How to spread Elastic Container Instance-based pods across zones and configure affinities - Elastic Container Instance

High availability and high performance are essential to distributed jobs. In Container Service for Kubernetes (ACK) Pro clusters or ACK Serverless Pro clusters, you can spread distributed tasks across zones based on Kubernetes-native scheduling semantics. You can also configure affinities to deploy distributed tasks in specific zones based on Kubernetes-native scheduling semantics. This improves the efficiency of task deployment. This topic describes how to spread Elastic Container Instance-based pods across zones and configure affinities for the pods.

Background information

In certain scenarios, you may need to deploy pods across multiple zones or to a specific zone to achieve high availability or high performance. In such cases, you can use Kubernetes-native scheduling features, such as pod topology spread constraints (topologySpreadConstraints), node affinity (nodeAffinity), and pod affinity (podAffinity).

Important

Elastic Container Instance-based pods can be spread across zones or affinities can be configured for pods only when the nodeAffinity, podAffinity, topologySpreadConstraints parameters are configured for the pods, or when the pods match an existing resource policy.

For more information, see the following official Kubernetes documentation:

Prerequisites

An ACK Pro cluster or ACK Serverless Pro cluster is created and the cluster meets the following requirements:
- The Kubernetes version of the cluster is 1.22 or later.
- The version of the ACK Virtual Node component in the cluster is 2.10.0 or later.
- The version of the kube-scheduler component in the cluster is 5.9 or later, and the virtual node-based pod scheduling feature is enabled for the cluster. For more information, see Enable the virtual node-based pod scheduling policy for an ACK cluster.
Multiple zones (vSwitches) are specified in the eci-profile. Pod can be scheduled to the multiple zones. For more information, see Configure multiple zones to create a pod.

Usage notes

You must set the topologyKey parameter to topology.kubernetes.io/zone.
The feature discussed in this topic will not take effect in the following scenarios:
- The k8s.aliyun.com/eci-schedule-strategy: "VSwitchOrdered" annotation is used to declare a multi-zone scheduling strategy for pods that follows a specified vSwitch order.
- The k8s.aliyun.com/eci-fail-strategy: "fail-fast" annotation is used to set the fault handling policy of the pod to fail-fast.

Configuration examples

In the following sections, an ACK Serverless Pro cluster whose Kubernetes version is 1.22 is used to explain how to spread pods across zones and configure affinity.

Example 1: Use topologySpreadConstraints to spread pods across zones

Note

The following example shows how to configure a topology spread constraint. By default, Scheduler evenly schedules all pods to all zones, but does not consider the production results of pods. For more information, see Strict Elastic Container Instance-based pod topology spread.

Add a topology spread constraint to the configuration of a workload.

Perform the following steps to specify a topology spread constraint in the Spec parameter in the configuration of a pod or in the Spec parameter in the configuration of a workload, such as a Deployment or Job.

  topologySpreadConstraints:
    - maxSkew: <integer>
      minDomains: <integer> # This parameter is optional and is in the Beta phase in Kubernetes 1.25 and later. 
      topologyKey: <string>
      whenUnsatisfiable: <string>
      labelSelector: <object>
      matchLabelKeys: <list> # This parameter is optional and is in the Beta phase in Kubernetes 1.27 and later. 
      nodeAffinityPolicy: [Honor|Ignore] # This parameter is optional and is in the Beta phase in Kubernetes 1.26 and later. 
      nodeTaintsPolicy: [Honor|Ignore] # This parameter is optional and is in the Beta phase in Kubernetes 1.26 and later.

In this example, a Deployment whose pods are evenly distributed to multiple zones is created. For more information about the parameters, see topologySpreadConstraints field. The following code block shows the YAML template of the Deployment:

Show YAML content

apiVersion: apps/v1
kind: Deployment
metadata:
  name: with-pod-topology-spread
  labels:
    app: with-pod-topology-spread
spec:
  replicas: 10
  selector:
    matchLabels:
      app: with-pod-topology-spread
  template:
    metadata:
      labels:
        app: with-pod-topology-spread
    spec:
      topologySpreadConstraints:
        - maxSkew: 1
          topologyKey: topology.kubernetes.io/zone
          whenUnsatisfiable: DoNotSchedule
          labelSelector:
            matchLabels:
              app: with-pod-topology-spread
      containers:
      - name: with-pod-topology-spread
        image: registry.k8s.io/pause:2.0
        resources:
          requests:
            cpu: "1"
            memory: "256Mi"

Create a workload.
Create a file named deployment.yaml and copy the preceding YAML template to the file. Then, run the following command to create a Deployment in the cluster:
```
kubectl apply -f deployment.yaml
```

Verify the scheduling result of the workload.

Run the following command to query the nodes on which the Deployment deploys the pods:

kubectl get po -lapp=with-pod-topology-spread -ocustom-columns=NAME:.metadata.name,NODE:.spec.nodeName --no-headers | grep -v "<none>"

Run the following command to query the number of pods that are created by the Deployment in each zone:

kubectl get po -lapp=with-pod-topology-spread -ocustom-columns=NODE:.spec.nodeName --no-headers | grep -v "<none>" | xargs -I {} kubectl get no {} -ojson | jq '.metadata.labels["topology.kubernetes.io/zone"]' | sort | uniq -c

Example 2: Use nodeAffinity and podAffinity to deploy pods to specific zones

Add affinities to the configuration of a workload.

In this example, a Deployment whose pods are deployed in a single zone is created. For more information about the parameters, see Node affinity. The following code block shows the YAML template of the Deployment:

Show YAML content

apiVersion: apps/v1
kind: Deployment
metadata:
  name: with-affinity
  labels:
    app: with-affinity
spec:
  replicas: 3
  selector:
    matchLabels:
      app: with-affinity
  template:
    metadata:
      labels:
        app: with-affinity
    spec:
      affinity:
        podAffinity:
          requiredDuringSchedulingIgnoredDuringExecution:
          - labelSelector:
              matchExpressions:
              - key: app
                operator: In
                values:
                - with-affinity
            topologyKey: topology.kubernetes.io/zone
      containers:
      - name: with-affinity
        image: registry.k8s.io/pause:2.0

If you want to deploy the pods in a specific zone, delete the podAffinity parameter and add the following constraint to the nodeAffinity parameter: The following configuration specifies that the pods must be deployed in Beijing Zone A.

requiredDuringSchedulingIgnoredDuringExecution:
  nodeSelectorTerms:
  - matchExpressions:
    - key: topology.kubernetes.io/zone
      operator: In
      values:
      - cn-beijing-a

The following code block shows the sample code that contains the nodeAffinity parameter. Pods are deployed only in Beijing Zone A.

Show YAML content

apiVersion: apps/v1
kind: Deployment
metadata:
  name: with-affinity
  labels:
    app: with-affinity
spec:
  replicas: 3
  selector:
    matchLabels:
      app: with-affinity
  template:
    metadata:
      labels:
        app: with-affinity
    spec:
      affinity:
        nodeAffinity:
          requiredDuringSchedulingIgnoredDuringExecution:
            nodeSelectorTerms:
            - matchExpressions:
              - key: topology.kubernetes.io/zone
                operator: In
                values:
                - cn-beijing-a
      containers:
      - name: with-affinity
        image: registry.k8s.io/pause:2.0

Create a workload.
Create a file named deployment.yaml and copy the preceding YAML template to the file. Then, run the following command to create a Deployment in the cluster:
```
kubectl apply -f deployment.yaml
```

Verify the scheduling result of the workload.

Run the following command to query the nodes on which the Deployment deploys the pods:

kubectl get po -lapp=with-affinity -ocustom-columns=NAME:.metadata.name,NODE:.spec.nodeName --no-headers | grep -v "<none>"

Run the following command to query the number of pods that are created by the Deployment in each zone:

kubectl get po -lapp=with-affinity -ocustom-columns=NODE:.spec.nodeName --no-headers | grep -v "<none>" | xargs -I {} kubectl get no {} -ojson | jq '.metadata.labels["topology.kubernetes.io/zone"]' | sort | uniq -c

Strict Elastic Container Instance-based pod topology spread

By default, if you force the system to spread Elastic Container Instance-based pods across zones, kube-scheduler evenly deploys the pods of a workload across all zones. However, Elastic Container Instance-based pods may fail to be created in some zones. The following figure shows the scheduling result when the maxSkew parameter is set to 1. For more information about maxSkew, see maxSkew.

If the Elastic Container Instance-based pods in Zone B and Zone C fail to be created, two Elastic Container Instance-based pods run in Zone A, and no Elastic Container Instance-based pod runs in Zone B or Zone C. This violates the constraint specified by the maxSkew parameter.

In an ACK Serverless Pro cluster, you can enable strict Elastic Container Instance-based pod topology spread to ensure that pods are strictly spread across zones. After you enable strict Elastic Container Instance-based pod topology spread, kube-scheduler first schedules a pod to each of Zone A, Zone B, and Zone C. kube-scheduler does not schedule pending pods until the scheduled pods are created, as shown in the following figure.

Even if Pod A1 is created, pending pods are not scheduled. This is because if the pod in Zone B or Zone C fails to be created, the constraint specified by the maxSkew parameter is violated. After Pod B1 is created, kube-scheduler schedules a pod to Zone C. Pods with green shading are created.

If you want to disable strict Elastic Container Instance-based pod topology spread, set the whenUnsatisfiable parameter to ScheduleAnyway. For more information, see Spread constraint definition.