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Kubernetes CRDs: Extending Kubernetes API For Your Needs


In this post, we’ll explore what custom resources are, why they need, and how you can use them to kubernetes for your specific use cases.

In Kubernetes, Custom Resources (CRs) are extensions of the Kubernetes API that allow you to define your own custom object types. These custom objects, referred to as CustomResourceDefinitions (CRDs), can be created and managed just like native Kubernetes resources such as Pods, Services, and Deployments.

Custom Resources give you the flexibility to define application-specific configurations, manage custom deployment strategies, and extend Kubernetes to handle your unique workloads.  But what if your unique applications demand more than the standard Kubernetes offerings? This is where Kubernetes Custom Resources comes into play.

What are Custom Resource Definitions ?

Custom Resource Definitions (CRDs) are a powerful extension mechanism in Kubernetes that allows you to define custom resources. These resources can be specific to your applications, making it easier to manage and interact with them within the Kubernetes. CRDs provide a way to extend the Kubernetes API, making it more adaptable to the demands of your specific use case. Once a CRD is created, instances of the custom resource can be created within a cluster.

Why Use CRDs ?

CRDs are addressing the need for greater extensibility and adaptability in Kubernetes, allowing users to handle specialized workloads, unique configurations, and specific operational needs that might not be addressed by the platform’s default resource types. They serve as a powerful tool for enhancing Kubernetes’s versatility, making it a more accommodating solution for a wide range of use cases. 

So, CRDs are your way of telling Kubernetes, “Hey, I need something special for my project.” It’s like adding a new tool to your toolbox, and it’s a handy trick for customizing Kubernetes to do exactly what you want.

There are many reasons to consider using CRDs in your Kubernetes deployments:

  • Customization: CRDs allow you to define and create custom resources that are tailored to your application’s requirements. This flexibility enables you to represent complex application configurations or stateful data stores as Kubernetes resources.
  • Declarative Management: With CRDs, you can apply the same Kubernetes principles of declarative resource management to your custom resources. This simplifies the deployment and scaling of applications while making your infrastructure more consistent and reliable.
  • Kubernetes Ecosystem Integration: By creating custom resources, you can seamlessly integrate your application with other Kubernetes components like Helm charts, kubectl, and operators. This enables a consistent user experience and simplifies interaction with your applications.
  • Automation: CRDs are the building blocks of Kubernetes Operators. Operators are specialized controllers designed to automate complex application lifecycle management tasks. By using CRDs and Operators, you can automate tasks such as database provisioning, scaling, backup, and recovery.
How to create and use CRDs ?
Defining Custom Resource Specifications:

Creating and using CRDs typically involves the following steps:

Structure and schema of custom resources:

  • Resource Kind: Every custom resource has a resource kind, which defines what the resource represents in the Kubernetes environment. It’s essential to choose a descriptive name that aligns with your use case.
  • API Version: Custom resources typically use the apiVersion field to specify which version of the custom resource definition they adhere to. It’s essential to maintain version compatibility when defining and using custom resources.
  • Metadata: Custom resources include metadata such as the resource name, namespace, labels, and annotations. Metadata helps in identifying and organizing custom resources within the Kubernetes cluster.
  • Spec and Status: Custom resources are commonly structured into two main sections: spec and status. The spec section contains the desired state of the resource, while the status section reflects the actual state.
  • Validation: It’s crucial to specify validation rules to ensure that the provided data for the custom resource adheres to the expected schema. Kubernetes provides various validation mechanisms, including schema validation and OpenAPI validation, to enforce rules.
  • Default Values: You can define default values for attributes in the spec section to simplify the resource creation process. This allows users to omit certain fields, relying on defaults when they create custom resources.
  • Create Custom Resources: Once the CRD is applied, you can create custom resources of that type. These resources can now be managed, scaled, and monitored like any other built-in Kubernetes resource.
  • Develop Custom Controllers: To make your custom resources do something meaningful, you’ll need to develop custom controllers (often referred to as Operators). These controllers watch for changes to your custom resources and take actions accordingly. This is where the real power of CRDs becomes apparent.
  • Deploy and Manage Your Application: With CRDs and custom controllers in place, you can now deploy, scale, and manage your application using Kubernetes-native tools and practices.

Define a CRD:

Start by defining a CRD in a YAML file. This YAML file specifies the structure and schema of your custom resource, including its properties and validation rules.

Here’s an example of a simple CRD definition for a custom resource named “MyApp”.

					apiVersion: apiextensions.k8s.io/v1
kind: CustomResourceDefinition
  name: myapps.example.com
  group: example.com
    - name: v1
      served: true
      storage: true
  scope: Namespaced
    plural: myapps
    singular: myapp
    kind: MyApp

Apply the CRD:

Once you’ve defined your CRD, use kubectl or another Kubernetes resource management tool to apply the CRD to your cluster. This step registers your custom resource type with the Kubernetes API server.

					kubectl apply -f myapp-crd.yaml
Create the Custom Resources:

With the CRD applied, you can now create custom resources of the type you defined. Custom resources are created just like built-in resources using YAML manifests. 

Here’s an example of a “MyApp” custom resource:

					apiVersion: example.com/v1
kind: MyApp
  name: myapp-instance
  replicas: 5
  version: 2.0.5

					kubectl apply -f my-app.yaml

Now, To check the created custom resource, it is look like this.

					kubectl get myapp
my-app  2.0.5     5s
Best Practices For Using Custom Resources

When working with custom resources, consider the following steps:

  • Security: Ensure that Custom Resources don’t introduce security vulnerabilities. Apply RBAC (Role-Based Access Control) and network policies to secure your CRs.

  • Scalability: Design CRs to scale efficiently, and monitor their performance as the number of CR instances grows.

  • Maintainability: Keep your CRDs and CRs well-documented and versioned. This is essential for future management and troubleshooting.


Kubernetes Custom Resource Definitions (CRDs) are a valuable tool for extending the Kubernetes API to meet the unique requirements of your applications. As your containerized applications become more complex, CRDs and custom controllers (Operators) can simplify their management and help ensure they operate reliably in your Kubernetes environment. Custom Resources empower you to take full advantage of Kubernetes’ flexibility, making it an even more versatile platform for deploying and managing containerized applications.

Are you ready to explore the world of Kubernetes Custom Resources and unlock the full potential of your Kubernetes clusters? Start by defining your first CustomResourceDefinition and see how Kubernetes can adapt to your unique needs.