Kubernetes Deployments

You only need a deployment to launch a container in Kubernetes. Deployments tell Kubernetes

• what container to run by specifying the Docker image name and tag
  • spec: template: spec: containers: – image:
• when to pull the image from the registry
  • spec: template: spec: containers: imagePullPolicy:
    • If the image is always rebuilt with the same version, like “latest”, set the policy to Always to disable the image caching
• what to do when the container crashes
  • spec: template: spec: containers: restartPolicy:
    • usually set to Always
• how many replicas to launch simultaneously
  • spec: replicas
• how to map this deployment to actual running containers
  • The label in spec: selector: matchLabels: connects the deployment to the pod specified in the deployment template via the same deployment’s spec: template: metadata: labels:
• the way Kubernetes should replace containers when we update the deployment
  • strategy:
    • rollingUpdate (default)
• the namespace
  • metadata: namespace:
    • if not specified, the “Default” namespace is used

apiVersion: apps/v1
kind: Deployment
metadata:
  name: app1-frontend-deployment
spec:
  replicas: 3
  selector:
    matchLabels:
      app: app1-frontend-template-label
  template:
    metadata:
      labels:
        app: app1-frontend-template-label
    spec:
      containers:
        - name: app1-frontend-container-label
          image: nginx:1.7.9
          ports:
          - containerPort: 80

To list all deployments

kubectl get deployments

To launch the container specified in the deployment file

kubectl apply -f ./MY_DEPLOYMENT_FILE.yml

Display information about the deployment

kubectl describe deployment MY_DEPLOYMENT

The deployment creates pods. A pod can contain one or multiple containers, usually one. To list the pods

kubectl get pods