Kubernetes Pods and Deployments

What is a Pod?#

A Pod is the smallest deployable unit in Kubernetes. It wraps one or more containers that share:

• The same network namespace (same IP, same ports)
• The same storage volumes
• The same lifecycle

Pod
├── Container 1 (app)
├── Container 2 (sidecar — e.g., log shipper)
└── Shared volume (/logs)

Creating Pods#

Imperatively (for quick testing)#

# Run a pod and attach to it
kubectl run nginx --image=nginx --port=80

# Run and immediately open a shell
kubectl run -it debugger --image=busybox --restart=Never -- sh

# Generate YAML without creating (dry-run)
kubectl run nginx --image=nginx --dry-run=client -o yaml

Declaratively (YAML manifest)#

# pod.yaml
apiVersion: v1
kind: Pod
metadata:
  name: nginx-pod
  labels:
    app: nginx
    environment: dev
spec:
  containers:
    - name: nginx
      image: nginx:1.25
      ports:
        - containerPort: 80
      resources:
        requests:
          memory: "64Mi"
          cpu: "100m"
        limits:
          memory: "128Mi"
          cpu: "250m"

kubectl apply -f pod.yaml
kubectl get pods
kubectl describe pod nginx-pod
kubectl logs nginx-pod
kubectl exec -it nginx-pod -- bash
kubectl delete pod nginx-pod

Pod Lifecycle#

Pending → Running → Succeeded / Failed
            ↕
         (Restart if restartPolicy allows)

Useful kubectl Commands#

# Inspect
kubectl get pods                      # All pods in current namespace
kubectl get pods -n kube-system       # Pods in kube-system namespace
kubectl get pods -o wide              # Shows node assignment + IP
kubectl get pods --watch              # Live updates
kubectl describe pod <name>           # Detailed events and config
kubectl logs <pod>                    # Container logs
kubectl logs <pod> -c <container>     # Specific container logs
kubectl logs <pod> --previous         # Logs from crashed container

# Execute
kubectl exec -it <pod> -- bash         # Attach a shell
kubectl exec <pod> -- cat /etc/hosts   # Run a single command

# Troubleshooting
kubectl get events --sort-by=.lastTimestamp
kubectl describe pod <name>           # Look at Events: section

ReplicaSets#

A ReplicaSet ensures a specified number of identical pod replicas are running at all times.

apiVersion: apps/v1
kind: ReplicaSet
metadata:
  name: nginx-rs
spec:
  replicas: 3
  selector:
    matchLabels:
      app: nginx
  template:
    metadata:
      labels:
        app: nginx
    spec:
      containers:
        - name: nginx
          image: nginx:1.25

Deployments#

A Deployment is the recommended way to manage stateless applications. It:

1. Creates a ReplicaSet
2. Manages rolling updates
3. Supports rollbacks

apiVersion: apps/v1
kind: Deployment
metadata:
  name: nginx-deployment
  labels:
    app: nginx
spec:
  replicas: 3
  selector:
    matchLabels:
      app: nginx
  strategy:
    type: RollingUpdate
    rollingUpdate:
      maxSurge: 1         # 1 extra pod during update
      maxUnavailable: 0   # 0 pods down during update
  template:
    metadata:
      labels:
        app: nginx
    spec:
      containers:
        - name: nginx
          image: nginx:1.25
          ports:
            - containerPort: 80

kubectl apply -f deployment.yaml
kubectl get deployment nginx-deployment
kubectl get replicasets                # See the RS created by the deployment
kubectl get pods -l app=nginx          # Pods managed by deployment

Rolling Updates#

# Update the image (triggers rolling update)
kubectl set image deployment/nginx-deployment nginx=nginx:1.26

# Watch the rollout
kubectl rollout status deployment/nginx-deployment

# Pause a rollout
kubectl rollout pause deployment/nginx-deployment

# Resume
kubectl rollout resume deployment/nginx-deployment

During a rolling update, Kubernetes:

1. Creates a new ReplicaSet with the new version
2. Scales up the new RS while scaling down the old one
3. Old RS remains at 0 replicas (for rollback)

Rollbacks#

# View rollout history
kubectl rollout history deployment/nginx-deployment

# Roll back to previous version
kubectl rollout undo deployment/nginx-deployment

# Roll back to a specific revision
kubectl rollout undo deployment/nginx-deployment --to-revision=2

Scaling#

# Scale imperatively
kubectl scale deployment/nginx-deployment --replicas=5

# Scale via YAML (edit and apply)
kubectl edit deployment nginx-deployment
# Change spec.replicas and save

# Auto-scaling (requires metrics server)
kubectl autoscale deployment nginx-deployment --min=2 --max=10 --cpu-percent=70

Common Interview Questions#

Q: What is the difference between a Pod and a Deployment? A Pod is the basic unit of execution — a single instance of your app. A Deployment manages multiple Pods via a ReplicaSet, ensuring the desired number are running and enabling rolling updates and rollbacks.

Q: What happens when you delete a Pod managed by a Deployment? The ReplicaSet detects a pod is missing and creates a new one to maintain the desired replica count. You cannot permanently delete a managed Pod without deleting the Deployment or scaling to 0.

Q: What is maxSurge and maxUnavailable in a Deployment? maxSurge is how many extra pods above the desired count can exist during a rolling update. maxUnavailable is how many pods below the desired count are tolerated. Together they control the speed and safety of the update.

Q: How do you roll back a Deployment?

kubectl rollout undo deployment/<name>
kubectl rollout undo deployment/<name> --to-revision=<n>

Common Mistakes#

• Editing a Pod spec directly — most fields are immutable on running pods. Delete and recreate, or use a Deployment
• Not setting resource requests/limits — without these, a Pod can consume all node resources (noisy neighbour problem)
• Using latest image tag — makes it impossible to know which version is running and breaks rollbacks
• Forgetting the selector must match template labels — if they don't match, the Deployment controller won't find its pods

What to Learn Next#

1. Kubernetes Services — exposing pods with ClusterIP, NodePort, LoadBalancer
2. Kubernetes ConfigMaps and Secrets — environment configuration
3. Kubernetes Persistent Storage — PVs, PVCs, StorageClasses