DevOps Bootcamp · Lesson

Init Containers and Startup Ordering

Learn how init containers run setup tasks before your main app containers start, and how they enforce ordering inside a Pod.

Init Containers and Startup Ordering is a free DevOps Bootcamp lesson on CoddyKit — lesson 4 of 4. You can read the complete lesson below for free — then practise it hands-on in the browser with a built-in code editor and a 24/7 AI tutor. It is part of the DevOps Bootcamp learning path, one of 4 lessons in the course, and your progress syncs across the web and the CoddyKit app.

What Are Init Containers?

An init container is a special container that runs to completion before the main app containers in a Pod start. Pods can have one or more of them.

They are perfect for one-time setup tasks: waiting for a dependency, cloning config, or running a database migration.

How They Differ from App Containers

Init containers always run to completion and must succeed before the next one starts. App containers, by contrast, run continuously.

Init containers run sequentially, one at a time
App containers run in parallel
If an init container fails, the Pod restarts it (per restartPolicy)

A Basic Init Container Spec

Init containers live under spec.initContainers, a sibling of spec.containers.

apiVersion: v1
kind: Pod
metadata:
  name: app-with-init
spec:
  initContainers:
  - name: wait-for-db
    image: busybox:1.36
    command: ['sh', '-c', 'echo waiting; sleep 5']
  containers:
  - name: app
    image: nginx:1.27

Why Ordering Matters

Kubernetes guarantees the order: every init container finishes successfully before the main containers even begin.

This lets you express dependencies declaratively instead of baking retry loops into your app image.

Waiting for a Service

A common pattern is to block startup until a dependent Service responds.

initContainers:
- name: wait-for-api
  image: busybox:1.36
  command:
  - sh
  - -c
  - 'until nslookup api-service; do echo waiting; sleep 2; done'

Multiple Init Containers

You can chain several init containers. They run in the order listed, each fully completing before the next begins.

initContainers:
- name: step-1-fetch-config
  image: busybox:1.36
  command: ['sh', '-c', 'echo fetching config']
- name: step-2-migrate
  image: busybox:1.36
  command: ['sh', '-c', 'echo running migration']

Sharing Data with emptyDir

Init containers often prepare files for the app container using a shared emptyDir volume.

volumes:
- name: shared
  emptyDir: {}
initContainers:
- name: setup
  image: busybox:1.36
  command: ['sh', '-c', 'echo hello > /work/index.html']
  volumeMounts:
  - name: shared
    mountPath: /work

Inspecting Init Status

While init containers run, the Pod shows a status like Init:0/2. Use kubectl to follow progress.

kubectl get pod app-with-init
kubectl logs app-with-init -c wait-for-db
kubectl describe pod app-with-init

Resource Usage Rules

Because init containers run sequentially, Kubernetes uses the highest resource request/limit among them (not the sum) when scheduling, then compares against app container needs.

Common Use Cases

Wait for a database or external API to be reachable
Run schema migrations before the app boots
Generate or download configuration files
Set file permissions on a mounted volume
Register the Pod with a service registry

Failure Behavior

If an init container fails and the Pod restartPolicy is Always or OnFailure, Kubernetes keeps retrying it until it succeeds. The main containers never start until then.

Quick Check

Test your understanding of init container ordering.

Recap

You learned that init containers run sequentially to completion before app containers start. They enforce startup ordering, prepare shared data via volumes, and handle one-time setup like migrations or dependency checks.

Next, you can combine init containers with sidecars for richer Pod startup patterns.

Frequently asked questions

Is the “Init Containers and Startup Ordering” lesson free?

Yes — the full text of “Init Containers and Startup Ordering” is free to read here on the web, and the DevOps Bootcamp course includes 4 lessons in total. To practise it interactively (a built-in code editor and a 24/7 AI tutor) and unlock the rest of the DevOps Bootcamp course, upgrade to CoddyKit PRO.

What will I learn in “Init Containers and Startup Ordering”?

Learn how init containers run setup tasks before your main app containers start, and how they enforce ordering inside a Pod. You practise DevOps Bootcamp with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.

Do I need any experience to start DevOps Bootcamp?

No prior experience is required. DevOps Bootcamp on CoddyKit is structured for beginners through advanced learners; this is — lesson 4 of 4, so you can start here or from the beginning and move at your own pace.

How long does the “Init Containers and Startup Ordering” lesson take?

Most CoddyKit lessons take about 5–10 minutes. Each one is bite-sized and interactive, so you make steady progress and pick up exactly where you left off across the web and the app.

Can I write and run code in this DevOps Bootcamp lesson?

Yes. Every DevOps Bootcamp lesson includes a built-in code editor, so you write and run real code right in your browser and get instant AI feedback — no local setup required.

All lessons in this course

← Back to DevOps Bootcamp