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Horizontal Pod Autoscaler

The Horizontal Pod Autoscaler automatically scales the number of pods in a replication controller, deployment or replica set based on observed CPU utilization (or, with custom metrics support, on some other application-provided metrics). Note that Horizontal Pod Autoscaling does not apply to objects that can’t be scaled, for example, DaemonSets.

The Horizontal Pod Autoscaler is implemented as a Kubernetes API resource and a controller. The resource determines the behavior of the controller. The controller periodically adjusts the number of replicas in a replication controller or deployment to match the observed average CPU utilization to the target specified by user.

• How does the Horizontal Pod Autoscaler work?
• API Object
• Support for Horizontal Pod Autoscaler in kubectl
• Autoscaling during rolling update
• Support for cooldown/delay
• Support for multiple metrics
• Support for custom metrics
• Support for metrics APIs
• What's next

How does the Horizontal Pod Autoscaler work?

The Horizontal Pod Autoscaler is implemented as a control loop, with a period controlled by the controller manager’s --horizontal-pod-autoscaler-sync-period flag (with a default value of 30 seconds).

During each period, the controller manager queries the resource utilization against the metrics specified in each HorizontalPodAutoscaler definition. The controller manager obtains the metrics from either the resource metrics API (for per-pod resource metrics), or the custom metrics API (for all other metrics).

• For per-pod resource metrics (like CPU), the controller fetches the metrics from the resource metrics API for each pod targeted by the HorizontalPodAutoscaler. Then, if a target utilization value is set, the controller calculates the utilization value as a percentage of the equivalent resource request on the containers in each pod. If a target raw value is set, the raw metric values are used directly. The controller then takes the mean of the utilization or the raw value (depending on the type of target specified) across all targeted pods, and produces a ratio used to scale the number of desired replicas.

Please note that if some of the pod’s containers do not have the relevant resource request set, CPU utilization for the pod will not be defined and the autoscaler will not take any action for that metric. See the autoscaling algorithm design document for further details about how the autoscaling algorithm works.

• For per-pod custom metrics, the controller functions similarly to per-pod resource metrics, except that it works with raw values, not utilization values.

• For object metrics, a single metric is fetched (which describes the object in question), and compared to the target value, to produce a ratio as above.

The HorizontalPodAutoscaler normally fetches metrics from a series of aggregated APIs (metrics.k8s.io, custom.metrics.k8s.io, and external.metrics.k8s.io). The metrics.k8s.io API is usually provided by metrics-server, which needs to be launched separately. See metrics-server for instructions. The HorizontalPodAutoscaler can also fetch metrics directly from Heapster.

FEATURE STATE: Kubernetes 1.11 deprecated

This feature is deprecated. For more information on this state, see the Kubernetes Deprecation Policy.

Fetching metrics from Heapster is deprecated as of Kubernetes 1.11.

See Support for metrics APIs for more details.

The autoscaler accesses corresponding scalable controllers (such as replication controllers, deployments, and replica sets) by using the scale sub-resource. Scale is an interface that allows you to dynamically set the number of replicas and examine each of their current states. More details on scale sub-resource can be found here.

API Object

The Horizontal Pod Autoscaler is an API resource in the Kubernetes autoscaling API group. The current stable version, which only includes support for CPU autoscaling, can be found in the autoscaling/v1 API version.

The beta version, which includes support for scaling on memory and custom metrics, can be found in autoscaling/v2beta1. The new fields introduced in autoscaling/v2beta1 are preserved as annotations when working with autoscaling/v1.

More details about the API object can be found at HorizontalPodAutoscaler Object.

Support for Horizontal Pod Autoscaler in kubectl

Horizontal Pod Autoscaler, like every API resource, is supported in a standard way by kubectl. We can create a new autoscaler using kubectl create command. We can list autoscalers by kubectl get hpa and get detailed description by kubectl describe hpa. Finally, we can delete an autoscaler using kubectl delete hpa.

In addition, there is a special kubectl autoscale command for easy creation of a Horizontal Pod Autoscaler. For instance, executing kubectl autoscale rs foo --min=2 --max=5 --cpu-percent=80 will create an autoscaler for replication set foo, with target CPU utilization set to 80% and the number of replicas between 2 and 5. The detailed documentation of kubectl autoscale can be found here.

Autoscaling during rolling update

Currently in Kubernetes, it is possible to perform a rolling update by managing replication controllers directly, or by using the deployment object, which manages the underlying replica sets for you. Horizontal Pod Autoscaler only supports the latter approach: the Horizontal Pod Autoscaler is bound to the deployment object, it sets the size for the deployment object, and the deployment is responsible for setting sizes of underlying replica sets.

Horizontal Pod Autoscaler does not work with rolling update using direct manipulation of replication controllers, i.e. you cannot bind a Horizontal Pod Autoscaler to a replication controller and do rolling update (e.g. using kubectl rolling-update). The reason this doesn’t work is that when rolling update creates a new replication controller, the Horizontal Pod Autoscaler will not be bound to the new replication controller.

Support for cooldown/delay

When managing the scale of a group of replicas using the Horizontal Pod Autoscaler, it is possible that the number of replicas keeps fluctuating frequently due to the dynamic nature of the metrics evaluated. This is sometimes referred to as thrashing.

Starting from v1.6, a cluster operator can mitigate this problem by tuning the global HPA settings exposed as flags for the kube-controller-manager component:

• --horizontal-pod-autoscaler-downscale-delay: The value for this option is a duration that specifies how long the autoscaler has to wait before another downscale operation can be performed after the current one has completed. The default value is 5 minutes (5m0s).

• --horizontal-pod-autoscaler-upscale-delay: The value for this option is a duration that specifies how long the autoscaler has to wait before another upscale operation can be performed after the current one has completed. The default value is 3 minutes (3m0s).

Note: When tuning these parameter values, a cluster operator should be aware of the possible consequences. If the delay (cooldown) value is set too long, there could be complaints that the Horizontal Pod Autoscaler is not responsive to workload changes. However, if the delay value is set too short, the scale of the replicas set may keep thrashing as usual.

Support for multiple metrics

Kubernetes 1.6 adds support for scaling based on multiple metrics. You can use the autoscaling/v2beta1 API version to specify multiple metrics for the Horizontal Pod Autoscaler to scale on. Then, the Horizontal Pod Autoscaler controller will evaluate each metric, and propose a new scale based on that metric. The largest of the proposed scales will be used as the new scale.

Support for custom metrics

Note: Kubernetes 1.2 added alpha support for scaling based on application-specific metrics using special annotations. Support for these annotations was removed in Kubernetes 1.6 in favor of the new autoscaling API. While the old method for collecting custom metrics is still available, these metrics will not be available for use by the Horizontal Pod Autoscaler, and the former annotations for specifying which custom metrics to scale on are no longer honored by the Horizontal Pod Autoscaler controller.

Kubernetes 1.6 adds support for making use of custom metrics in the Horizontal Pod Autoscaler. You can add custom metrics for the Horizontal Pod Autoscaler to use in the autoscaling/v2beta1 API. Kubernetes then queries the new custom metrics API to fetch the values of the appropriate custom metrics.

See Support for metrics APIs for the requirements.

Support for metrics APIs

By default, the HorizontalPodAutoscaler controller retrieves metrics from a series of APIs. In order for it to access these APIs, cluster administrators must ensure that:

• The API aggregation layer is enabled.

• The corresponding APIs are registered:

  • For resource metrics, this is the metrics.k8s.io API, generally provided by metrics-server. It can be launched as a cluster addon.

  • For custom metrics, this is the custom.metrics.k8s.io API. It’s provided by “adapter” API servers provided by metrics solution vendors. Check with your metrics pipeline, or the list of known solutions. If you would like to write your own, check out the boilerplate to get started.

  • For external metrics, this is the external.metrics.k8s.io API. It may be provided by the custom metrics adapters provided above.

• The --horizontal-pod-autoscaler-use-rest-clients is true or unset. Setting this to false switches to Heapster-based autoscaling, which is deprecated.

What's next

• Design documentation: Horizontal Pod Autoscaling.
• kubectl autoscale command: kubectl autoscale.
• Usage example of Horizontal Pod Autoscaler.