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20210310-opt-in-autoscaling-from-zero.md

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title authors reviewers creation-date last-updated status
Opt-in Autoscaling from Zero
@elmiko
@fabriziopandini
@sbueringer
@marcelmue
@alexander-demichev
@enxebre
@mrajashree
@arunmk
@randomvariable
@joelspeed
2021-03-10
2023-01-31
implementable

Opt-in Autoscaling from Zero

Table of Contents

Glossary

  • Node Group This term has special meaning within the cluster autoscaler, it refers to collections of nodes, and related physical hardware, that are organized within the autoscaler for scaling operations. These node groups do not have a direct relation to specific CRDs within Kubernetes, and may be handled differently by each autoscaler cloud implementation. In the case of Cluster API, node groups correspond directly to MachineSets and MachineDeployments that are marked for autoscaling.

Refer to the Cluster API Book Glossary.

Summary

The Kubernetes cluster autoscaler currently supports scaling on Cluster API deployed clusters for MachineSets and MachineDeployments. One feature that is missing from this integration is the ability to scale down to, and up from, a MachineSet or MachineDeployment with zero replicas.

This proposal describes opt-in mechanisms whereby Cluster API users and infrastructure providers can define the specific resource requirements for each Infrastructure Machine Template they create. In situations where there are zero nodes in the node group, and thus the autoscaler does not have information about the nodes, the resource requests are utilized to predict the number of nodes needed. This information is only used by the autoscaler when it is scaling from zero.

Motivation

Allowing the cluster autoscaler to scale down its node groups to zero replicas is a common feature implemented for many of the integrated infrastructure providers. It is a popular feature that has been requested for Cluster API on multiple occasions. This feature empowers users to reduce their operational resource needs, and likewise reduce their operating costs.

Given that Cluster API is an abstraction point that provides access to multiple concrete cloud implementations, this feature might not make sense in all scenarios. To accomodate the wide range of deployment options in Cluster API, the scale to zero feature will be optional for users and infrastructure providers.

Goals

  • Provide capability for Cluster API MachineSets and MachineDeployments to be auto scaled from and to zero replicas.
  • Create an optional API contract in the Infrastructure Machine Template that allows infrastructure providers to specify instance resource requirements that will be utilized by the cluster autoscaler.
  • Provide a mechanism for users to override the defined instance resource requirements on any given MachineSet or MachineDeployment.

Non-Goals/Future Work

  • Create an API contract that infrastructure providers must follow.
  • Create an API that replicates Taint and Label information from Machines to MachineSets and MachineDeployments.
  • Support for MachinePools, either with the cluster autoscaler or using infrastructure provider native implementations (eg AWS AutoScalingGroups).
  • Create an autoscaling custom resource for Cluster API.

Proposal

To facilitate scaling from zero replicas, the minimal information needed by the cluster autoscaler is the CPU and memory resources for nodes within the target node group that will be scaled. The autoscaler uses this information to create a prediction about how many nodes should be created when scaling. In most situations this information can be directly read from the nodes that are running within a node group. But, during a scale from zero situation (ie when a node group has zero replicas) the autoscaler needs to acquire this information from the infrastructure provider.

An optional status field is proposed on the Infrastructure Machine Template which will be populated by infrastructure providers to contain the CPU, memory, and GPU capacities for machines described by that template. The cluster autoscaler will then utilize this information by reading the appropriate infrastructure reference from the resource it is scaling (MachineSet or MachineDeployment).

A user may override the field in the associated infrastructure template by applying annotations to the MachineSet or MachineDeployment in question. This annotation mechanism also provides users an opportunity to utilize scaling from zero even in situations where the infrastructure provider has not given the information in the Infrastructure Machine Template. In these cases the autoscaler will evaluate the annotation information in favor of reading the information from the status.

User Stories

Story 1

As a cloud administrator, I would like to reduce my operating costs by scaling down my workload cluster when they are not in use. Using the cluster autoscaler with a minimum size of zero for a MachineSet or MachineDeployment will allow me to automate the scale down actions for my clusters.

Story 2

As an application developer, I would like to have special resource nodes (eg GPU enabled) provided when needed by workloads without the need for human intervention. As these nodes might be more expensive, I would also like to return them when not in use. By using the cluster autoscaler with a zero-sized MachineSet or MachineDeployment, I can automate the creation of nodes that will not consume resources until they are required by applications on my cluster.

Story 3

As a cluster operator, I would like to have access to the scale from zero feature but my infrastructure provider has not yet implemented the status field updates. By using annotations on my MachineSets or MachineDeployments, I can utilize this feature until my infrastructure provider has completed updating their Cluster API implementation.

Implementation Details/Notes/Constraints

There are 2 methods described for informing the cluster autoscaler about the resource needs of the nodes in each node group: through a status field on Infrastructure Machine Templates, and through annotations on MachineSets or MachineDeployments. The first method requires updates to a infrastructure provider's controllers and will require more coordination between developers and users. The second method requires less direct intervention from infrastructure providers and puts more resposibility on users, for this additional responsibility the users gain immediate access to the feature. These methods are mutually exclusive, and the annotations will take preference when specified.

It is worth noting that the implmentation definitions for the annotations will be owned and maintained by the cluster autoscaler. They will not be defined within the cluster-api project. The reasoning for this is to establish the API contract with the cluster autoscaler and not the cluster-api.

Infrastructure Machine Template Status Updates

Infrastructure providers should add a field to the status of any Infrastructure Machine Template they reconcile. This field will contain the CPU, memory, and GPU resources associated with the machine described by the template. Internally, this field will be represented by a Go map type utilizing named constants for the keys and k8s.io/apimachinery/pkg/api/resource.Quantity as the values (similar to how resource limits and requests are handled for pods).

It is worth mentioning that the Infrastructure Machine Templates are not usually reconciled by themselves. Each infrastructure provider will be responsible for determining the best implementation for adding the status field based on the information available on their platform.

Example implementation in Docker provider

// these constants will be carried in Cluster API, but are repeated here for clarity
const (
    AutoscalerResourceCPU corev1.ResourceName = "cpu"
    AutoscalerResourceMemory corev1.ResourceName = "memory"
)

// DockerMachineTemplateStatus defines the observed state of a DockerMachineTemplate
type DockerMachineTemplateStatus struct {
    Capacity corev1.ResourceList `json:"capacity,omitempty"`
}

// DockerMachineTemplate is the Schema for the dockermachinetemplates API.
type DockerMachineTemplate struct {
    metav1.TypeMeta   `json:",inline"`
    metav1.ObjectMeta `json:"metadata,omitempty"`

    Spec DockerMachineTemplateSpec     `json:"spec,omitempty"`
    Status DockerMachineTemplateStatus `json:"status,omitempty"`
}

Note: the ResourceList and ResourceName referenced are from k8s.io/api/core/v1`

When used as a manifest, it would look like this:

apiVersion: infrastructure.cluster.x-k8s.io/v1alpha4
kind: DockerMachineTemplate
metadata:
  name: workload-md-0
  namespace: default
spec:
  template:
    spec: {}
status:
  capacity:
    memory: 500mb
    cpu: "1"
    nvidia.com/gpu: "1"

MachineSet and MachineDeployment Annotations

In cases where a user needs to provide specific resource information for a MachineSet or MachineDeployment, or in cases where an infrastructure provider has not yet added the Infrastructure Machine Template status changes, they may use annotations to provide the information. The annotation values match the API that is used in the Infrastructure Machine Template, e.g. the memory and cpu annotations allow a resource.Quantity value and the two gpu annotations allow for the count and type of GPUs per instance.

If a user wishes to specify the resource capacity through annotations, they may do by adding the following to any MachineSet or MachineDeployment (it is not required on both) that are participating in autoscaling:

kind: <MachineSet or MachineDeployment>
metadata:
  annotations:
      capacity.cluster-autoscaler.kubernetes.io/gpu-count: "1"
      capacity.cluster-autoscaler.kubernetes.io/gpu-type: "nvidia.com/gpu"
      capacity.cluster-autoscaler.kubernetes.io/memory: "500mb"
      capacity.cluster-autoscaler.kubernetes.io/cpu: "1"
      capacity.cluster-autoscaler.kubernetes.io/ephemeral-disk: "100Gi"

Note: the annotations will be defined in the cluster autoscaler, not in cluster-api.

Node Labels and Taints

When a user would like to signal that the node being created from a MachineSet or MachineDeployment will have specific taints or labels on it, they can use the following annotations to specify that information.

kind: <MachineSet or MachineDeployment>
metadata:
  annotations:
    capacity.cluster-autoscaler.kubernetes.io/labels: "key1=value1,key2=value2"
    capacity.cluster-autoscaler.kubernetes.io/taints: "key1=value1:NoSchedule,key2=value2:NoExecute"

Security Model

This feature will require the service account associated with the cluster autoscaler to have the ability to get and list the Cluster API machine template infrastructure objects.

Beyond the permissions change, there should be no impact on the security model over current cluster autoscaler usages.

Risks and Mitigations

One risk for this process is that infrastructure providers will need to figure out when and where to reconcile the Infrastructure Machine Templates. This is not something that is done currently and there will need to be some thought and design work to make this accessible for all providers.

Another risk is that the annotation mechanism is not the best user experience. Users will need to manage these annotations themselves, and it will require some upkeep with respect to the infrastructure resources that are deployed. This risk is relatively minor as users will already be managing the general cluster autoscaler annotations.

Creating clear documentation about the flow information, and the action of the cluster autoscaler will be the first line of mitigating the confusion around this process. Additionally, adding examples in the Docker provider and the cluster autoscaler will help to clarify the usage.

Alternatives

An alternative approach would be to reconcile the information from the machine templates into the MachineSet and MachineDeployment statuses. This would make the permissions and implementation on the cluster autoscaler lighter. The trade off for making things easier on the cluster autoscaler is that the process of exposing this information becomes more convoluted and the Cluster API controllers will need to synchronize this data.

A much larger alternative would be to create a new custom resource that would act as an autoscaling abstraction. This new resource would be accessed by both the cluster autoscaler and the Cluster API controllers, as well as potentially another operator to own its lifecycle. This approach would provide the cleanest separation between the components, and allow for future features in a contained environment. The downside is that this approach requires the most engineering and design work to accomplish.

Upgrade Strategy

As this field is optional, it should not negatively affect upgrades. That said, care should be taken to ensure that this field is copied during any object upgrade as its absence will create unexpected behavior for end users.

In general, it should be safe for users to run the cluster autoscaler while performing an upgrade, but this should be tested more and documented clearly in the autoscaler and Cluster API references.

Additional Details

Test Plan

The cluster autoscaler tests for Cluster API integration do not currently exist outside of the downstream testing done by Red Hat on the OpenShift platform. There have talks over the last year to improve this situation, but it is slow moving currently.

The end goal for testing is to contribute the scale from zero tests that currently exist for OpenShift to the wider Kubernetes community. This will not be possible until the testing infrastructure around the cluster autoscaler and Cluster API have resolved more.

Implementation History