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4.01 Rolling Updates and Rollbacks

Kubernetes Deployments provide built‑in support for safe upgrades and fast recovery using:

  • ✅ Rolling Updates — upgrade without downtime
  • ⏪ Rollbacks — revert to a previous working version

Each change to a Deployment creates a new revision, so Kubernetes can track versions and switch back if needed.

🎯 What Is a Rollout

A rollout is triggered whenever a Deployment is created or its Pod template changes.

A rollout happens when you:

  • Create a Deployment
  • Change container image
  • Modify labels / env / command
  • Change pod template fields

Each rollout creates:

  • A new ReplicaSet
  • A new revision number

Note

Think of a rollout as a new version release of your Deployment inside the cluster.

🧱 Deployment Revision Model

Every Deployment keeps a history of ReplicaSets (versions).

Deployment
   │
   ├─ Revision 1 → ReplicaSet A → Pods (v1)
   ├─ Revision 2 → ReplicaSet B → Pods (v2)
   └─ Revision 3 → ReplicaSet C → Pods (v3)
  • Only one ReplicaSet serves full traffic after rollout completes
  • Older ReplicaSets are kept for rollback

Tip

ReplicaSets are the “version history” behind a Deployment.

🚀 Deployment Strategies

Kubernetes supports two upgrade strategies.

🔴 Recreate Strategy

All old pods are deleted first, then new pods are created.

Flow

Old Pods → deleted → ❌ downtime → New Pods created

Behavior

  • Simple
  • Causes application downtime
  • Not default

Warning

Users will experience outage during upgrade with Recreate strategy.

🟢 Rolling Update Strategy (Default)

Pods are replaced gradually.

Flow

Old: █████
Step1: ████░ + ░
Step2: ███░░ + ██
Step3: ██░░░ + ███
New:  ░░░░░ + █████

Behavior

  • Old and new pods run together temporarily
  • No downtime (if readiness probes are correct)
  • Default strategy

Success

RollingUpdate is used automatically if no strategy is specified.

⚙️ Strategy Configuration (YAML)

strategy:
  type: RollingUpdate

strategy:
  type: Recreate

🔄 How to Update a Deployment

You can trigger a new rollout in two main ways.

Method 1 — Update YAML and Apply

Edit Deployment YAML, then:

kubectl apply -f deployment.yaml

Kubernetes will:

  • Create new ReplicaSet
  • Start rollout
  • Create new revision

Method 2 — Update Image Directly

kubectl set image deployment/myapp nginx=nginx:1.9.1

Tip

Fast for quick updates, but your YAML file will not reflect this change automatically.

📊 Rollout Status and History

Check rollout progress

kubectl rollout status deployment/myapp

View revision history

kubectl rollout history deployment/myapp

Example:

REVISION   CHANGE-CAUSE
1          initial deploy
2          image update

Example

Useful to identify which revision to roll back to.

🧩 What Happens During Rolling Update

Behind the scenes Kubernetes creates a new ReplicaSet and shifts traffic gradually.

Start:
ReplicaSet A → 5 pods (v1)

During:
ReplicaSet A → 3 pods
ReplicaSet B → 2 pods

Later:
ReplicaSet A → 1 pod
ReplicaSet B → 4 pods

End:
ReplicaSet A → 0 pods
ReplicaSet B → 5 pods (v2)

🔍 View ReplicaSets

kubectl get replicasets

You will observe:

  • Old ReplicaSet scaled down
  • New ReplicaSet scaled up

⏪ Rollback a Deployment

If a new version is broken, revert instantly.

kubectl rollout undo deployment/myapp

Kubernetes will:

  • Scale down new ReplicaSet
  • Scale up previous ReplicaSet
  • Restore last working version

Success

Rollback is fast because old ReplicaSet already exists.

⏪ Rollback Flow

Current → ReplicaSet B → bad version → 5 pods

Rollback →

ReplicaSet B → scaled to 0
ReplicaSet A → scaled to 5

Previous version restored

🛠️ Command Cheat Sheet

Create

kubectl create -f deployment.yaml

List

kubectl get deployments

Update

kubectl apply -f deployment.yaml
kubectl set image deployment/myapp nginx=nginx:1.9.1

Status

kubectl rollout status deployment/myapp
kubectl rollout history deployment/myapp

Rollback

kubectl rollout undo deployment/myapp

🧠 Exam Tips

Question

Default deployment strategy?

RollingUpdate

Question

What creates a new revision?

Any Pod template change

Question

Does rollback create new pods?

Yes — previous ReplicaSet pods are scaled up again.

✅ Quick Summary

Summary

  • Rollout = new Deployment revision
  • Each rollout creates a new ReplicaSet
  • RollingUpdate is default
  • Recreate causes downtime
  • rollout status shows progress
  • rollout history shows revisions
  • rollout undo restores previous version