A `PersistentVolume` (PV) is a storage resource provisioned by an administrator. It is analogous to a node. See [Persistent Volume Guide](../user-guide/persistent-volumes/) for how to use it.
`PersistentVolumeClaimBinder` is a singleton running in master that watches all PersistentVolumeClaims in the system and binds them to the closest matching available PersistentVolume. The volume manager watches the API for newly created volumes to manage.
`PersistentVolumeClaimVolumeSource` references the user's PVC in the same namespace. This volume finds the bound PV and mounts that volume for the pod. A `PersistentVolumeClaimVolumeSource` is, essentially, a wrapper around another type of volume that is owned by someone else (the system).
Cluster administrators use the API to manage *PersistentVolumes*. A custom store `NewPersistentVolumeOrderedIndex` will index volumes by access modes and sort by storage capacity. The `PersistentVolumeClaimBinder` watches for new claims for storage and binds them to an available volume by matching the volume's characteristics (AccessModes and storage size) to the user's request.
Kubernetes users request persistent storage for their pod by creating a ```PersistentVolumeClaim```. Their request for storage is described by their requirements for resources and mount capabilities.
| CREATE | POST | /api/{version}/namespaces/{ns}/persistentvolumeclaims/ | Create instance of PersistentVolumeClaim in namespace {ns} |
| GET | GET | /api/{version}/namespaces/{ns}/persistentvolumeclaims/{name} | Get instance of PersistentVolumeClaim in namespace {ns} with {name} |
| UPDATE | PUT | /api/{version}/namespaces/{ns}/persistentvolumeclaims/{name} | Update instance of PersistentVolumeClaim in namespace {ns} with {name} |
| DELETE | DELETE | /api/{version}/namespaces/{ns}/persistentvolumeclaims/{name} | Delete instance of PersistentVolumeClaim in namespace {ns} with {name} |
| LIST | GET | /api/{version}/namespaces/{ns}/persistentvolumeclaims | List instances of PersistentVolumeClaim in namespace {ns} |
| WATCH | GET | /api/{version}/watch/namespaces/{ns}/persistentvolumeclaims | Watch for changes to PersistentVolumeClaim in namespace {ns} |
Scheduling constraints are to be handled similar to pod resource constraints. Pods will need to be annotated or decorated with the number of resources it requires on a node. Similarly, a node will need to list how many it has used or available.
The implementation of persistent storage will not require events to communicate to the user the state of their claim. The CLI for bound claims contains a reference to the backing persistent volume. This is always present in the API and CLI, making an event to communicate the same unnecessary.
Events that communicate the state of a mounted volume are left to the volume plugins.
An administrator provisions storage by posting PVs to the API. Various way to automate this task can be scripted. Dynamic provisioning is a future feature that can maintain levels of PVs.
A user requests storage by posting a PVC to the API. Their request contains the AccessModes they wish their volume to have and the minimum size needed.
The user must be within a namespace to create PVCs.
The ```PersistentVolumeClaimBinder``` attempts to find an available volume that most closely matches the user's request. If one exists, they are bound by putting a reference on the PV to the PVC. Requests can go unfulfilled if a suitable match is not found.
The claim holder can use their claim as a volume. The ```PersistentVolumeClaimVolumeSource``` knows to fetch the PV backing the claim and mount its volume for a pod.
The claim holder owns the claim and its data for as long as the claim exists. The pod using the claim can be deleted, but the claim remains in the user's namespace. It can be used again and again by many pods.
