Why we need annotations ?

We can use either labels or annotations to attach metadata to Kubernetes objects. Labels can be used to select objects and to find collections of objects that satisfy certain conditions. In contrast, annotations are not used to identify and select objects. The metadata in an annotation can be small or large, structured or unstructured, and can include characters not permitted by labels.

Annotate Pod

Lets say , if you want to add a download URL to our running pod e.g webserver pod.

$ kubectl annotate pod webserver url=https://hub.docker.com/_/nginx/

View annotations

$ kubectl describe pod webserver

You will see Annotations filed contains the above URL.

Remove annotation

Use same annotate command and mention only key with a dash (-) at the end of the key . Below command will remove the annotation from Pod.

$ kubectl annotate pod webserver-

How can i create labels and annotations declaratively

  • Create a manifest file like below
apiVersion: v1
kind: Pod
metadata:
  name: nginx
  labels:
     app: nginx
  annotations:
    build: demo
    environment: dev
    owner: Orphan
spec:
  containers:
  - name: nginx-demo
    image: nginx:1.9.1
    ports:
    - containerPort: 80

  • Save it as a yaml file e.g annotate-pod.yaml

  • Deploy Pod and Describe pod to see annotation.

$ kubectl -f annotate-pod.yaml
  • Lets describe the pod to check the annotation and labels.
$ kubectl describe pod nginx

What are the use-cases for Annotations?

  • Annotations are used for non-identifying information for the tools or services which are not used internally by k8s.

  • You can’t specify selectors over them within Kubernetes, but they can be used by external tools and libraries.

  • For example If you plan to use AWS ALB as ingress controller add the alb.ingress.kubernetes.io/actions.${action-name} annotation

If you want expose the metrics of your application to the Prometheus, then you can prometheus.io/scrape: “true” annotation annotation to your pod configuration.