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What is a ReplicaSet?

Learn that a ReplicaSet ensures Pod replicas.

What is a ReplicaSet?

A ReplicaSet's only job is to make sure that, at every moment, exactly replicas pods matching its selector exist. If some are missing it creates new ones from the template; if there are too many it deletes some. It knows nothing of version transitions or rollbacks; the Deployment adds those.

$ kubectl get rs
NAME            DESIRED   CURRENT   READY   AGE
web-7c5b9f6d8   3         3         3       2h
web-6b4d8c7f5   0         0         0       3h     # old version, for rollback

That is why ReplicaSets are used not directly but through a Deployment.

The trouble with a bare ReplicaSet

If you change the image in the template of a directly created ReplicaSet, the existing pods aren't updated; only pods created from then on get the new image. A Deployment in this case opens a new ReplicaSet and replaces all the pods in an orderly way.

kubectl set image rs/api api=api:2.0
kubectl get pods -l app=api -o jsonpath='{.items[*].spec.containers[0].image}'
# api:1.0 api:1.0 api:1.0   <- still old

Practice Inspect the ReplicaSets

Write two commands: list the ReplicaSets labelled app=web; then show the history of the web deployment.

Starter code

terminal
# buraya yaz

Sample solution

terminal
kubectl get rs -l app=web
kubectl rollout history deployment/web

Expected output

NAME            DESIRED   CURRENT   READY   AGE
web-7c5b9f6d8   3         3         3       2h
web-6b4d8c7f5   0         0         0       3h
REVISION  CHANGE-CAUSE
1         <none>
2         nginx 1.27.3

Questions and answers

Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.

1 Why isn't the old ReplicaSet with DESIRED 0 deleted?

  • It can't be deleted
  • It remained because of an error
  • It creates new pods
  • It is kept in the Deployment history for rollback Correct answer

Explanation: revisionHistoryLimit determines how many are kept.

2 List the ReplicaSets.

Question 
kubectl get ____
Answer 
kubectl get rs

Explanation: rs is short for replicasets.

3 Match.

Term Meaning
DESIRED Desired replicas
CURRENT Existing replicas
READY Ready replicas
DESIRED 0 Old version

All lessons in Intermediate

1 What is Kubernetes?

  1. What is Kubernetes?
  2. Why Kubernetes?
  3. Cluster architecture
  4. Control plane
  5. Worker node
  6. Summary - What is Kubernetes

2 Minikube

  1. What is Minikube?
  2. Installing Minikube
  3. minikube start
  4. minikube status
  5. minikube dashboard
  6. Summary - Minikube

3 kubectl basics

  1. What is kubectl?
  2. kubectl get
  3. kubectl describe
  4. kubectl apply
  5. kubectl delete
  6. Summary - kubectl

4 Pod

  1. What is a Pod?
  2. Pod YAML
  3. Creating a Pod
  4. Pod status
  5. Multi-container Pod
  6. Summary - Pod

5 Deployment

  1. What is a Deployment?
  2. Deployment YAML
  3. Replica count
  4. Rolling update
  5. Rollback
  6. Summary - Deployment

6 ReplicaSet

  1. What is a ReplicaSet?
  2. ReplicaSet YAML
  3. ReplicaSet selector
  4. Scaling
  5. Self-healing
  6. Summary - ReplicaSet

7 Service

  1. What is a Service?
  2. ClusterIP
  3. NodePort
  4. LoadBalancer
  5. Service selector
  6. Summary - Service

8 ConfigMap

  1. What is a ConfigMap?
  2. ConfigMap YAML
  3. Environment variable
  4. Mounting as a volume
  5. kubectl create configmap
  6. Summary - ConfigMap

9 Secret

  1. What is a Secret?
  2. Secret types
  3. Secret YAML
  4. Mounting a Secret
  5. kubectl create secret
  6. Summary - Secret

10 Namespace

  1. What is a Namespace?
  2. kubectl get namespaces
  3. Creating a Namespace
  4. The -n flag
  5. Default Namespace
  6. Summary - Namespace

11 Labels and selectors

  1. What is a label?
  2. Adding labels
  3. Label selector
  4. kubectl get -l
  5. Equality and set selectors
  6. Summary - Labels

12 Project: Multi-tier app

  1. Project design
  2. Database Deployment
  3. Web Deployment
  4. ConfigMap and Secret
  5. Putting it together - multi-tier

13 Troubleshooting

  1. kubectl logs
  2. kubectl exec
  3. kubectl describe pod
  4. CrashLoopBackOff
  5. Pending Pod
  6. Summary - Troubleshooting

14 Intermediate final

  1. Challenge - Pod YAML
  2. Challenge - Deployment
  3. Challenge - Service
  4. Challenge - ConfigMap
  5. Challenge - Intermediate project