replicaset controller是kube-controller-manager组件中众多控制器中的一个，是 replicaset 资源对象的控制器，其通过对replicaset、pod 2种资源的监听，完成replicaset期望副本数的调谐。

replicaset controller分析

replicaset controller简介

replicaset controller是kube-controller-manager组件中众多控制器中的一个，是 replicaset 资源对象的控制器，其通过对replicaset、pod 2种资源的监听，当这2种资源发生变化时会触发 replicaset controller 对相应的replicaset对象进行调谐操作，从而完成replicaset期望副本数的调谐，当实际pod的数量未达到预期时创建pod，当实际pod的数量超过预期时删除pod。

replicaset controller主要作用是根据replicaset对象所期望的pod数量与现存pod数量做比较，然后根据比较结果创建/删除pod，最终使得replicaset对象所期望的pod数量与现存pod数量相等。

replicaset controller架构图

replicaset controller的大致组成和处理流程如下图，replicaset controller对pod和replicaset对象注册了event handler，当有事件时，会watch到然后将对应的replicaset对象放入到queue中，然后syncReplicaSet方法为replicaset controller调谐replicaset对象的核心处理逻辑所在，从queue中取出replicaset对象，做调谐处理。

replicaset controller分析分为3大块进行，分别是：
（1）replicaset controller初始化和启动分析；
（2）replicaset controller核心处理逻辑分析；
（3）replicaset controller expectations机制分析。

本篇博客进行replicaset controller核心处理逻辑分析。

replicaset controller核心处理逻辑分析

基于v1.17.4

经过前面分析的replicaset controller的初始化与启动，知道了replicaset controller监听replicaset、pod对象的add、update与delete事件，然后对replicaset对象做相应的调谐处理，这里来接着分析replicaset controller的调谐处理（核心处理）逻辑，从rsc.syncHandler作为入口进行分析。

rsc.syncHandler

rsc.syncHandler即rsc.syncReplicaSet方法，主要逻辑：
（1）获取replicaset对象以及关联的pod对象列表；
（2）调用rsc.expectations.SatisfiedExpectations，判断上一轮对replicaset期望副本的创删操作是否完成，也可以认为是判断上一次对replicaset对象的调谐操作中，调用的rsc.manageReplicas方法是否执行完成；
（3）如果上一轮对replicaset期望副本的创删操作已经完成，且replicaset对象的DeletionTimestamp字段为nil，则调用rsc.manageReplicas做replicaset期望副本的核心调谐处理，即创删pod；
（4）调用calculateStatus计算replicaset的status，并更新。

// syncReplicaSet will sync the ReplicaSet with the given key if it has had its expectations fulfilled,
// meaning it did not expect to see any more of its pods created or deleted. This function is not meant to be
// invoked concurrently with the same key.
func (rsc *ReplicaSetController) syncReplicaSet(key string) error {
	startTime := time.Now()
	defer func() {
		klog.V(4).Infof("Finished syncing %v %q (%v)", rsc.Kind, key, time.Since(startTime))
	}()

	namespace, name, err := cache.SplitMetaNamespaceKey(key)
	if err != nil {
		return err
	}
	rs, err := rsc.rsLister.ReplicaSets(namespace).Get(name)
	if errors.IsNotFound(err) {
		klog.V(4).Infof("%v %v has been deleted", rsc.Kind, key)
		rsc.expectations.DeleteExpectations(key)
		return nil
	}
	if err != nil {
		return err
	}

	rsNeedsSync := rsc.expectations.SatisfiedExpectations(key)
	selector, err := metav1.LabelSelectorAsSelector(rs.Spec.Selector)
	if err != nil {
		utilruntime.HandleError(fmt.Errorf("error converting pod selector to selector: %v", err))
		return nil
	}

	// list all pods to include the pods that don't match the rs`s selector
	// anymore but has the stale controller ref.
	// TODO: Do the List and Filter in a single pass, or use an index.
	allPods, err := rsc.podLister.Pods(rs.Namespace).List(labels.Everything())
	if err != nil {
		return err
	}
	// Ignore inactive pods.
	filteredPods := controller.FilterActivePods(allPods)

	// NOTE: filteredPods are pointing to objects from cache - if you need to
	// modify them, you need to copy it first.
	filteredPods, err = rsc.claimPods(rs, selector, filteredPods)
	if err != nil {
		return err
	}

	var manageReplicasErr error
	if rsNeedsSync && rs.DeletionTimestamp == nil {
		manageReplicasErr = rsc.manageReplicas(filteredPods, rs)
	}
	rs = rs.DeepCopy()
	newStatus := calculateStatus(rs, filteredPods, manageReplicasErr)

	// Always updates status as pods come up or die.
	updatedRS, err := updateReplicaSetStatus(rsc.kubeClient.AppsV1().ReplicaSets(rs.Namespace), rs, newStatus)
	if err != nil {
		// Multiple things could lead to this update failing. Requeuing the replica set ensures
		// Returning an error causes a requeue without forcing a hotloop
		return err
	}
	// Resync the ReplicaSet after MinReadySeconds as a last line of defense to guard against clock-skew.
	if manageReplicasErr == nil && updatedRS.Spec.MinReadySeconds > 0 &&
		updatedRS.Status.ReadyReplicas == *(updatedRS.Spec.Replicas) &&
		updatedRS.Status.AvailableReplicas != *(updatedRS.Spec.Replicas) {
		rsc.queue.AddAfter(key, time.Duration(updatedRS.Spec.MinReadySeconds)*time.Second)
	}
	return manageReplicasErr
}

1 rsc.expectations.SatisfiedExpectations

该方法主要是判断上一轮对replicaset期望副本的创删操作是否完成，也可以认为是判断上一次对replicaset对象的调谐操作中，调用的rsc.manageReplicas方法是否执行完成。待上一次创建删除pod的操作完成后，才能进行下一次的rsc.manageReplicas方法调用。

若某replicaset对象的调谐中从未调用过rsc.manageReplicas方法，或上一轮调谐时创建/删除pod的数量已达成或调用rsc.manageReplicas后已达到超时期限（超时时间5分钟），则返回true，代表上一次创建删除pod的操作完成，可以进行下一次的rsc.manageReplicas方法调用，否则返回false。

expectations记录了replicaset对象在某一次调谐中期望创建/删除的pod数量，pod创建/删除完成后，该期望数会相应的减少，当期望创建/删除的pod数量小于等于0时，说明上一次调谐中期望创建/删除的pod数量已经达到，返回true。

关于Expectations机制后面会做详细分析。

// pkg/controller/controller_utils.go
func (r *ControllerExpectations) SatisfiedExpectations(controllerKey string) bool {
	if exp, exists, err := r.GetExpectations(controllerKey); exists {
		if exp.Fulfilled() {
			klog.V(4).Infof("Controller expectations fulfilled %#v", exp)
			return true
		} else if exp.isExpired() {
			klog.V(4).Infof("Controller expectations expired %#v", exp)
			return true
		} else {
			klog.V(4).Infof("Controller still waiting on expectations %#v", exp)
			return false
		}
	} else if err != nil {
		klog.V(2).Infof("Error encountered while checking expectations %#v, forcing sync", err)
	} else {
		// When a new controller is created, it doesn't have expectations.
		// When it doesn't see expected watch events for > TTL, the expectations expire.
		//	- In this case it wakes up, creates/deletes controllees, and sets expectations again.
		// When it has satisfied expectations and no controllees need to be created/destroyed > TTL, the expectations expire.
		//	- In this case it continues without setting expectations till it needs to create/delete controllees.
		klog.V(4).Infof("Controller %v either never recorded expectations, or the ttl expired.", controllerKey)
	}
	// Trigger a sync if we either encountered and error (which shouldn't happen since we're
	// getting from local store) or this controller hasn't established expectations.
	return true
}

func (exp *ControlleeExpectations) isExpired() bool {
	return clock.RealClock{}.Since(exp.timestamp) > ExpectationsTimeout // ExpectationsTimeout = 5 * time.Minute
}

2 核心创建删除pod方法-rsc.manageReplicas

核心创建删除pod方法，主要是根据replicaset所期望的pod数量与现存pod数量做比较，然后根据比较结果来创建/删除pod，最终使得replicaset对象所期望的pod数量与现存pod数量相等，需要特别注意的是，每一次调用rsc.manageReplicas方法，创建/删除pod的个数上限为500。

在replicaset对象的调谐中，rsc.manageReplicas方法不一定每一次都会调用执行，只有当rsc.expectations.SatisfiedExpectations方法返回true，且replicaset对象的DeletionTimestamp属性为空时，才会进行rsc.manageReplicas方法的调用。

先简单的看一下代码，代码后面会做详细的逻辑分析。

// pkg/controller/replicaset/replica_set.go
func (rsc *ReplicaSetController) manageReplicas(filteredPods []*v1.Pod, rs *apps.ReplicaSet) error {
	diff := len(filteredPods) - int(*(rs.Spec.Replicas))
	rsKey, err := controller.KeyFunc(rs)
	if err != nil {
		utilruntime.HandleError(fmt.Errorf("Couldn't get key for %v %#v: %v", rsc.Kind, rs, err))
		return nil
	}
	if diff < 0 {
		diff *= -1
		if diff > rsc.burstReplicas {
			diff = rsc.burstReplicas
		}
		// TODO: Track UIDs of creates just like deletes. The problem currently
		// is we'd need to wait on the result of a create to record the pod's
		// UID, which would require locking *across* the create, which will turn
		// into a performance bottleneck. We should generate a UID for the pod
		// beforehand and store it via ExpectCreations.
		rsc.expectations.ExpectCreations(rsKey, diff)
		glog.V(2).Infof("Too few replicas for %v %s/%s, need %d, creating %d", rsc.Kind, rs.Namespace, rs.Name, *(rs.Spec.Replicas), diff)
		// Batch the pod creates. Batch sizes start at SlowStartInitialBatchSize
		// and double with each successful iteration in a kind of "slow start".
		// This handles attempts to start large numbers of pods that would
		// likely all fail with the same error. For example a project with a
		// low quota that attempts to create a large number of pods will be
		// prevented from spamming the API service with the pod create requests
		// after one of its pods fails.  Conveniently, this also prevents the
		// event spam that those failures would generate.
		successfulCreations, err := slowStartBatch(diff, controller.SlowStartInitialBatchSize, func() error {
			boolPtr := func(b bool) *bool { return &b }
			controllerRef := &metav1.OwnerReference{
				APIVersion:         rsc.GroupVersion().String(),
				Kind:               rsc.Kind,
				Name:               rs.Name,
				UID:                rs.UID,
				BlockOwnerDeletion: boolPtr(true),
				Controller:         boolPtr(true),
			}
			err := rsc.podControl.CreatePodsWithControllerRef(rs.Namespace, &rs.Spec.Template, rs, controllerRef)
			if err != nil && errors.IsTimeout(err) {
				// Pod is created but its initialization has timed out.
				// If the initialization is successful eventually, the
				// controller will observe the creation via the informer.
				// If the initialization fails, or if the pod keeps
				// uninitialized for a long time, the informer will not
				// receive any update, and the controller will create a new
				// pod when the expectation expires.
				return nil
			}
			return err
		})

		// Any skipped pods that we never attempted to start shouldn't be expected.
		// The skipped pods will be retried later. The next controller resync will
		// retry the slow start process.
		if skippedPods := diff - successfulCreations; skippedPods > 0 {
			glog.V(2).Infof("Slow-start failure. Skipping creation of %d pods, decrementing expectations for %v %v/%v", skippedPods, rsc.Kind, rs.Namespace, rs.Name)
			for i := 0; i < skippedPods; i++ {
				// Decrement the expected number of creates because the informer won't observe this pod
				rsc.expectations.CreationObserved(rsKey)
			}
		}
		return err
	} else if diff > 0 {
		if diff > rsc.burstReplicas {
			diff = rsc.burstReplicas
		}
		glog.V(2).Infof("Too many replicas for %v %s/%s, need %d, deleting %d", rsc.Kind, rs.Namespace, rs.Name, *(rs.Spec.Replicas), diff)

		// Choose which Pods to delete, preferring those in earlier phases of startup.
		podsToDelete := getPodsToDelete(filteredPods, diff)

		// Snapshot the UIDs (ns/name) of the pods we're expecting to see
		// deleted, so we know to record their expectations exactly once either
		// when we see it as an update of the deletion timestamp, or as a delete.
		// Note that if the labels on a pod/rs change in a way that the pod gets
		// orphaned, the rs will only wake up after the expectations have
		// expired even if other pods are deleted.
		rsc.expectations.ExpectDeletions(rsKey, getPodKeys(podsToDelete))

		errCh := make(chan error, diff)
		var wg sync.WaitGroup
		wg.Add(diff)
		for _, pod := range podsToDelete {
			go func(targetPod *v1.Pod) {
				defer wg.Done()
				if err := rsc.podControl.DeletePod(rs.Namespace, targetPod.Name, rs); err != nil {
					// Decrement the expected number of deletes because the informer won't observe this deletion
					podKey := controller.PodKey(targetPod)
					glog.V(2).Infof("Failed to delete %v, decrementing expectations for %v %s/%s", podKey, rsc.Kind, rs.Namespace, rs.Name)
					rsc.expectations.DeletionObserved(rsKey, podKey)
					errCh <- err
				}
			}(pod)
		}
		wg.Wait()

		select {
		case err := <-errCh:
			// all errors have been reported before and they're likely to be the same, so we'll only return the first one we hit.
			if err != nil {
				return err
			}
		default:
		}
	}

	return nil
}

diff = 现存pod数量 - 期望的pod数量

diff := len(filteredPods) - int(*(rs.Spec.Replicas))

（1）当现存pod数量比期望的少时，需要创建pod，进入创建pod的逻辑代码块。
（2）当现存pod数量比期望的多时，需要删除pod，进入删除pod的逻辑代码块。

一次同步操作中批量创建或删除pod的个数上限为rsc.burstReplicas，即500个。

// pkg/controller/replicaset/replica_set.go
const (
	// Realistic value of the burstReplica field for the replica set manager based off
	// performance requirements for kubernetes 1.0.
	BurstReplicas = 500

	// The number of times we retry updating a ReplicaSet's status.
	statusUpdateRetries = 1
)

if diff > rsc.burstReplicas {
		diff = rsc.burstReplicas
	}

接下来分析一下创建/删除pod的逻辑代码块。

2.1 创建pod逻辑代码块

主要逻辑：
（1）运算获取需要创建的pod数量，并设置数量上限500；
（2）调用rsc.expectations.ExpectCreations，将本轮调谐期望创建的pod数量设置进expectations；
（3）调用slowStartBatch函数来对pod进行创建逻辑处理；
（4）调用slowStartBatch函数完成后，计算获取创建失败的pod的数量，然后调用相应次数的rsc.expectations.CreationObserved方法，减去本轮调谐中期望创建的pod数量。
为什么要减呢？因为expectations记录了replicaset对象在某一次调谐中期望创建/删除的pod数量，pod创建/删除完成后，replicaset controller会watch到pod的创建/删除事件，从而调用rsc.expectations.CreationObserved方法来使期望创建/删除的pod数量减少。当有相应数量的pod创建/删除失败后，replicaset controller是不会watch到相应的pod创建/删除事件的，所以必须把本轮调谐期望创建/删除的pod数量做相应的减法，否则本轮调谐中的期望创建/删除pod数量永远不可能小于等于0，这样的话，rsc.expectations.SatisfiedExpectations方法就只会等待expectations超时期限到达才会返回true了。

diff *= -1
		if diff > rsc.burstReplicas {
			diff = rsc.burstReplicas
		}
		
		rsc.expectations.ExpectCreations(rsKey, diff)
		glog.V(2).Infof("Too few replicas for %v %s/%s, need %d, creating %d", rsc.Kind, rs.Namespace, rs.Name, *(rs.Spec.Replicas), diff)
		
        successfulCreations, err := slowStartBatch(diff, controller.SlowStartInitialBatchSize, func() error {
			boolPtr := func(b bool) *bool { return &b }
			controllerRef := &metav1.OwnerReference{
				APIVersion:         rsc.GroupVersion().String(),
				Kind:               rsc.Kind,
				Name:               rs.Name,
				UID:                rs.UID,
				BlockOwnerDeletion: boolPtr(true),
				Controller:         boolPtr(true),
			}
			err := rsc.podControl.CreatePodsWithControllerRef(rs.Namespace, &rs.Spec.Template, rs, controllerRef)
			if err != nil && errors.IsTimeout(err) {
				// Pod is created but its initialization has timed out.
				// If the initialization is successful eventually, the
				// controller will observe the creation via the informer.
				// If the initialization fails, or if the pod keeps
				// uninitialized for a long time, the informer will not
				// receive any update, and the controller will create a new
				// pod when the expectation expires.
				return nil
			}
			return err
		})

		if skippedPods := diff - successfulCreations; skippedPods > 0 {
			glog.V(2).Infof("Slow-start failure. Skipping creation of %d pods, decrementing expectations for %v %v/%v", skippedPods, rsc.Kind, rs.Namespace, rs.Name)
			for i := 0; i < skippedPods; i++ {
				// Decrement the expected number of creates because the informer won't observe this pod
				rsc.expectations.CreationObserved(rsKey)
			}
		}
		return err

2.1.1 slowStartBatch

来看到slowStartBatch，可以看到创建pod的算法为：
（1）每次批量创建的 pod 数依次为 1、2、4、8......，呈指数级增长，起与要创建的pod数量相同的goroutine来负责创建pod。
（2）创建pod按1、2、4、8...的递增趋势分多批次进行，若某批次创建pod有失败的（如apiserver限流，丢弃请求等，注意：超时除外，因为initialization处理有可能超时），则后续批次不再进行，结束本次函数调用。

// pkg/controller/replicaset/replica_set.go
// slowStartBatch tries to call the provided function a total of 'count' times,
// starting slow to check for errors, then speeding up if calls succeed.
//
// It groups the calls into batches, starting with a group of initialBatchSize.
// Within each batch, it may call the function multiple times concurrently.
//
// If a whole batch succeeds, the next batch may get exponentially larger.
// If there are any failures in a batch, all remaining batches are skipped
// after waiting for the current batch to complete.
//
// It returns the number of successful calls to the function.
func slowStartBatch(count int, initialBatchSize int, fn func() error) (int, error) {
	remaining := count
	successes := 0
	for batchSize := integer.IntMin(remaining, initialBatchSize); batchSize > 0; batchSize = integer.IntMin(2*batchSize, remaining) {
		errCh := make(chan error, batchSize)
		var wg sync.WaitGroup
		wg.Add(batchSize)
		for i := 0; i < batchSize; i++ {
			go func() {
				defer wg.Done()
				if err := fn(); err != nil {
					errCh <- err
				}
			}()
		}
		wg.Wait()
		curSuccesses := batchSize - len(errCh)
		successes += curSuccesses
		if len(errCh) > 0 {
			return successes, <-errCh
		}
		remaining -= batchSize
	}
	return successes, nil
}

rsc.podControl.CreatePodsWithControllerRef

前面定义的创建pod时调用的方法为rsc.podControl.CreatePodsWithControllerRef。

func (r RealPodControl) CreatePodsWithControllerRef(namespace string, template *v1.PodTemplateSpec, controllerObject runtime.Object, controllerRef *metav1.OwnerReference) error {
	if err := validateControllerRef(controllerRef); err != nil {
		return err
	}
	return r.createPods("", namespace, template, controllerObject, controllerRef)
}

func (r RealPodControl) createPods(nodeName, namespace string, template *v1.PodTemplateSpec, object runtime.Object, controllerRef *metav1.OwnerReference) error {
	pod, err := GetPodFromTemplate(template, object, controllerRef)
	if err != nil {
		return err
	}
	if len(nodeName) != 0 {
		pod.Spec.NodeName = nodeName
	}
	if len(labels.Set(pod.Labels)) == 0 {
		return fmt.Errorf("unable to create pods, no labels")
	}
	newPod, err := r.KubeClient.CoreV1().Pods(namespace).Create(pod)
	if err != nil {
		// only send an event if the namespace isn't terminating
		if !apierrors.HasStatusCause(err, v1.NamespaceTerminatingCause) {
			r.Recorder.Eventf(object, v1.EventTypeWarning, FailedCreatePodReason, "Error creating: %v", err)
		}
		return err
	}
	accessor, err := meta.Accessor(object)
	if err != nil {
		klog.Errorf("parentObject does not have ObjectMeta, %v", err)
		return nil
	}
	klog.V(4).Infof("Controller %v created pod %v", accessor.GetName(), newPod.Name)
	r.Recorder.Eventf(object, v1.EventTypeNormal, SuccessfulCreatePodReason, "Created pod: %v", newPod.Name)

	return nil
}

2.2 删除逻辑代码块

主要逻辑：
（1）运算获取需要删除的pod数量，并设置数量上限500；
（2）根据要缩容删除的pod数量，先调用getPodsToDelete函数找出需要删除的pod列表；
（3）调用rsc.expectations.ExpectCreations，将本轮调谐期望删除的pod数量设置进expectations；
（4）每个pod拉起一个goroutine，调用rsc.podControl.DeletePod来删除该pod；
（5）对于删除失败的pod，会调用rsc.expectations.DeletionObserved方法，减去本轮调谐中期望创建的pod数量。
至于为什么要减，原因跟上面创建逻辑代码块中分析的一样。
（6）等待所有gorouutine完成，return返回。

if diff > rsc.burstReplicas {
			diff = rsc.burstReplicas
		}
		glog.V(2).Infof("Too many replicas for %v %s/%s, need %d, deleting %d", rsc.Kind, rs.Namespace, rs.Name, *(rs.Spec.Replicas), diff)

		// Choose which Pods to delete, preferring those in earlier phases of startup.
		podsToDelete := getPodsToDelete(filteredPods, diff)

		rsc.expectations.ExpectDeletions(rsKey, getPodKeys(podsToDelete))

		errCh := make(chan error, diff)
		var wg sync.WaitGroup
		wg.Add(diff)
		for _, pod := range podsToDelete {
			go func(targetPod *v1.Pod) {
				defer wg.Done()
				if err := rsc.podControl.DeletePod(rs.Namespace, targetPod.Name, rs); err != nil {
					// Decrement the expected number of deletes because the informer won't observe this deletion
					podKey := controller.PodKey(targetPod)
					glog.V(2).Infof("Failed to delete %v, decrementing expectations for %v %s/%s", podKey, rsc.Kind, rs.Namespace, rs.Name)
					rsc.expectations.DeletionObserved(rsKey, podKey)
					errCh <- err
				}
			}(pod)
		}
		wg.Wait()

		select {
		case err := <-errCh:
			// all errors have been reported before and they're likely to be the same, so we'll only return the first one we hit.
			if err != nil {
				return err
			}
		default:
		}

2.2.1 getPodsToDelete

getPodsToDelete：根据要缩容删除的pod数量，然后返回需要删除的pod列表。

// pkg/controller/replicaset/replica_set.go
func getPodsToDelete(filteredPods, relatedPods []*v1.Pod, diff int) []*v1.Pod {
	// No need to sort pods if we are about to delete all of them.
	// diff will always be <= len(filteredPods), so not need to handle > case.
	if diff < len(filteredPods) {
		podsWithRanks := getPodsRankedByRelatedPodsOnSameNode(filteredPods, relatedPods)
		sort.Sort(podsWithRanks)
	}
	return filteredPods[:diff]
}

func getPodsRankedByRelatedPodsOnSameNode(podsToRank, relatedPods []*v1.Pod) controller.ActivePodsWithRanks {
	podsOnNode := make(map[string]int)
	for _, pod := range relatedPods {
		if controller.IsPodActive(pod) {
			podsOnNode[pod.Spec.NodeName]++
		}
	}
	ranks := make([]int, len(podsToRank))
	for i, pod := range podsToRank {
		ranks[i] = podsOnNode[pod.Spec.NodeName]
	}
	return controller.ActivePodsWithRanks{Pods: podsToRank, Rank: ranks}
}

筛选要删除的pod逻辑

按照下面的排序规则，从上到下进行排序，各个条件相互互斥，符合其中一个条件则排序完成：
（1）优先删除没有绑定node的pod；
（2）优先删除处于Pending状态的pod，然后是Unknown，最后才是Running；
（3）优先删除Not ready的pod，然后才是ready的pod；
（4）按同node上所属replicaset的pod数量排序，优先删除所属replicaset的pod数量多的node上的pod；
（5）按pod ready的时间排序，优先删除ready时间最短的pod；
（6）优先删除pod中容器重启次数较多的pod；
（7）按pod创建时间排序，优先删除创建时间最短的pod。

// pkg/controller/controller_utils.go
func (s ActivePodsWithRanks) Less(i, j int) bool {
	// 1. Unassigned < assigned
	// If only one of the pods is unassigned, the unassigned one is smaller
	if s.Pods[i].Spec.NodeName != s.Pods[j].Spec.NodeName && (len(s.Pods[i].Spec.NodeName) == 0 || len(s.Pods[j].Spec.NodeName) == 0) {
		return len(s.Pods[i].Spec.NodeName) == 0
	}
	// 2. PodPending < PodUnknown < PodRunning
	if podPhaseToOrdinal[s.Pods[i].Status.Phase] != podPhaseToOrdinal[s.Pods[j].Status.Phase] {
		return podPhaseToOrdinal[s.Pods[i].Status.Phase] < podPhaseToOrdinal[s.Pods[j].Status.Phase]
	}
	// 3. Not ready < ready
	// If only one of the pods is not ready, the not ready one is smaller
	if podutil.IsPodReady(s.Pods[i]) != podutil.IsPodReady(s.Pods[j]) {
		return !podutil.IsPodReady(s.Pods[i])
	}
	// 4. Doubled up < not doubled up
	// If one of the two pods is on the same node as one or more additional
	// ready pods that belong to the same replicaset, whichever pod has more
	// colocated ready pods is less
	if s.Rank[i] != s.Rank[j] {
		return s.Rank[i] > s.Rank[j]
	}
	// TODO: take availability into account when we push minReadySeconds information from deployment into pods,
	//       see https://github.com/kubernetes/kubernetes/issues/22065
	// 5. Been ready for empty time < less time < more time
	// If both pods are ready, the latest ready one is smaller
	if podutil.IsPodReady(s.Pods[i]) && podutil.IsPodReady(s.Pods[j]) {
		readyTime1 := podReadyTime(s.Pods[i])
		readyTime2 := podReadyTime(s.Pods[j])
		if !readyTime1.Equal(readyTime2) {
			return afterOrZero(readyTime1, readyTime2)
		}
	}
	// 6. Pods with containers with higher restart counts < lower restart counts
	if maxContainerRestarts(s.Pods[i]) != maxContainerRestarts(s.Pods[j]) {
		return maxContainerRestarts(s.Pods[i]) > maxContainerRestarts(s.Pods[j])
	}
	// 7. Empty creation time pods < newer pods < older pods
	if !s.Pods[i].CreationTimestamp.Equal(&s.Pods[j].CreationTimestamp) {
		return afterOrZero(&s.Pods[i].CreationTimestamp, &s.Pods[j].CreationTimestamp)
	}
	return false
}

2.2.2 rsc.podControl.DeletePod

删除pod的方法。

// pkg/controller/controller_utils.go
func (r RealPodControl) DeletePod(namespace string, podID string, object runtime.Object) error {
	accessor, err := meta.Accessor(object)
	if err != nil {
		return fmt.Errorf("object does not have ObjectMeta, %v", err)
	}
	klog.V(2).Infof("Controller %v deleting pod %v/%v", accessor.GetName(), namespace, podID)
	if err := r.KubeClient.CoreV1().Pods(namespace).Delete(podID, nil); err != nil && !apierrors.IsNotFound(err) {
		r.Recorder.Eventf(object, v1.EventTypeWarning, FailedDeletePodReason, "Error deleting: %v", err)
		return fmt.Errorf("unable to delete pods: %v", err)
	}
	r.Recorder.Eventf(object, v1.EventTypeNormal, SuccessfulDeletePodReason, "Deleted pod: %v", podID)

	return nil
}

3 calculateStatus

calculateStatus函数计算并返回replicaset对象的status。

怎么计算status呢？
（1）根据现存pod数量、Ready状态的pod数量、availabel状态的pod数量等，给replicaset对象的status的Replicas、ReadyReplicas、AvailableReplicas等字段赋值；
（2）根据replicaset对象现有status中的condition配置以及前面调用rsc.manageReplicas方法后是否有错误，来决定给status新增condition或移除condition，conditionType为ReplicaFailure。

当调用rsc.manageReplicas方法出错，且replicaset对象的status中，没有conditionType为ReplicaFailure的condition，则新增conditionType为ReplicaFailure的condition，表示该replicaset创建/删除pod出错；
当调用rsc.manageReplicas方法没有任何错误，且replicaset对象的status中，有conditionType为ReplicaFailure的condition，则去除该condition，表示该replicaset创建/删除pod成功。

func calculateStatus(rs *apps.ReplicaSet, filteredPods []*v1.Pod, manageReplicasErr error) apps.ReplicaSetStatus {
	newStatus := rs.Status
	// Count the number of pods that have labels matching the labels of the pod
	// template of the replica set, the matching pods may have more
	// labels than are in the template. Because the label of podTemplateSpec is
	// a superset of the selector of the replica set, so the possible
	// matching pods must be part of the filteredPods.
	fullyLabeledReplicasCount := 0
	readyReplicasCount := 0
	availableReplicasCount := 0
	templateLabel := labels.Set(rs.Spec.Template.Labels).AsSelectorPreValidated()
	for _, pod := range filteredPods {
		if templateLabel.Matches(labels.Set(pod.Labels)) {
			fullyLabeledReplicasCount++
		}
		if podutil.IsPodReady(pod) {
			readyReplicasCount++
			if podutil.IsPodAvailable(pod, rs.Spec.MinReadySeconds, metav1.Now()) {
				availableReplicasCount++
			}
		}
	}

	failureCond := GetCondition(rs.Status, apps.ReplicaSetReplicaFailure)
	if manageReplicasErr != nil && failureCond == nil {
		var reason string
		if diff := len(filteredPods) - int(*(rs.Spec.Replicas)); diff < 0 {
			reason = "FailedCreate"
		} else if diff > 0 {
			reason = "FailedDelete"
		}
		cond := NewReplicaSetCondition(apps.ReplicaSetReplicaFailure, v1.ConditionTrue, reason, manageReplicasErr.Error())
		SetCondition(&newStatus, cond)
	} else if manageReplicasErr == nil && failureCond != nil {
		RemoveCondition(&newStatus, apps.ReplicaSetReplicaFailure)
	}

	newStatus.Replicas = int32(len(filteredPods))
	newStatus.FullyLabeledReplicas = int32(fullyLabeledReplicasCount)
	newStatus.ReadyReplicas = int32(readyReplicasCount)
	newStatus.AvailableReplicas = int32(availableReplicasCount)
	return newStatus
}

4 updateReplicaSetStatus

主要逻辑：
（1）判断新计算出来的status中的各个属性如Replicas、ReadyReplicas、AvailableReplicas以及Conditions是否与现存replicaset对象的status中的一致，一致则不用做更新操作，直接return；
（2）调用c.UpdateStatus更新replicaset的status。

// pkg/controller/replicaset/replica_set_utils.go
func updateReplicaSetStatus(c appsclient.ReplicaSetInterface, rs *apps.ReplicaSet, newStatus apps.ReplicaSetStatus) (*apps.ReplicaSet, error) {
	// This is the steady state. It happens when the ReplicaSet doesn't have any expectations, since
	// we do a periodic relist every 30s. If the generations differ but the replicas are
	// the same, a caller might've resized to the same replica count.
	if rs.Status.Replicas == newStatus.Replicas &&
		rs.Status.FullyLabeledReplicas == newStatus.FullyLabeledReplicas &&
		rs.Status.ReadyReplicas == newStatus.ReadyReplicas &&
		rs.Status.AvailableReplicas == newStatus.AvailableReplicas &&
		rs.Generation == rs.Status.ObservedGeneration &&
		reflect.DeepEqual(rs.Status.Conditions, newStatus.Conditions) {
		return rs, nil
	}

	// Save the generation number we acted on, otherwise we might wrongfully indicate
	// that we've seen a spec update when we retry.
	// TODO: This can clobber an update if we allow multiple agents to write to the
	// same status.
	newStatus.ObservedGeneration = rs.Generation

	var getErr, updateErr error
	var updatedRS *apps.ReplicaSet
	for i, rs := 0, rs; ; i++ {
		klog.V(4).Infof(fmt.Sprintf("Updating status for %v: %s/%s, ", rs.Kind, rs.Namespace, rs.Name) +
			fmt.Sprintf("replicas %d->%d (need %d), ", rs.Status.Replicas, newStatus.Replicas, *(rs.Spec.Replicas)) +
			fmt.Sprintf("fullyLabeledReplicas %d->%d, ", rs.Status.FullyLabeledReplicas, newStatus.FullyLabeledReplicas) +
			fmt.Sprintf("readyReplicas %d->%d, ", rs.Status.ReadyReplicas, newStatus.ReadyReplicas) +
			fmt.Sprintf("availableReplicas %d->%d, ", rs.Status.AvailableReplicas, newStatus.AvailableReplicas) +
			fmt.Sprintf("sequence No: %v->%v", rs.Status.ObservedGeneration, newStatus.ObservedGeneration))

		rs.Status = newStatus
		updatedRS, updateErr = c.UpdateStatus(rs)
		if updateErr == nil {
			return updatedRS, nil
		}
		// Stop retrying if we exceed statusUpdateRetries - the replicaSet will be requeued with a rate limit.
		if i >= statusUpdateRetries {
			break
		}
		// Update the ReplicaSet with the latest resource version for the next poll
		if rs, getErr = c.Get(rs.Name, metav1.GetOptions{}); getErr != nil {
			// If the GET fails we can't trust status.Replicas anymore. This error
			// is bound to be more interesting than the update failure.
			return nil, getErr
		}
	}

	return nil, updateErr
}

c.UpdateStatus

// staging/src/k8s.io/client-go/kubernetes/typed/apps/v1/replicaset.go
func (c *replicaSets) UpdateStatus(replicaSet *v1.ReplicaSet) (result *v1.ReplicaSet, err error) {
	result = &v1.ReplicaSet{}
	err = c.client.Put().
		Namespace(c.ns).
		Resource("replicasets").
		Name(replicaSet.Name).
		SubResource("status").
		Body(replicaSet).
		Do().
		Into(result)
	return
}

总结

replicaset controller架构图

replicaset controller的大致组成和处理流程如下图，replicaset controller对pod和replicaset对象注册了event handler，当有事件时，会watch到然后将对应的replicaset对象放入到queue中，然后syncReplicaSet方法为replicaset controller调谐replicaset对象的核心处理逻辑所在，从queue中取出replicaset对象，做调谐处理。

replicaset controller核心处理逻辑

replicaset controller的核心处理逻辑是根据replicaset对象里期望的pod数量以及现存pod数量的比较，当期望pod数量比现存pod数量多时，调用创建pod算法创建出新的pod，直至达到期望数量；当期望pod数量比现存pod数量少时，调用删除pod算法，并根据一定的策略对现存pod列表做排序，从中按顺序选择多余的pod然后删除，直至达到期望数量。

replicaset controller创建pod算法

replicaset controller创建pod的算法是，按1、2、4、8...的递增趋势分多批次进行（每次调谐中创建pod的数量上限为500个，超过上限的会在下次调谐中再创建），若某批次创建pod有失败的（如apiserver限流，丢弃请求等，注意：超时除外，因为initialization处理有可能超时），则后续批次的pod创建不再进行，需等待该repliaset对象下次调谐时再触发该pod创建算法，进行pod的创建，直至达到期望数量。

replicaset controller删除pod算法

replicaset controller删除pod的算法是，先根据一定的策略将现存pod列表做排序，然后按顺序从中选择指定数量的pod，拉起与要删除的pod数量相同的goroutine来删除pod（每次调谐中删除pod的数量上限为500个），并等待所有goroutine执行完成。删除pod有失败的（如apiserver限流，丢弃请求）或超过500上限的部分，需等待该repliaset对象下次调谐时再触发该pod删除算法，进行pod的删除，直至达到期望数量。

筛选要删除的pod逻辑

按照下面的排序规则，从上到下进行排序，各个条件相互互斥，符合其中一个条件则排序完成：
（1）优先删除没有绑定node的pod；
（2）优先删除处于Pending状态的pod，然后是Unknown，最后才是Running；
（3）优先删除Not ready的pod，然后才是ready的pod；
（4）按同node上所属replicaset的pod数量排序，优先删除所属replicaset的pod数量多的node上的pod；
（5）按pod ready的时间排序，优先删除ready时间最短的pod；
（6）优先删除pod中容器重启次数较多的pod；
（7）按pod创建时间排序，优先删除创建时间最短的pod。

expectations机制

关于expectations机制的分析，会在下一篇博客中进行。