Imagine a platform team is building an application hosting and CI/CD platform that manages the entire application lifecycle.
This is not a separate process, but a module integrated into kube-apiserver. CRD operations, however, are handled inside kube-apiserver by the apiextensions-apiserver module. The API server is the front end for the Kubernetes control plane. The API server is a component of the Kubernetes control plane that exposes the Kubernetes API. This functionality interacts well with CRDs, and using these two together, IT teams can implement some relatively advanced features and functionality. These enable IT admins to extend the behavior of Kubernetes without modifications to the underlying code. However, to bring more advanced functionality for these custom resources, implement controllers or operators. CRDs do not have any logic attached, nor any special behavior once they are created, modified or removed, they take no actions on their own. CRDs eliminate the overhead of self-directed implementation as well.ĬRDs are, by themselves, just blobs of data: Their primary purpose is to provide a mechanism to create, store and expose Kubernetes API objects that contain data that suits any requirements not satisfied by default. The custom resource is also stored in the etcd cluster with proper replication and lifecycle management. A Kubernetes CRD acts like any other Kubernetes object: It uses all the features of the Kubernetes ecosystem - for example, its command-line interface (CLI), security, API services and role-based access control. CRDs enable IT admins to introduce unique objects or types into the Kubernetes cluster to meet their custom requirements. The standard Kubernetes distribution ships with many built-in API objects and resources. What are custom resource definitions?Ī custom resource definition (CRD) is a powerful feature introduced in Kubernetes 1.7.
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Once the custom resource is registered, end users can create, update and delete its object using kubectl, similar to how users interact with built-in resources, like pods, deployments and services.Ĭustom resources are used for small, in-house configuration objects without any corresponding controller logic - and are, therefore, defined declaratively. But, unlike a normal resource, custom resources are not necessarily available in a default Kubernetes installation.Ĭustom resources are instead registered dynamically to a cluster. In a nutshell custom resources are extensions of the Kubernetes API. Kubernetes comes with several resources by default, such as pods, deployments and ReplicaSets. This declarative records paradigm is complemented by practices like GitOps, where the entire cluster state and operations can be performed by making changes to these records stored in a version control repository.Īlso, Kubernetes stores these records of resources and makes them available via a RESTful HTTP API exposing create, read, update, delete cycle semantics for these objects and resources out of the box.Ī Kubernetes resource is a collection of similar objects accessible via the Kubernetes API. controllers, which are always active, read these records and create, update or delete resources.record of intent created and persisted in the data store and.
Kubernetes exposes a powerful declarative API system, where the record of intent or desired state is specified by cluster operators in a YAML file or via the REST API, and the controllers work in a control loop to converge intent with the observed state.Īt a high level, typically, the Kubernetes resources operate in the following way:
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