labels.md

Labels and Selectors

Labels are the mechanism you use to organize Kubernetes objects.

A label is a key-value pair that is meaningful and relevant to users with certain restrictions concerning length and allowed values.

Exercise 1: Labels in pods

1. Save the following as label-pod.yaml

   cat > label-pod.yaml <<EOF
   apiVersion: v1
   kind: Pod
   metadata:
     name: label-pod
     labels:
       env: development
   spec:
     containers:
     - name: sise
       image: mhausenblas/simpleservice:0.5.0
       ports:
       - containerPort: 9876
   EOF

2. Run the following command to create the pod

   kubectl create -f label-pod.yaml

3. Run the following to list the pod, noting the new column

   kubectl get pods --show-labels
   
   NAME       READY     STATUS    RESTARTS   AGE    LABELS
   label-pod  1/1       Running   0          10m    env=development

4. Add an additional label to the pod by running

   kubectl label pods label-pod owner=you
   
   kubectl get pods --show-labels
   
   NAME        READY     STATUS    RESTARTS   AGE    LABELS
   label-pod   1/1       Running   0          16m    env=development,owner=you

Exercise 2: Using Selectors

We can use labels for filtering a list.

1. To list all pods labeled as owner=you, we can use the --selector option:

   kubectl get pods --selector owner=you
   
   NAME       READY     STATUS    RESTARTS   AGE
   label-pod  1/1       Running   0          27m

   Note: The --selector option can be abbreviated to -l

2. To list pods labeled with env=development we can run:

   kubectl get pods -l env=development
   
   NAME       READY     STATUS    RESTARTS   AGE
   label-pod  1/1       Running   0          27m

3. Save the following as label-pod2.yaml

   cat > label-pod2.yaml <<EOF
   apiVersion: v1
   kind: Pod
   metadata:
     name: label-pod2
     labels:
       env: production
       owner: you
   spec:
     containers:
     - name: sise
       image: mhausenblas/simpleservice:0.5.0
       ports:
       - containerPort: 9876
   EOF

4. Create the above pod using

   kubectl create -f label-pod2.yaml

5. To list all pods labeled with env=development or env=production

   kubectl get pods -l 'env in (production, development)'
   
   NAME           READY     STATUS    RESTARTS   AGE
   label-pod      1/1       Running   0          43m
   label-pod2     1/1       Running   0          3m

6. Other verbs also support label selection, for example, you could remove both of these pods with:

   kubectl delete pods -l 'env in (production, development)'

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Note: Labels are not restricted to pods. In fact, you can apply them to all sorts of objects, such as nodes or services.

Excercise 3: Using Annotations

In this excercise you will add the phone number of responsible person to the running pod. Phone number contains symbols that can't be used in the label field. So you will add a new phone field to the annotations.

1. Edit annotation-pod.yaml

   cat > annotation-pod.yaml <<EOF
   apiVersion: v1
   kind: Pod
   metadata:
     name: annotation-pod
     labels:
       env: production
       owner: you
       phone: "+1 (123) 456-78-90"
   spec:
     containers:
     - name: sise
       image: mhausenblas/simpleservice:0.5.0
       ports:
       - containerPort: 9876
   EOF

2. Try to create the pod

   kubectl apply -f annotation-pod.yaml
   

You will get an error as phone number does not satisfy formatting requirements for a label field.

3. Change phone field to annotation

   cat > annotation-pod.yaml <<EOF
   apiVersion: v1
   kind: Pod
   metadata:
     name: annotation-pod
     labels:
       env: production
       owner: you
     annotations:
       phone: "+1 (123) 456-78-90"
   spec:
     containers:
     - name: sise
       image: mhausenblas/simpleservice:0.5.0
       ports:
       - containerPort: 9876
   EOF

4. Create the pod

   kubectl apply -f annotation-pod.yaml
   

5. Check the annotation was stored in the pods metadata

   kubectl get pods --selector owner=you -o jsonpath='{.items[*].metadata.annotations.phone}'
   

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Exercise 4 (Optional): Using Selectors

1. Deploy 3 pods; each one with different labels: version=1, version=2 and version=3

2. List the pods using selectors that will return all pods with versions not equal to 3

Refer to the documentation to review selector syntax.

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Next: Services