概念及原理
利用Horizontal Pod Autoscaling（HPA），kubernetes能够根据监测到的CPU利用率自动的扩缩容 replication controller，deployment和replica set中pod的数量。
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HPA作为kubernetes API resource和controller 的实现。Resource确定controller的行为。Controller 会根据监测到用户指定的目标的 CPU 利用率周期性地调整 replication controller 或 deployment 的 replica 数量。
HPA由一个控制循环实现，循环周期由controller manager 中的 –horizontal-pod-autoscaler-sync-period标志指定。在每个周期内，controller manager会查询HPA中定义的metric的资源利用率。Controller manager 从 resource metric API（每个 pod 的 resource metric）或者自定义 metric API（所有的metric）中获取 metric。
Install Metrics-Server
usage
# kubectl autoscale sts apache2 --cpu-percent=50 --min=1 --max=3
# kubectl get hpa
NAME REFERENCE TARGETS MINPODS MAXPODS REPLICAS AGE
apache2 StatefulSet/apache2 5%/10% 1 2 2 33m
# kubectl describe hpa apache2
Name: apache2
Namespace: default
Labels: <none>
Annotations: <none>
CreationTimestamp: Wed, 11 Sep 2019 10:47:42 +0800
Reference: StatefulSet/apache2
Metrics: ( current / target )
resource cpu on pods (as a percentage of request): 5% (14m) / 10%
Min replicas: 1
Max replicas: 2
StatefulSet pods: 2 current / 2 desired
Conditions:
Type Status Reason Message
---- ------ ------ -------
AbleToScale True ScaleDownStabilized recent recommendations were higher than current one, applying the highest recent recommendation
ScalingActive True ValidMetricFound the HPA was able to successfully calculate a replica count from cpu resource utilization (percentage of request)
ScalingLimited False DesiredWithinRange the desired count is within the acceptable range
Events:
Type Reason Age From Message
---- ------ ---- ---- -------
Normal SuccessfulRescale 89s horizontal-pod-autoscaler New size: 2; reason: cpu resource utilization (percentage of request) above target
- AbleToScale：表明HPA是否 可以获取和更新伸缩信息，以及是否存在阻止伸缩的各种回退条件
- ScalingActive：表明HPA是否被启用（即目标的副本数量不为零） 以及是否能够完成伸缩计算。当这一状态为False时，通常表明获取度量指标存在问题。
- ScalingLimitted：表明所需伸缩的值被HorizontalPodAutoscaler所定义的最大或者最小值所限制（即已经达到最大或者最小伸缩值）。这通常表明您可能需要调整HorizontalPodAutoscaler 所定义的最大或者最小副本数量的限制了。
Demo
第一步：部署pod、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
第二步：创建Horizontal Pod Autoscaler：
$ kubectl autoscale deployment php-apache --cpu-percent=50 --min=1 --max=10
deployment "php-apache" autoscaled
$ kubectl get hpa
NAME REFERENCE TARGET MINPODS MAXPODS REPLICAS AGE
php-apache Deployment/php-apache/scale 0% / 50% 1 10 1
第三步：增加负载：
$ 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
$ kubectl get hpa
NAME REFERENCE TARGET CURRENT MINPODS MAXPODS REPLICAS AGE
php-apache Deployment/php-apache/scale 305% / 50% 305% 1 10 1 3m
$ kubectl get deployment php-apache
NAME DESIRED CURRENT UP-TO-DATE AVAILABLE AGE
php-apache 7 7 7 7 19m
第四步：停止负载：
$ kubectl get hpa
NAME REFERENCE TARGET MINPODS MAXPODS REPLICAS AGE
php-apache Deployment/php-apache/scale 0% / 50% 1 10 1 11m
$ kubectl get deployment php-apache
NAME DESIRED CURRENT UP-TO-DATE AVAILABLE AGE
php-apache 1 1 1 1 27m
详细文档：https://jimmysong.io/kubernetes-handbook/concepts/horizontal-pod-autoscaling.html
