Init Containers

Overview

Init containers run before app containers start. They must complete successfully before the main container starts.

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Key Characteristics

Feature	Init Container
Run order	Sequential, before app containers
Must complete	Yes, must exit 0
Restart on failure	Yes, pod restarts
Share volumes	Yes, with app containers
Resource limits	Calculated separately

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Use Cases

• Wait for a service to be ready
• Clone a git repo
• Register with external service
• Download/prepare configuration
• Run database migrations

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Basic Example

apiVersion: v1
kind: Pod
metadata:
  name: init-demo
spec:
  initContainers:
  - name: wait-for-service
    image: busybox:1.36
    command: ['sh', '-c', 'until nslookup myservice; do echo waiting; sleep 2; done']
  - name: init-data
    image: busybox:1.36
    command: ['sh', '-c', 'echo "initialized" > /data/ready']
    volumeMounts:
    - name: shared-data
      mountPath: /data
  containers:
  - name: app
    image: nginx
    volumeMounts:
    - name: shared-data
      mountPath: /usr/share/nginx/html
  volumes:
  - name: shared-data
    emptyDir: {}

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Multiple Init Containers

Run in order - each must succeed before next starts:

spec:
  initContainers:
  - name: init-1    # Runs first
    image: busybox
    command: ['echo', 'step 1']
  - name: init-2    # Runs after init-1 completes
    image: busybox
    command: ['echo', 'step 2']
  - name: init-3    # Runs after init-2 completes
    image: busybox
    command: ['echo', 'step 3']

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Wait for Service Example

initContainers:
- name: wait-for-db
  image: busybox:1.36
  command:
  - sh
  - -c
  - |
    until nc -z postgres-svc 5432; do
      echo "waiting for postgres..."
      sleep 2
    done

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Clone Git Repo

initContainers:
- name: git-clone
  image: alpine/git
  command:
  - git
  - clone
  - https://github.com/example/repo.git
  - /app
  volumeMounts:
  - name: app-volume
    mountPath: /app

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Check Init Container Status

# See init container status
kubectl get pod <pod-name>

# STATUS shows init progress: Init:0/2, Init:1/2, PodInitializing, Running

# Describe for details
kubectl describe pod <pod-name>

# Logs from init container
kubectl logs <pod-name> -c <init-container-name>

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Debugging Failed Init

# Check which init failed
kubectl describe pod <pod-name> | grep -A10 "Init Containers"

# Get logs from failed init
kubectl logs <pod-name> -c <init-container-name>

# Check events
kubectl get events --field-selector involvedObject.name=<pod-name>