Exercise 1: Stateful Applications

Task 1 - Understanding StatefulSets

1. StatefulSets are Kubernetes controllers that manage the deployment and scaling of a set of pods, providing guarantees about the ordering and uniqueness of these pods.

2. Key characteristics of StatefulSets:

   • Stable, unique network identifiers
   • Stable, persistent storage
   • Ordered, graceful deployment and scaling
   • Ordered, automated rolling updates

Task 2 - Deploy MongoDB Using StatefulSets

1. Create the MongoDB StatefulSet manifest:

   ​

   AZ CLI PowerShell AZ CLI Bash

   $statefulsetYaml = @"
   apiVersion: v1
   kind: Service
   metadata:
     name: mongodb-service
     labels:
       app: mongodb
   spec:
     clusterIP: None
     selector:
       app: mongodb
     ports:
     - port: 27017
       targetPort: 27017
   ---
   apiVersion: apps/v1
   kind: StatefulSet
   metadata:
     name: mongodb
   spec:
     serviceName: "mongodb-service"
     replicas: 3
     selector:
       matchLabels:
         app: mongodb
     template:
       metadata:
         labels:
           app: mongodb
       spec:
         containers:
         - name: mongodb
           image: k8sonazureworkshoppublic.azurecr.io/mongo:4.4
           ports:
           - containerPort: 27017
             name: mongodb
           resources:
             requests:
               memory: "256Mi"
               cpu: "250m"
             limits:
               memory: "512Mi"
               cpu: "500m"

   cat << EOF | kubectl apply -f -
   apiVersion: v1
   kind: Service
   metadata:
     name: mongodb-service
     labels:
       app: mongodb
   spec:
     clusterIP: None
     selector:
       app: mongodb
     ports:
     - port: 27017
       targetPort: 27017
   ---
   apiVersion: apps/v1
   kind: StatefulSet
   metadata:
     name: mongodb
   spec:
     serviceName: "mongodb-service"
     replicas: 3
     selector:
       matchLabels:
         app: mongodb
     template:
       metadata:
         labels:
           app: mongodb
       spec:
         containers:
         - name: mongodb
           image: k8sonazureworkshoppublic.azurecr.io/mongo:4.4
           ports:
           - containerPort: 27017
             name: mongodb
           resources:
             requests:
               memory: "256Mi"
               cpu: "250m"
             limits:
               memory: "512Mi"
               cpu: "500m"
   EOF

   This manifest defines:

   • A headless service (clusterIP: None)
   • A StatefulSet with 3 replicas
   • Resource requests and limits

2. Verify the StatefulSet deployment:

   ​

   AZ CLI PowerShell AZ CLI Bash

   kubectl get statefulset

   kubectl get statefulset

   You should see output similar to:

   

   You may need to wait a few minutes for all pods to start.

Task 3 - Observe StatefulSet Behavior

1. List the pods created by the StatefulSet:

   ​

   AZ CLI PowerShell AZ CLI Bash

   kubectl get pods -l app=mongodb

   kubectl get pods -l app=mongodb

   Notice the predictable pod names: mongodb-0, mongodb-1, mongodb-2

Task 4 - Test StatefulSet Ordering

1. Delete a pod to observe the recreation behaviour:

   ​

   AZ CLI PowerShell AZ CLI Bash

   kubectl delete pod mongodb-1

   kubectl delete pod mongodb-1

2. Watch the pod recreation:

   ​

   AZ CLI PowerShell AZ CLI Bash

   kubectl get pods -l app=mongodb --watch

   kubectl get pods -l app=mongodb --watch

   Notice that Kubernetes recreates the exact same pod with the same name, maintaining the StatefulSet’s guarantee of identity.

Task 5 - Scale the StatefulSet

1. Scale the StatefulSet to 4 replicas:

   ​

   AZ CLI PowerShell AZ CLI Bash

   kubectl scale statefulset mongodb --replicas=4

   kubectl scale statefulset mongodb --replicas=4

2. Observe the scaling process:

   ​

   AZ CLI PowerShell AZ CLI Bash

   kubectl get pods -l app=mongodb 

   kubectl get pods -l app=mongodb 

   Notice that pods are created one at a time, in order.

3. Scale back down to 2 replicas:

   ​

   AZ CLI PowerShell AZ CLI Bash

   kubectl scale statefulset mongodb --replicas=2

   kubectl scale statefulset mongodb --replicas=2

4. Observe the termination order:

   ​

   AZ CLI PowerShell AZ CLI Bash

   kubectl get pods -l app=mongodb

   kubectl get pods -l app=mongodb

   Notice that pods are terminated in reverse order (highest to lowest index).

Task 6 - Clean Up

1. Delete the StatefulSet and associated resources:

   kubectl delete statefulset mongodb
   kubectl delete service mongodb-service