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In Kubernetes, Deployments automate application updates, versioning, and rollbacks. This guide covers:
  • How rollouts create revisions
  • Deployment update strategies
  • Applying and inspecting updates
  • Undoing changes

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Rollouts and Versioning

Whenever you create or modify a Deployment, Kubernetes starts a new rollout, creating a revision:
The image shows a diagram titled "Rollout and Versioning" with two revisions of Nginx versions, 1.7.0 and 1.7.1, represented by icons.
To monitor your rollout and review history: 
kubectl rollout status deployment/myapp-deployment
# Waiting for rollout to finish: 0 of 10 updated replicas are available...
# ...
kubectl rollout history deployment/myapp-deployment
# REVISION  CHANGE-CAUSE
# 1         initial create
# 2         updated image to nginx:1.7.1
Use kubectl rollout status to ensure a smooth update before proceeding with any dependent tasks.

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Deployment Strategies

Kubernetes supports two primary update strategies:

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Recreate

This strategy terminates all existing Pods before creating new ones—resulting in downtime.
The image illustrates a deployment strategy showing a transition from version 1.7.0 to 1.7.1 of nginx, with an application downtime indicated during the process.
Recreate will interrupt service during the update. Use only when downtime is acceptable.

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RollingUpdate (default)

With RollingUpdate, old Pods are replaced incrementally, ensuring continuous availability.
The image illustrates two deployment strategies: "Recreate," which results in application downtime, and "Rolling Update," which maintains application availability by updating instances incrementally.
If you omit a strategy in your Deployment spec, RollingUpdate applies by default.

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Applying Updates

You can update a Deployment by editing its YAML or using kubectl set image. deployment.yaml: 
apiVersion: apps/v1
kind: Deployment
metadata:
  name: myapp-deployment
  labels:
    app: myapp
    tier: frontend
spec:
  replicas: 3
  selector:
    matchLabels:
      app: myapp
      tier: frontend
  template:
    metadata:
      labels:
        app: myapp
        tier: frontend
    spec:
      containers:
        - name: nginx-container
          image: nginx:1.7.0
Apply the manifest: 
kubectl apply -f deployment.yaml
# deployment "myapp-deployment" configured
Or update the image directly: 
kubectl set image deployment/myapp-deployment \
  nginx-container=nginx:1.9.1
# deployment "myapp-deployment" image updated
Each change creates a new revision and triggers a rollout.

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Inspecting Deployment Details

To see strategy settings, ReplicaSet events, and scaling details: 
kubectl describe deployment myapp-deployment
Example for Recreate: 
Name:                   myapp-deployment
StrategyType:           Recreate
Replicas:               5 desired | 5 total | 5 available
Events:
  Type    Reason             Age   From                   Message
  ----    ------             ----  ----                   -------
  Normal  ScalingReplicaSet  11m   deployment-controller  Scaled up replica set myapp-deployment-6795… to 5
Example for RollingUpdate: 
Name:                   myapp-deployment
StrategyType:           RollingUpdate
RollingUpdateStrategy:  25% max unavailable, 25% max surge
Replicas:               5 desired | 6 total | 4 available | 2 unavailable
Events:
  Type    Reason             Age  From                   Message
  ----    ------             ---- ----                   -------
  Normal  ScalingReplicaSet  1m   deployment-controller  Scaled up replica set myapp-deployment-67c7… to 5
Under the hood, each Deployment manages ReplicaSets:
The image is a diagram illustrating a Kubernetes deployment with two replica sets. Replica Set 1 contains five pods, while Replica Set 2 is empty.
List ReplicaSets: 
kubectl get replicasets
# NAME                         DESIRED CURRENT READY AGE
# myapp-deployment-67c749c58c  0       0       0     22m
# myapp-deployment-75d7bdbd8d  5       5       5     20m

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Rolling Back

To revert a faulty rollout: 
kubectl rollout undo deployment/myapp-deployment
# deployment "myapp-deployment" rolled back
After rollback, ReplicaSet counts swap: 
kubectl get replicasets
# NAME                         DESIRED CURRENT READY AGE
# myapp-deployment-67c749c58c  5       5       5     22m
# myapp-deployment-75d7bdbd8d  0       0       0     20m

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Creating a Deployment with kubectl run

Although kubectl run nginx --image=nginx creates a Deployment by default: 
kubectl run nginx --image=nginx
# deployment "nginx" created
Using manifest files ensures version control, repeatability, and easier collaboration.

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Summary of Key Commands

CommandDescription
kubectl create deploymentCreate a new Deployment
kubectl get deploymentsList Deployments
kubectl apply -f deployment.yamlApply or update a Deployment manifest
kubectl set imageUpdate container image in a Deployment
kubectl rollout statusMonitor rollout progress
kubectl rollout historyView revision history
kubectl rollout undoRoll back to the previous revision
kubectl describe deploymentShow detailed Deployment settings and events

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References

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