Solving Kubernetes Object Deletion Issues: Managing Owner References and Finalizers

Learn how to effectively manage object dependencies in `Kubernetes` using owner references and finalizers, ensuring smooth deletions and preventing orphaned resources.
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This video is based on the question https://stackoverflow.com/q/70758219/ asked by the user 'cosimoth' ( https://stackoverflow.com/u/12745566/ ) and on the answer https://stackoverflow.com/a/75309524/ provided by the user 'Klaus Telenius' ( https://stackoverflow.com/u/10619390/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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Managing Kubernetes Object Dependencies: Owner References and Finalizers

Kubernetes is a powerful platform for managing containerized applications, but it can present challenges when dealing with object dependencies. In this guide, we will explore a common problem faced by developers: the cascading deletion of Kubernetes objects, particularly when using owner references. We'll break down the solution into manageable sections, focusing on how to utilize finalizers effectively to handle dependencies between custom resources.

The Problem: Orphaned Resources in Kubernetes

Let's say you've implemented a controller for a custom resource (CR) called A, which depends on another resource B. When you set an ownerReference on B instances in your controller, it allows your controller to "watch" B for updates. In a typical scenario, when A is deleted, you would expect the corresponding ownerReference on B to be cleared to prevent it from being orphaned.

However, there’s a catch. The Kubernetes API server may detect B as orphaned and delete it immediately, even before your controller can clear the ownerReference. This leaves you with unexpected behavior and data loss. You may try adding a finalizer on B, but this can also fail if both the ownerReference and finalizer are removed during the deletion process.

So, what can you do to prevent this cascading deletion issue?

The Solution: Adding Finalizers to Object A

To manage dependencies effectively in this scenario, you can add a finalizer to A instead of B. Here's how it works:

Step 1: Understanding Finalizers

A finalizer is a mechanism that allows you to extend the deletion process of a Kubernetes object. By adding a finalizer to A, you can control what happens right before A is deleted and ensure that its dependencies are handled correctly.

Step 2: Implementing the Finalizer Logic

Add a Finalizer to A: Modify the metadata of your CR A to include a finalizer. This will prevent A from being deleted immediately until the finalizer logic is executed.

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Implement the Finalizer Logic in the Controller: In your controller's Reconcile function, check if A has been marked for deletion. If it has, proceed to clear the ownerReference from B before allowing A to be deleted:

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Step 3: Removing the Finalizer After Processing

Once you’ve cleared the ownerReference, you need to remove the finalizer from A. This will allow A to be deleted without causing B to be orphaned.

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Conclusion: Best Practices for Handling Dependencies

Managing dependencies in Kubernetes can be tricky, but understanding how to leverage owner references and finalizers can help you avoid common pitfalls. Here are some best practices to keep in mind:

Always consider object ownership: Be mindful of how your resources are linked to one another through owner references.

Utilize finalizers: Implement finalizers to gain more control over the deletion flow of your resources.

Test your controller logic: Regularly test the deletion flow in various scenarios to ensure that your logic handles dependencies correctly.

By following these best practices, you can ensure that your Kubernetes applications maintain the integrity of their resources, even in complex scenarios.

Now, you have the tools you need to manage Kubernetes object dependencies effectively!