04-Labels-and-Selectors.md

Labels and Selectors

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In this section, we will take a look at Labels and Selectors

Labels and Selectors are standard methods to group things together.

Labels are properties attached to each item.

Selectors help you to filter these items

How are labels and selectors are used in kubernetes?

• We have created different types of objects in kubernetes such as PODs, ReplicaSets, Deployments etc.

  

How do you specify labels?

 apiVersion: v1
 kind: Pod
 metadata:
  name: simple-webapp
  labels:
    app: App1
    function: Front-end
 spec:
  containers:
  - name: simple-webapp
    image: simple-webapp
    ports:
    - containerPort: 8080

Once the pod is created, to select the pod with labels run the below command

$ kubectl get pods --selector app=App1

Kubernetes uses labels to connect different objects together

 apiVersion: apps/v1
 kind: ReplicaSet
 metadata:
   name: simple-webapp
   labels:
     app: App1
     function: Front-end
 spec:
  replicas: 3
  selector:
    matchLabels:
     app: App1
  template:
    metadata:
      labels:
        app: App1
        function: Front-end
    spec:
      containers:
      - name: simple-webapp
        image: simple-webapp   

For services

  ```
  apiVersion: v1
  kind: Service
  metadata:
   name: my-service
  spec:
   selector:
     app: App1
   ports:
   - protocol: TCP
     port: 80
     targetPort: 9376 
   ```

Annotations

• While labels and selectors are used to group objects, annotations are used to record other details for informative purpose.

  apiVersion: apps/v1
  kind: ReplicaSet
  metadata:
    name: simple-webapp
    labels:
      app: App1
      function: Front-end
    annotations:
       buildversion: 1.34
  spec:
   replicas: 3
   selector:
     matchLabels:
      app: App1
  template:
    metadata:
      labels:
        app: App1
        function: Front-end
    spec:
      containers:
      - name: simple-webapp
        image: simple-webapp   
  

  

K8s Reference Docs:

• https://kubernetes.io/docs/concepts/overview/working-with-objects/labels/