What are Init Containers in Kubernetes & Where Should You Use them?

Kubernetes provides a highly scalable and flexible platform for deploying microservices, and its popularity has grown rapidly in recent years.

However, managing containerized applications is not always straightforward, especially regarding dependencies and initialization. That's where Init Containers come in.

This blog will deeply dive into what Init Containers are in Kubernetes (K8s) and explore their purpose, benefits, and use cases. We'll cover everything you need to know to start with Init Containers, including how they work, how to create them, and where to use them.

So, whether you're a Kubernetes beginner or an experienced user, this blog is for you. Let's start and learn how to make the most of Init Containers in Kubernetes.

What are Init Containers - Understanding the Basics

In Kubernetes, an Init Container is a special type of container that runs before the main application container and is used to initialize or prepare dependencies required by the application.

Init Containers run to completion before the actual application containers are started, ensuring that any dependencies or prerequisites are in place before the application starts running.

Think of an Init Container as your application's assistant, taking care of all the necessary preparations so your application container can focus on its main job.

With Init Containers, you can ensure that your application container is lightweight and only includes the necessary components to run the application.

So, if you want happy application containers ready to go from the moment they start running, add an Init Container to your Kubernetes deployment.

Features of Init Containers

One key feature of Init Containers in Kubernetes is their ability to ensure that all necessary resources are in place before an application container starts running. They do this by waiting until networking and storage are ready and executing in the order they appear in the Pod's specification.

Init Containers must complete successfully before the next container starts. You cannot really consider a Pod ready until all Init Containers have succeeded.

Additionally, changes to the Init Container spec are limited to the container image field, and the code should be idempotent to account for possible restarts or retries.

Finally, teams can use activeDeadlineSeconds to prevent an initcontainer from failing indefinitely, but this should only be done in the context of a Job deployment to avoid interfering with running Pods.

Also Read: Kubernetes Best Practices to Follow

Use Case Of Init Containers

Containing Utilities or Custom Code

One of the most common use cases for Init Containers is to contain utilities or custom codes for the setup that are not present in an app image. Init Containers can perform initialization tasks specific to the environment in which your application is running, such as installing dependencies or configuring settings.

You can use an Init Container to install a required tool or utility before running your application. This ensures that your application has all the necessary components to function properly, no matter the environment it is deployed in.

By including custom code and utilities within the Init Container, you can guarantee a consistent and reliable setup process for your application.

Accessing Secrets that App Containers Can't Access

In Kubernetes, Secrets store sensitive information such as passwords, API keys, and certificates. By default, app containers are not able to access Secrets directly.

However, with the use of Init Containers, it is possible to give them access to these Secrets. The Init Container can pull the required Secrets from the Kubernetes API server and write them to a shared volume. The app container can then mount this volume and access the Secrets as environment variables or configuration files.

Cloning Git Repositories

Another common use case for Init Containers is to clone a Git repository into a shared volume. This can be useful when you need to include specific files or configuration files in your application image that are not part of the application code.

Using an Init Container to clone the repository into a shared volume, you can ensure that the required files are available to all app containers using the same volume. This can help simplify your deployment process and ensure your application is always running with the correct configuration.

Waiting for a Service to Start

Sometimes, the main app may need to connect to a database or other external service before it can start processing requests. The app may fail to start or behave unpredictably if the service is unavailable.

Using an Init Container to wait for the service to become available, you can ensure that the main app only starts once all required dependencies are ready. The Init Container can use a tool like a curl or nc to periodically check if the service is up and running and only exit once it receives a successful response.

Init Container Example

Here's a code snippet for a simple Kubernetes Pod with an init container:

In this example, we have a Pod with two containers: a main container and an init container. The main container is running an application called my-app, and the init container is running a simple busybox image.

The init container in the YAML above runs a shell command that simply echoes "Initializing..." and then sleeps for 5 seconds. Once the init container completes its tasks, the main container will start running.

Init Containers vs Other Common Terms

Init Containers vs Containers in Kubernetes

Init containers and containers are both important components in Kubernetes. While containers are the main workloads that run in a Pod, init containers are executed before the main containers to perform setup tasks.

Init containers run to completion before the app containers are started, while regular containers run continuously until terminated.

The key difference is that init containers ensure the environment is set up correctly for the main containers to run successfully.

Init Containers vs Sidecar

Init Containers perform a specific task before the main container starts, such as populating a database or loading configuration files.

On the other hand, sidecar containers run alongside the main container and provide complementary services, such as logging or monitoring.

K8s Job vs Init Container

K8s Jobs are utilized for executing batch processes that complete their run, while Init Containers are employed to perform a specific task before the main container is launched.

Init Containers are a part of the Pod's lifecycle, whereas K8s Jobs creates a new pod for each execution.

Init Containers are the way to go if you need to perform a pre-start task, such as initializing a database or loading configuration files. And, if you need to run a task that completes and exits, then Jobs is the better option.

Init Container vs Readiness Probe

Init Containers in Kubernetes (K8s) execute a specific task before the main container starts, while Readiness Probes are employed to check if the container is capable of serving traffic.

Init Containers perform a specific task before the main container starts, such as initializing the application by downloading and setting up dependencies, while Readiness Probes are used to check the application's health status.

Init Container vs Startup Probe

Init Containers perform a specific task before the main container starts, such as database initialization or loading configuration files. On the other hand, startup probes are used to determine when the application inside a container has started and is ready to serve traffic.

Init Containers serve the purpose of setting up the environment for the application, while Startup Probes are used to determine the readiness of the application.

Init Container vs Poststart Hook

Init Containers perform a specific task before the main container starts, such as initializing a database or downloading dependencies.

In Kubernetes, PostStart Hooks serve the purpose of performing specific tasks after the main container has started, such as running health checks or registering with a service discovery mechanism.

Also Read: Tools for Cost Optimization in K8s

Frequently Asked Questions

What are the different types of containers in Kubernetes?

There are two main types of containers in Kubernetes: Init Containers and Application Containers.

Init Containers perform specific tasks before the main application container starts.

Application Containers run the main application or service in the Pod.

In addition, there are also Sidecar Containers and Helper Containers that provide additional functionality to the main container.

What is exit code 1 in the init container?

An exit code of 1 in an init container indicates that the container has failed to complete its task. This can be due to an error in the container's code or an issue with the container's environment or configuration.

What is the lifecycle of an init container?

The lifecycle of an init container in Kubernetes consists of the following steps:

• In Kubernetes, Kubernetes creates and starts the init container before starting the main container(s) in the pod.

• The init container runs to completion and either succeeds or fails with an exit code.

• If the init container succeeds, Kubernetes starts the main container(s) in the pod.

• When the init container fails, Kubernetes marks the pod as failed and may attempt to restart the pod based on its restart policy.

Further Reading