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Volume Mounter trait

The volume mounter trait is responsible for attaching a Kubernetes PersistentVolume Claim (PVC) to a component.

Installation

None. The volume mounter trait has no external dependencies.

Supported workload types

  • core.oam.dev/v1alpha1.Server
  • core.oam.dev/v1alpha1.SingletonServer
  • core.oam.dev/v1alpha1.Task
  • core.oam.dev/v1alpha1.SingletonTask
  • core.oam.dev/v1alpha1.Worker
  • core.oam.dev/v1alpha1.SingletonWorker
  • harmonycloud.cn/v1alpha1.MysqlCluster

Properties

Name Description Allowable values Required Default
volumeName The name of the volume this backs. string. Matches the volume name declared in ComponentSchematic. ☑
storageClass The storage class that a PVC requires. string. According to the available StorageClasses(s) (kubectl get storageclass) in your cluster and/or default ☑

Usage

This is usage of how to attach a storage volume to your container:

apiVersion: core.oam.dev/v1alpha1
kind: ComponentSchematic
metadata:
  name: server-with-volume
spec:
  workloadType: core.oam.dev/v1alpha1.Server
  containers:
    - name: server
      image: nginx:latest
      resources:
        volumes:
          - name: myvol
            mountPath: /myvol
            disk:
              required: "50M"
              ephemeral: true

In the component schematic volumes section, one volume is specified. It must be at least 50M in size. It is ephemeral, which means that the component author does not expect the data to persist if the pod is destroyed.

Sometimes, components need to persist data. In such cases, the ephemeral flag should be set to false:

apiVersion: core.oam.dev/v1alpha1
kind: ComponentSchematic
metadata:
  name: server-with-volume
spec:
  workloadType: core.oam.dev/v1alpha1.Server
  containers:
    - name: server
      image: nginx:latest
      resources:
        volumes:
          - name: myvol
            mountPath: /myvol
            disk:
              required: "50M"
              ephemeral: false

In the Kubernetes implementation of OAM, a Persistent Volume Claim (PVC) is used to satisfy the non-ephemeral case. However. A trait must be applied that will indicate how the PVC is created:

apiVersion: core.oam.dev/v1alpha1
kind: ApplicationConfiguration
metadata:
  name: example-server-with-volume
spec:
  components:
    - componentName: server-with-volume-v1
      instanceName: example-server-with-volume
      traits:
        - name: volume-mounter
          properties:
            volumename: myvol
            storageClass: default

The volume-mounter trait ensures that a PVC is created with the given name (myvol) using the given storage class (default). Typically, the volumeName should match the resources.volumes[].name field from the ComponentSchematic. Thus myvol above will match the volume declared in the volumes section of server-with-volume-v1.

When this request is processed by oam-controller, it will first create the Kubernetes PVC named myvol and then create a Kubernetes pod that attaches that PVC as a volumeMount.

Attaching PVCs to Pods may take extra time, as the underlying system must first provision storage.

Example

$ kubectl get storageclass
NAME                       PROVISIONER                                       AGE
default                    nfs-client-provisioner-default                    124d
$
$ kubectl apply -f component-schematics.yaml 
componentschematic.core.oam.dev/server-with-volume created
$ kubectl apply -f application-configurations.yaml 
applicationconfiguration.core.oam.dev/example-server-with-volume created
$
$ kubectl get pvc,deploy,pod
NAME                          STATUS   VOLUME                                     CAPACITY   ACCESS MODES   STORAGECLASS   AGE
persistentvolumeclaim/myvol   Bound    pvc-0966159f-6202-11ea-81d4-005056b71aad   50M        RWO            default        55s

NAME                                               DESIRED   CURRENT   UP-TO-DATE   AVAILABLE   AGE
deployment.extensions/example-server-with-volume   1         1         1            1           55s

NAME                                              READY   STATUS   RESTARTS   AGE
pod/example-server-with-volume-57c488c79b-259rb   1/1     Running    0          55s
$ kubectl get pv pvc-0966159f-6202-11ea-81d4-005056b71aad
NAME                                       CAPACITY   ACCESS MODES   RECLAIM POLICY   STATUS   CLAIM            STORAGECLASS   REASON   AGE
pvc-0966159f-6202-11ea-81d4-005056b71aad   50M        RWO            Delete           Bound    oam-test/myvol   default                 55s