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Deployment / ReplicaSet

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Architecture

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Detailed Description

Kubernetes Deployments serve as a blueprint for running your application in a cluster. Building on ReplicaSets, they ensure your application remains in the desired state by maintaining the defined number of instances.

ReplicaSets take care of the following:

  • Ensuring the desired number of Pods are always running
  • Replacing failed Pods automatically to maintain the specified replicas

On top of that, a Deployment adds powerful features, such as:

  • Automatically rolling out new versions of your application
  • Rolling back to a previous version if something goes wrong
  • Managing updates with strategies like rolling updates or recreating Pods

Command Reference Guide

Remeber to use dry-run and tee to check the configuration of each command first.
--dry-run=client -o yaml | tee nginx-deployment.yaml

Create a ReplicaSet using a YAML file (declarative method)
apiVersion: apps/v1
kind: ReplicaSet
metadata:
  name: nginx-replicaset
spec:
  replicas: 3
  selector:
    matchLabels:
      app: nginx
  template:
    metadata:
      labels:
        app: nginx
    spec:
      containers:
      - name: nginx
        image: nginxdemos/hello
        ports:
        - containerPort: 80


# Apply replicaset
kubectl apply -f nginx-replicaset.yaml

# Get ReplicaSet informatoin
kubectl get replicaset nginx-replicaset -o wide

# Get detailed ReplicaSet information
kubectl describe replicaset/nginx-replicaset

# Check the current Pods running
kubectl get pods

# Delete a Pod of the ReplicaSet
kubectl delete pod nginx-replicaset-7x6gl

# Recheck running Pods
kubectl get pods

# Change image in yaml to nginxdemos/hello:v0.2 and apply replicaset again
kubectl apply -f nginx-replicaset.yaml


# You will encounter that the replicaset was updated - but the pods are still using the old image
kubectl describe replicaset/nginx-replicaset
kubectl describe pod/<podname>

# You have to kill and recreate the pods, so the new ones will be created with the new image. (see Hint section)
kubectl scale rs nginx-replicaset --replicas=0
kubectl scale rs nginx-replicaset --replicas=3
kubectl describe pod/<podname>

Create a Deplyoment (imperative method)
# Create nginx deployment with the default of one replica
kubectl create deployment nginx-deplyoment --image=nginxdemos/hello --port=80

# Create nginx deployment with three replicas
kubectl create deployment nginx-deplyoment --image=nginxdemos/hello --port=80  --replicas=3


# Expose application as ClusterIP with port 8080 (ClusterIP is the default if not defined)
kubectl expose deployment nginx --type=ClusterIP --port=8080 --target-port=80
# --port=8080: The port exposed by the service (used internally to access the deployment)
# --target-port=80: The port on the pods where the application is running

# Get services
kubectl get service nginx -o wide

# Get full resource description using describe
kubectl describe service/nginx

# Get created endpoints
kubectl get endpoints

# curl by default service DNS entry
# Each curl request gets a different hostname due to Kubernetes' Kube-Proxy load balancing
curl nginx.default.svc.cluster.local

# Delete service
kubectl delete service/nginx
Hints

Deployments manage ReplicaSets, primarily due to historical reasons. There is no practical need to manually create ReplicaSets (or previously, ReplicationControllers), as Deployments, built on top of ReplicaSets, offer a more user-friendly and feature-rich abstraction for managing the application lifecycle, including replication, updates, and rollbacks.

ReplicaSets do not support auto updates. As long as required number of pods exist matching the selector labels, replicaset's jobs is done.


Open questions

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My personal summary

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