Overview

Pod affinity and pod anti-affinity allow you to specify rules about how pods should be placed relative to other pods. The rules are defined using custom labels on nodes and label selectors specified in pods. Pod affinity/anti-affinity allows a pod to specify an affinity (or anti-affinity) towards a group of pods it can be placed with. The node does not have control over the placement.

For example, using affinity rules, you could spread or pack pods within a service or relative to pods in other services. Anti-affinity rules allow you to prevent pods of a particular service from scheduling on the same nodes as pods of another service that are known to interfere with the performance of the pods of the first service. Or, you could spread the pods of a service across nodes or availability zones to reduce correlated failures.

Pod affinity/anti-affinity allows you to constrain which nodes your pod is eligible to be scheduled on based on the labels on other pods. A label is a key/value pair.

  • Pod affinity can tell the scheduler to locate a new pod on the same node as other pods if the label selector on the new pod matches the label on the current pod.

  • Pod anti-affinity can prevent the scheduler from locating a new pod on the same node as pods with the same labels if the label selector on the new pod matches the label on the current pod.

There are two types of pod affinity rules: required and preferred.

Required rules must be met before a pod can be scheduled on a node. Preferred rules specify that, if the rule is met, the scheduler tries to enforce the rules, but does not guarantee enforcement.

Depending on your pod priority and preemption settings, the scheduler might not be able to find an appropriate node for a pod without violating affinity requirements. If so, a pod might not be scheduled.

To prevent this situation, carefully configure pod affinity with equal-priority pods.

Configuring Pod Affinity and Anti-affinity

You configure pod affinity/anti-affinity through the pod specification files. You can specify a required rule, a preferred rule, or both. If you specify both, the node must first meet the required rule, then attempts to meet the preferred rule.

The following example shows a pod specification configured for pod affinity and anti-affinity.

In this example, the pod affinity rule indicates that the pod can schedule onto a node only if that node has at least one already-running pod with a label that has the key security and value S1. The pod anti-affinity rule says that the pod prefers to not schedule onto a node if that node is already running a pod with label having key security and value S2.

Sample pod config file with pod affinity
apiVersion: v1
kind: Pod
metadata:
  name: with-pod-affinity
spec:
  affinity:
    podAffinity: (1)
      requiredDuringSchedulingIgnoredDuringExecution: (2)
      - labelSelector:
          matchExpressions:
          - key: security (3)
            operator: In (4)
            values:
            - S1 (3)
        topologyKey: failure-domain.beta.kubernetes.io/zone
  containers:
  - name: with-pod-affinity
    image: docker.io/ocpqe/hello-pod
1 Stanza to configure pod affinity.
2 Defines a required rule.
3 The key and value (label) that must be matched to apply the rule.
4 The operator represents the relationship between the label on the existing pod and the set of values in the matchExpression parameters in the specification for the new pod. Can be In, NotIn, Exists, or DoesNotExist.
Sample pod config file with pod anti-affinity
apiVersion: v1
kind: Pod
metadata:
  name: with-pod-antiaffinity
spec:
  affinity:
    podAntiAffinity: (1)
      preferredDuringSchedulingIgnoredDuringExecution: (2)
      - weight: 100  (3)
        podAffinityTerm:
          labelSelector:
            matchExpressions:
            - key: security (4)
              operator: In (5)
              values:
              - S2
          topologyKey: kubernetes.io/hostname
  containers:
  - name: with-pod-affinity
    image: docker.io/ocpqe/hello-pod
1 Stanza to configure pod anti-affinity.
2 Defines a preferred rule.
3 Specifies a weight for a preferred rule. The node with the highest weight is preferred.
4 Description of the pod label that determines when the anti-affinity rule applies. Specify a key and value for the label.
5 The operator represents the relationship between the label on the existing pod and the set of values in the matchExpression parameters in the specification for the new pod. Can be In, NotIn, Exists, or DoesNotExist.

If labels on a node change at runtime such that the affinity rules on a pod are no longer met, the pod continues to run on the node.

Configuring an Affinity Rule

The following steps demonstrate a simple two-pod configuration that creates pod with a label and a pod that uses affinity to allow scheduling with that pod.

  1. Create a pod with a specific label in the pod specification:

    $ cat team4.yaml
    apiVersion: v1
    kind: Pod
    metadata:
      name: security-s1
      labels:
        security: S1
    spec:
      containers:
      - name: security-s1
        image: docker.io/ocpqe/hello-pod
  2. When creating other pods, edit the pod specification as follows:

    1. Use the podAffinity stanza to configure the requiredDuringSchedulingIgnoredDuringExecution parameter or preferredDuringSchedulingIgnoredDuringExecution parameter:

    2. Specify the key and value that must be met. If you want the new pod to be scheduled with the other pod, use the same key and value parameters as the label on the first pod.

          podAffinity:
            requiredDuringSchedulingIgnoredDuringExecution:
            - labelSelector:
                matchExpressions:
                - key: security
                  operator: In
                  values:
                  - S1
              topologyKey: failure-domain.beta.kubernetes.io/zone
    3. Specify an operator. The operator can be In, NotIn, Exists, or DoesNotExist. For example, use the operator In to require the label to be in the node.

    4. Specify a topologyKey, which is a prepopulated Kubernetes label that the system uses to denote such a topology domain.

  3. Create the pod.

    $ oc create -f <pod-spec>.yaml

Configuring an Anti-affinity Rule

The following steps demonstrate a simple two-pod configuration that creates pod with a label and a pod that uses an anti-affinity preferred rule to attempt to prevent scheduling with that pod.

  1. Create a pod with a specific label in the pod specification:

    $ cat team4.yaml
    apiVersion: v1
    kind: Pod
    metadata:
      name: security-s2
      labels:
        security: S2
    spec:
      containers:
      - name: security-s2
        image: docker.io/ocpqe/hello-pod
  2. When creating other pods, edit the pod specification to set the following parameters:

  3. Use the podAffinity stanza to configure the requiredDuringSchedulingIgnoredDuringExecution parameter or preferredDuringSchedulingIgnoredDuringExecution parameter:

    1. Specify a weight for the node, 1-100. The node that with highest weight is preferred.

    2. Specify the key and values that must be met. If you want the new pod to not be scheduled with the other pod, use the same key and value parameters as the label on the first pod.

          podAntiAffinity:
            preferredDuringSchedulingIgnoredDuringExecution:
            - weight: 100
              podAffinityTerm:
                labelSelector:
                  matchExpressions:
                  - key: security
                    operator: In
                    values:
                    - S2
                topologyKey: kubernetes.io/hostname
    3. For a preferred rule, specify a weight, 1-100.

    4. Specify an operator. The operator can be In, NotIn, Exists, or DoesNotExist. For example, use the operator In to require the label to be in the node.

  4. Specify a topologyKey, which is a prepopulated Kubernetes label that the system uses to denote such a topology domain.

  5. Create the pod.

    $ oc create -f <pod-spec>.yaml

Examples

The following examples demonstrate pod affinity and pod anti-affinity.

Pod Affinity

The following example demonstrates pod affinity for pods with matching labels and label selectors.

  • The pod team4 has the label team:4.

    $ cat team4.yaml
    apiVersion: v1
    kind: Pod
    metadata:
      name: team4
      labels:
         team: "4"
    spec:
      containers:
      - name: ocp
        image: docker.io/ocpqe/hello-pod
  • The pod team4a has the label selector team:4 under podAffinity.

    $ cat pod-team4a.yaml
    apiVersion: v1
    kind: Pod
    metadata:
      name: team4a
    spec:
      affinity:
        podAffinity:
          requiredDuringSchedulingIgnoredDuringExecution:
          - labelSelector:
              matchExpressions:
              - key: team
                operator: In
                values:
                - "4"
            topologyKey: kubernetes.io/hostname
      containers:
      - name: pod-affinity
        image: docker.io/ocpqe/hello-pod
  • The team4a pod is scheduled on the same node as the team4 pod.

Pod Anti-affinity

The following example demonstrates pod anti-affinity for pods with matching labels and label selectors.

  • The pod pod-s1 has the label security:s1.

    cat pod-s1.yaml
    apiVersion: v1
    kind: Pod
    metadata:
      name: s1
      labels:
        security: s1
    spec:
      containers:
      - name: ocp
        image: docker.io/ocpqe/hello-pod
  • The pod pod-s2 has the label selector security:s1 under podAntiAffinity.

    cat pod-s2.yaml
    apiVersion: v1
    kind: Pod
    metadata:
      name: pod-s2
    spec:
      affinity:
        podAntiAffinity:
          requiredDuringSchedulingIgnoredDuringExecution:
          - labelSelector:
              matchExpressions:
              - key: security
                operator: In
                values:
                - s1
            topologyKey: kubernetes.io/hostname
      containers:
      - name: pod-antiaffinity
        image: docker.io/ocpqe/hello-pod
  • The pod pod-s2 is not scheduled unless there is a node with a pod that has the security:s2 label. If there is no other pod with that label, the new pod remains in a pending state:

    NAME      READY     STATUS    RESTARTS   AGE       IP        NODE
    pod-s2    0/1       Pending   0          32s       <none>

Pod Affinity with no Matching Labels

The following example demonstrates pod affinity for pods without matching labels and label selectors.

  • The pod pod-s1 has the label security:s1.

    $ cat pod-s1.yaml
    apiVersion: v1
    kind: Pod
    metadata:
      name: pod-s1
      labels:
        security: s1
    spec:
      containers:
      - name: ocp
        image: docker.io/ocpqe/hello-pod
  • The pod pod-s2 has the label selector security:s2.

    $ cat pod-s2.yaml
    apiVersion: v1
    kind: Pod
    metadata:
      name: pod-s2
    spec:
      affinity:
        podAffinity:
          requiredDuringSchedulingIgnoredDuringExecution:
          - labelSelector:
              matchExpressions:
              - key: security
                operator: In
                values:
                - s2
            topologyKey: kubernetes.io/hostname
      containers:
      - name: pod-affinity
        image: docker.io/ocpqe/hello-pod
  • The pod pod-s2 cannot be scheduled on the same node as pod-s1.