使用 Kubernetes 进行容器编排的主要优点之一是，它可以非常轻松地对我们的应用程序进行水平扩展。Pod 水平自动缩放（HPA）可以根据 CPU 和内存使用量来扩展应用，前面讲解的 HPA 章节 我们只演示了基于 CPU 的自动缩放，在更复杂的情况下，我们可能还需要基于内存或者基于某些自定义的指标来进行扩缩容。

HorizontalPodAutoscaler 是 Kubernetes autoscaling API 组的资源，在当前稳定版本 autoscaling/v1 中只支持基于 CPU 指标的缩放。在 Beta 版本 autoscaling/v2beta2，引入了基于内存和自定义指标的缩放。所以我们这里需要使用 Beta 版本的 API。

内存

要使用基于内存或者自定义指标进行扩缩容，现在的版本都必须依赖 metrics-server 这个项目，如果没有安装的需要提前安装该项目，而且还需要开启聚合 API 层，前面 HPA 章节 都已经讲解过。

现在我们用 Deployment 来创建一个 Nginx Pod，然后利用 HPA 来进行自动扩缩容。资源清单如下所示：（hpa-mem-demo.yaml）

apiVersion: apps/v1
kind: Deployment
metadata:
  name: hpa-mem-demo
spec:
  selector:
    matchLabels:
      app: nginx
  template:
    metadata:
      labels:
        app: nginx
    spec:
      volumes:
      - name: increase-mem-script
        configMap:
          name: increase-mem-config
      containers:
      - name: nginx
        image: nginx
        ports:
        - containerPort: 80
        volumeMounts:
        - name: increase-mem-script
          mountPath: /etc/script
        resources:
          requests:
            memory: 50Mi
            cpu: 50m
        securityContext:
          privileged: true

这里和前面普通的应用有一些区别，我们将一个名为 increase-mem-config 的 ConfigMap 资源对象挂载到了容器中，该配置文件是用于后面增加容器内存占用的脚本，配置文件如下所示：（increase-mem-cm.yaml）

apiVersion: v1
kind: ConfigMap
metadata:
  name: increase-mem-config
data:
  increase-mem.sh: |
    #!/bin/bash  
    mkdir /tmp/memory  
    mount -t tmpfs -o size=40M tmpfs /tmp/memory  
    dd if=/dev/zero of=/tmp/memory/block  
    sleep 60 
    rm /tmp/memory/block  
    umount /tmp/memory  
    rmdir /tmp/memory

由于这里增加内存的脚本需要使用到 mount 命令，这需要声明为特权模式，所以我们添加了 securityContext.privileged=true 这个配置。现在我们直接创建上面的资源对象即可：

$ kubectl apply -f increase-mem-cm.yaml
$ kubectl apply -f hpa-mem-demo.yaml 
$ kubectl get pods -l app=nginx
NAME                            READY   STATUS    RESTARTS   AGE
hpa-mem-demo-66944b79bf-tqrn9   1/1     Running   0          35s

然后需要创建一个基于内存的 HPA 资源对象：（hpa-mem.yaml）

apiVersion: autoscaling/v2beta1
kind: HorizontalPodAutoscaler
metadata:
  name: nginx-hpa
spec:
  scaleTargetRef:
    apiVersion: apps/v1
    kind: Deployment
    name: hpa-mem-demo
  minReplicas: 1
  maxReplicas: 5
  metrics:
  - type: Resource
    resource:
      name: memory
      targetAverageUtilization: 60

要注意这里使用的 apiVersion 是 autoscaling/v2beta1，然后 metrics 属性里面指定的是内存的配置，直接创建上面的资源对象即可：

$ kubectl apply -f hpa-mem.yaml 
horizontalpodautoscaler.autoscaling/nginx-hpa created
$ kubectl get hpa
NAME        REFERENCE                 TARGETS   MINPODS   MAXPODS   REPLICAS   AGE
nginx-hpa   Deployment/hpa-mem-demo   2%/60%    1         5         1          12s

到这里证明 HPA 资源对象已经部署成功了，接下来我们对应用进行压测，将内存压上去，直接执行上面我们挂载到容器中的 increase-mem.sh 脚本即可：

$ kubectl exec -it hpa-mem-demo-66944b79bf-tqrn9 /bin/bash
root@hpa-mem-demo-66944b79bf-tqrn9:/# ls /etc/script/
increase-mem.sh
root@hpa-mem-demo-66944b79bf-tqrn9:/# source /etc/script/increase-mem.sh 
dd: writing to '/tmp/memory/block': No space left on device
81921+0 records in
81920+0 records out
41943040 bytes (42 MB, 40 MiB) copied, 0.584029 s, 71.8 MB/s

然后打开另外一个终端观察 HPA 资源对象的变化情况：

$ kubectl get hpa
NAME        REFERENCE                 TARGETS   MINPODS   MAXPODS   REPLICAS   AGE
nginx-hpa   Deployment/hpa-mem-demo   83%/60%   1         5         1          5m3s
$ kubectl describe hpa nginx-hpa
Name:                                                     nginx-hpa
Namespace:                                                default
Labels:                                                   
Annotations:                                              kubectl.kubernetes.io/last-applied-configuration:
                                                            {"apiVersion":"autoscaling/v2beta1","kind":"HorizontalPodAutoscaler","metadata":{"annotations":{},"name":"nginx-hpa","namespace":"default"...
CreationTimestamp:                                        Tue, 07 Apr 2020 13:13:59 +0800
Reference:                                                Deployment/hpa-mem-demo
Metrics:                                                  ( current / target )
  resource memory on pods  (as a percentage of request):  3% (1740800) / 60%
Min replicas:                                             1
Max replicas:                                             5
Deployment 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 memory resource utilization (percentage of request)
  ScalingLimited  False   DesiredWithinRange   the desired count is within the acceptable range
Events:
  Type     Reason                        Age                    From                       Message
  ----     ------                        ----                   ----                       -------
  Warning  FailedGetResourceMetric       5m26s (x3 over 5m58s)  horizontal-pod-autoscaler  unable to get metrics for resource memory: no metrics returned from resource metrics API
  Warning  FailedComputeMetricsReplicas  5m26s (x3 over 5m58s)  horizontal-pod-autoscaler  invalid metrics (1 invalid out of 1), first error is: failed to get memory utilization: unable to get metrics for resource memory: no metrics returned from resource metrics API
  Normal   SuccessfulRescale             77s                    horizontal-pod-autoscaler  New size: 2; reason: memory resource utilization (percentage of request) above target
$ kubectl top pod hpa-mem-demo-66944b79bf-tqrn9
NAME                            CPU(cores)   MEMORY(bytes)
hpa-mem-demo-66944b79bf-tqrn9   0m           41Mi

可以看到内存使用已经超过了我们设定的 60% 这个阈值了，HPA 资源对象也已经触发了自动扩容，变成了两个副本了：

$ kubectl get pods -l app=nginx
NAME                            READY   STATUS    RESTARTS   AGE
hpa-mem-demo-66944b79bf-8m4d9   1/1     Running   0          2m51s
hpa-mem-demo-66944b79bf-tqrn9   1/1     Running   0          8m11s

当内存释放掉后，controller-manager 默认5分钟过后会进行缩放，到这里就完成了基于内存的 HPA 操作。

自定义指标

除了基于 CPU 和内存来进行自动扩缩容之外，我们还可以根据自定义的监控指标来进行。这个我们就需要使用 Prometheus Adapter，Prometheus 用于监控应用的负载和集群本身的各种指标，Prometheus Adapter 可以帮我们使用 Prometheus 收集的指标并使用它们来制定扩展策略，这些指标都是通过 APIServer 暴露的，而且 HPA 资源对象也可以很轻易的直接使用。

首先，我们部署一个示例应用，在该应用程序上测试 Prometheus 指标自动缩放，资源清单文件如下所示：（hpa-prome-demo.yaml）

apiVersion: apps/v1
kind: Deployment
metadata:
  name: hpa-prom-demo
spec:
  selector:
    matchLabels:
      app: nginx-server
  template:
    metadata:
      labels:
        app: nginx-server
    spec:
      containers:
      - name: nginx-demo
        image: cnych/nginx-vts:v1.0
        resources:
          limits:
            cpu: 50m
          requests:
            cpu: 50m
        ports:
        - containerPort: 80
          name: http
---
apiVersion: v1
kind: Service
metadata:
  name: hpa-prom-demo
  annotations:
    prometheus.io/scrape: "true"
    prometheus.io/port: "80"
    prometheus.io/path: "/status/format/prometheus"
spec:
  ports:
  - port: 80
    targetPort: 80
    name: http
  selector:
    app: nginx-server
  type: NodePort

这里我们部署的应用是在 80 端口的 /status/format/prometheus 这个端点暴露 nginx-vts 指标的，前面我们已经在 Prometheus 中配置了 Endpoints 的自动发现，所以我们直接在 Service 对象的 annotations 中进行配置，这样我们就可以在 Prometheus 中采集该指标数据了。为了测试方便，我们这里使用 NodePort 类型的 Service，现在直接创建上面的资源对象即可：

$ kubectl apply -f hpa-prome-demo.yaml
deployment.apps/hpa-prom-demo created
service/hpa-prom-demo created
$ kubectl get pods -l app=nginx-server 
NAME                             READY   STATUS    RESTARTS   AGE
hpa-prom-demo-755bb56f85-lvksr   1/1     Running   0          4m52s
$ kubectl get svc 
NAME            TYPE        CLUSTER-IP       EXTERNAL-IP   PORT(S)        AGE
hpa-prom-demo   NodePort    10.101.210.158           80:32408/TCP   5m44s
......

部署完成后我们可以使用如下命令测试应用是否正常，以及指标数据接口能够正常获取：

$ curl http://k8s.qikqiak.com:32408







Welcome to nginx!
If you see this page, the nginx web server is successfully installed and
working. Further configuration is required.

For online documentation and support please refer to
nginx.org.

Commercial support is available at
nginx.com.

Thank you for using nginx.


$ curl http://k8s.qikqiak.com:32408/status/format/prometheus 
# HELP nginx_vts_info Nginx info
# TYPE nginx_vts_info gauge
nginx_vts_info{hostname="hpa-prom-demo-755bb56f85-lvksr",version="1.13.12"} 1
# HELP nginx_vts_start_time_seconds Nginx start time
# TYPE nginx_vts_start_time_seconds gauge
nginx_vts_start_time_seconds 1586240091.623
# HELP nginx_vts_main_connections Nginx connections
# TYPE nginx_vts_main_connections gauge
nginx_vts_main_connections{status="accepted"} 5
nginx_vts_main_connections{status="active"} 3
nginx_vts_main_connections{status="handled"} 5
nginx_vts_main_connections{status="reading"} 0
nginx_vts_main_connections{status="requests"} 33
nginx_vts_main_connections{status="waiting"} 2
nginx_vts_main_connections{status="writing"} 1
# HELP nginx_vts_main_shm_usage_bytes Shared memory [ngx_http_vhost_traffic_status] info
# TYPE nginx_vts_main_shm_usage_bytes gauge
nginx_vts_main_shm_usage_bytes{shared="max_size"} 1048575
nginx_vts_main_shm_usage_bytes{shared="used_size"} 3510
nginx_vts_main_shm_usage_bytes{shared="used_node"} 1
# HELP nginx_vts_server_bytes_total The request/response bytes
# TYPE nginx_vts_server_bytes_total counter
# HELP nginx_vts_server_requests_total The requests counter
# TYPE nginx_vts_server_requests_total counter
# HELP nginx_vts_server_request_seconds_total The request processing time in seconds
# TYPE nginx_vts_server_request_seconds_total counter
# HELP nginx_vts_server_request_seconds The average of request processing times in seconds
# TYPE nginx_vts_server_request_seconds gauge
# HELP nginx_vts_server_request_duration_seconds The histogram of request processing time
# TYPE nginx_vts_server_request_duration_seconds histogram
# HELP nginx_vts_server_cache_total The requests cache counter
# TYPE nginx_vts_server_cache_total counter
nginx_vts_server_bytes_total{host="_",direction="in"} 8025
nginx_vts_server_bytes_total{host="_",direction="out"} 121608
nginx_vts_server_requests_total{host="_",code="1xx"} 0
nginx_vts_server_requests_total{host="_",code="2xx"} 32
nginx_vts_server_requests_total{host="_",code="3xx"} 0
nginx_vts_server_requests_total{host="_",code="4xx"} 0
nginx_vts_server_requests_total{host="_",code="5xx"} 0
nginx_vts_server_requests_total{host="_",code="total"} 32
nginx_vts_server_request_seconds_total{host="_"} 0.000
nginx_vts_server_request_seconds{host="_"} 0.000
nginx_vts_server_cache_total{host="_",status="miss"} 0
nginx_vts_server_cache_total{host="_",status="bypass"} 0
nginx_vts_server_cache_total{host="_",status="expired"} 0
nginx_vts_server_cache_total{host="_",status="stale"} 0
nginx_vts_server_cache_total{host="_",status="updating"} 0
nginx_vts_server_cache_total{host="_",status="revalidated"} 0
nginx_vts_server_cache_total{host="_",status="hit"} 0
nginx_vts_server_cache_total{host="_",status="scarce"} 0
nginx_vts_server_bytes_total{host="*",direction="in"} 8025
nginx_vts_server_bytes_total{host="*",direction="out"} 121608
nginx_vts_server_requests_total{host="*",code="1xx"} 0
nginx_vts_server_requests_total{host="*",code="2xx"} 32
nginx_vts_server_requests_total{host="*",code="3xx"} 0
nginx_vts_server_requests_total{host="*",code="4xx"} 0
nginx_vts_server_requests_total{host="*",code="5xx"} 0
nginx_vts_server_requests_total{host="*",code="total"} 32
nginx_vts_server_request_seconds_total{host="*"} 0.000
nginx_vts_server_request_seconds{host="*"} 0.000
nginx_vts_server_cache_total{host="*",status="miss"} 0
nginx_vts_server_cache_total{host="*",status="bypass"} 0
nginx_vts_server_cache_total{host="*",status="expired"} 0
nginx_vts_server_cache_total{host="*",status="stale"} 0
nginx_vts_server_cache_total{host="*",status="updating"} 0
nginx_vts_server_cache_total{host="*",status="revalidated"} 0
nginx_vts_server_cache_total{host="*",status="hit"} 0
nginx_vts_server_cache_total{host="*",status="scarce"} 0

上面的指标数据中，我们比较关心的是 nginx_vts_server_requests_total 这个指标，表示请求总数，是一个 Counter 类型的指标，我们将使用该指标的值来确定是否需要对我们的应用进行自动扩缩容。

接下来我们将 Prometheus-Adapter 安装到集群中，并添加一个规则来跟踪 Pod 的请求，我们可以将 Prometheus 中的任何一个指标都用于 HPA，但是前提是你得通过查询语句将它拿到（包括指标名称和其对应的值）。

这里我们定义一个如下所示的规则：

rules:
- seriesQuery: 'nginx_vts_server_requests_total'
  seriesFilters: []
  resources:
    overrides:
      kubernetes_namespace:
        resource: namespace
      kubernetes_pod_name:
        resource: pod
  name:
    matches: "^(.*)_total"
    as: "${1}_per_second"
  metricsQuery: (sum(rate(<<.Series>>{<<.LabelMatchers>>}[1m])) by (<<.GroupBy>>))

这是一个带参数的 Prometheus 查询，其中：

• seriesQuery：查询 Prometheus 的语句，通过这个查询语句查询到的所有指标都可以用于 HPA
• seriesFilters：查询到的指标可能会存在不需要的，可以通过它过滤掉。
• resources：通过 seriesQuery 查询到的只是指标，如果需要查询某个 Pod 的指标，肯定要将它的名称和所在的命名空间作为指标的标签进行查询，resources 就是将指标的标签和 k8s 的资源类型关联起来，最常用的就是 pod 和 namespace。有两种添加标签的方式，一种是 overrides，另一种是 template。
  • overrides：它会将指标中的标签和 k8s 资源关联起来。上面示例中就是将指标中的 pod 和 namespace 标签和 k8s 中的 pod 和 namespace 关联起来，因为 pod 和 namespace 都属于核心 api 组，所以不需要指定 api 组。当我们查询某个 pod 的指标时，它会自动将 pod 的名称和名称空间作为标签加入到查询条件中。比如 nginx: {group: "apps", resource: "deployment"} 这么写表示的就是将指标中 nginx 这个标签和 apps 这个 api 组中的 deployment 资源关联起来；
  • template：通过 go 模板的形式。比如template: "kube_<<.Group>>_<<.Resource>>" 这么写表示，假如 <<.Group>> 为 apps，<<.Resource>> 为 deployment，那么它就是将指标中 kube_apps_deployment 标签和 deployment 资源关联起来。
• name：用来给指标重命名的，之所以要给指标重命名是因为有些指标是只增的，比如以 total 结尾的指标。这些指标拿来做 HPA 是没有意义的，我们一般计算它的速率，以速率作为值，那么此时的名称就不能以 total 结尾了，所以要进行重命名。
  • matches：通过正则表达式来匹配指标名，可以进行分组
  • as：默认值为 $1，也就是第一个分组。as 为空就是使用默认值的意思。
• metricsQuery：这就是 Prometheus 的查询语句了，前面的 seriesQuery 查询是获得 HPA 指标。当我们要查某个指标的值时就要通过它指定的查询语句进行了。可以看到查询语句使用了速率和分组，这就是解决上面提到的只增指标的问题。
  • Series：表示指标名称
  • LabelMatchers：附加的标签，目前只有 pod 和 namespace 两种，因此我们要在之前使用 resources 进行关联
  • GroupBy：就是 pod 名称，同样需要使用 resources 进行关联。

接下来我们通过 Helm Chart 来部署 Prometheus Adapter，新建 hpa-prome-adapter-values.yaml 文件覆盖默认的 Values 值，内容如下所示：

rules:
  default: false
  custom:
  - seriesQuery: 'nginx_vts_server_requests_total'
    resources: 
      overrides:
        kubernetes_namespace:
          resource: namespace
        kubernetes_pod_name:
          resource: pod
    name:
      matches: "^(.*)_total"
      as: "${1}_per_second"
    metricsQuery: (sum(rate(<<.Series>>{<<.LabelMatchers>>}[1m])) by (<<.GroupBy>>))

prometheus:
  url: http://thanos-querier.kube-mon.svc.cluster.local

这里我们添加了一条 rules 规则，然后指定了 Prometheus 的地址，我们这里是使用了 Thanos 部署的 Promethues 集群，所以用 Querier 的地址。使用下面的命令一键安装：

$ helm install prometheus-adapter stable/prometheus-adapter -n kube-mon -f hpa-prome-adapter-values.yaml
NAME: prometheus-adapter
LAST DEPLOYED: Tue Apr  7 15:26:36 2020
NAMESPACE: kube-mon
STATUS: deployed
REVISION: 1
TEST SUITE: None
NOTES:
prometheus-adapter has been deployed.
In a few minutes you should be able to list metrics using the following command(s):

  kubectl get --raw /apis/custom.metrics.k8s.io/v1beta1

等一小会儿，安装完成后，可以使用下面的命令来检测是否生效了：

$ kubectl get pods -n kube-mon -l app=prometheus-adapter
NAME                                  READY   STATUS    RESTARTS   AGE
prometheus-adapter-58b559fc7d-l2j6t   1/1     Running   0          3m21s
$  kubectl get --raw="/apis/custom.metrics.k8s.io/v1beta1" | jq
{
  "kind": "APIResourceList",
  "apiVersion": "v1",
  "groupVersion": "custom.metrics.k8s.io/v1beta1",
  "resources": [
    {
      "name": "namespaces/nginx_vts_server_requests_per_second",
      "singularName": "",
      "namespaced": false,
      "kind": "MetricValueList",
      "verbs": [
        "get"
      ]
    },
    {
      "name": "pods/nginx_vts_server_requests_per_second",
      "singularName": "",
      "namespaced": true,
      "kind": "MetricValueList",
      "verbs": [
        "get"
      ]
    }
  ]
}

我们可以看到 nginx_vts_server_requests_per_second 指标可用。 现在，让我们检查该指标的当前值：

$ kubectl get --raw "/apis/custom.metrics.k8s.io/v1beta1/namespaces/default/pods/*/nginx_vts_server_requests_per_second" | jq .
{
  "kind": "MetricValueList",
  "apiVersion": "custom.metrics.k8s.io/v1beta1",
  "metadata": {
    "selfLink": "/apis/custom.metrics.k8s.io/v1beta1/namespaces/default/pods/%2A/nginx_vts_server_requests_per_second"
  },
  "items": [
    {
      "describedObject": {
        "kind": "Pod",
        "namespace": "default",
        "name": "hpa-prom-demo-755bb56f85-lvksr",
        "apiVersion": "/v1"
      },
      "metricName": "nginx_vts_server_requests_per_second",
      "timestamp": "2020-04-07T09:45:45Z",
      "value": "527m",
      "selector": null
    }
  ]
}

出现类似上面的信息就表明已经配置成功了，接下来我们部署一个针对上面的自定义指标的 HAP 资源对象，如下所示：(hpa-prome.yaml)

apiVersion: autoscaling/v2beta1
kind: HorizontalPodAutoscaler
metadata:
  name: nginx-custom-hpa
spec:
  scaleTargetRef:
    apiVersion: apps/v1
    kind: Deployment
    name: hpa-prom-demo
  minReplicas: 2
  maxReplicas: 5
  metrics:
  - type: Pods
    pods:
      metricName: nginx_vts_server_requests_per_second
      targetAverageValue: 10

如果请求数超过每秒10个，则将对应用进行扩容。直接创建上面的资源对象：

$ kubectl apply -f hpa-prome.yaml
horizontalpodautoscaler.autoscaling/nginx-custom-hpa created
$ kubectl describe hpa nginx-custom-hpa
Name:                                              nginx-custom-hpa
Namespace:                                         default
Labels:                                            
Annotations:                                       kubectl.kubernetes.io/last-applied-configuration:
                                                     {"apiVersion":"autoscaling/v2beta1","kind":"HorizontalPodAutoscaler","metadata":{"annotations":{},"name":"nginx-custom-hpa","namespace":"d...
CreationTimestamp:                                 Tue, 07 Apr 2020 17:54:55 +0800
Reference:                                         Deployment/hpa-prom-demo
Metrics:                                           ( current / target )
  "nginx_vts_server_requests_per_second" on pods:   / 10
Min replicas:                                      2
Max replicas:                                      5
Deployment pods:                                   1 current / 2 desired
Conditions:
  Type         Status  Reason            Message
  ----         ------  ------            -------
  AbleToScale  True    SucceededRescale  the HPA controller was able to update the target scale to 2
Events:
  Type    Reason             Age   From                       Message
  ----    ------             ----  ----                       -------
  Normal  SuccessfulRescale  7s    horizontal-pod-autoscaler  New size: 2; reason: Current number of replicas below Spec.MinReplicas

可以看到 HPA 对象已经生效了，会应用最小的副本数2，所以会新增一个 Pod 副本：

$ kubectl get pods -l app=nginx-server
NAME                             READY   STATUS    RESTARTS   AGE
hpa-prom-demo-755bb56f85-s5dzf   1/1     Running   0          67s
hpa-prom-demo-755bb56f85-wbpfr   1/1     Running   0          3m30s

接下来我们同样对应用进行压测：

$ while true; do wget -q -O- http://k8s.qikqiak.com:32408; done

打开另外一个终端观察 HPA 对象的变化：

$ kubectl get hpa
NAME               REFERENCE                  TARGETS     MINPODS   MAXPODS   REPLICAS   AGE
nginx-custom-hpa   Deployment/hpa-prom-demo   14239m/10   2         5         2          4m27s
$ kubectl describe hpa nginx-custom-hpa
Name:                                              nginx-custom-hpa
Namespace:                                         default
Labels:                                            
Annotations:                                       kubectl.kubernetes.io/last-applied-configuration:
                                                     {"apiVersion":"autoscaling/v2beta1","kind":"HorizontalPodAutoscaler","metadata":{"annotations":{},"name":"nginx-custom-hpa","namespace":"d...
CreationTimestamp:                                 Tue, 07 Apr 2020 17:54:55 +0800
Reference:                                         Deployment/hpa-prom-demo
Metrics:                                           ( current / target )
  "nginx_vts_server_requests_per_second" on pods:  14308m / 10
Min replicas:                                      2
Max replicas:                                      5
Deployment pods:                                   3 current / 3 desired
Conditions:
  Type            Status  Reason              Message
  ----            ------  ------              -------
  AbleToScale     True    ReadyForNewScale    recommended size matches current size
  ScalingActive   True    ValidMetricFound    the HPA was able to successfully calculate a replica count from pods metric nginx_vts_server_requests_per_second
  ScalingLimited  False   DesiredWithinRange  the desired count is within the acceptable range
Events:
  Type    Reason             Age   From                       Message
  ----    ------             ----  ----                       -------
  Normal  SuccessfulRescale  5m2s  horizontal-pod-autoscaler  New size: 2; reason: Current number of replicas below Spec.MinReplicas
  Normal  SuccessfulRescale  61s   horizontal-pod-autoscaler  New size: 3; reason: pods metric nginx_vts_server_requests_per_second above target

可以看到指标 nginx_vts_server_requests_per_second 的数据已经超过阈值了，触发扩容动作了，副本数变成了3，但是后续很难继续扩容了，这是因为上面我们的 while 命令并不够快，3个副本完全可以满足每秒不超过 10 个请求的阈值。

如果需要更好的进行测试，我们可以使用一些压测工具，比如 ab、fortio 等工具。当我们中断测试后，默认5分钟过后就会自动缩容：

$ kubectl describe hpa nginx-custom-hpa
Name:                                              nginx-custom-hpa
Namespace:                                         default
Labels:                                            
Annotations:                                       kubectl.kubernetes.io/last-applied-configuration:
                                                     {"apiVersion":"autoscaling/v2beta1","kind":"HorizontalPodAutoscaler","metadata":{"annotations":{},"name":"nginx-custom-hpa","namespace":"d...
CreationTimestamp:                                 Tue, 07 Apr 2020 17:54:55 +0800
Reference:                                         Deployment/hpa-prom-demo
Metrics:                                           ( current / target )
  "nginx_vts_server_requests_per_second" on pods:  533m / 10
Min replicas:                                      2
Max replicas:                                      5
Deployment pods:                                   2 current / 2 desired
Conditions:
  Type            Status  Reason            Message
  ----            ------  ------            -------
  AbleToScale     True    ReadyForNewScale  recommended size matches current size
  ScalingActive   True    ValidMetricFound  the HPA was able to successfully calculate a replica count from pods metric nginx_vts_server_requests_per_second
  ScalingLimited  True    TooFewReplicas    the desired replica count is less than the minimum replica count
Events:
  Type    Reason             Age   From                       Message
  ----    ------             ----  ----                       -------
  Normal  SuccessfulRescale  23m   horizontal-pod-autoscaler  New size: 2; reason: Current number of replicas below Spec.MinReplicas
  Normal  SuccessfulRescale  19m   horizontal-pod-autoscaler  New size: 3; reason: pods metric nginx_vts_server_requests_per_second above target
  Normal  SuccessfulRescale  4m2s  horizontal-pod-autoscaler  New size: 2; reason: All metrics below target

到这里我们就完成了使用自定义的指标对应用进行自动扩缩容的操作。如果 Prometheus 安装在我们的 Kubernetes 集群之外，则只需要确保可以从集群访问到查询的端点，并在 adapter 的部署清单中对其进行更新即可。在更复杂的场景中，可以获取多个指标结合使用来制定扩展策略。