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.
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).
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 a series of aggregated APIs (
external.metrics.k8s.io). It can also fetch metrics directly
from Heapster. 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.
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
are preserved as annotations when working with
More details about the API object can be found at HorizontalPodAutoscaler Object.
Horizontal Pod Autoscaler, like every API resource, is supported in a standard way by
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 rc foo --min=2 --max=5 --cpu-percent=80
will create an autoscaler for replication controller foo, with target CPU utilization set to
and the number of replicas between 2 and 5.
The detailed documentation of
kubectl autoscale can be found here.
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
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.
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
--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 (
--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 (
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.
Kubernetes 1.6 adds support for scaling based on multiple metrics. You can use the
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.
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
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.
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.
true or unset. Setting this to false switches to Heapster-based autoscaling, which is deprecated.