Introduction

Extending the Kubernetes API is a powerful technique, but building a successful extension requires careful planning and execution. What are the key considerations for building a robust and maintainable Kubernetes API extension? This book aims to answer that question, providing practical guidance for every stage of the development lifecycle.

While the initial setup of a Custom Resource Definition (CRD) and a basic reconciler can be surprisingly quick, developing a well-designed and maintainable Kubernetes extension is a significantly more complex undertaking. Many developers find themselves navigating a landscape with limited guidance on best practices, particularly when it comes to long-term maintenance.

Part of the challenge stems from the flexibility of popular controller frameworks. While this flexibility allows for rapid prototyping, it can also lead to the development of non-standard patterns that exhibit limited interoperability with existing tooling and often deviate from the intended implicit design principles of Kubernetes extensions.

Even the core Kubernetes project itself, while a valuable resource, doesn't always provide the ideal model for CRD design. The core Kubernetes API was not built under the same constraints as CRDs, and with the benefit of hindsight and a decade of experience, we now understand better patterns in platform development. It's likely the core API of Kubernetes would evolve differently if designed today. However, the extensive ecosystem built upon the core Kubernetes APIs necessitates a cautious and deliberate approach to API evolution.

This book offers a collection of design document-like chapters that delve into the practical realities of designing and maintaining Custom Resources for the Kubernetes API. Drawing from years of experience building and maintaining CRDs, leveraging several reconciliation frameworks (including a custom-built framework for Knative), we'll explore topics ranging from API design and versioning strategies to testing methodologies and operational considerations. We'll examine the nuances of reconciler and controller development, providing practical guidance on building extensions that are not only functional but also robust, scalable, and easy to maintain.