What is kubernetes custom resource definition (CRD)?

Custom resource definition (CRD) yes Kubernetes API Expand , You can define new object types .Pod、ReplicaSet、ConfigMap and Ingress Is an example of common built-in resources .CRD Allows you to add new types to this list , Then use the familiar Kubernetes Tools （ for example Kubectl） Manage them .

CRD Mechanisms are deliberately Abstract , It can be used to store data and build new functions in many ways . You will find custom resources in many popular community tools ：Cert-Manager Defines the representation SSL Objects of certificates and issuers , and Helm take Charts Expressed as their own CRD.

What is resource ？

Kubernetes Resources define the types of data that can be stored in the cluster . They are through Kubernetes API Access to the , The API Provides for creating 、 List and modify the endpoints of the project .
You can add custom resources to store your own arbitrary data in the cluster . The project you create will be created by etcd Control plane components are stored with instances of built-in resources .API Custom resources will be automatically displayed , So you don't need to set up your own tools to create project instances .

By default ,CRD Act as a simple data structure . Although wild CRD Usually have their own behavior , But it's not CRD The mechanism provides . Customize Kubernetes Controllers and operators can be used to implement functions around custom resources . If there is no controller ,CRD The project of will always exist as data in the static cluster , You can Kubernetes API Provided CRUD The endpoint interacts with it .

CRD It is a dynamic component that can be created and deleted at any time .Kubernetes Some object types contained in are also implemented as CRD, Thus, more modularity is provided in the core of the cluster .

establish CRD