Kubernetes Classroom Series – 16/Oct/2019, Labels, Selectors and Annotations

Challenges With Multiple Pods

Challenge with Continuous Deployments

To Solve above Challenges

• Controllers
• Services
• Labels

Labels

• Labels are key-value pairs.
• They can be applied to any k8s resources.
• Labels can be used retreive by using key-value pairs

Give me the pods which have label app=tomcat
Give me the pods which have label app=tomcat and env=production

• Labels are used in k8s by various resources
• In this example today we would see the usage by Replication Controller

Sample Commands to know the Label and RC connection

• Create a pod1.yaml file for Pod using

---
apiVersion: v1
kind: Pod
metadata:
  name: mytomcat
  labels:
    env: dev
    app: tomcat
    ver: "8"
spec:
  containers:
    - image: tomcat:8
      name: tomcat
      ports:
        - containerPort: 8080
          protocol: TCP

• Execute kubectl apply -f pod1.yml
• Create a pod2.yaml file for Pod using

---
apiVersion: v1
kind: Pod
metadata:
  name: mytomcattest
  labels:
    env: test
    app: tomcat
    ver: "8"
spec:
  containers:
    - image: tomcat:8
      name: tomcat
      ports:
        - containerPort: 8080
          protocol: TCP

• Execute kubectl apply -f pod2.yml
• Execute following commands

kubectl get pods
# observe labels
kubectl get pod mytomcat -oyaml
kubectl get pod mytomcattest -oyaml

• Now lets create rc.yml with two replicas of tomcat

---
apiVersion: v1
kind: ReplicationController
metadata:
  name: tomcat-rc
spec:
  replicas: 2
  selector:
    app: tomcat
  template:
    metadata:
      labels:
        app: tomcat
    spec:
      containers:
        - image: tomcat:8
          name: tomcat
          ports:
            - containerPort: 8080
              protocol: TCP

• Execute the following commands

kubectl apply -f rc.yml
kubectl get pods

• No new pods are created by RC as there are two pods already existing with matching labels.
• To test this, lets make a small change

---
apiVersion: v1
kind: ReplicationController
metadata:
  name: tomcat-rc
spec:
  replicas: 2
  selector:
    app: tomcat
    env: dev
  template:
    metadata:
      labels:
        app: tomcat
    spec:
      containers:
        - image: tomcat:8
          name: tomcat
          ports:
            - containerPort: 8080
              protocol: TCP

• Observe the selector section, this matches only with one existing pod. Now if you apply you should see 3 pods in total in your k8s cluster

kubectl apply -f rc.yml
kubectl get pods

Annotations

• Annoatations are also key values pairs.
• Annotations are for external usage. Sample is

---
apiVersion: v1
kind: Pod
metadata:
  name: mytomcattest
  labels:
    env: test
    app: tomcat
    ver: "8"
  annotations:
    commit-id: 0c345f
    author: khaja
    organization: qualitythought
spec:
  containers:
    - image: tomcat:8
      name: tomcat
      ports:
        - containerPort: 8080
          protocol: TCP

Controller Daemonset.

• Create a yaml file for daemon set.
• Daemon set create a pod on every node