labels, selectors, and annotations

Labels are key-value pairs attached to Kubernetes objects. They are used for identifying various attributes of objects which can in turn be used to select and group various subsets of those objects. We can attach labels to objects by listing them in the metadata.labels section of an object descriptor:

my-production-label-pod

apiVersion: v1
kind: Pod
metadata:
  name: my-production-label-pod
  labels:
    app: my-app
    environment: production
spec:
  containers:
  - name: nginx
    image: nginx

Selectors are used for identifying and selecting a specific group of objects using their labels. One way to use selectors is to use them with kubectl get to retrieve a specific list of objects. We can specify a selector using the -l flag.

kubectl get pods -l app=my-app

Annotations are similar to labels in that they can be used to store custom metadata about objects. However, unlike labels, annotations cannot be used to select or group objects in Kubernetes. External tools can read, write, and interact with annotation.

apiVersion: v1
kind: Pod
metadata:
  name: annotations-demo
  annotations:
    imageregistry: "https://hub.docker.com/"
spec:
  containers:
  - name: nginx
    image: nginx:1.14.2
    ports:
    - containerPort: 80