Getting Started with Kubernetes Deployments

Kubernetes runs containers for you and keeps them running. You declare the desired state, and the control loop works to match it. A Deployment is the object you use for stateless apps.

The object hierarchy

A Deployment manages a ReplicaSet, which manages the Pods that actually run your containers.

graph TD;
    A[Deployment] --> B[ReplicaSet];
    B --> C[Pod 1];
    B --> D[Pod 2];
    B --> E[Pod 3];
    C --> F[Container];
    D --> G[Container];
    E --> H[Container];

Declaring a Deployment

The manifest says which image to run and how many replicas to keep alive:

yamlk8s/deployment.yaml

apiVersion: apps/v1
kind: Deployment
metadata:
  name: web
spec:
  replicas: 3
  selector:
    matchLabels:
      app: web
  template:
    metadata:
      labels:
        app: web
    spec:
      containers:
        - name: web
          image: registry.example.com/web:1.4.0
          ports:
            - containerPort: 8080

Applying and watching

Apply the manifest and watch the rollout converge:

kubectl apply -f k8s/deployment.yaml
kubectl rollout status deployment/web

Rolling updates come free

Change the image tag and re-apply. Kubernetes replaces Pods gradually, so there is no downtime if your readiness probe is honest.

A Service to expose it

Pairing the Deployment with a Service manifest gives it a stable address:

	class HelloWorld
	def initialize(name)
	@name = name.capitalize
	end
	def sayHi
	puts "Hello #{@name}!"
	end
	end

	hello = HelloWorld.new("World")
	hello.sayHi

view raw hello_world.rb hosted with ❤ by GitHub

Declare what you want, let the control loop reconcile, and lean on rolling updates and probes for safe releases.