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Self-healing

Learn that ReplicaSet recreates deleted Pods.

Self-healing

The ReplicaSet controller runs a continuous reconciliation loop: it observes the actual pod count, compares it with the desired one, and closes the gap. If a pod is deleted, a node crashes or a pod is evicted, the missing pod is recreated:

$ kubectl delete pod web-7c5b9f6d8-abcde
$ kubectl get events --field-selector involvedObject.kind=ReplicaSet | tail -2
Normal  SuccessfulDelete  replicaset/web-7c5b9f6d8  Deleted pod: web-7c5b9f6d8-abcde
Normal  SuccessfulCreate  replicaset/web-7c5b9f6d8  Created pod: web-7c5b9f6d8-mnopq

Container restarts

There are two levels of healing. If a container crashes, the kubelet restarts the container inside the same pod (restartPolicy: Always) and the RESTARTS counter goes up. If the pod itself disappears, the ReplicaSet creates a new pod. The log of the previously crashed container is seen with --previous:

$ kubectl get pod api-5d8f7c9b4-qrstu
NAME                  READY   STATUS    RESTARTS      AGE
api-5d8f7c9b4-qrstu   1/1     Running   3 (2m ago)    20m
$ kubectl logs api-5d8f7c9b4-qrstu --previous

Practice Observe the healing

Write three commands: delete the web-7c5b9f6d8-abcde pod; list the pods labelled app=web; show the previous container log of the api-5d8f7c9b4-qrstu pod.

Starter code

terminal
# buraya yaz

Sample solution

terminal
kubectl delete pod web-7c5b9f6d8-abcde
kubectl get pods -l app=web
kubectl logs api-5d8f7c9b4-qrstu --previous

Expected output

pod "web-7c5b9f6d8-abcde" deleted
NAME                  READY   STATUS    RESTARTS   AGE
web-7c5b9f6d8-fghij   1/1     Running   0          2h
web-7c5b9f6d8-mnopq   1/1     Running   0          4s
panic: runtime error: invalid memory address

Questions and answers

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

1 What does self-healing not cover?

  • A deleted pod
  • A pod on a crashed node
  • An evicted pod
  • A bug in the application; the new pod crashes with the same bug Correct answer

Explanation: A constantly crashing pod falls into CrashLoopBackOff.

2 Show the log of the previous (crashed) container.

Question 
kubectl logs api-5d8f7c9b4-qrstu ____
Answer 
kubectl logs api-5d8f7c9b4-qrstu --previous

Explanation: --previous (-p) is the previous container instance's log.

3 Match.

Term Meaning
kubelet Restarts the container
ReplicaSet Recreates the pod
RESTARTS Container restart count
--previous Log of the crashed one

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