What is Init Container in Kubernetes?

An Init Container is a special container that runs before your main application container starts.

👉 Think of it like setup/initialization step
Before your app starts, something must be ready → init container handles that.

Key Points (Very Important)

• Runs before main container
• Runs only once and completes
• Runs in sequence (if multiple init containers)
• If it fails → main container will NOT start

Why we use Init Containers (Real Use Cases)

Real-world use cases (very useful in your EKS/Kubernetes work):

1. Wait for dependency (most common)

Example:

• Wait until DB is up before app starts

2. Create config files

• Download config from Git / S3

3. Prepare volume/data

• Create directories
• Copy files

4. Security / permissions setup

• Set file permissions before app runs

👉 Basically:
👉 “Do setup → THEN start app”

YAML Example

apiVersion: v1
kind: Pod
metadata:
  name: init-demo
spec:
  initContainers:
  - name: init-myservice
    image: alpine
    command: ['sh', '-c', 'echo "Preparing data..."; sleep 5; mkdir -p /data/logs']
    volumeMounts:
    - name: shared-data
      mountPath: /data
  containers:
  - name: main-app
    image: nginx
    ports:
    - containerPort: 80
    volumeMounts:
    - name: shared-data
      mountPath: /data
  volumes:
  - name: shared-data
    emptyDir: {}

What happens here?

Step-by-step:

1. Init container runs first
   → prints message
   → waits 5 sec
   → creates /data/logs
2. After success ✅
   → main container (nginx) starts
3. Both share same volume (emptyDir)
   → data created is available to app

Commands to Run & Verify

1. Apply YAML

kubectl apply -f init-demo.yaml

2. Check Pod Status

kubectl get pods

3. Describe Pod (Detailed)

kubectl describe pod init-demo

4. Check Init Container Logs

kubectl logs init-demo -c init-myservice

5. After success

kubectl get pods

Another Real Example

initContainers:
- name: wait-for-db
  image: busybox
  command:
  - sh
  - -c
  - |
    until nc -z mydb 5432; do
      echo "Waiting for DB...";
      sleep 2;
    done

Important Difference (Init vs Normal Container)