How to fix Kubernetes CrashLoopBackOff

 How to debug/troubleshoot and fix Kubernetes CrashLoopBackOff?

Step 1:

run kubectl describe pod <REPLACE YOUR POD NAME HERE> will give us more information on that pod:

kubectl describe pod nginx-5796d5bc7c-xsl6p --namespace nginx-crashloop

Name:           nginx-5796d5bc7c-xsl6p

Namespace:      nginx-crashloop

Node:           ip-10-0-9-132.us-east-2.compute.internal/10.0.9.132

Start Time:     Tue, 11 Aug 2021 19:11:05 +0200

Labels:         app=nginx-crashloop

                name=nginx

                pod-template-hash=1352816737

                role=app

Annotations:    kubernetes.io/created-by={"kind":"SerializedReference","apiVersion":"v1","reference":{"kind":"ReplicaSet","namespace":"nginx-crashloop","name":"nginx-5796d5bc7c","uid":"fb9e9518-f542-11e7-a8f2-065cff0...

Status:         Running

IP:             10.47.0.15

Controlled By:  ReplicaSet/nginx-5796d5bc7c

Containers:

  nginx:

    Container ID:   docker://513cab3de8be8754d054a4eff45e291d33b63e11b2143d0ff782dccc286ba05e

    Image:          nginx

    Image ID:       docker-pullable://nginx@sha256:c4ee0ecb376636258447e1d8effb56c09c75fe7acf756bf7c13efadf38aa0aca

    Port:           <none>

    State:          Waiting

      Reason:       CrashLoopBackOff

    Last State:     Terminated

      Reason:       Error

      Exit Code:    1

      Started:      Tue, 11 Aug 2021 19:13:15 +0200

That will help to understand the issue.

Most of the cases you will find are related to configuration issues in deployment.yaml file.

For I found the reference key for the database password was wrong in myapp.yaml file.

I fixed and redeployed I got fixed this issue. here is hilighted key that was wrong earlier.

apiVersion: apps/v1

kind: Deployment

metadata: 

  name: mongo-express

  labels:

    app: mongo-express

spec:

  replicas: 2

  selector:

    matchLabels:

      app: mongo-express    

  template:

    metadata:

      labels:

        app: mongo-express

    spec:

      containers:

      - image: mongo-express

        imagePullPolicy: IfNotPresent

        name: mongo-express

        ports:

        - containerPort: 8081

        env:

        - name: ME_CONFIG_MONGODB_ADMINUSERNAME

          valueFrom:

            secretKeyRef:

              name: mongodb-secrete

              key: mongo-root-username

        - name: ME_CONFIG_MONGODB_ADMINPASSWORD 

          valueFrom:

            secretKeyRef:

              name: mongodb-secrete

              key: mongo-root-password

        - name: ME_CONFIG_MONGODB_SERVER 

          valueFrom:

            configMapKeyRef:

              name: mongodb-map

              key: database_url

---

apiVersion: v1

kind: Service

metadata:  

  name: mongo-express-service

spec:

  selector:

    app: mongo-express

  type: LoadBalancer

  ports:

    - protocol: TCP

      port: 8081

      targetPort: 8081

      nodePort: 30002