CKAD studying - application deployment

Deployments

• deployment - Kubernetes object that defines a desired state for a set of replica Pods using a Pod template

  • Kubernetes actively maintains that desired state by creating/deleting/replacing those Pods
  • Pod template provides the Pod configuration that the Deployment will use to create new Pods

• replicas field sets the number of replicas - this value can be changed to scale up or down

sample Pod template, more reading -> https://kubernetes.io/docs/concepts/workloads/controllers/deployment/

apiVersion: apps/v1
kind: Deployment
metadata:
  name: nginx-deployment
  labels:
    app: nginx
spec:
  replicas: 3
  selector:
    matchLabels:
      app: nginx
  template:
    metadata:
      labels:
        app: nginx
    spec:
      containers:
      - name: nginx
        image: nginx:1.14.2
        ports:
        - containerPort: 80

^ run with kubectl apply -f <file_name>

to scale, use kubectl scale

kubectl scale --replicas=3 rs/foo                                 # Scale a replicaset named 'foo' to 3
kubectl scale --replicas=3 -f foo.yaml                            # Scale a resource specified in "foo.yaml" to 3
kubectl scale --replicas=3 deployment/mysql                       # Scale deployment/mysql (Deployment named mysql) to 3
kubectl scale --current-replicas=2 --replicas=3 deployment/mysql  # If the deployment named mysql's current size is 2, scale mysql to 3
kubectl scale --replicas=5 rc/foo rc/bar rc/baz                   # Scale multiple replication controllers

can also scale by using kubectl edit to edit the deployment

Rolling Updates

• Rolling updates allow Deployments' update to take place with zero downtime by incrementally updating old Pod instances with new ones

• read more -> https://kubernetes.io/docs/concepts/workloads/controllers/deployment/#updating-a-deployment

• rollout triggered if and only if the Deployment's Pod template is changed, e.g. if the labels or container images of the template are updated

  • Other updates, such as scaling the Deployment, do not trigger a rollout
  • trigger with kubectl set image to change the image version/tag
  • trigger with kubectl edit to edit the deployment
  • check status with kubectl rollout status
  • roll back an update with kubectl rollout undo

Deployment Strategies

• deployment strategy - method of rolling out new code

Blue/Green

• uses 2 identical production environments; new code is rolled out to the 2nd environment (green) and validated before redirecting traffic to the 2nd environment from the 1st (blue)

• blue - active environment; green - new environment

• you can use multiple Deployments to set up blue/green environments

  • use labels and selectors on Services to direct user traffic to different Pods

Canary

• uses 2 production environments; portion of user base is directed to the environment with the new code (canary) to validate before rolling out all users

• simple way to set up a canary environment is using a Service that selects Pods from 2 different Deployments

  • vary the number of replicas to direct fewer users to the canary

• read more -> https://kubernetes.io/docs/concepts/workloads/controllers/deployment/#canary-deployment

Helm

• Helm is a package manager for Kubernetes applications; it allows easy installation of software in your cluster
• Helm commands -> https://helm.sh/docs/helm/

Helm Charts

• Helm software package, contains all the Kubernetes resource definitions needed to get applications up and running in the cluster
• install Charts with helm install
• uninstall Charts with helm uninstall

Helm Repository

• collection of available Charts
• add repositories with helm repo add
• update your repositories after adding one with helm repo update
• list all packages in a repository with helm search repo