10. Replication Controller, ReplicaSet & Deployments

☸️ Day 10: Replication Controller, ReplicaSet & Deployments

Video Overview

This note covers how Kubernetes manages workloads using:

• Replication Controllers
• ReplicaSets
• Deployments
• Selectors (Equality & Set-based)

📌 Core idea: Maintain desired number of Pods automatically

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📦 Concept 1: Why Controllers?

Important

Pods alone are NOT reliable

❌ Problem:

• Pod crashes → no auto-restart
• No scaling
• No management

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✅ Solution:

• Use Controllers
• Ensure:
  • High availability
  • Self-healing
  • Scaling

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🔁 Concept 2: Replication Controller (RC)

Info

First method to manage multiple pods

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🧠 What it does:

• Maintains desired number of pods
• If pod dies → creates new one

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📄 RC YAML Structure

apiVersion: v1
kind: ReplicationController
 
metadata:
  name: nginx-rc
 
spec:
  replicas: 3
 
  selector:
    app: nginx
 
  template:
    metadata:
      labels:
        app: nginx
 
    spec:
      containers:
        - name: nginx-container
          image: nginx

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🎯 Key Concept: Labels & Selectors

Important

RC works ONLY based on labels

• Selector:

app: nginx

• It manages:
  • Pods having label app=nginx

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⚠️ Important Behavior

Warning

• Existing pods with matching labels are also managed
• RC may NOT create new pods if matching ones already exist

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⚙️ Useful Commands (RC)

kubectl get rc
kubectl describe rc nginx-rc
kubectl scale rc nginx-rc --replicas=4
kubectl set image rc nginx-rc nginx-container=nginx:1.22
kubectl edit rc nginx-rc

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🚀 Concept 3: ReplicaSet (RS)

Important

Modern replacement of Replication Controller

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🧠 Key Difference:

• Supports Set-based selectors (more powerful)

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📄 RS YAML

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-container
          image: nginx

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🔍 Concept 4: Types of Selectors

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🔹 1. Equality-Based Selector

Example

matchLabels:
  app: nginx

• Exact match required

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🔹 2. Set-Based Selector

Example

matchExpressions:
  - key: app
    operator: In
    values:
      - nginx
      - apache

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🔑 Operators:

Operator	Meaning
In	Value must be in list
NotIn	Value must NOT be in list
Exists	Key must exist
DoesNotExist	Key must NOT exist

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💡 Example (Advanced)

matchExpressions:
  - key: app
    operator: In
    values: [nginx, apache]
 
  - key: environment
    operator: NotIn
    values: [development]
 
  - key: tier
    operator: Exists
 
  - key: debug
    operator: DoesNotExist

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Important

ALL conditions must be satisfied (AND logic)

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⚠️ RC vs RS

Feature	RC	RS
Equality Selector	✅ Yes	✅ Yes
Set-Based Selector	❌ No	✅ Yes
Usage Today	❌ Deprecated	⚠️ Rarely used

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🚀 Concept 5: Deployment (MOST IMPORTANT)

Success

Deployment = Highest-level controller (used in real world)

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🧠 What it does:

• Manages ReplicaSets
• Provides:
  • Rolling updates
  • Rollbacks
  • Version control

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📄 Deployment YAML

apiVersion: apps/v1
kind: Deployment
 
metadata:
  name: nginx-deployment
  annotations:
    kubernetes.io/change-cause: "Initial release"
 
spec:
  replicas: 3
 
  selector:
    matchLabels:
      app: nginx
 
  template:
    metadata:
      labels:
        app: nginx
 
    spec:
      containers:
        - name: nginx-container
          image: nginx:1.19

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🔄 Concept 6: Rolling Updates

Important

Zero-downtime updates

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🔧 Update Image

kubectl set image deployment nginx-deployment nginx-container=nginx:1.20

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📜 Check History

kubectl rollout history deployment nginx-deployment

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🏁 Output Example

• Revision 1 → nginx:1.19
• Revision 2 → nginx:1.20

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🔁 Concept 7: Rollback

Important

Rollback to Previous

kubectl rollout undo deployment nginx-deployment

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Rollback to Specific Version

kubectl rollout undo deployment nginx-deployment --to-revision=1

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🧠 How Deployment Works Internally

Info

• Each update creates:
  • New ReplicaSet
• Old ReplicaSet:
  • Scaled down to 0
• New ReplicaSet:
  • Scaled up

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📊 Example

Revision	Image	Replicas
RS1	nginx:1.19	0
RS2	nginx:1.20	0
RS3	nginx:1.21	3

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⚠️ Imperative vs Declarative (Reminder)

Warning

• Imperative → quick fixes
• Declarative → production

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🧠 Final Takeaways

Summary

• RC → old method
• RS → improved (selectors)
• Deployment → real-world usage
• Labels + Selectors = core concept
• Deployment manages ReplicaSets

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📌 One-Line Summary

Quote

Deployment → manages ReplicaSets → manages Pods ☸️

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