Understanding Init Containers in Kubernetes: Use Cases, Examples, and Best Practices

Introduction

Kubernetes gives you a powerful way to manage your containerised applications but what happens when your app needs a little prep work before it runs? That’s where init containers come in.

Think of init containers as setup assistants. They handle important tasks like waiting for a database, setting permissions, or pulling down config files—before your main app even starts. In this guide, you’ll learn what init containers are, when to use them, how to write them, and the best ways to keep them secure and reliable.

Whether you’re just getting started with Kubernetes or you're building more complex deployments, init containers can help you build apps that start right—and stay running.

What Are Init Containers and Where Are They Used?

Init containers are special containers that run before your main application containers in a Kubernetes pod. Their job is to get everything ready before your app kicks off.

Common Use Cases:

• Setting up configs: Pulling configuration files from a repo or service
• Database readiness: Waiting for a database to become available
• Secrets: Grabbing secrets from tools like HashiCorp Vault
• Permissions: Fixing file permissions before your app starts
• Data sync: Downloading large assets or syncing from a repo
• Health checks: Making sure a service is reachable before launching

Each init container runs one at a time, in the order you define. If any of them fail, your main app won’t start—so you can be sure everything is ready.

When Should You Use Init Containers?

Init containers are useful when:

• Your app needs something else to be ready (like a database).
• You want to keep setup logic separate from your app.
• The setup requires tools not included in your main container.
• Tasks need to run in a specific order.
• You want to keep your app image clean and lightweight.

When Not to Use Them

• For background tasks that need to run alongside your app.
• For anything that needs to keep running or restart if it fails.
• For simple tasks that don’t affect whether your app can start.

How to Use Init Containers (With Example)

Here’s a practical example of using init containers to wait for a database and clone config files before starting the app -

apiVersion: v1
kind: Pod
metadata:
  name: myapp-pod
  labels:
    app: myapp
spec:
  containers:
  - name: myapp-container
    image: myapp:1.0
    ports:
    - containerPort: 80
    volumeMounts:
    - name: config-volume
      mountPath: /etc/config
  initContainers:
  - name: init-db
    image: busybox:1.28
    command: ['sh', '-c', 'until nc -z database-service 3306; do echo waiting for database; sleep 2; done;']
  - name: init-config
    image: alpine/git
    command: ['git', 'clone', 'https://github.com/myorg/config-repo.git', '/etc/config']
    volumeMounts:
    - name: config-volume
      mountPath: /etc/config
  volumes:
  - name: config-volume
    emptyDir: {}

What’s happening here:

1. init-db waits until the database is up and reachable.
2. init-config clones a Git repo into a shared volume.
3. Both steps must succeed before the main container starts.
4. The config is shared using a volume mounted in both containers.

Benefits of Using Init Containers

Here’s why init containers are worth using:
✅ Keep things separate: Setup logic stays out of your main app

✅ Order matters: Ensure steps run in the right sequence

✅ Improve security: Run init containers with limited permissions

✅ Lightweight images: Use special-purpose images for setup tasks

✅ Better reliability: Don’t start your app unless everything’s ready

✅ Easier debugging: If something breaks, you’ll know it’s in the setup

✅ Flexible tools: Use different languages, packages, or shells

Potential Backdoors and Security Considerations

Init containers can introduce risks if not handled carefully:
Watch Out For:

• Too many privileges: Avoid running init containers as root unless absolutely needed.
• Secret exposure: Be careful when fetching and storing secrets.
• Bad images: Vulnerable or untrusted images can open up your cluster.
• Persistent changes: Anything written to shared volumes will stick around.

Mitigation Tips:

• Give init containers only the permissions they need.
• Use read-only filesystems if possible.
• Scan init container images for vulnerabilities regularly.
• Use PodSecurityPolicies (or PodSecurity Standards) to control what containers can do.
• For temporary debugging, use ephemeral containers instead.

Conclusion

Init containers give you control and flexibility when starting up your applications in Kubernetes. They make sure everything your app needs is ready before it even starts.

Key Takeaways:

• Use them for one-off setup tasks
• Keep setup logic clean and separate
• Use secure, minimal images
• Monitor their execution for early warning signs

If you’re just starting with Kubernetes, init containers might feel like “extra work.” But once you use them, you’ll see how they can simplify your app logic, boost reliability, and give you a cleaner deployment process.