What is Kubernetes sessions - 8: Extensibility

This blog highlights Kubernetes' flexibility through extensibility, including plugins, Custom Resource Definitions (CRDs), and Custom Controllers. These features enable customizations without altering core code, automate processes, and streamline app-specific tasks.

In today's fast-paced tech environment, flexibility and customization are key. That's where Kubernetes (K8s) extensibility comes into play. In the previous articles we addressed the native building blocks of Kubernetes. But many organizations may need to integrate parts of their existing ecosystem, going beyond the capabilities at hand. This article will explore the various ways in which K8s allows users to do so.

The power of plugins and add-ons

One of the most attractive features of Kubernetes is its ability to support plugins and add-ons. These tools allow users to extend the platform's capabilities without having to modify the core K8s codebase. This flexibility enables users to address unique requirements and integrate with other systems in their infrastructure.

Examples of popular plugins and add-ons include:

  • Metrics server: collects container resource metrics for autoscaling
  • Cluster autoscaler: adds nodes to the cluster based on metrics
  • Overlay networks: a network that allows all the parts to talk to each other
  • Ingress controllers: to manage cloud ingress resources, like load balancers and api gateways

Some clouds allow you to specify addons at cluster creation time, and install the most needed ones by default. Yet other clouds, like AWS, instruct you to deploy any addons yourself afterwards. Failing to do so may lead to headaches, and so it is advised to run a tool like Sonobuoy to check if your cluster conforms to the requirements you defined up front.

CRDs: Tailoring Kubernetes to your needs

Kubernetes allows you to create Custom Resource Definitions (CRDs), which are a way to extend the platform with new, custom resource types. CRDs enable users to define their application-specific resources that K8s can understand and manage, giving them more control over their deployments and configurations.

For example, suppose you're managing a multi-tenant application that needs to handle varying configurations for each tenant. In that case, you could create a CRD representing a tenant configuration, which would include details such as authentication settings, storage quotas, and custom feature flags. This approach allows for managing tenant configurations directly within the K8s ecosystem, streamlining the deployment process and making it easier to manage multiple tenants.

Moreover, CRDs can be used to extend Kubernetes functionality by introducing resources that interact with third-party APIs. For example, a custom resource could be created to represent a cloud-based machine learning model, with the custom controller managing the interactions between the K8s cluster and the external machine learning service. This integration simplifies the management of machine learning models and allows for seamless scaling of model deployments.

Custom Controllers: The art of automation

Custom Controllers are another way to extend the functionality of Kubernetes. They work in tandem with custom resources to automate processes and orchestrate workflows specific to your applications. Essentially, custom controllers are like "robots" that watch for changes in your custom resources and react accordingly, based on the logic you define.

By combining custom resources and custom controllers, you can create what's called an "Operator." Operators enable you to manage application-specific operations and automate complex tasks, making it easier to deploy, configure, and manage your applications on Kubernetes.

One very useful innovation that came out of this is the Operator Lifecycle Manager. It is a generic top level operator that can deploy domain specific operators listed in operator catalogs found in the cluster, or in external catalogs, such as operatorhub.io. Please check its extensive list of operators and their maturity level. Level 5 means it is fully autonomous and will do its own lifecycle maintenance. Awesome!


Kubernetes' extensibility is one of its most powerful features. By leveraging plugins, add-ons, custom resources, aggregated API servers, and custom controllers, users can customize their systems to meet unique needs and streamline operations.

Ultimately, Kubernetes' extensibility allows organizations to focus on building innovative applications and solutions, rather than getting bogged down in the nitty-gritty details of managing infrastructure. So go ahead, unlock the full potential of Kubernetes, and embrace the endless possibilities of its extensible ecosystem.

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