Why labels are mentioned three times in a single deployment

Question

I've gone over the following docomentation page: https://kubernetes.io/docs/concepts/workloads/controllers/deployment/

The example deployment yaml is as follows:

apiVersion: apps/v1
kind: Deployment
metadata:
  name: nginx-deployment
  labels:
    app: nginx
spec:
  replicas: 3
  selector:
    matchLabels:
      app: nginx
  template:
    metadata:
      labels:
        app: nginx
    spec:
      containers:
      - name: nginx
        image: nginx:1.7.9
        ports:
        - containerPort: 80

We can see here three different times where the label app: nginx is mentioned.

Why do we need each of them? I had a hard time understanding it from the official documentation.

Answer 1

As we know it, the labels are to identify the resources,

• First label identifies the Deployment itself
• Third one is falls under the Pod template section. So, this one is specific to the Pod.
• Second one i.e the matchLabels is used to tell Services, ReplicaSet and other resources to act on the resources on the specified label conditions.

While first and third ones are label assignment to Deployment and Pods respectively, the second one is matching condition expression rather than assignment.

Though all 3 have same labels in the real world examples, First one can be different than second and third ones. But, second and third one usually to be identical as the second is the conditional expression that acts upon third one.