Deployments

A Deployment is a higher-level controller that manages ReplicaSets, which in turn manage Pods.

Deployments provide declarative updates, scaling, and rollback.

Deployments vs ReplicaSets vs Pods

Object	Responsibility
Pod	Runs containers
ReplicaSet	Ensures Pod count
Deployment	Manages ReplicaSets and updates

Hierarchy:

Deployment → ReplicaSet → Pods

You should never manage ReplicaSets directly.

Why Deployments Exist

Deployments solve:

• Zero-downtime updates
• Version tracking
• Safe rollbacks
• Horizontal scaling

Without Deployments, updates are destructive.

Deployment Strategies

Strategy	Behavior
RollingUpdate	Gradual replacement (default)
Recreate	Stop all, then start new

RollingUpdate parameters:

• maxSurge
• maxUnavailable

Hands-on: Deploy nginx

Create Deployment

Input:

kubectl create deployment nginx --image=nginx:1.25

Output:

deployment.apps/nginx created

This creates a Deployment with one ReplicaSet.

Inspect Deployment

Input:

kubectl get deployment nginx

Output:

NAME    READY   UP-TO-DATE   AVAILABLE
nginx   1/1     1            1

This shows desired vs available replicas.

Inspect ReplicaSets

Input:

kubectl get rs

Output:

NAME               DESIRED   CURRENT
nginx-6c8bdfc8c9   1         1

ReplicaSet is automatically created.

Scaling Applications

Scale Up

Input:

kubectl scale deployment nginx --replicas=3

Output:

deployment.apps/nginx scaled

This increases Pod replicas.

Verify Pods

Input:

kubectl get pods -l app=nginx

Output:

nginx-xxxx   Running
nginx-yyyy   Running
nginx-zzzz   Running

Deployment maintains replica count.

Scale Down

Input:

kubectl scale deployment nginx --replicas=1

Output:

deployment.apps/nginx scaled

Extra Pods are terminated gracefully.

Rolling Update

Update Image Version

Input:

kubectl set image deployment/nginx nginx=nginx:1.26

Output:

deployment.apps/nginx image updated

This triggers a rolling update.

Watch Rollout

Input:

kubectl rollout status deployment/nginx

Output:

Waiting for deployment "nginx" rollout to finish: 1 old replicas are pending termination...
deployment "nginx" successfully rolled out

Pods are replaced gradually.

Check ReplicaSets After Update

Input:

kubectl get rs

Output:

nginx-7f9cbbf9f6   1   1
nginx-6c8bdfc8c9   0   0

Old ReplicaSet is scaled down.

Rollback Deployment

Rollback to Previous Version

Input:

kubectl rollout undo deployment/nginx

Output:

deployment.apps/nginx rolled back

This restores the previous ReplicaSet.

Verify Image

Input:

kubectl describe deployment nginx | grep Image

Output:

Image: nginx:1.25

Rollback completed successfully.

Deployment YAML Structure

Core fields:

Field	Purpose
replicas	Desired Pod count
selector	Pod label selector
template	Pod spec
strategy	Update strategy

Example:

apiVersion: apps/v1
kind: Deployment
metadata:
  name: nginx
spec:
  replicas: 3
  strategy:
    type: RollingUpdate
  selector:
    matchLabels:
      app: nginx
  template:
    metadata:
      labels:
        app: nginx
    spec:
      containers:
      - name: nginx
        image: nginx:1.25

Key Takeaways

Concept	Meaning
Deployment	Application lifecycle manager
ReplicaSet	Pod count enforcer
Rolling update	Zero-downtime upgrade
Rollback	Safe version recovery
Scaling	Declarative replica control

Deployments turn manual releases into repeatable, safe operations.