Lab 08 - ReplicaSets and Deployments

Goal: watch the reconciliation loop, see the selector do the binding, and run your first Deployment. Chapter: ReplicaSets and Deployments

ReplicaSet

# 1 - a ReplicaSet by hand (the only time you will write one)
cd ~
cat > web-rs.yaml <<'YAML'
apiVersion: apps/v1
kind: ReplicaSet
metadata:
  name: web-rs
spec:
  replicas: 3
  selector:
    matchLabels:
      app: web
  template:
    metadata:
      labels:
        app: web
    spec:
      containers:
        - name: web
          image: nginx:1.27
YAML
kubectl apply -f web-rs.yaml
kubectl get rs,pods -l app=web -o wide

# 2 - self-healing
kubectl delete pod "$(kubectl get pods -l app=web -o name | head -1)"
kubectl get pods -l app=web

The same loop that created the Pods counted two, wanted three, created one.

# 3 - scaling is the same loop with a different number
kubectl scale rs web-rs --replicas=5
kubectl scale rs web-rs --replicas=2
kubectl get pods -l app=web

# 4 - the selector binds Pods, not the name
kubectl run stray --image nginx:1.27 --labels app=web
kubectl get pods -l app=web

stray was adopted - the ReplicaSet counted three and deleted one. An accidental label match is an accidental takeover.

# 5 - a changed template does NOT touch running Pods
sed -i 's/nginx:1.27/nginx:1.28/' web-rs.yaml
kubectl apply -f web-rs.yaml
kubectl get pods -l app=web -o jsonpath='{.items[*].spec.containers[0].image}{"\n"}'
kubectl delete -f web-rs.yaml

Still 1.27 - no rollout logic. That gap is what a Deployment fills.

Deployment

# 6 - generate, apply, look at the three layers
kubectl create deploy web --image nginx:1.27 --replicas 3 \
  --dry-run=client -o yaml > web-deploy.yaml
kubectl apply -f web-deploy.yaml
kubectl get deploy,rs,pods -l app=web

# 7 - follow the ownership chain Pod -> ReplicaSet -> Deployment
POD=$(kubectl get pods -l app=web -o name | head -1)
kubectl get $POD -o jsonpath='{.metadata.ownerReferences[0].kind}/{.metadata.ownerReferences[0].name}{"\n"}'
RS=$(kubectl get rs -l app=web -o name | head -1)
kubectl get $RS -o jsonpath='{.metadata.ownerReferences[0].kind}/{.metadata.ownerReferences[0].name}{"\n"}'

# 8 - change the image: a new ReplicaSet appears
kubectl set image deploy/web nginx=nginx:1.28
kubectl rollout status deploy/web
kubectl get rs -l app=web

Old ReplicaSet at 0, new at 3. Strategies, history and rollback: Day 3.

Discovery

Infodiscover

Task A. Scale web to 5 replicas by editing web-deploy.yaml, not with kubectl scale.

Show one answer

sed -i 's/replicas: 3/replicas: 5/' web-deploy.yaml
kubectl apply -f web-deploy.yaml
kubectl get deploy web -o wide

Look at the image: back to 1.27. Step 8 changed the live object, not the file - apply rolled it back (drift, Lab 04).

Task B. Delete the Deployment but keep its ReplicaSets and Pods running. What owns the Pods afterwards?

Show one answer

kubectl delete deploy web --cascade=orphan
kubectl get rs,pods -l app=web
kubectl get rs -l app=web -o jsonpath='{.items[*].metadata.ownerReferences}{"\n"}'

The ReplicaSets have no owner now; the Pods are still owned by their ReplicaSet.

Task C. Re-apply web-deploy.yaml. Does the Deployment create new Pods or adopt the orphans?

Show one answer

kubectl apply -f web-deploy.yaml
kubectl get rs -l app=web

It adopts the ReplicaSet whose template matches - no new Pods for that revision.

Clean up

kubectl delete -f web-deploy.yaml
kubectl delete rs -l app=web --ignore-not-found
kubectl delete pod stray --ignore-not-found