Overview
- Custom controllers 允许用户自定义控制器的逻辑，基于已有的资源定义更高阶的控制器，实现 Kubernetes 集群原生不支持的功能
- Custom controllers 的逻辑其实是很简单的：watch CRD 实例（以及关联的资源）的 CRUD 事件，然后开始执行相应的业务逻辑
Controller model
一个无限循环不断地对比实际状态和期望状态，如果有出入则进行调谐逻辑将实际状态调整为期望状态，最终达到与申明一致
- 实际状态: 来自于 Kubernetes 集群本身
- 期望状态: 来自于 用户提交的 YAML 文件
for {
实际状态 := 获取集群中对象 X 的实际状态（Actual State）
期望状态 := 获取集群中对象 X 的期望状态（Desired State）
if 实际状态 == 期望状态{
// do nothing
} else {
// reconcile loop (执行编排动作，将实际状态调整为期望状态)
}
}
Informer
- Informer 与 API 对象是一一对应的
- Informer 其实就是一个带有本地缓存和索引机制的、可以注册 EventHandler 的 client。它是自定义控制器跟 APIServer 进行数据同步的重要组件。
- Informer 通过一种叫作 ListAndWatch 的方法，把 APIServer 中的 API 对象缓存在了本地，并负责更新和维护这个缓存。
Workflow
- 编写 CRD
- 编写 Custom Controller
CRD
apiVersion: apiextensions.k8s.io/v1beta1
kind: CustomResourceDefinition
metadata:
name: foobars.k8s.io
spec:
group: k8s.io
names:
kind: FooBar
listKind: FooBarList
plural: foobars
singular: foobar
scope: Namespaced
validation:
openAPIV3Schema:
description: FooBar is the Schema for the foobars API
properties:
apiVersion:
description: 'APIVersion defines the versioned schema of this representation
of an object. Servers should convert recognized schemas to the latest
internal value, and may reject unrecognized values. More info: https://git.k8s.io/community/contributors/devel/api-conventions.md#resources'
type: string
kind:
description: 'Kind is a string value representing the REST resource this
object represents. Servers may infer this from the endpoint the client
submits requests to. Cannot be updated. In CamelCase. More info: https://git.k8s.io/community/contributors/devel/api-conventions.md#types-kinds'
type: string
metadata:
type: object
spec:
description: FooBarSpec defines the desired state of FooBar
type: object
status:
description: FooBarStatus defines the observed state of FooBar
type: object
type: object
version: v1
versions:
- name: v1
served: true
storage: true
status:
acceptedNames:
kind: ""
plural: ""
conditions: []
storedVersions: []CR
apiVersion: k8s.io/v1
kind: FooBar
metadata:
name: foobar-sample
spec:
foo: bar
barz: "false"
command: "echo foobar"