How to Provision Node Labels and Selectors in Kubernetes? (K8s)

Published in

AVM Consulting Blog

4 min readDec 5, 2020

Labels selectors are core grouping primitive in Kubernetes. They are used by the users to select a set of objects. Kubernetes API currently supports two types of selectors − Equality-based selectors.

You can constrain a pod to run only on particular nodes. The recommended approach to do this is to use label selectors to make the selection.

Types of Kubernetes Selector

Following are the types of Kubernetes selector.

1. Label Selector

We apply labels to the Kubernetes objects to organize or select a group of objects. Labels can be attached at creation time or added and modified at any time.

apiVersion: v1
kind: Pod
metadata:
name: apache-web-server
labels:
env: development
app: apache-web
spec:
containers:
- name: apache
image: apache
ports:
- containerPort: 80

List all the pods with labels using the below command:


kubectl get pods --show-labelsNAME        READY     STATUS    RESTARTS    AGE      LABELS
apache-web  1/1       Running    0          10m    env=development

You can add a label to the pod as:

kubectl label pods apache-web owner=devopskubectl get pods — show-labels
NAME        READY   STATUS  RESTARTS   AGE    LABELS          apache-web   1/1    Running   0        16 env=development,owner=devops

The --selector the option can be abbreviated to -l, so to select pods that are labeled with env=development, do:

kubectl get pods -l env=development
NAME        READY     STATUS    RESTARTS   AGE 
apache-web   1/1       Running   0          27m

2. Field Selector

Field selector is used to querying Kubernetes objects based on the value of one or more resource fields that we need to define when we create the objects.

$kubectl get pods — field-selector=metadata.name=apache-web
$kubectl get pods — field-selector=metadata.namespace=defaultThis kubectl command selects all Pods for which the value of the status.phase field is Running:kubectl get pods --field-selector status.phase=Running

3. Node Selector

The node selector is used when we have to deploy a pod or group of pods on a specific group of nodes that passed the criteria defined in the configuration file.

$kubectl label <object_type> <object_name> key=value
$kubectl label node server1 disk=hdd

Explanation: In the above example, rename the node ‘server1’ with the key is equal to “disk” and the value for this key is “HDD”. If the key is already assigned with a different value and want to change it then need to use the option ‘–overwrite’ as below: –

$kubectl lable node server1 disk=hdd --overwrite

Yaml file

--- 
apiVersion: v1
app: apache-web
containers: 
  - 
    name: apache
disk: hdd
env: prod
image: apache
kind: Pod
labels: ~
metadata: ~
name: apache-web-hdd
nodeSelector: ~
ports: 
  - 
    containerPort: 80
spec: ~

GKE (Google Kubernetes Engine)

You can attach labels to nodes using the “kubectl label nodes” command.

List all nodes:

kubectl get nodesNAME                         STATUS ROLES     AGE VERSIONgke-cluster-1-75a9c0b4-83dn   Ready  < none >  3h  v1.11.6-gke.2gke-cluster-1-75a9c0b4-cgzw   Ready  < none >  3h  v1.11.6-gke.2gke-cluster-1-75a9c0b4-rd54   Ready  < none >  3h  v1.11.6-gke.2

Add a label to a node:

kubectl label nodes gke-cluster-1-75a9c0b4-83dn env=prod node "gke-cluster-1-75a9c0b4-83dn" labeled

You can verify that it worked by running

kubectl get nodes --show-labels

or you can also use

kubectl describe node gke-cluster-1-75a9c0b4-83dnName:   gke-cluster-1-75a9c0b4-83dnRoles:  < none >Labels: beta.kubernetes.io/arch=amd64        beta.kubernetes.io/fluentd-ds-ready=true        beta.kubernetes.io/instance-type=n1-standard-1        beta.kubernetes.io/os=linux        cloud.google.com/gke-nodepool=default-pool        cloud.google.com/gke-os-distribution=cos        env=prod        failure-domain.beta.kubernetes.io/region=us-central1............

Built-in node labels

In addition to the labels you attach, nodes will have a standard set of labels.

Some of the labels are:

kubernetes.io/hostname
failure-domain.beta.kubernetes.io/zone
failure-domain.beta.kubernetes.io/region
beta.kubernetes.io/instance-type
beta.kubernetes.io/os
beta.kubernetes.io/arch

Node Selectors

nodeSelector is the simplest recommended form of node selection constraint.

nodeSelector is a field of PodSpec. It specifies a map of key-value pairs.

Pod manifest :

--- 
apiVersion: v1
kind: Pod
metadata: 
  labels: 
    env: test
  name: nginx
spec: 
  containers: 
    - 
      image: nginx
      name: nginx
  nodeSelector: 
    env: prod

If you are using workload controller for your application, you have to specify nodeSelector in pod template (spec. template. spec. nodeSelector).

--- 
spec: 
  replicas: 3
  selector: 
    matchLabels: 
      app: webapp
      tier: frontend
  template: 
    metadata: 
      labels: 
        app: webapp
        tier: frontend
    spec: 
      containers: 
        - 
          image: "webapp:1.0"
          name: webapp
          ports: 
            - 
              containerPort: 80
      nodeSelector: 
        env: prodenv: prod