Kubernetes Labels and Selectors

Labels

Labels are kind of tags that can be assigned to any object in kubernetes.
A label of the mechanism you use to organize Kubernetes object.
A label is a key value pair without any predefined meaning that can be attached to the object
Labels are similar to tags in AWS or git where you use a name for quick reference
So you are free to choose labels as you need it to refer an environment which is used for dev or testing or production, refer a product group like Department A department B etc.
Multiple labels can be added to a single object.
Unlike name or UID's, labels do not provide uniqueness, as in general, we can expect many objects to carry the same label.

Label Selectors

Selectors are the way to filter objects using labels also called label selectors.
The API currently support two types of selectors that is equality based and set based
A label selector can be made of multiple requirements which are comma separated

Type of selector	Operator	Example
Equality based	`=`,`!=`	`name=example`
Set based	`notin`, `in`, `exist`	`env in (production, dev)`

More examples of set-based selectors i.e., match multiple values

kubectl get pods -l 'env in (development, testing)'

kubectl get pods -l 'env notin (development, testing)'

kubectl get pods -l class=pods,env=testing

Example of labels

Labels are always written in metadata

k8s_example_5.yml

kind: Pod
apiVersion: v1
metadata:
  name: example-5
  labels:
    env: development
    class: pods
spec:
  containers:
    - name: c00
      image: ubuntu
      command:
        [
          "/bin/bash",
          "-c",
          "while true; do echo The hostname is $(hostname); sleep 5; done",
        ]

Commands

Create pod

kubectl apply -f  k8s_example_5.yml

Output

controlplane $ kubectl apply -f  k8s_example_5.yml
pod/example-5 created

Adding a label to an existing pod

kubectl label pods example-5 group=example

Here example-5 is a pod name. group=example is the label

key	value	Description
`group`	`example`	Here in the label it is in form of Key-Pair

Output

controlplane $ kubectl label pods example-5 group=example
pod/example-5 labeled

Show labels

kubectl get pods --show-labels

Output

controlplane $ kubectl get pods --show-labels
NAME        READY   STATUS    RESTARTS   AGE   LABELS
example-5   1/1     Running   0          46s   class=pods,env=development,group=example
example-6   2/2     Running   0          21m   class=pods,env=development
example-7   2/2     Running   0          36m   class=pods,env=development

Filter pods using labels

kubectl get pods -l group=example
kubectl get pods -l group!=example

Output

controlplane $ kubectl get pods -l group=example
NAME        READY   STATUS    RESTARTS   AGE
example-5   1/1     Running   0          24m
controlplane $ kubectl get pods -l group!=example
NAME        READY   STATUS    RESTARTS   AGE
example-6   2/2     Running   0          29m
example-7   2/2     Running   0          32m

Delete pods using labels

kubectl delete pods -l group=example

Output

controlplane $ kubectl delete pods -l group=example
pod "example-5" deleted

Pods Node Selectors