Kubernetes - Node Selectors and Affinity 🔘

Description 👀

You have different types of workloads running on your cluster, and you want to make sure that the right workloads are running on the right nodes. For example, you might want to run a set of pods on a high CPU node, and run another set of pods on a low CPU node. You can use node selectors to make sure that the right pods are running on the right nodes.

• Node Selectors

  Node selectors are constrained to node labels. Node labels are key-value pairs that are attached to nodes. Node labels can be used in node selectors specified in pod definitions. Node selectors work by matching node labels with pod labels. A pod can only be scheduled onto a node if the node labels match the pod labels.

• Node Affinity

  Instead you can use Node Affinity to perform the same task. Node Affinity is a field in pod definitions. It specifies a set of rules that are used to match nodes with pods. Node Affinity can be used in conjunction with node selectors. Node Affinity can also be used to specify a preferred node that should be used to run a pod. Node Affinity is more expressive than node selectors. Node Affinity can specify weight for each rule. Node Affinity can specify whether or not a rule is required. Node Affinity can specify how many rules must be satisfied for the pod to be scheduled onto a node.

Basic Commands 📝

• to label a node

    kubectl label nodes <node-name> <label-key>=<label-value>
  

Examples 🧩

• sample pod definition with node selector

    apiVersion: v1
    kind: Pod
    metadata:
        name: my-app
    spec:
        containers:
          - name: nginx
            image: nginx
        nodeSelector:
            size: Large
  

• sample pod definition with node affinity

    apiVersion: v1
    kind: Pod
    metadata:
        name: my-app
    spec:
        containers:
          - name: nginx
            image: nginx
        affinity:
            nodeAffinity:
                requiredDuringSchedulingIgnoredDuringExecution:
                    nodeSelectorTerms:
                      - matchExpressions:
                          - key: size
                            operator: In
                            values:
                              - Large
  

Red and Blue Example 🔴🟦

• red deployment definition 🔴

    apiVersion: apps/v1
    kind: Deployment
    metadata:
        name: red
    spec:
        replicas: 2
        selector:
            matchLabels:
            app: red
        template:
            metadata:
                name: red
                labels:
                    app: red
            spec:
                containers:
                  - image: nginx
                    name: nginx
                affinity:
                    nodeAffinity:
                        requiredDuringSchedulingIgnoredDuringExecution:
                            nodeSelectorTerms:
                              - matchExpressions:
                                  - key: color
                                    operator: In
                                    values:
                                      - red
  

• blue deployment definition 🟦

    apiVersion: apps/v1
    kind: Deployment
    metadata:
        name: blue
    spec:
        replicas: 3
        selector: 
            matchLabels:
                app: nginx
        template:
            metadata:
                name: blue
                labels:
                    type: pod
                    app: nginx
            spec:
                containers:
                  - name: nginx
                    image: nginx
                affinity:
                    nodeAffinity:
                        requiredDuringSchedulingIgnoredDuringExecution:
                            nodeSelectorTerms:
                              - matchExpressions:
                                  - key: color
                                    operator: In
                                    values:
                                      - blue
  

↩️