Why Are Labels Required?

As resources increase, how to classify and manage resources becomes important. Kubernetes provides a mechanism to classify resources, that is, using labels. Labels are simple but powerful. Almost all resources in the Kubernetes can be organized by labels.

A label is a key-value pair, which can be set when a resource is created, or can be added or modified later.

Taking pods as an example, as the number of pods increases, pods become cluttered and difficult to manage, as shown in the following figure.

Figure 1 Pods without classification

If we attach different labels to the pods, the situation is totally different, as shown in the following figure.

Figure 2 Pods organized with labels

Adding a Label

A label is a key-value pair. As shown below, two labels app=nginx and env=prod are set for a pod.

apiVersion: v1
kind: Pod
metadata:
  name: nginx
  labels:                     # Set two labels for the pod
    app: nginx    
    env: prod
spec:
  containers:
  - image: nginx:latest
    name: container-0
    resources:
      limits:
        cpu: 500m
        memory: 1024Mi
      requests:
        cpu: 500m
        memory: 1024Mi
  imagePullSecrets:
  - name: imagepull-secret

When a pod has labels, you can view the label of the pod by using --show-labels when querying the pod.

$ kubectl get pod --show-labels -n $namespace_name
NAME              READY   STATUS    RESTARTS   AGE   LABELS
nginx             1/1     Running   0          50s   app=nginx,env=prod

You can also use -L to query only certain labels.

$ kubectl get pod -L app,env -n $namespace_name
NAME              READY   STATUS    RESTARTS   AGE   APP     ENV
nginx             1/1     Running   0          1m    nginx   prod

For an existing pod, you can directly run the kubectl label command to add a label.

$ kubectl label po nginx creation_method=manual -n $namespace_name
pod "nginx" labeled

$ kubectl get pod --show-labels -n $namespace_name
NAME              READY   STATUS    RESTARTS   AGE   LABELS
nginx             1/1     Running   0          50s   app=nginx,env=prod,creation_method=manual

Modifying a Label

If you want to modify an existing label, you need to add --overwrite to the command, as shown below:

$ kubectl label po nginx env=debug --overwrite -n $namespace_name
pod "nginx" labeled

$ kubectl get pod --show-labels -n $namespace_name
NAME              READY   STATUS    RESTARTS   AGE   LABELS
nginx             1/1     Running   0          50s   app=nginx,env=debug,creation_method=manual