Configuring Workload Affinity or Anti-affinity Scheduling (podAffinity or podAntiAffinity)

In this example, a backend workload with the app=backend label has been created in the cluster. This label can be used for workload affinity or anti-affinity scheduling, allowing the newly created frontend workload labeled with app=frontend to be deployed on the same node as the backend workload. Both workloads run in topology domain kubernetes.io/hostname. When you use kubernetes.io/hostname for topology classification, only one node can be assigned to each topology domain because each node has a unique hostname. This enables workloads to be scheduled on the same node when workload affinity is enabled.

1. Log in to the CCE console.
2. Click the cluster name to go to the cluster console, choose Workloads in the navigation pane, and click Create Workload in the upper right corner.
3. In Advanced Settings, choose Scheduling and select a policy for Load Affinity. In this example, Customize affinity is selected. For details about how to create a workload, see Creating a Workload.

   

   Table 1 Scheduling policies

   Parameter	Description	Example
   Load affinity	Not configured: No load affinity policy is configured. Preferred multi-AZ deployment: Workload pods are preferentially scheduled to nodes in different AZs through pod anti-affinity. The AZs serve as topology domains in this process. Forcible multi-AZ deployment: Workload pods are forcibly scheduled to nodes in different AZs through pod anti-affinity. The AZs serve as topology domains in this process. When this scheduling policy is used, if there are fewer nodes than pods or node resources are insufficient, the extra pods will fail to run. Customize affinity: You can configure flexible scheduling policies using pod labels to forcibly or preferentially deploy pods to specific AZs or nodes. You can also use label selectors to customize affinity and anti-affinity rules that control whether pods must run on, prefer, or avoid nodes matching specific conditions. For details about the supported scheduling policies, see Table 2. Select a proper policy type and add a policy. For details about the parameters, see Table 3.	Customize affinity

4. Select a proper load affinity rule and click to add a scheduling policy. In this example, a scheduling policy is added in the Required area under Workload Affinity. This policy specifies that the newly created workload can only be scheduled to a node if a workload with specific labels is already running on that node.

   Table 2 Load affinity policies

   Policy	Rule Type	Description	Example
   Workload affinity	Required	Hard constraint, which corresponds to requiredDuringSchedulingIgnoredDuringExecution in YAML for specifying the conditions that must be met. Select pods that require affinity by label. If such pods already run on a node in the topology domain, the scheduler will forcibly schedule the created pods to that topology domain. NOTE: If multiple affinity rules are configured, multiple labels will be used to filter pods that require affinity, and the newly created pods must have affinity with all pods that meet the label filtering conditions. In this way, all pods that meet the label filtering conditions locate in the same topology domain for scheduling.	Required
   	Preferred	Soft constraint, which corresponds to preferredDuringSchedulingIgnoredDuringExecution in YAML for specifying the conditions that need to be met as much as possible. Select pods that require affinity by label. If such pods already run on a node in the topology domain, the scheduler will preferentially schedule the created pods to that topology domain. NOTE: If multiple affinity rules are configured, multiple labels will be used to filter pods that require affinity, and the newly created pods will be preferentially to have affinity with multiple pods that meet the label filtering conditions. However, even if no pod meets the label filter conditions, a topology domain will be selected for scheduling.
   Workload anti-affinity	Required	Hard constraint, which corresponds to requiredDuringSchedulingIgnoredDuringExecution in YAML for specifying the conditions that must be met. Select one or more pods that require anti-affinity by label. If such pods already run on a node in the topology domain, the scheduler will not schedule the created pods to that topology domain. NOTE: If multiple anti-affinity rules are configured, multiple labels will be used to filter pods that require anti-affinity, and the newly created pods must be anti-affinity with all pods that meet the label filtering conditions. In this way, all the topology domains where the pods that meet the label filtering conditions locate will not be scheduled.	None
   	Preferred	Soft constraint, which corresponds to preferredDuringSchedulingIgnoredDuringExecution in YAML for specifying the conditions that need to be met as much as possible. Select one or more pods that require anti-affinity by label. If such pods already run on a node in the topology domain, the scheduler will preferentially schedule the created pods to other topology domains. NOTE: If multiple anti-affinity rules are configured, multiple labels will be used to filter pods that require anti-affinity, and the newly created pods will be preferentially to be anti-affinity with multiple pods that meet the label filtering conditions. However, even if all topology domains involve the pods that require anti-affinity, a topology domain will be selected for scheduling.

5. In the dialog box that is displayed on the right, click Add Policy to configure rules for filtering node labels.

   Table 3 Parameters for configuring load affinity/anti-affinity scheduling policies

   Parameter	Description	Example
   Weight	This parameter is available only in a preferred scheduling policy. Weights range from 1 to 100 and are taken into account as an extra scoring factor during scheduling. The scheduler combines the weight with other priority functions of the node to determine the final score and then assigns pods to the node with the highest total score.	None
   Namespace	Namespace for which the scheduling policy takes effect.	default
   Topology Domain	A topology domain (topologyKey) determines the range of nodes to be scheduled based on node labels, identifies affinity/anti-affinity objects based on the labels and operators, and performs scheduling based on the topology domain where the target object is located. For example, if the node label is kubernetes.io/hostname, the label value will be a Kubernetes node name. Kubernetes nodes with different names are assigned to different topology domains. This allows for workload affinity scheduling on a single node, as each topology domain contains only one node. If the specified label is kubernetes.io/os, the label value will be a node OS. Nodes running different OSs are assigned to different topology domains. This allows for workload affinity scheduling on multiple nodes, as each topology domain contains multiple nodes. For example, if pods that meet the load affinity rule are running on a node in a topology domain, all nodes in the topology domain can be scheduled.	kubernetes.io/hostname
   Label Key	When configuring a workload affinity or anti-affinity policy, enter the workload label to be matched. Both default labels and custom labels are supported.	app
   Operator	The following operators are supported: In: The label of the affinity or anti-affinity object is in the label value list (values field). NotIn: The label of the affinity or anti-affinity object is not in the label value list (values field). Exists: The affinity or anti-affinity object has a specified label key. DoesNotExist: The affinity or anti-affinity object does not have a specified label key.	In
   Label Value	When configuring a workload affinity or anti-affinity policy, enter the value of the workload label.	backend

6. After the scheduling policy is added, click Create Workload.
7. Verify that all pods run on the target node.
   1. In the navigation pane, choose Workloads.
   2. Click the workload name to enter its details page. On the Pods tab, verify that the new pod and the existing backend pod run on the same node.

      

• Workload affinity

  Kubernetes supports affinity between pods, which allows the frontend and backend pods of an application to be deployed together to minimize access latency.

  Assume that the backend pods of an application have been created with label app=backend. You can use .spec.affinity.podAffinity to configure workload affinity so that the frontend pods (labeled app=frontend) and backend pods (labeled app=backend) can be deployed together.

  apiVersion: apps/v1
  kind: Deployment
  metadata:
    name: frontend
    labels:
      app: frontend
  spec:
    selector:
      matchLabels:
        app: frontend
    replicas: 3
    template:
      metadata:
        labels:
          app: frontend
      spec:
        containers:
        - image: nginx:alpine
          name: frontend
          resources:
            requests:
              cpu: 100m
              memory: 200Mi
            limits:
              cpu: 100m
              memory: 200Mi
        imagePullSecrets:
        - name: default-secret
        affinity:  # Configure a scheduling policy.
          podAffinity:  # Workload affinity scheduling rule
            requiredDuringSchedulingIgnoredDuringExecution:   # Scheduling policy that must be met
            - topologyKey: prefer    # Topology domains are divided based on node labels, among which prefer is a custom label.
              labelSelector:  # Select workloads that meet the requirements based on workload labels.
                matchExpressions: # Workload label matching rule
                - key: app # The key of the workload label is app.
                  operator: In # The rule is met if a value exists in the value list.
                  values: # Workload label values
                  - backend
            preferredDuringSchedulingIgnoredDuringExecution:    # Scheduling policy that is met as much as possible
            - weight: 100  # Priority that can be configured when the best-effort policy is used. The value ranges from 1 to 100. A larger value indicates a higher priority.
              podAffinityTerm:  # Affinity configuration when the best-effort policy is used
                topologyKey: topology.kubernetes.io/zone   # Topology domains are divided based on node labels by node AZ.
                labelSelector:
                  matchExpressions:
                  - key: app
                    operator: In
                    values:
                    - backend

  During workload scheduling in the preceding example, node topology domains are divided based on the prefer label using the rule that must be met. If backend pods (labeled app=backend) are running on a node in the topology domain, frontend pods (labeled app=frontend) will also be deployed in that topology domain, even if not all nodes in the topology domain are running the backend pods. According to the best-effort rule, topology domains are divided based on topology.kubernetes.io/zone by node AZ. This ensures that the frontend and backend pods are deployed on nodes within the same AZ as much as possible.

  

  For workload affinity, topologyKey cannot be left blank when requiredDuringSchedulingIgnoredDuringExecution and preferredDuringSchedulingIgnoredDuringExecution are used.

  topologyKey is used to divide topology domains based on the labels of nodes. Nodes with the same labels are grouped into the same topology domain. The scheduler then selects the topology domain to be scheduled based on the workload label. A topology domain can consist of multiple nodes. If a workload that meets a label selection rule runs on a node in a topology domain, all nodes in the topology domain can be scheduled.

  For example, if the topologyKey label is set to topology.kubernetes.io/zone, nodes' AZs will be used as the topology domains, and workloads will be scheduled by AZ during deployment.

• Workload anti-affinity

  In some cases, pods need to be deployed separately. This is because deploying them together can negatively impact performance.

  Assume that the frontend pods of an application have been created with label app=frontend. To ensure that pods are deployed on different nodes and multiple AZs are preferred, you can use .spec.affinity.podAntiAffinity to configure workload anti-affinity.

  apiVersion: apps/v1
  kind: Deployment
  metadata:
    name:   frontend
    labels:
      app:  frontend
  spec:
    selector:
      matchLabels:
        app: frontend
    replicas: 5
    template:
      metadata:
        labels:
          app:  frontend
      spec:
        containers:
        - image:  nginx:alpine
          name:  frontend
          resources:
            requests:
              cpu:  100m
              memory:  200Mi
            limits:
              cpu:  100m
              memory:  200Mi
        imagePullSecrets:
        - name: default-secret
        affinity:
          podAntiAffinity:  # Workload anti-affinity scheduling rule
            requiredDuringSchedulingIgnoredDuringExecution:   # Scheduling policy that must be met
            - topologyKey: kubernetes.io/hostname    # Topology domains are divided based on node labels.
              labelSelector:    # Pod label matching rule
                matchExpressions:  # The key of the workload label is app.
                - key: app  # The key of the workload label is app.
                  operator: In  # The rule is met if a value exists in the value list.
                  values:  # Workload label values
                  - frontend
            preferredDuringSchedulingIgnoredDuringExecution:    # Scheduling policy that is met as much as possible
            - weight: 100  # Priority that can be configured when the best-effort policy is used. The value ranges from 1 to 100. A larger value indicates a higher priority.
              podAffinityTerm:  # Affinity configuration when the best-effort policy is used
                topologyKey: topology.kubernetes.io/zone   # Topology domains are divided based on node labels by node AZ.
                labelSelector:
                  matchExpressions:
                  - key: app
                    operator: In
                    values:
                    - frontend

  In the preceding example, anti-affinity rules are configured. The rule that must be met indicates that node topology domains are divided based on kubernetes.io/hostname. Nodes with the kubernetes.io/hostname label have different label values. Therefore, there is only one node in each topology domain. If a topology domain contains only one node where a frontend pod already exists, pods with the same label will not be scheduled to that topology domain. According to the best-effort rule, topology domains are divided based on topology.kubernetes.io/zone by node AZ. This ensures that the pods are deployed on nodes in different AZs as much as possible.

  

  For workload anti-affinity, when requiredDuringSchedulingIgnoredDuringExecution is used, the default access controller LimitPodHardAntiAffinityTopology of Kubernetes requires that topologyKey can only be kubernetes.io/hostname. To use other custom topology logic, modify or disable the access controller.