在不同应用业务环境下，对于平台可能有一些特殊的需求，这些需求可以抽象为 Kubernetes 的扩展资源，而 Kubernetes 的 CRD (CustomResourceDefinition)为这样的需求提供了轻量级的机制，保证新的资源的快速注册和使用。
我们希望在执行 kubectl create -f kafka-source.yaml 之后，在 kubernetes 里会启动一个 pod，这个 pod 会做下面的事情：它会从地址是 my-cluster-kafka-bootstrap.kafka:9092，topic 是 knative- demo-topic 的 kafka 机群里读取消;将读到的消息，发送到 kubernetes 的一个 Service 去执行;Kuberentes 里并没有 KafkaSource 这个资源可以使用，所以直接执行 kubectl create -f kafka-source.yaml 的时候，会出错。但是 kubernetes 提供的 CRD 机制可以让我们轻松的把上述功能添加到 kubernetes 里。
CRD 机制以上述 Kafkasource 为例，如下：(1)需要把 KafkaSource 这个资源注册到 kubernetes 中，这样 kubernetes 才会知道这个资源(2)注册之后，还需要开发一个 controller 组件，来监听用户是否创建了 KafkaSource，也就是部署、更新或者删除如上的 yaml 文件。(3)Controller 监听到有用户创建了 KafkaSource，就会创建一个 pod 来做相应的工作
归纳一下就是：用户向 Kubernetes API 服务注册一个带特定 schema 的资源，并定义相关 API;将扩展资源的数据存储到 Kubernetes 的 etcd 集群;借助 Kubernetes 提供的 controller 模式开发框架，实现新的 controller，并借助 APIServer 监听 etcd 集群关于该资源的状态并定义状态变化的处理逻辑.informer 会借助 APIServer 跟踪该扩展资源定义的变化，一旦被触发就会调用回调函数，并把变更的具体内容放到 Workqueue 中，自定义 controller 里面的 worker会获取Workqueue 里面内容，并进行相应的业务处理。
k8s官方有个crd的例子可以供我们研究学习：
代码语言：javascript复制https://github.com/kubernetes/sample-controller1，编译：
代码语言：javascript复制% go build -o sample-controller .
../../../../go/pkg/mod/sigs.k8s.io/json@v0.0.0-20211020170558-c049b76a60c6/internal/golang/encoding/json/encode.go:1255:18: sf.IsExported undefined (type reflect.StructField has no field or method IsExported)原因是最新版本用的是go 1.17，json包不兼容；
代码语言：javascript复制% git checkout release-1.21重新编译，问题解决。
2，启动controller：
代码语言：javascript复制 % ./sample-controller -kubeconfig=$HOME/.kube/config
I1104 13:44:56.114207 58916 controller.go:115] Setting up event handlers
I1104 13:44:56.114648 58916 controller.go:156] Starting Foo controller
I1104 13:44:56.114654 58916 controller.go:159] Waiting for informer caches to sync
E1104 13:44:56.132686 58916 reflector.go:138] pkg/generated/informers/externalversions/factory.go:117: Failed to watch *v1alpha1.Foo: failed to list *v1alpha1.Foo: the server could not find the requested resource (get foos.samplecontroller.k8s.io)
E1104 13:44:57.427687 58916 reflector.go:138] pkg/generated/informers/externalversions/factory.go:117: Failed to watch *v1alpha1.Foo: failed to list *v1alpha1.Foo: the server启动成功，中间报错，是因为我们没有通过kubectl apply向etcd里注册对应的资源，所以informers说没有对应的资源。
3，创建我们的foo资源
代码语言：javascript复制% kubectl create -f artifacts/examples/crd.yaml
Warning: apiextensions.k8s.io/v1beta1 CustomResourceDefinition is deprecated in v1.16 , unavailable in v1.22 ; use apiextensions.k8s.io/v1 CustomResourceDefinition
customresourcedefinition.apiextensions.k8s.io/foos.samplecontroller.k8s.io created% kubectl create -f artifacts/examples/example-foo.yaml
foo.samplecontroller.k8s.io/example-foo created4，查看资源：
代码语言：javascript复制 % kubectl get deployments
NAME READY UP-TO-DATE AVAILABLE AGE
example-foo 0/1 1 0 14s已经成功将foo注册进去了。
5，回头看我们controller的日志，已经不报错了
代码语言：javascript复制^@^@^@I1104 13:48:10.365898 58916 controller.go:228] Successfully synced 'default/example-foo'
I1104 13:48:10.365938 58916 event.go:291] "Event occurred" object="default/example-foo" kind="Foo" apiVersion="samplecontroller.k8s.io/v1alpha1" type="Normal" reason="Synced" message="Foo synced successfully"6，我们尝试着获取资源
代码语言：javascript复制 % kubectl get CustomResourceDefinition foos.samplecontroller.k8s.io
NAME CREATED AT
foos.samplecontroller.k8s.io 2021-11-04T05:46:53Z% kubectl get Foo example-foo
NAME AGE
example-foo 3m11s说明已经成功运行了，我们定义的资源是否正常写入etcd了呢？根据上一讲的内容，我们尝试获取下：
7，查看etcd内容：
代码语言：javascript复制% ETCDCTL_API=3 etcdctl --cacert=./etcd/ca.crt
--cert=./etcd/peer.crt
--key=./etcd/peer.key
--endpoints=https://127.0.0.1:32389
get "" --prefix --keys-only |grep foo
/registry/apiextensions.k8s.io/customresourcedefinitions/foos.samplecontroller.k8s.io
/registry/deployments/default/example-foo
/registry/events/default/example-foo-9bbb75dc8-v7dzv.16b441bc25c1f3ba
/registry/events/default/example-foo-9bbb75dc8-v7dzv.16b441bc4b1a9b8f
/registry/events/default/example-foo-9bbb75dc8-v7dzv.16b441cbaffd4dc2
/registry/events/default/example-foo-9bbb75dc8-v7dzv.16b441cbbc571207
/registry/events/default/example-foo-9bbb75dc8-v7dzv.16b441cbc264acc2
/registry/events/default/example-foo-9bbb75dc8.16b441bc25872175
/registry/events/default/example-foo.16b441bc243a6ed8
/registry/events/default/example-foo.16b441bc24a13b4a
/registry/pods/default/example-foo-9bbb75dc8-v7dzv
/registry/replicasets/default/example-foo-9bbb75dc8
/registry/samplecontroller.k8s.io/foos/default/example-foo查看key对应的具体内容：
代码语言：javascript复制% ETCDCTL_API=3 etcdctl --cacert=./etcd/ca.crt
--cert=./etcd/peer.crt
--key=./etcd/peer.key
--endpoints=https://127.0.0.1:32389
get /registry/pods/default/example-foo-9bbb75dc8-v7dzv -w json
{"header":{"cluster_id":4202879228857769416,"member_id":16554063148076462710,"revision":2058014,"raft_term":18},"kvs":[{"key":"L3JlZ2lzdHJ5L3BvZHMvZGVmYXVsdC9leGFtcGxlLWZvby05YmJiNzVkYzgtdjdkenY=","create_revision":2057101,"mod_revision":2057210,"version":4,"value":"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"}],"count":1}至此我们的demo已经正常运行了。
