Horizontal Pod Autoscaling automatically scales the number of pods in a replication controller, deployment or replica set based on observed CPU utilization (or, with alpha support, on some other, application-provided metrics).
In this document we explain how this feature works by walking you through an example of enabling Horizontal Pod Autoscaling for the php-apache server.
Prerequisites
This example requires a running Kubernetes cluster and kubectl, version 1.2 or later. Heapster monitoring needs to be deployed in the cluster as Horizontal Pod Autoscaler uses it to collect metrics (if you followed getting started on GCE guide, heapster monitoring will be turned-on by default).
Step One: Run & expose php-apache server
To demonstrate Horizontal Pod Autoscaler we will use a custom docker image based on the php-apache image. The image can be found here. It defines an index.php page which performs some CPU intensive computations.
First, we will start a deployment running the image and expose it as a service:
$ kubectl run php-apache --image=gcr.io/google_containers/hpa-example --requests=cpu=200m --expose --port=80
service "php-apache" created
deployment "php-apache" created
Step Two: Create Horizontal Pod Autoscaler
Now that the server is running, we will create the autoscaler using kubectl autoscale. The following command will create a Horizontal Pod Autoscaler that maintains between 1 and 10 replicas of the Pods controlled by the php-apache deployment we created in the first step of these instructions. Roughly speaking, HPA will increase and decrease the number of replicas (via the deployment) to maintain an average CPU utilization across all Pods of 50% (since each pod requests 200 milli-cores by kubectl run, this means average CPU usage of 100 milli-cores). See here for more details on the algorithm.
$ kubectl autoscale deployment php-apache --cpu-percent=50 --min=1 --max=10
deployment "php-apache" autoscaled
We may check the current status of autoscaler by running:
$ kubectl get hpa
NAME REFERENCE TARGET CURRENT MINPODS MAXPODS AGE
php-apache Deployment/php-apache/scale 50% 0% 1 10 18s
Please note that the current CPU consumption is 0% as we are not sending any requests to the server
(the CURRENT column shows the average across all the pods controlled by the corresponding deployment).
Step Three: Increase load
Now, we will see how the autoscaler reacts to increased load. We will start a container, and send an infinite loop of queries to the php-apache service (please run it in a different terminal):
$ kubectl run -i --tty load-generator --image=busybox /bin/sh
Hit enter for command prompt
$ while true; do wget -q -O- http://php-apache.default.svc.cluster.local; done
Within a minute or so, we should see the higher CPU load by executing:
$ kubectl get hpa
NAME REFERENCE TARGET CURRENT MINPODS MAXPODS AGE
php-apache Deployment/php-apache/scale 50% 305% 1 10 3m
Here, CPU consumption has increased to 305% of the request. As a result, the deployment was resized to 7 replicas:
$ kubectl get deployment php-apache
NAME DESIRED CURRENT UP-TO-DATE AVAILABLE AGE
php-apache 7 7 7 7 19m
Note Sometimes it may take a few minutes to stabilize the number of replicas. Since the amount of load is not controlled in any way it may happen that the final number of replicas will differ from this example.
Step Four: Stop load
We will finish our example by stopping the user load.
In the terminal where we created the container with busybox image, terminate
the load generation by typing <Ctrl> + C.
Then we will verify the result state (after a minute or so):
$ kubectl get hpa
NAME REFERENCE TARGET CURRENT MINPODS MAXPODS AGE
php-apache Deployment/php-apache/scale 50% 0% 1 10 11m
$ kubectl get deployment php-apache
NAME DESIRED CURRENT UP-TO-DATE AVAILABLE AGE
php-apache 1 1 1 1 27m
Here CPU utilization dropped to 0, and so HPA autoscaled the number of replicas back down to 1.
Note autoscaling the replicas may take a few minutes.
Appendix: Other possible scenarios
Creating the autoscaler from a .yaml file
Instead of using kubectl autoscale command we can use the hpa-php-apache.yaml file, which looks like this:
apiVersion: autoscaling/v1
kind: HorizontalPodAutoscaler
metadata:
name: php-apache
namespace: default
spec:
scaleTargetRef:
apiVersion: extensions/v1beta1
kind: Deployment
name: php-apache
minReplicas: 1
maxReplicas: 10
targetCPUUtilizationPercentage: 50
We will create the autoscaler by executing the following command:
$ kubectl create -f docs/user-guide/horizontal-pod-autoscaling/hpa-php-apache.yaml
horizontalpodautoscaler "php-apache" created