If you are using a released version of Kubernetes, you should refer to the docs that go with that version.
The latest release of this document can be found [here](http://releases.k8s.io/release-1.1/docs/user-guide/horizontal-pod-autoscaler.md).Documentation for other releases can be found at releases.k8s.io.
This document describes the current state of Horizontal Pod Autoscaler in Kubernetes.
Horizontal pod autoscaling allows the number of pods in a replication controller or deployment to scale automatically based on observed CPU utilization. It is a beta feature in Kubernetes 1.1.
The autoscaler is implemented as a Kubernetes API resource and a controller. The resource describes 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 autoscaler is implemented as a control loop.
It periodically queries CPU utilization for the pods it targets.
(The period of the autoscaler is controlled by --horizontal-pod-autoscaler-sync-period flag of controller manager.
The default value is 30 seconds).
Then, it compares the arithmetic mean of the pods' CPU utilization with the target and adjust the number of replicas if needed.
CPU utilization is the recent CPU usage of a pod divided by the sum of CPU requested by the pod's containers. Please note that if some of the pod's containers do not have CPU request set, CPU utilization for the pod will not be defined and the autoscaler will not take any action. Further details of the autoscaling algorithm are given here.
Autoscaler uses heapster to collect CPU utilization. Therefore, it is required to deploy heapster monitoring in your cluster for autoscaling to work.
Autoscaler accesses corresponding replication controller or deployment by scale sub-resource. Scale is an interface which allows to dynamically set the number of replicas and to learn the current state of them. More details on scale sub-resource can be found here.
Horizontal pod autoscaler is a top-level resource in the Kubernetes REST API (currently in beta). 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 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 that allows for easy creation of 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 80%
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 replication controllers 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 replication controllers.
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.
- Design documentation: Horizontal Pod Autoscaling.
- Manual of autoscale command in kubectl: kubectl autoscale.
- Usage example of Horizontal Pod Autoscaler.