Use ack-autoscaling-placeholder to scale pods in seconds

Procedure

1. Log on to the ACK console.
2. In the left-side navigation pane of the ACK console, choose Marketplace > App Catalog.
3. On the App Catalog page, find and click ack-autoscaling-placeholder. On the App Catalog - ack-autoscaling-placeholder page, select the cluster and namespace where you want to deploy the application and click Create.
4. Configure ack-autoscaling-placeholder.
   1. On the Clusters page, find the cluster that you want to manage and click the name of the cluster or click Details in the Actions column. The details page of the cluster appears.
   2. In the left-side navigation pane of the details page, choose Applications > Helm.
   3. On the page that appears, find ack-autoscaling-placeholder and click Update in the Actions column.

      nameOverride: ""
      fullnameOverride: ""
      ##
      priorityClassDefault:
        enabled: true
        name: default-priority-class
        value: -1
      
      ##
      deployments:
         - name: ack-place-holder
           replicaCount: 1
           containers:
             - name: placeholder
               image: registry-vpc.cn-shenzhen.aliyuncs.com/acs/pause:3.1
               pullPolicy: IfNotPresent
               resources:
                 requests:
                   cpu: cpu: 4                  # Occupy 4 cores and 8 GB of memory.
                   memory: 8               
           imagePullSecrets: {}
           annotations: {}
           nodeSelector:                   # Select nodes.
             demo: "yes"  
           tolerations: []
           affinity: {}
           labels: {}

   4. Create a PriorityClass for the workload.

      In this example, a PriorityClass that grants a high priority is created.

      kubectl apply -f priorityClass.yaml

      Expected output:

      apiVersion: scheduling.k8s.io/v1
      kind: PriorityClass
      metadata:
        name: high-priority
      value: value: 1000000              # Specify the priority. 
      globalDefault: false
      description: "This priority class should be used for XYZ service pods only."

   5. Deploy the workload.

      kubectl apply -f workload.yaml

      apiVersion: apps/v1 
      kind: Deployment
      metadata:
        name: placeholder-test
        labels:
          app: nginx
      spec:
        replicas: 1
        selector:
          matchLabels:
            app: nginx
        template:
          metadata:
            labels:
              app: nginx
          spec:
            nodeSelector:                        # Select nodes.
              demo: "yes"
            priorityClassName: high-priority     # Specify the name of the PriorityClass that is created in Step 3. 
            containers:
            - name: nginx
              image: nginx:1.7.9 
              ports:
              - containerPort: 80
              resources:       
                requests:      
                  cpu: cpu: 3                         # Specify the amount of resources required by the workload. 
                  memory: 5

      A PriorityClass that grants a higher priority than other pods is created for the workload, as shown in the following figure. When node resources are insufficient, the placeholder pod named placeHolder is evicted and its state changes to Pending. After the placeholder pod changes to the Pending state, a scale-out activity is triggered in the cluster because Auto Scaling is activated for the cluster. Consequently, pods are scaled out within seconds for the workload.

How it works

A placeholder pod with an extremely low priority (a negative value) is created to occupy a certain amount of computing resources for other pods with higher priorities. If the computing resources are insufficient, the placeholder pod is evicted to release the occupied computing resources for the workload. This way, pods can be launched in seconds. cluster-autoscaler is also used to scale nodes in the cluster.