apply -f https://k8s.io/examples/application/mysql/mysql-pv.yaml
Deploy the contents of the YAML file:
kubectl apply -f https://k8s.io/examples/application/mysql/mysql-deployment.yaml
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
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
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.
- 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. TheRecreate
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
Learn more about Deployment objects.
Learn more about Deploying applications
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