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A resource is an endpoint in the Kubernetes API that stores a collection of API objects of a certain kind. For example, the built-in pods resource contains a collection of Pod objects. A custom resource is an extension of the Kubernetes API that is not necessarily available in a default Kubernetes installation. It represents a customization of a particular Kubernetes installation. However, many core Kubernetes functions are now built using custom resources, making Kubernetes more modular. Custom resources can appear and disappear in a running cluster through dynamic registration, and cluster admins can update custom resources independently of the cluster itself. Once a custom resource is installed, users can create and access its objects using kubectl, just as they do for built-in resources like Pods.
ResourcesとCustom Controllersによって実現されます。 Custom controllers On their own, custom resources simply let you store and retrieve structured data. When you combine a custom resource with a custom controller, custom resources provide a true declarative API. A declarative API allows you to declare or specify the desired state of your resource and tries to keep the current state of Kubernetes objects in sync with the desired state. The controller interprets the structured data as a record of the user’s desired state, and continually maintains this state. You can deploy and update a custom controller on a running cluster, independently of the cluster’s own lifecycle. Custom controllers can work with any kind of resource, but they are especially effective when combined with custom resources. The Operator pattern combines custom resources and custom controllers. You can use custom controllers to encode domain knowledge for speciﬁc applications into an extension of the Kubernetes API. 引用元：https://kubernetes.io/docs/concepts/extend-kubernetes/api-extension/custom-resources/#custom-controllers
Site Reliability Engineer (SRE) is a person that operates an application by writing software. They are an engineer, a developer, who knows how to develop software speciﬁcally for a particular application domain. The resulting piece of software has an application's operational domain knowledge programmed into it. Our team has been busy in the Kubernetes community designing and implementing this concept to reliably create, conﬁgure, and manage complex application instances atop Kubernetes. We call this new class of software Operators. An Operator is an application-speciﬁc controller that extends the Kubernetes API to create, conﬁgure, and manage instances of complex stateful applications on behalf of a Kubernetes user. It builds upon the basic Kubernetes resource and controller concepts but includes domain or application-speciﬁc knowledge to automate common tasks.. 引用元：https://coreos.com/blog/introducing-operators.html