Dynamic Resource Allocation

Dynamic resource allocation is an API for requesting and sharing resources between pods and containers inside a pod. It is a generalization of the persistent volumes API for generic resources. Typically those resources are devices like GPUs.

Third-party resource drivers are responsible for tracking and preparing resources, with allocation of resources handled by Kubernetes via structured parameters (introduced in Kubernetes 1.30). Different kinds of resources support arbitrary parameters for defining requirements and initialization.

Kubernetes v1.26 through to 1.31 included an (alpha) implementation of classic DRA, which is no longer supported. This documentation, which is for Kubernetes v1.32, explains the current approach to dynamic resource allocation within Kubernetes.

Before you begin

Kubernetes v1.32 includes cluster-level API support for dynamic resource allocation, but it needs to be enabled explicitly. You also must install a resource driver for specific resources that are meant to be managed using this API. If you are not running Kubernetes v1.32, check the documentation for that version of Kubernetes.

API

The resource.k8s.io/v1beta1 API group provides these types:

ResourceClaim

Describes a request for access to resources in the cluster, for use by workloads. For example, if a workload needs an accelerator device with specific properties, this is how that request is expressed. The status stanza tracks whether this claim has been satisfied and what specific resources have been allocated.

ResourceClaimTemplate

Defines the spec and some metadata for creating ResourceClaims. Created by a user when deploying a workload. The per-Pod ResourceClaims are then created and removed by Kubernetes automatically.

DeviceClass

Contains pre-defined selection criteria for certain devices and configuration for them. DeviceClasses are created by a cluster administrator when installing a resource driver. Each request to allocate a device in a ResourceClaim must reference exactly one DeviceClass.

ResourceSlice

Used by DRA drivers to publish information about resources that are available in the cluster.

All parameters that select devices are defined in the ResourceClaim and DeviceClass with in-tree types. Configuration parameters can be embedded there. Which configuration parameters are valid depends on the DRA driver -- Kubernetes only passes them through without interpreting them.

The core/v1 PodSpec defines ResourceClaims that are needed for a Pod in a resourceClaims field. Entries in that list reference either a ResourceClaim or a ResourceClaimTemplate. When referencing a ResourceClaim, all Pods using this PodSpec (for example, inside a Deployment or StatefulSet) share the same ResourceClaim instance. When referencing a ResourceClaimTemplate, each Pod gets its own instance.

The resources.claims list for container resources defines whether a container gets access to these resource instances, which makes it possible to share resources between one or more containers.

Here is an example for a fictional resource driver. Two ResourceClaim objects will get created for this Pod and each container gets access to one of them.

apiVersion: resource.k8s.io/v1beta1
kind: DeviceClass
name: resource.example.com
spec:
  selectors:
  - cel:
      expression: device.driver == "resource-driver.example.com"
---
apiVersion: resource.k8s.io/v1beta1
kind: ResourceClaimTemplate
metadata:
  name: large-black-cat-claim-template
spec:
  spec:
    devices:
      requests:
      - name: req-0
        deviceClassName: resource.example.com
        selectors:
        - cel:
           expression: |-
              device.attributes["resource-driver.example.com"].color == "black" &&
              device.attributes["resource-driver.example.com"].size == "large"              
–--
apiVersion: v1
kind: Pod
metadata:
  name: pod-with-cats
spec:
  containers:
  - name: container0
    image: ubuntu:20.04
    command: ["sleep", "9999"]
    resources:
      claims:
      - name: cat-0
  - name: container1
    image: ubuntu:20.04
    command: ["sleep", "9999"]
    resources:
      claims:
      - name: cat-1
  resourceClaims:
  - name: cat-0
    resourceClaimTemplateName: large-black-cat-claim-template
  - name: cat-1
    resourceClaimTemplateName: large-black-cat-claim-template

Scheduling

The scheduler is responsible for allocating resources to a ResourceClaim whenever a pod needs them. It does so by retrieving the full list of available resources from ResourceSlice objects, tracking which of those resources have already been allocated to existing ResourceClaims, and then selecting from those resources that remain.

The only kind of supported resources at the moment are devices. A device instance has a name and several attributes and capacities. Devices get selected through CEL expressions which check those attributes and capacities. In addition, the set of selected devices also can be restricted to sets which meet certain constraints.

The chosen resource is recorded in the ResourceClaim status together with any vendor-specific configuration, so when a pod is about to start on a node, the resource driver on the node has all the information it needs to prepare the resource.

By using structured parameters, the scheduler is able to reach a decision without communicating with any DRA resource drivers. It is also able to schedule multiple pods quickly by keeping information about ResourceClaim allocations in memory and writing this information to the ResourceClaim objects in the background while concurrently binding the pod to a node.

Monitoring resources

The kubelet provides a gRPC service to enable discovery of dynamic resources of running Pods. For more information on the gRPC endpoints, see the resource allocation reporting.

Pre-scheduled Pods

When you - or another API client - create a Pod with spec.nodeName already set, the scheduler gets bypassed. If some ResourceClaim needed by that Pod does not exist yet, is not allocated or not reserved for the Pod, then the kubelet will fail to run the Pod and re-check periodically because those requirements might still get fulfilled later.

Such a situation can also arise when support for dynamic resource allocation was not enabled in the scheduler at the time when the Pod got scheduled (version skew, configuration, feature gate, etc.). kube-controller-manager detects this and tries to make the Pod runnable by reserving the required ResourceClaims. However, this only works if those were allocated by the scheduler for some other pod.

It is better to avoid bypassing the scheduler because a Pod that is assigned to a node blocks normal resources (RAM, CPU) that then cannot be used for other Pods while the Pod is stuck. To make a Pod run on a specific node while still going through the normal scheduling flow, create the Pod with a node selector that exactly matches the desired node:

apiVersion: v1
kind: Pod
metadata:
  name: pod-with-cats
spec:
  nodeSelector:
    kubernetes.io/hostname: name-of-the-intended-node
  ...

You may also be able to mutate the incoming Pod, at admission time, to unset the .spec.nodeName field and to use a node selector instead.

Admin access

You can mark a request in a ResourceClaim or ResourceClaimTemplate as having privileged features. A request with admin access grants access to devices which are in use and may enable additional permissions when making the device available in a container:

apiVersion: resource.k8s.io/v1beta1
kind: ResourceClaimTemplate
metadata:
  name: large-black-cat-claim-template
spec:
  spec:
    devices:
      requests:
      - name: req-0
        deviceClassName: resource.example.com
        adminAccess: true

If this feature is disabled, the adminAccess field will be removed automatically when creating such a ResourceClaim.

Admin access is a privileged mode which should not be made available to normal users in a multi-tenant cluster. Cluster administrators can restrict usage of this feature by installing a validating admission policy similar to the following example. Cluster administrators need to adapt at least the names and replace "dra.example.com".

# Permission to use admin access is granted only in namespaces which have the
# "admin-access.dra.example.com" label. Other ways of making that decision are
# also possible.

apiVersion: admissionregistration.k8s.io/v1
kind: ValidatingAdmissionPolicy
metadata:
  name: resourceclaim-policy.dra.example.com
spec:
  failurePolicy: Fail
  matchConstraints:
    resourceRules:
    - apiGroups:   ["resource.k8s.io"]
      apiVersions: ["v1alpha3", "v1beta1"]
      operations:  ["CREATE", "UPDATE"]
      resources:   ["resourceclaims"]
  validations:
    - expression: '! object.spec.devices.requests.exists(e, has(e.adminAccess) && e.adminAccess)'
      reason: Forbidden
      messageExpression: '"admin access to devices not enabled"'
---
apiVersion: admissionregistration.k8s.io/v1
kind: ValidatingAdmissionPolicyBinding
metadata:
  name: resourceclaim-binding.dra.example.com
spec:
  policyName:  resourceclaim-policy.dra.example.com
  validationActions: [Deny]
  matchResources:
    namespaceSelector:
      matchExpressions:
      - key: admin-access.dra.example.com
        operator: DoesNotExist
---
apiVersion: admissionregistration.k8s.io/v1
kind: ValidatingAdmissionPolicy
metadata:
  name: resourceclaimtemplate-policy.dra.example.com
spec:
  failurePolicy: Fail
  matchConstraints:
    resourceRules:
    - apiGroups:   ["resource.k8s.io"]
      apiVersions: ["v1alpha3", "v1beta1"]
      operations:  ["CREATE", "UPDATE"]
      resources:   ["resourceclaimtemplates"]
  validations:
    - expression: '! object.spec.spec.devices.requests.exists(e, has(e.adminAccess) && e.adminAccess)'
      reason: Forbidden
      messageExpression: '"admin access to devices not enabled"'
---
apiVersion: admissionregistration.k8s.io/v1
kind: ValidatingAdmissionPolicyBinding
metadata:
  name: resourceclaimtemplate-binding.dra.example.com
spec:
  policyName:  resourceclaimtemplate-policy.dra.example.com
  validationActions: [Deny]
  matchResources:
    namespaceSelector:
      matchExpressions:
      - key: admin-access.dra.example.com
        operator: DoesNotExist

ResourceClaim Device Status

The drivers can report driver-specific device status data for each allocated device in a resource claim. For example, IPs assigned to a network interface device can be reported in the ResourceClaim status.

The drivers setting the status, the accuracy of the information depends on the implementation of those DRA Drivers. Therefore, the reported status of the device may not always reflect the real time changes of the state of the device.

When the feature is disabled, that field automatically gets cleared when storing the ResourceClaim.

A ResourceClaim device status is supported when it is possible, from a DRA driver, to update an existing ResourceClaim where the status.devices field is set.

Enabling dynamic resource allocation

Dynamic resource allocation is a beta feature which is off by default and only enabled when the DynamicResourceAllocation feature gate and the resource.k8s.io/v1beta1 API group are enabled. For details on that, see the --feature-gates and --runtime-config kube-apiserver parameters. kube-scheduler, kube-controller-manager and kubelet also need the feature gate.

When a resource driver reports the status of the devices, then the DRAResourceClaimDeviceStatus feature gate has to be enabled in addition to DynamicResourceAllocation.

A quick check whether a Kubernetes cluster supports the feature is to list DeviceClass objects with:

kubectl get deviceclasses

If your cluster supports dynamic resource allocation, the response is either a list of DeviceClass objects or:

No resources found

If not supported, this error is printed instead:

error: the server doesn't have a resource type "deviceclasses"

The default configuration of kube-scheduler enables the "DynamicResources" plugin if and only if the feature gate is enabled and when using the v1 configuration API. Custom configurations may have to be modified to include it.

In addition to enabling the feature in the cluster, a resource driver also has to be installed. Please refer to the driver's documentation for details.

Enabling admin access

Admin access is an alpha feature and only enabled when the DRAAdminAccess feature gate is enabled in the kube-apiserver and kube-scheduler.

Enabling Device Status

ResourceClaim Device Status is an alpha feature and only enabled when the DRAResourceClaimDeviceStatus feature gate is enabled in the kube-apiserver.

What's next

• For more information on the design, see the Dynamic Resource Allocation with Structured Parameters KEP.