client-go源码解析(一)：client-go客户端对象

client-go的重要性

client-go是对K8s集群的二次开发工具，所以client-go是k8s开发者的必备工具之一。

本篇目的

本篇的目的旨在教会大家k8s集群的客户端有哪些种类，大家可能之前都使用过client-go去操作k8s中的资源对象，但是对于其中原理可能一知半解，client-go github那边只是给出了简单的几个例子，没有太多深入讲解。本讲就先跟大家介绍下，client-go到底有几种客户端对象去和集群交互，这些客户端对象各自有什么用途，他们交互的原理是什么？

客户端对象

client-go的客户端对象有4个，作用各有不同：

• RESTClient： 是对HTTP Request进行了封装，实现了RESTful风格的API。其他客户端都是在RESTClient基础上的实现。可与用于k8s内置资源和CRD资源
• ClientSet:是对k8s内置资源对象的客户端的集合，默认情况下，不能操作CRD资源，但是通过client-gen代码生成的话，也是可以操作CRD资源的。
• DynamicClient:不仅能对K8S内置资源进行处理，还可以对CRD资源进行处理，不需要client-gen生成代码即可实现。
• DiscoveryClient：用于发现kube-apiserver所支持的资源组、资源版本、资源信息（即Group、Version、Resources）。
  这小节总结下：RESTCLient、ClientSet和DynamicClient都可以对K8S内置资源和CRD资源进行操作。只是clientSet需要生成代码才能操作CRD资源。

RESTClient

RESTClient是对HTTP Request 进行了封装，是实现了RESTful风格的API封装，使得使用者在获取K8S资源时不需要使用原始API的方式（例如请求https://1.142.113.45:6443/api/v1/pods这样的原始形式）。 ClientSet的客户端使得代码更有阅读性，更易使用。下面举个例子：

func GetRestClient(kubeconfig string) (error, *rest.RESTClient){
	config, err := clientcmd.BuildConfigFromFlags("", kubeconfig)
	if err != nil {
		return err, nil
	}
	config.APIPath = "api"
	config.GroupVersion = &corev1.SchemeGroupVersion
	config.NegotiatedSerializer = scheme.Codecs

	restClient, err := rest.RESTClientFor(config)
	if err != nil {
		return err, nil
	}
	return nil, restClient
}

这边是先给了kubeconfig文件，然后clientcmd.BuildConfigFromFlags("", kubeconfig)加载配置文件并生成config对象，看下ClientConfig对象的生成：

func BuildConfigFromFlags(masterUrl, kubeconfigPath string) (*restclient.Config, error) {
	if kubeconfigPath == "" && masterUrl == "" {
		klog.Warningf("Neither --kubeconfig nor --master was specified.  Using the inClusterConfig.  This might not work.")
		kubeconfig, err := restclient.InClusterConfig()
		if err == nil {
			return kubeconfig, nil
		}
		klog.Warning("error creating inClusterConfig, falling back to default config: ", err)
	}
	return NewNonInteractiveDeferredLoadingClientConfig(
		&ClientConfigLoadingRules{ExplicitPath: kubeconfigPath},
		&ConfigOverrides{ClusterInfo: clientcmdapi.Cluster{Server: masterUrl}}).ClientConfig()
}

这个ClientConfig()函数负责将Kuebconfig文件加载后的对象NewNonInteractiveDeferredLoadingClientConfig转换成restclient.Config对象：

func (config *DirectClientConfig) ClientConfig() (*restclient.Config, error) {
	// check that getAuthInfo, getContext, and getCluster do not return an error.
	// Do this before checking if the current config is usable in the event that an
	// AuthInfo, Context, or Cluster config with user-defined names are not found.
	// This provides a user with the immediate cause for error if one is found
	configAuthInfo, err := config.getAuthInfo()
	if err != nil {
		return nil, err
	}

	_, err = config.getContext()
	if err != nil {
		return nil, err
	}

	configClusterInfo, err := config.getCluster()
	if err != nil {
		return nil, err
	}

	if err := config.ConfirmUsable(); err != nil {
		return nil, err
	}

	clientConfig := &restclient.Config{}
	clientConfig.Host = configClusterInfo.Server

	if len(config.overrides.Timeout) > 0 {
		timeout, err := ParseTimeout(config.overrides.Timeout)
		if err != nil {
			return nil, err
		}
		clientConfig.Timeout = timeout
	}

	if u, err := url.ParseRequestURI(clientConfig.Host); err == nil && u.Opaque == "" && len(u.Path) > 1 {
		u.RawQuery = ""
		u.Fragment = ""
		clientConfig.Host = u.String()
	}
	if len(configAuthInfo.Impersonate) > 0 {
		clientConfig.Impersonate = restclient.ImpersonationConfig{
			UserName: configAuthInfo.Impersonate,
			Groups:   configAuthInfo.ImpersonateGroups,
			Extra:    configAuthInfo.ImpersonateUserExtra,
		}
	}

	// only try to read the auth information if we are secure
	if restclient.IsConfigTransportTLS(*clientConfig) {
		var err error
		var persister restclient.AuthProviderConfigPersister
		if config.configAccess != nil {
			authInfoName, _ := config.getAuthInfoName()
			persister = PersisterForUser(config.configAccess, authInfoName)
		}
		userAuthPartialConfig, err := config.getUserIdentificationPartialConfig(configAuthInfo, config.fallbackReader, persister)
		if err != nil {
			return nil, err
		}
		mergo.MergeWithOverwrite(clientConfig, userAuthPartialConfig)

		serverAuthPartialConfig, err := getServerIdentificationPartialConfig(configAuthInfo, configClusterInfo)
		if err != nil {
			return nil, err
		}
		mergo.MergeWithOverwrite(clientConfig, serverAuthPartialConfig)
	}

	return clientConfig, nil
}

clientConfig就是最后生成的config对象。这边使用给出的kubeconfig文件的路径来初始化一个clientset。当然，client-go可以使用in-cluster config的方式来初始化这些客户端。只不过一般我们调试的时候使用kubeconfig文件来调试。
那么我们来看下RESTClient客户端的实现以此来深入理解：
先看RESTClientFor函数的实现：

func RESTClientFor(config *Config) (*RESTClient, error) {
// 基础配置检验下，没有就直接报错退出
	if config.GroupVersion == nil {
		return nil, fmt.Errorf("GroupVersion is required when initializing a RESTClient")
	}
	if config.NegotiatedSerializer == nil {
		return nil, fmt.Errorf("NegotiatedSerializer is required when initializing a RESTClient")
	}
	// 有些配置没给配置，就默认配置下。
	qps := config.QPS
	if config.QPS == 0.0 {
		qps = DefaultQPS
	}
	burst := config.Burst
	if config.Burst == 0 {
		burst = DefaultBurst
	}

	baseURL, versionedAPIPath, err := defaultServerUrlFor(config) // 获取基本的URL和apiPath，这个行程了基础的url，一会跟一下代码
	if err != nil {
		return nil, err
	}

	transport, err := TransportFor(config)
	if err != nil {
		return nil, err
	}

	var httpClient *http.Client
	if transport != http.DefaultTransport {
		httpClient = &http.Client{Transport: transport}
		if config.Timeout > 0 {//是否设置超时时间，没有就给默认值
			httpClient.Timeout = config.Timeout
		}
	}

	return NewRESTClient(baseURL, versionedAPIPath, config.ContentConfig, qps, burst, config.RateLimiter, httpClient) // 新建的NewRESTClient
}

先看下defaultServerUrlFor函数的代码：

func defaultServerUrlFor(config *Config) (*url.URL, string, error) {
	// TODO: move the default to secure when the apiserver supports TLS by default
	// config.Insecure is taken to mean "I want HTTPS but don't bother checking the certs against a CA."
	hasCA := len(config.CAFile) != 0 || len(config.CAData) != 0
	hasCert := len(config.CertFile) != 0 || len(config.CertData) != 0
	defaultTLS := hasCA || hasCert || config.Insecure
	host := config.Host // 拿出hosts
	if host == "" {
		host = "localhost"
	}

	if config.GroupVersion != nil { // groupversion
		return DefaultServerURL(host, config.APIPath, *config.GroupVersion, defaultTLS)
	}
	return DefaultServerURL(host, config.APIPath, schema.GroupVersion{}, defaultTLS)
}

看下NewRESTClient函数里面有什么：

func NewRESTClient(baseURL *url.URL, versionedAPIPath string, config ContentConfig, maxQPS float32, maxBurst int, rateLimiter flowcontrol.RateLimiter, client *http.Client) (*RESTClient, error) {
	base := *baseURL
	if !strings.HasSuffix(base.Path, "/") {
		base.Path += "/"
	}
	base.RawQuery = ""
	base.Fragment = ""

	if config.GroupVersion == nil {
		config.GroupVersion = &schema.GroupVersion{}
	}
	if len(config.ContentType) == 0 {
		config.ContentType = "application/json"
	}
	serializers, err := createSerializers(config)
	if err != nil {
		return nil, err
	}

	var throttle flowcontrol.RateLimiter
	if maxQPS > 0 && rateLimiter == nil {
		throttle = flowcontrol.NewTokenBucketRateLimiter(maxQPS, maxBurst)
	} else if rateLimiter != nil {
		throttle = rateLimiter
	}
	return &RESTClient{
		base:             &base,
		versionedAPIPath: versionedAPIPath,
		contentConfig:    config,
		serializers:      *serializers,
		createBackoffMgr: readExpBackoffConfig,
		Throttle:         throttle,
		Client:           client,
	}, nil
}

看了写这个函数只是做了基本的封装。下面讲下如何使用RESTClient操作资源对象：

restClient.Get().Namespace("default").Resource("pods").Do()

Get()函数的返回,是个request对象:

func (c *RESTClient) Get() *Request {
	return c.Verb("GET")
}

Namespace函数，只是将namespace设置进request字段：

func (r *Request) Namespace(namespace string) *Request {
	if r.err != nil {
		return r
	}
	if r.namespaceSet {
		r.err = fmt.Errorf("namespace already set to %q, cannot change to %q", r.namespace, namespace)
		return r
	}
	if msgs := IsValidPathSegmentName(namespace); len(msgs) != 0 {
		r.err = fmt.Errorf("invalid namespace %q: %v", namespace, msgs)
		return r
	}
	r.namespaceSet = true
	r.namespace = namespace // 在request对象中设置namespace
	return r
}

Resource函数：

func (r *Request) Resource(resource string) *Request {
	if r.err != nil {
		return r
	}
	if len(r.resource) != 0 {
		r.err = fmt.Errorf("resource already set to %q, cannot change to %q", r.resource, resource)
		return r
	}
	if msgs := IsValidPathSegmentName(resource); len(msgs) != 0 {
		r.err = fmt.Errorf("invalid resource %q: %v", resource, msgs)
		return r
	}
	r.resource = resource // 在request对象中设置resource
	return r
}

Do函数：

func (r *Request) Do() Result {
	r.tryThrottle()

	var result Result
	err := r.request(func(req *http.Request, resp *http.Response) {
		result = r.transformResponse(resp, req)
	})
	if err != nil {
		return Result{err: err}
	}
	return result
}

着重看下里面的request函数：

func (r *Request) request(fn func(*http.Request, *http.Response)) error {
	//Metrics for total request latency
	start := time.Now()
	defer func() {
		metrics.RequestLatency.Observe(r.verb, r.finalURLTemplate(), time.Since(start))
	}()

	if r.err != nil {
		klog.V(4).Infof("Error in request: %v", r.err)
		return r.err
	}

	// TODO: added to catch programmer errors (invoking operations with an object with an empty namespace)
	// 校验参数是否合理
	if (r.verb == "GET" || r.verb == "PUT" || r.verb == "DELETE") && r.namespaceSet && len(r.resourceName) > 0 && len(r.namespace) == 0 {
		return fmt.Errorf("an empty namespace may not be set when a resource name is provided")
	}
	if (r.verb == "POST") && r.namespaceSet && len(r.namespace) == 0 {
		return fmt.Errorf("an empty namespace may not be set during creation")
	}

	client := r.client
	if client == nil {
		client = http.DefaultClient
	}

	// Right now we make about ten retry attempts if we get a Retry-After response.
	maxRetries := 10
	retries := 0
	for {
		url := r.URL().String()// 需要着重看下这个URL函数
		req, err := http.NewRequest(r.verb, url, r.body)
		if err != nil {
			return err
		}
		if r.timeout > 0 {
			if r.ctx == nil {
				r.ctx = context.Background()
			}
			var cancelFn context.CancelFunc
			r.ctx, cancelFn = context.WithTimeout(r.ctx, r.timeout)
			defer cancelFn()
		}
		if r.ctx != nil {
			req = req.WithContext(r.ctx)
		}
		req.Header = r.headers

		r.backoffMgr.Sleep(r.backoffMgr.CalculateBackoff(r.URL()))
		if retries > 0 {
			// We are retrying the request that we already send to apiserver
			// at least once before.
			// This request should also be throttled with the client-internal throttler.
			r.tryThrottle()
		}
		resp, err := client.Do(req)
		updateURLMetrics(r, resp, err)
		if err != nil {
			r.backoffMgr.UpdateBackoff(r.URL(), err, 0)
		} else {
			r.backoffMgr.UpdateBackoff(r.URL(), err, resp.StatusCode)
		}
		if err != nil {
			// "Connection reset by peer" is usually a transient error.
			// Thus in case of "GET" operations, we simply retry it.
			// We are not automatically retrying "write" operations, as
			// they are not idempotent.
			if !net.IsConnectionReset(err) || r.verb != "GET" {
				return err
			}
			// For the purpose of retry, we set the artificial "retry-after" response.
			// TODO: Should we clean the original response if it exists?
			resp = &http.Response{
				StatusCode: http.StatusInternalServerError,
				Header:     http.Header{"Retry-After": []string{"1"}},
				Body:       ioutil.NopCloser(bytes.NewReader([]byte{})),
			}
		}

		done := func() bool {
			// Ensure the response body is fully read and closed
			// before we reconnect, so that we reuse the same TCP
			// connection.
			defer func() {
				const maxBodySlurpSize = 2 << 10
				if resp.ContentLength <= maxBodySlurpSize {
					io.Copy(ioutil.Discard, &io.LimitedReader{R: resp.Body, N: maxBodySlurpSize})
				}
				resp.Body.Close()
			}()

			retries++
			if seconds, wait := checkWait(resp); wait && retries < maxRetries {
				if seeker, ok := r.body.(io.Seeker); ok && r.body != nil {
					_, err := seeker.Seek(0, 0)
					if err != nil {
						klog.V(4).Infof("Could not retry request, can't Seek() back to beginning of body for %T", r.body)
						fn(req, resp)
						return true
					}
				}

				klog.V(4).Infof("Got a Retry-After %ds response for attempt %d to %v", seconds, retries, url)
				r.backoffMgr.Sleep(time.Duration(seconds) * time.Second)
				return false
			}
			fn(req, resp)
			return true
		}()
		if done {
			return nil
		}
	}
}

着重看下r.URL函数，它是生成URL用的,这边你就会发现之前在request中设置的resource,namepspace之类的都用在这边拼接url上了：

func (r *Request) URL() *url.URL {
	p := r.pathPrefix
	if r.namespaceSet && len(r.namespace) > 0 {
		p = path.Join(p, "namespaces", r.namespace)
	}
	if len(r.resource) != 0 {
		p = path.Join(p, strings.ToLower(r.resource))
	}
	// Join trims trailing slashes, so preserve r.pathPrefix's trailing slash for backwards compatibility if nothing was changed
	if len(r.resourceName) != 0 || len(r.subpath) != 0 || len(r.subresource) != 0 {
		p = path.Join(p, r.resourceName, r.subresource, r.subpath)
	}

	finalURL := &url.URL{}
	if r.baseURL != nil {
		*finalURL = *r.baseURL
	}
	finalURL.Path = p

	query := url.Values{}
	for key, values := range r.params {
		for _, value := range values {
			query.Add(key, value)
		}
	}

	// timeout is handled specially here.
	if r.timeout != 0 {
		query.Set("timeout", r.timeout.String())
	}
	finalURL.RawQuery = query.Encode()
	return finalURL
}

总结下这个小节：api-server暴露了很多接口，而RESTClient就会对这些api的封装，所以我们看到源码中有生成对应的URL之类的。

ClientSet客户端

ClientSet客户端默认是对k8s内置资源对象客户端的集合，通过ClientSet客户端可以操作k8s的内置资源对象。下面会介绍为什么clientset是内置资源对象客户端的集合的。

来段代码做引入:

func getClientSet(kubeconfig string) {
	config, err := clientcmd.BuildConfigFromFlags("", kubeconfig)
	if err != nil {
		panic(err.Error())
	}

	// create the clientset
	clientset, err := kubernetes.NewForConfig(config)
	if err != nil {
		panic(err.Error())
	}
	for {
		pods, err := clientset.CoreV1().Pods("").List(metav1.ListOptions{})
		if err != nil {
			panic(err.Error())
		}
		fmt.Printf("There are %d pods in the cluster\n", len(pods.Items))

		// Examples for error handling:
		// - Use helper functions like e.g. errors.IsNotFound()
		// - And/or cast to StatusError and use its properties like e.g. ErrStatus.Message
		namespace := "default"
		pod := "example-xxxxx"
		_, err = clientset.CoreV1().Pods(namespace).Get(pod, metav1.GetOptions{})
		if errors.IsNotFound(err) {
			fmt.Printf("Pod %s in namespace %s not found\n", pod, namespace)
		} else if statusError, isStatus := err.(*errors.StatusError); isStatus {
			fmt.Printf("Error getting pod %s in namespace %s: %v\n",
				pod, namespace, statusError.ErrStatus.Message)
		} else if err != nil {
			panic(err.Error())
		} else {
			fmt.Printf("Found pod %s in namespace %s\n", pod, namespace)
		}

		time.Sleep(10 * time.Second)
	}
}

clientcmd.BuildConfigFromFlags函数上面已经分析过了，这边就分析一下不一样的地方，生成clientset的函数kubernetes.NewForConfig,这边我们可以看到cs这是了很多k8s的内置资源客户端对象，还有设置了一个discoveryClient：

func NewForConfig(c *rest.Config) (*Clientset, error) {
	configShallowCopy := *c
	if configShallowCopy.RateLimiter == nil && configShallowCopy.QPS > 0 {
		configShallowCopy.RateLimiter = flowcontrol.NewTokenBucketRateLimiter(configShallowCopy.QPS, configShallowCopy.Burst)
	}
	var cs Clientset
	var err error
	// 这边开始会初始化k8s内置的资源的clientset
	cs.admissionregistrationV1beta1, err = admissionregistrationv1beta1.NewForConfig(&configShallowCopy)
	if err != nil {
		return nil, err
	}
	cs.appsV1, err = appsv1.NewForConfig(&configShallowCopy)
	if err != nil {
		return nil, err
	}
	cs.appsV1beta1, err = appsv1beta1.NewForConfig(&configShallowCopy)
	if err != nil {
		return nil, err
	}
	cs.appsV1beta2, err = appsv1beta2.NewForConfig(&configShallowCopy)
	if err != nil {
		return nil, err
	}
	cs.auditregistrationV1alpha1, err = auditregistrationv1alpha1.NewForConfig(&configShallowCopy)
	if err != nil {
		return nil, err
	}
	cs.authenticationV1, err = authenticationv1.NewForConfig(&configShallowCopy)
	if err != nil {
		return nil, err
	}
	cs.authenticationV1beta1, err = authenticationv1beta1.NewForConfig(&configShallowCopy)
	if err != nil {
		return nil, err
	}
	cs.authorizationV1, err = authorizationv1.NewForConfig(&configShallowCopy)
	if err != nil {
		return nil, err
	}
	cs.authorizationV1beta1, err = authorizationv1beta1.NewForConfig(&configShallowCopy)
	if err != nil {
		return nil, err
	}
	cs.autoscalingV1, err = autoscalingv1.NewForConfig(&configShallowCopy)
	if err != nil {
		return nil, err
	}
	cs.autoscalingV2beta1, err = autoscalingv2beta1.NewForConfig(&configShallowCopy)
	if err != nil {
		return nil, err
	}
	cs.autoscalingV2beta2, err = autoscalingv2beta2.NewForConfig(&configShallowCopy)
	if err != nil {
		return nil, err
	}
	cs.batchV1, err = batchv1.NewForConfig(&configShallowCopy)
	if err != nil {
		return nil, err
	}
	cs.batchV1beta1, err = batchv1beta1.NewForConfig(&configShallowCopy)
	if err != nil {
		return nil, err
	}
	cs.batchV2alpha1, err = batchv2alpha1.NewForConfig(&configShallowCopy)
	if err != nil {
		return nil, err
	}
	cs.certificatesV1beta1, err = certificatesv1beta1.NewForConfig(&configShallowCopy)
	if err != nil {
		return nil, err
	}
	cs.coordinationV1beta1, err = coordinationv1beta1.NewForConfig(&configShallowCopy)
	if err != nil {
		return nil, err
	}
	cs.coordinationV1, err = coordinationv1.NewForConfig(&configShallowCopy)
	if err != nil {
		return nil, err
	}
	cs.coreV1, err = corev1.NewForConfig(&configShallowCopy)
	if err != nil {
		return nil, err
	}
	cs.eventsV1beta1, err = eventsv1beta1.NewForConfig(&configShallowCopy)
	if err != nil {
		return nil, err
	}
	cs.extensionsV1beta1, err = extensionsv1beta1.NewForConfig(&configShallowCopy)
	if err != nil {
		return nil, err
	}
	cs.networkingV1, err = networkingv1.NewForConfig(&configShallowCopy)
	if err != nil {
		return nil, err
	}
	cs.networkingV1beta1, err = networkingv1beta1.NewForConfig(&configShallowCopy)
	if err != nil {
		return nil, err
	}
	cs.nodeV1alpha1, err = nodev1alpha1.NewForConfig(&configShallowCopy)
	if err != nil {
		return nil, err
	}
	cs.nodeV1beta1, err = nodev1beta1.NewForConfig(&configShallowCopy)
	if err != nil {
		return nil, err
	}
	cs.policyV1beta1, err = policyv1beta1.NewForConfig(&configShallowCopy)
	if err != nil {
		return nil, err
	}
	cs.rbacV1, err = rbacv1.NewForConfig(&configShallowCopy)
	if err != nil {
		return nil, err
	}
	cs.rbacV1beta1, err = rbacv1beta1.NewForConfig(&configShallowCopy)
	if err != nil {
		return nil, err
	}
	cs.rbacV1alpha1, err = rbacv1alpha1.NewForConfig(&configShallowCopy)
	if err != nil {
		return nil, err
	}
	cs.schedulingV1alpha1, err = schedulingv1alpha1.NewForConfig(&configShallowCopy)
	if err != nil {
		return nil, err
	}
	cs.schedulingV1beta1, err = schedulingv1beta1.NewForConfig(&configShallowCopy)
	if err != nil {
		return nil, err
	}
	cs.schedulingV1, err = schedulingv1.NewForConfig(&configShallowCopy)
	if err != nil {
		return nil, err
	}
	cs.settingsV1alpha1, err = settingsv1alpha1.NewForConfig(&configShallowCopy)
	if err != nil {
		return nil, err
	}
	cs.storageV1beta1, err = storagev1beta1.NewForConfig(&configShallowCopy)
	if err != nil {
		return nil, err
	}
	cs.storageV1, err = storagev1.NewForConfig(&configShallowCopy)
	if err != nil {
		return nil, err
	}
	cs.storageV1alpha1, err = storagev1alpha1.NewForConfig(&configShallowCopy)
	if err != nil {
		return nil, err
	}

	cs.DiscoveryClient, err = discovery.NewDiscoveryClientForConfig(&configShallowCopy) // 注意啦，clientset里面还放了一个DiscoverClient
	if err != nil {
		return nil, err
	}
	return &cs, nil
}

然后追进NewForConfig看下,看到rest.RESTClientFor就知道是啥了吧，这函数里面就是RESTClient的实现了：

func NewForConfig(c *rest.Config) (*AppsV1beta1Client, error) {
	config := *c
	if err := setConfigDefaults(&config); err != nil {
		return nil, err
	}
	client, err := rest.RESTClientFor(&config)
	if err != nil {
		return nil, err
	}
	return &AppsV1beta1Client{client}, nil
}

现在理解了，为啥clientset客户端是每个k8s内置资源客户端对象的集合了吧。

DynamicClient

DynamicClient的特点就是除了可以使用k8s内置资源外，还可以使用CRD资源。dynamicClient的原理就是传入的资源数据都是使用map[string]interface{}结构。

func getDynamicClientExample(kubeconfig string) {
	config, err := clientcmd.BuildConfigFromFlags("", kubeconfig)
	if err != nil {
		panic(err)
	}
	client, err := dynamic.NewForConfig(config)
	if err != nil {
		panic(err)
	}

	deploymentRes := schema.GroupVersionResource{Group: "apps", Version: "v1", Resource: "deployments"}

	deployment := &unstructured.Unstructured{
		Object: map[string]interface{}{
			"apiVersion": "apps/v1",
			"kind":       "Deployment",
			"metadata": map[string]interface{}{
				"name": "demo-deployment",
			},
			"spec": map[string]interface{}{
				"replicas": 2,
				"selector": map[string]interface{}{
					"matchLabels": map[string]interface{}{
						"app": "demo",
					},
				},
				"template": map[string]interface{}{
					"metadata": map[string]interface{}{
						"labels": map[string]interface{}{
							"app": "demo",
						},
					},

					"spec": map[string]interface{}{
						"containers": []map[string]interface{}{
							{
								"name":  "web",
								"image": "nginx:1.12",
								"ports": []map[string]interface{}{
									{
										"name":          "http",
										"protocol":      "TCP",
										"containerPort": 80,
									},
								},
							},
						},
					},
				},
			},
		},
	}

	// Create Deployment
	fmt.Println("Creating deployment...")
	result, err := client.Resource(deploymentRes).Namespace("").Create(deployment, metav1.CreateOptions{})
	if err != nil {
		panic(err)
	}
	fmt.Printf("Created deployment %q.\n", result.GetName())
}

直接从dynamic.NewForConfig(config)开始：

func NewForConfig(inConfig *rest.Config) (Interface, error) {
	config := rest.CopyConfig(inConfig)
	// for serializing the options
	config.GroupVersion = &schema.GroupVersion{}
	config.APIPath = "/if-you-see-this-search-for-the-break"
	config.AcceptContentTypes = "application/json"
	config.ContentType = "application/json"
	config.NegotiatedSerializer = basicNegotiatedSerializer{} // this gets used for discovery and error handling types
	if config.UserAgent == "" {
		config.UserAgent = rest.DefaultKubernetesUserAgent()
	}

	restClient, err := rest.RESTClientFor(config) //还是去新建了一个RESTClient客户端
	if err != nil {
		return nil, err
	}

	return &dynamicClient{client: restClient}, nil
}

从代码中看dynamicClient内部还是restClient.

DiscoveryClient

DiscoveryClient是发现客户端，主要用于发现k8s api-server所支持的资源组、资源版本及资源信息。

先来个damo:

func getDisCoveryClient(kubeconfig string) {
	config, err := clientcmd.BuildConfigFromFlags("", kubeconfig)
	if err != nil {
		panic(err)
	}
	discoveryClient, err := discovery.NewDiscoveryClientForConfig(config)
	if err != nil {
		panic(err)
	}
	_, APIResourceList, err := discoveryClient.ServerGroupsAndResources()\
	if err != nil {
		panic(err)
	}
	for _, list := range APIResourceList {

	}
}

直接看这个函数discovery.NewDiscoveryClientForConfig(config)吧，下面也可以看到就是使用了RESTClient：

func NewDiscoveryClientForConfig(c *restclient.Config) (*DiscoveryClient, error) {
	config := *c
	// 设置一些默认的参数
	if err := setDiscoveryDefaults(&config); err != nil {
		return nil, err
	}
	// 类似RESTClient客户端
	client, err := restclient.UnversionedRESTClientFor(&config)
	return &DiscoveryClient{restClient: client, LegacyPrefix: "/api"}, err
}

疑惑点

问：4种客户端其实都使用了restClient这个基础的客户端，但是有必要分的这么清吗？4种客户端有同时存在的必要吗？

答：我之前感觉只要有RESTClient就够了，至少没必要同时存在clientSet和DynamicClient,因为clientset客户端通过代码生成也能操作CRD资源。后来仔细想想，对于未知对象的获取，dynamicClient客户端就有用了，也就是说，clientset客户端只针对你知道是什么类型的对象是有用的，对于你不知道的类型，你只能时使用dynamicClient。

总结

client-go客户端是客户端访问k8s资源的一种RESTFul API封装,使得用户与k8s交互更方便，代码更容易理解。当然,client-go提供给用户的远不止这些。本节主要介绍了4中客户端形式，比较常用的还是能够直接操作k8s内置资源的clientset。client-go的github仓库上有官方提供的例子（https://github.com/kubernetes/client-go/blob/master/examples/in-cluster-client-configuration/main.go）

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