Introduction to Deployments

A Deployment is a higher-level abstraction that sits on top of ReplicaSets. It provides declarative updates for Pods and ReplicaSets. You describe a desired state in a Deployment manifest, and the Deployment controller manages the transition from the current state to the desired state.

Deployment Strategies

When updating container versions (e.g. changing the image tag), Deployments use two main strategies:

• Recreate: Terminates all running Pods first, before spinning up the new version. This results in service downtime, but guarantees that two different versions of the code never run simultaneously.
• RollingUpdate (Default): Gradually replaces old Pods with new ones, ensuring zero downtime. You configure rolling updates using two parameters:
  • maxSurge: The maximum number of Pods that can be created above the desired replica count during the update (e.g. 25%).
  • maxUnavailable: The maximum number of Pods that can be unavailable during the update process (e.g. 25%).

Under the Hood: Double ReplicaSet Swapping

When you update a Deployment's container configuration, the Deployment controller automatically creates a new ReplicaSet representing the new version. It then scales up the new ReplicaSet (adding new pods) while scaling down the old ReplicaSet (removing old pods).

Rollback and History Management CLI

If an update introduces a bug or causes container crashes, you can inspect rollout history and roll back changes instantly:

# View the active status of a rollout update
kubectl rollout status deployment/web-deployment
View the deployment's rollout revision history
kubectl rollout history deployment/web-deployment
Roll back the deployment to the immediately preceding version
kubectl rollout undo deployment/web-deployment
Roll back to a specific revision from the history log
kubectl rollout undo deployment/web-deployment --to-revision=2

Deployment Manifest Example

apiVersion: apps/v1
kind: Deployment
metadata:
  name: api-deployment
spec:
  replicas: 4
  strategy:
    type: RollingUpdate
    rollingUpdate:
      maxSurge: 1
      maxUnavailable: 0
  selector:
    matchLabels:
      app: api-server
  template:
    metadata:
      labels:
        app: api-server
    spec:
      containers:
      - name: main-api
        image: my-registry/api:v2.0.0
        ports:
        - containerPort: 8080