Use Cases: Install applications on Kubernetes with Ondat
Ondat can be used to provide permanent storage for other applications running in Kubernetes or other Orchestrators that are derived from Kubernetes such as OpenShift or Rancher. This is useful for running stateful applications, such as databases or CI/CD applications, under the control of Kubernetes as Kubernetes can make scheduling decisions without the application data being lost.
What we have outlined in the cookbooks below are some quick deployments of stateful applications into a Kubernetes cluster. These examples are not production ready but have been provided to give you some insight into how to use Ondat with stateful applications.
The examples we have provided use StatefulSets as a way to deploy applications into Kubernetes. The reason for this is that the StatefulSet controller is designed to manage stateful applications and it “provides guarantees about the ordering and uniqueness” of sets of pods.
Practically this means that when a StatefulSet scales, pods are created in order from 0-(N-1), if a StatefulSet scales down then pods are deleted in reverse order from (N-1)-0.
Secondly, it means that the behaviour of the StatefulSet controller differs from that of Deployment and ReplicaSet controllers. For Deployment and ReplicaSet controllers “… at many points in the lifetime of a replica set there will be 2 copies of a pod’s processes running concurrently”. Having two different pods mount a volume at the same time can cause corruption of data. Currently Kubernetes accessModes only apply restrictions to nodes mounting volumes rather than pods, so it is important that StatefulSets are used with Ondat volumes so the necessary pod uniqueness guarantees are maintained.
The Ondat specific part of the Kubernetes manifests for these examples lies in the VolumeClaimTemplate that’s part of the statefulset definition.
apiVersion: apps/v1 kind: StatefulSet metadata: name: mssql spec: selector: matchLabels: app: mssql env: prod serviceName: mssql replicas: 1 template: metadata: labels: app: mssql env: prod spec: serviceAccountName: mssql containers: - name: foo image: bar volumeMounts: - name: baz mountPath: /var/opt/bar envFrom: - configMapRef: name: mssql volumeClaimTemplates: - metadata: name: baz labels: env: prod spec: accessModes: ["ReadWriteOnce"] storageClassName: "fast" # Ondat storageClass resources: requests: storage: 5Gi
In the StatefulSet definition above the container has a volume mount defined called baz. The definition for this volume is found in the VolumeClaimTemplate where the fast storageClass will be used to dynamically provision storage if the persistent volume does not already exist.