Controlling pod placement on nodes using node affinity rules

You can use a node affinity to control which nodes your pod can be scheduled on based on node labels. Node affinity helps you ensure your applications run on nodes with specific capabilities or configurations.

In OpenShift Container Platform node affinity is a set of rules used by the scheduler to determine where a pod can be placed. The rules are defined using custom labels on the nodes and label selectors specified in pods.

Understanding node affinity

To specify a preference towards a group of nodes that a pod can be placed on, you can use a node affinity in the pod spec. For example, you could configure a pod to run only on a node with a specific CPU or in a specific availability zone. The node does not have control over the placement.

There are two types of node 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.

note

If labels on a node change at runtime that results in an node affinity rule on a pod no longer being met, the pod continues to run on the node.

You configure node affinity through the Pod spec file. 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 is a Pod spec with a rule that requires the pod be placed on a node with a label whose key is e2e-az-NorthSouth and whose value is either e2e-az-North or e2e-az-South:

Example pod configuration file with a node affinity required rule
apiVersion: v1
kind: Pod
metadata:
  name: with-node-affinity
spec:
  securityContext:
    runAsNonRoot: true
    seccompProfile:
      type: RuntimeDefault
  affinity:
    nodeAffinity:
      requiredDuringSchedulingIgnoredDuringExecution:
        nodeSelectorTerms:
        - matchExpressions:
          - key: e2e-az-NorthSouth
            operator: In
            values:
            - e2e-az-North
            - e2e-az-South
  containers:
  - name: with-node-affinity
    image: docker.io/ocpqe/hello-pod
    securityContext:
      allowPrivilegeEscalation: false
      capabilities:
        drop: [ALL]
# ...

where:

spec.affinity.nodeAffinity: Specifies the stanza to configure node affinity.
spec.affinity.nodeAffinity.requiredDuringSchedulingIgnoredDuringExecution: Specifies a required rule. Configure the following nodeSelectorTerms.matchExpressions parameters:
key: Specifies the key of the key/value pair (label) that must be matched to apply the rule.
operator: Specifies the relationship between the label on the node and the set of values in the matchExpression parameters in the Pod spec. This value can be In, NotIn, Exists, or DoesNotExist, Lt, or Gt.
values: Specifies the value of the key/value pair (label) that must be matched to apply the rule.

The following example is a node specification with a preferred rule that a node with a label whose key is e2e-az-EastWest and whose value is either e2e-az-East or e2e-az-West is preferred for the pod:

Example pod configuration file with a node affinity preferred rule
apiVersion: v1
kind: Pod
metadata:
  name: with-node-affinity
spec:
  securityContext:
    runAsNonRoot: true
    seccompProfile:
      type: RuntimeDefault
  affinity:
    nodeAffinity:
      preferredDuringSchedulingIgnoredDuringExecution:
      - weight: 1
        preference:
          matchExpressions:
          - key: e2e-az-EastWest
            operator: In
            values:
            - e2e-az-East
            - e2e-az-West
  containers:
  - name: with-node-affinity
    image: docker.io/ocpqe/hello-pod
    securityContext:
      allowPrivilegeEscalation: false
      capabilities:
        drop: [ALL]
# ...

where:

spec.affinity.nodeAffinity: Specifies the stanza to configure node affinity.
spec.affinity.nodeAffinity.preferredDuringSchedulingIgnoredDuringExecution: Specifies a preferred rule. Configure a weight and the following preference.matchExpression parameters:
weight: Specifies a weight for a preferred rule. The node with highest weight is preferred.
key: Specifies the key of the key/value pair (label) that must be matched to apply the rule.
operator: Specifies the relationship between the label on the node and the set of values in the matchExpression parameters in the Pod spec. This value can be In, NotIn, Exists, or DoesNotExist, Lt, or Gt. There is no explicit node anti-affinity concept, but using the NotIn or DoesNotExist operator replicates that behavior.
values: Specifies the value of the key/value pair (label) that must be matched to apply the rule.

note

If you are using node affinity and node selectors in the same pod configuration, note the following:

If you configure both nodeSelector and nodeAffinity, both conditions must be satisfied for the pod to be scheduled onto a candidate node.
If you specify multiple nodeSelectorTerms associated with nodeAffinity types, then the pod can be scheduled onto a node if one of the nodeSelectorTerms is satisfied.
If you specify multiple matchExpressions associated with nodeSelectorTerms, then the pod can be scheduled onto a node only if all matchExpressions are satisfied.

Configure a required node affinity rule

You can use a required rule to instruct the scheduler that the rules must be met before a pod can be scheduled on a node.

The following steps demonstrate a simple configuration that creates a node and a pod that the scheduler is required to place on the node.

Procedure

Add a label to a node using the oc label node command:

$ oc label node node1 e2e-az-name=e2e-az1

tip

You can alternatively apply the following YAML to add the label:

kind: Node
apiVersion: v1
metadata:
  name: <node_name>
  labels:
    e2e-az-name: e2e-az1
#...

Create a pod with a specific label in the pod spec:
1. Create a YAML file with the following content:
  
  note
  You cannot add an affinity directly to a scheduled pod.
  Example output
```
apiVersion: v1
kind: Pod
metadata:
  name: s1
spec:
  affinity:
    nodeAffinity:
      requiredDuringSchedulingIgnoredDuringExecution:
        nodeSelectorTerms:
        - matchExpressions:
          - key: e2e-az-name
            values:
            - e2e-az1
            - e2e-az2
            operator: In
#...
```
  where:
  
  spec.affinity.nodeAffinity
  
  Specifies the stanza to configure node affinity.
  
  spec.affinity.nodeAffinity.requiredDuringSchedulingIgnoredDuringExecution
  
  Specifies a required rule. Configure the following nodeSelectorTerms.matchExpressions parameters:
  
  key
  
  Specifies the key of the key/value pair (label) that must be matched to apply the rule.
  
  operator
  
  Specifies the relationship between the label on the node and the set of values in the matchExpression parameters in the Pod spec. This value can be In, NotIn, Exists, or DoesNotExist, Lt, or Gt. There is no explicit node anti-affinity concept, but using the NotIn or DoesNotExist operator replicates that behavior.
  
  values
  
  Specifies the value of the key/value pair (label) that must be matched to apply the rule.
2. Create the pod:
```
$ oc create -f <file-name>.yaml
```

Configure a preferred node affinity rule

You can use a preferred rule to instruct the scheduler that if a matching node is not available, schedule the pod on a different node to ensure the workload application runs.

For a preferred rule, the scheduler tries to enforce the rule, but does not guarantee enforcement.

The following procedure demonstrates a simple configuration that creates a node and a pod that the scheduler tries to place on the node.

Procedure

Add a label to a node using the oc label node command:
```
$ oc label node node1 e2e-az-name=e2e-az3
```
Create a pod with a specific label:
1. Create a YAML file with the following content:
  
  note
  You cannot add an affinity directly to a scheduled pod.
```
apiVersion: v1
kind: Pod
metadata:
  name: s1
spec:
  affinity:
    nodeAffinity:
      preferredDuringSchedulingIgnoredDuringExecution:
      - weight:
        preference:
          matchExpressions:
          - key: e2e-az-name
            values:
            - e2e-az3
            operator: In
#...
```
  where:
  
  spec.affinity.nodeAffinity
  
  Specifies the stanza to configure node affinity.
  
  spec.affinity.nodeAffinity.preferredDuringSchedulingIgnoredDuringExecution
  
  Specifies a preferred rule. Configure a weight and the following preference.matchExpressions parameters. If you want the new pod to be scheduled on the node you edited, use the same key and values parameters as the label in the node.
  
  weight
  
  Specifies a weight for the node, as a number 1-100. The node with highest weight is preferred.
  
  key
  
  Specifies the key of the key/value pair (label) that must be matched to apply the rule.
  
  operator
  
  Specifies the relationship between the label on the node and the set of values in the matchExpression parameters in the Pod spec. This value can be In, NotIn, Exists, or DoesNotExist, Lt, or Gt. There is no explicit node anti-affinity concept, but using the NotIn or DoesNotExist operator replicates that behavior.
  
  values
  
  Specifies the value of the key/value pair (label) that must be matched to apply the rule.
2. Create the pod.
```
$ oc create -f <file-name>.yaml
```

Sample node affinity rules

To use node affinity, the pod spec that you want to schedule on a node must have a node selector that matches a label on the node.

The following examples demonstrate node affinity with or without matching labels.

Node affinity with matching labels

The following example demonstrates node affinity for a node and pod with matching labels:

The Node1 node has the label zone:us:

$ oc label node node1 zone=us

tip

You can alternatively apply the following YAML to add the label:

kind: Node
apiVersion: v1
metadata:
  name: <node_name>
  labels:
    zone: us
#...

The pod-s1 pod has the zone and us key/value pair under a required node affinity rule:

$ cat pod-s1.yaml

Example output
apiVersion: v1
kind: Pod
metadata:
  name: pod-s1
spec:
  securityContext:
    runAsNonRoot: true
    seccompProfile:
      type: RuntimeDefault
  containers:
    - image: "docker.io/ocpqe/hello-pod"
      name: hello-pod
      securityContext:
        allowPrivilegeEscalation: false
        capabilities:
          drop: [ALL]
  affinity:
    nodeAffinity:
      requiredDuringSchedulingIgnoredDuringExecution:
        nodeSelectorTerms:
          - matchExpressions:
            - key: "zone"
              operator: In
              values:
              - us
#...

The pod-s1 pod can be scheduled on Node1:

$ oc get pod -o wide

Example output
NAME     READY     STATUS       RESTARTS   AGE      IP      NODE
pod-s1   1/1       Running      0          4m       IP1     node1

Node affinity with no matching labels

The following example demonstrates node affinity for a node and pod without matching labels:

The Node1 node has the label zone:emea:

$ oc label node node1 zone=emea

tip

You can alternatively apply the following YAML to add the label:

kind: Node
apiVersion: v1
metadata:
  name: <node_name>
  labels:
    zone: emea
#...

The pod-s1 pod has the zone and us key/value pair under a required node affinity rule:

$ cat pod-s1.yaml

Example output
apiVersion: v1
kind: Pod
metadata:
  name: pod-s1
spec:
  securityContext:
    runAsNonRoot: true
    seccompProfile:
      type: RuntimeDefault
  containers:
    - image: "docker.io/ocpqe/hello-pod"
      name: hello-pod
      securityContext:
        allowPrivilegeEscalation: false
        capabilities:
          drop: [ALL]
  affinity:
    nodeAffinity:
      requiredDuringSchedulingIgnoredDuringExecution:
        nodeSelectorTerms:
          - matchExpressions:
            - key: "zone"
              operator: In
              values:
              - us
#...

The pod-s1 pod cannot be scheduled on Node1:

$ oc describe pod pod-s1

Example output
...

Events:
 FirstSeen LastSeen Count From              SubObjectPath  Type                Reason
 --------- -------- ----- ----              -------------  --------            ------
 1m        33s      8     default-scheduler Warning        FailedScheduling    No nodes are available that match all of the following predicates:: MatchNodeSelector (1).

Using node affinity to control where an Operator is installed

You can use affinities to schedule an Operator pod on a specific node or set of nodes.

By default, when you install an Operator, OpenShift Container Platform installs the Operator pod on one of your compute nodes randomly. However, the following examples describe situations where you might want to schedule an Operator pod to a specific node or set of nodes:

If an Operator requires a particular platform, such as amd64 or arm64
If an Operator requires a particular operating system, such as Linux or Windows
If you want Operators that work together scheduled on the same host or on hosts located on the same rack
If you want Operators dispersed throughout the infrastructure to avoid downtime due to network or hardware issues

You can control where an Operator pod is installed by adding a node affinity constraints to the Operator’s Subscription object.

The following examples show how to use node affinity to install an instance of the Custom Metrics Autoscaler Operator to a specific node in the cluster:

The following node affinity example places the Operator pod on a specific node:

apiVersion: operators.coreos.com/v1alpha1
kind: Subscription
metadata:
  name: openshift-custom-metrics-autoscaler-operator
  namespace: openshift-keda
spec:
  name: my-package
  source: my-operators
  sourceNamespace: operator-registries
  config:
    affinity:
      nodeAffinity:
        requiredDuringSchedulingIgnoredDuringExecution:
          nodeSelectorTerms:
          - matchExpressions:
            - key: kubernetes.io/hostname
              operator: In
              values:
              - ip-10-0-163-94.us-west-2.compute.internal
#...

This node affinity requires the Operator’s pod be scheduled on a node named ip-10-0-163-94.us-west-2.compute.internal.

The following node affinity example places the Operator pod on a node with a specific platform:

apiVersion: operators.coreos.com/v1alpha1
kind: Subscription
metadata:
  name: openshift-custom-metrics-autoscaler-operator
  namespace: openshift-keda
spec:
  name: my-package
  source: my-operators
  sourceNamespace: operator-registries
  config:
    affinity:
      nodeAffinity:
        requiredDuringSchedulingIgnoredDuringExecution:
          nodeSelectorTerms:
          - matchExpressions:
            - key: kubernetes.io/arch
              operator: In
              values:
              - arm64
            - key: kubernetes.io/os
              operator: In
              values:
              - linux
#...

This node affinity requires the Operator’s pod be scheduled on a node with the kubernetes.io/arch=arm64 and kubernetes.io/os=linux labels.

To control the placement of an Operator pod, complete the following steps.

Procedure

Install the Operator as usual.
If needed, ensure that your nodes are labeled to properly respond to the affinity.

Edit the Operator Subscription object to add an affinity:

apiVersion: operators.coreos.com/v1alpha1
kind: Subscription
metadata:
  name: openshift-custom-metrics-autoscaler-operator
  namespace: openshift-keda
spec:
  name: my-package
  source: my-operators
  sourceNamespace: operator-registries
  config:
    affinity:
      nodeAffinity:
        requiredDuringSchedulingIgnoredDuringExecution:
          nodeSelectorTerms:
          - matchExpressions:
            - key: kubernetes.io/hostname
              operator: In
              values:
              - ip-10-0-185-229.ec2.internal
#...

where:

spec.config.affinity: Specifies a nodeAffinity.

Verification

To ensure that the pod is deployed on the specific node, run the following command:

$ oc get pods -o wide

Example output
NAME                                                  READY   STATUS    RESTARTS   AGE   IP            NODE                           NOMINATED NODE   READINESS GATES
custom-metrics-autoscaler-operator-5dcc45d656-bhshg   1/1     Running   0          50s   10.131.0.20   ip-10-0-185-229.ec2.internal   <none>           <none>

Additional resources

Understanding how to update labels on nodes

Understanding node affinity​

Configure a required node affinity rule​

Configure a preferred node affinity rule​

Sample node affinity rules​

Node affinity with matching labels​

Node affinity with no matching labels​

Using node affinity to control where an Operator is installed​

Understanding node affinity

Configure a required node affinity rule

Configure a preferred node affinity rule

Sample node affinity rules

Node affinity with matching labels

Node affinity with no matching labels

Using node affinity to control where an Operator is installed