一、概念

informer 是 client-go 中的核心工具包，在kubernetes中，各个组件通过HTTP协议跟 API Server 进行通信。如果各组件每次都直接和API Server 进行交互，会给API Server 和ETCD造成非常大的压力。在不依赖任何中间件的情况下，通过informer保证了消息的实时性、可靠性和顺序性。

二、架构设计

informer运行原理

三、源码分析

3.1 informer启动

informer启动有以下步骤：

1. 注册及启动processLoop和reflector
2. reflector开始LIST和WATCH，watch到的数据进行对比处理，存入到queue中
3. processLoop开始循环pop队列数据

	factory := informers.NewSharedInformerFactory(clientset, 0)podInformer := factory.Core().V1().Pods().Informer()podInformer.AddEventHandler(cache.ResourceEventHandlerFuncs{AddFunc: func(obj interface{}) {mObj := obj.(v1.Object)log.Printf("New pod added: %s", mObj.GetName())},UpdateFunc: func(oldObj, newObj interface{}) {oObj := oldObj.(v1.Object)nObj := newObj.(v1.Object)log.Printf("%s pod updated to %s", oObj.GetName(), nObj.GetName())},DeleteFunc: func(obj interface{}) {mObj := obj.(v1.Object)log.Printf("pod deleted from store: %s", mObj.GetName())},})//启动informerpodInformer.Run(stopCh)

func (s *sharedIndexInformer) Run(stopCh <-chan struct{}) {......fifo := NewDeltaFIFOWithOptions(DeltaFIFOOptions{KnownObjects:          s.indexer,EmitDeltaTypeReplaced: true,})cfg := &Config{Queue:            fifo,ListerWatcher:    s.listerWatcher,ObjectType:       s.objectType,FullResyncPeriod: s.resyncCheckPeriod,RetryOnError:     false,ShouldResync:     s.processor.shouldResync,//注册回调函数HandleDeltas，后面从queue弹出数据的时候要用到Process:           s.HandleDeltas,WatchErrorHandler: s.watchErrorHandler,}......s.controller.Run(stopCh)
}

代码位置：client-go/tools/cache/controller.go

func (c *controller) Run(stopCh <-chan struct{}) {defer utilruntime.HandleCrash()go func() {<-stopChc.config.Queue.Close()}()r := NewReflector(c.config.ListerWatcher,c.config.ObjectType,c.config.Queue,c.config.FullResyncPeriod,)// 省略代码......var wg wait.Group//启动reflectorwg.StartWithChannel(stopCh, r.Run)//启动processLoopwait.Until(c.processLoop, time.Second, stopCh)wg.Wait()
}

reflector开始list and watch,代码位置：client-go/tools/cache/reflector.go

func (r *Reflector) Run(stopCh <-chan struct{}) {klog.V(3).Infof("Starting reflector %s (%s) from %s", r.expectedTypeName, r.resyncPeriod, r.name)wait.BackoffUntil(func() {if err := r.ListAndWatch(stopCh); err != nil {r.watchErrorHandler(r, err)}}, r.backoffManager, true, stopCh)klog.V(3).Infof("Stopping reflector %s (%s) from %s", r.expectedTypeName, r.resyncPeriod, r.name)
}

			switch event.Type {//watch到add事件case watch.Added:err := r.store.Add(event.Object)if err != nil {utilruntime.HandleError(fmt.Errorf("%s: unable to add watch event object (%#v) to store: %v", r.name, event.Object, err))}//watch到modified事件case watch.Modified:err := r.store.Update(event.Object)if err != nil {utilruntime.HandleError(fmt.Errorf("%s: unable to update watch event object (%#v) to store: %v", r.name, event.Object, err))}//watch到delete事件case watch.Deleted:// TODO: Will any consumers need access to the "last known// state", which is passed in event.Object? If so, may need// to change this.err := r.store.Delete(event.Object)if err != nil {utilruntime.HandleError(fmt.Errorf("%s: unable to delete watch event object (%#v) from store: %v", r.name, event.Object, err))}case watch.Bookmark:// A `Bookmark` means watch has synced here, just update the resourceVersiondefault:utilruntime.HandleError(fmt.Errorf("%s: unable to understand watch event %#v", r.name, event))}

以update为例

func (f *DeltaFIFO) Update(obj interface{}) error {f.lock.Lock()defer f.lock.Unlock()f.populated = truereturn f.queueActionLocked(Updated, obj)
}

func (f *DeltaFIFO) queueActionLocked(actionType DeltaType, obj interface{}) error {id, err := f.KeyOf(obj)if err != nil {return KeyError{obj, err}}oldDeltas := f.items[id]newDeltas := append(oldDeltas, Delta{actionType, obj})newDeltas = dedupDeltas(newDeltas)if len(newDeltas) > 0 {if _, exists := f.items[id]; !exists {//将key放入到queuef.queue = append(f.queue, id)}//将newDeltas放入到items中f.items[id] = newDeltas//事件到达广播f.cond.Broadcast()} else {// This never happens, because dedupDeltas never returns an empty list// when given a non-empty list (as it is here).// If somehow it happens anyway, deal with it but complain.if oldDeltas == nil {klog.Errorf("Impossible dedupDeltas for id=%q: oldDeltas=%#+v, obj=%#+v; ignoring", id, oldDeltas, obj)return nil}klog.Errorf("Impossible dedupDeltas for id=%q: oldDeltas=%#+v, obj=%#+v; breaking invariant by storing empty Deltas", id, oldDeltas, obj)f.items[id] = newDeltasreturn fmt.Errorf("Impossible dedupDeltas for id=%q: oldDeltas=%#+v, obj=%#+v; broke DeltaFIFO invariant by storing empty Deltas", id, oldDeltas, obj)}return nil
}

DeltaFIFO的数据结构如下：

type DeltaFIFO struct {// lock/cond protects access to 'items' and 'queue'.lock sync.RWMutexcond sync.Cond// `items` maps a key to a Deltas.// Each such Deltas has at least one Delta.items map[string]Deltas// `queue` maintains FIFO order of keys for consumption in Pop().// There are no duplicates in `queue`.// A key is in `queue` if and only if it is in `items`.queue []string// populated is true if the first batch of items inserted by Replace() has been populated// or Delete/Add/Update/AddIfNotPresent was called first.populated bool// initialPopulationCount is the number of items inserted by the first call of Replace()initialPopulationCount int// keyFunc is used to make the key used for queued item// insertion and retrieval, and should be deterministic.keyFunc KeyFunc// knownObjects list keys that are "known" --- affecting Delete(),// Replace(), and Resync()knownObjects KeyListerGetter// Used to indicate a queue is closed so a control loop can exit when a queue is empty.// Currently, not used to gate any of CRUD operations.closed bool// emitDeltaTypeReplaced is whether to emit the Replaced or Sync// DeltaType when Replace() is called (to preserve backwards compat).emitDeltaTypeReplaced bool
}

到这里，已经将最新的数据推送到了DeltaFIFO的queue中，接下来看下怎么处理queue中的数据。

queue出队：
回到之前注册的processLoop

func (c *controller) processLoop() {for {//从queue弹出数据,交由process处理，也就是之前注册的handleDeltasobj, err := c.config.Queue.Pop(PopProcessFunc(c.config.Process))if err != nil {if err == ErrFIFOClosed {return}if c.config.RetryOnError {// This is the safe way to re-enqueue.// 重新入队queuec.config.Queue.AddIfNotPresent(obj)}}}
}

func (f *DeltaFIFO) Pop(process PopProcessFunc) (interface{}, error) {f.lock.Lock()defer f.lock.Unlock()for {for len(f.queue) == 0 {// When the queue is empty, invocation of Pop() is blocked until new item is enqueued.// When Close() is called, the f.closed is set and the condition is broadcasted.// Which causes this loop to continue and return from the Pop().if f.closed {return nil, ErrFIFOClosed}//如果queue中没有数据，阻塞等待f.cond.Wait()}id := f.queue[0]f.queue = f.queue[1:]depth := len(f.queue)if f.initialPopulationCount > 0 {f.initialPopulationCount--}item, ok := f.items[id]if !ok {// This should never happenklog.Errorf("Inconceivable! %q was in f.queue but not f.items; ignoring.", id)continue}delete(f.items, id)// Only log traces if the queue depth is greater than 10 and it takes more than// 100 milliseconds to process one item from the queue.// Queue depth never goes high because processing an item is locking the queue,// and new items can't be added until processing finish.// https://github.com/kubernetes/kubernetes/issues/103789if depth > 10 {trace := utiltrace.New("DeltaFIFO Pop Process",utiltrace.Field{Key: "ID", Value: id},utiltrace.Field{Key: "Depth", Value: depth},utiltrace.Field{Key: "Reason", Value: "slow event handlers blocking the queue"})defer trace.LogIfLong(100 * time.Millisecond)}//处理数据，重点看下这个方法，进入HandleDeltaserr := process(item)if e, ok := err.(ErrRequeue); ok {f.addIfNotPresent(id, item)err = e.Err}// Don't need to copyDeltas here, because we're transferring// ownership to the caller.return item, err}
}

代码位置 client-go/tools/cache/shared_informer.go

func (s *sharedIndexInformer) HandleDeltas(obj interface{}) error {s.blockDeltas.Lock()defer s.blockDeltas.Unlock()// from oldest to newestfor _, d := range obj.(Deltas) {switch d.Type {case Sync, Replaced, Added, Updated:s.cacheMutationDetector.AddObject(d.Object)//从本地缓存indexer中查询数据是否存在if old, exists, err := s.indexer.Get(d.Object); err == nil && exists {//如果存在，则更新indexer中该数据if err := s.indexer.Update(d.Object); err != nil {return err}isSync := falseswitch {case d.Type == Sync:// Sync events are only propagated to listeners that requested resyncisSync = truecase d.Type == Replaced:if accessor, err := meta.Accessor(d.Object); err == nil {if oldAccessor, err := meta.Accessor(old); err == nil {// Replaced events that didn't change resourceVersion are treated as resync events// and only propagated to listeners that requested resyncisSync = accessor.GetResourceVersion() == oldAccessor.GetResourceVersion()}}}//分发监听者，通知监听updates.processor.distribute(updateNotification{oldObj: old, newObj: d.Object}, isSync)} else {//如果不存在，则在indexer中添加该数据if err := s.indexer.Add(d.Object); err != nil {return err}//分发监听者，通知监听adds.processor.distribute(addNotification{newObj: d.Object}, false)}case Deleted:if err := s.indexer.Delete(d.Object); err != nil {return err}//分发监听者，通知监听deletes.processor.distribute(deleteNotification{oldObj: d.Object}, false)}}return nil
}

func (p *sharedProcessor) distribute(obj interface{}, sync bool) {p.listenersLock.RLock()defer p.listenersLock.RUnlock()if sync {for _, listener := range p.syncingListeners {//往监听者加入数据listener.add(obj)}} else {for _, listener := range p.listeners {//往监听者加入数据listener.add(obj)}}
}

func (p *processorListener) add(notification interface{}) {p.addCh <- notification
}

数据分发到了监听者，那么监听者是什么时候注册的，又是怎么工作的呢？
联系到前面informer注册的eventHandler

podInformer.AddEventHandler(cache.ResourceEventHandlerFuncs{AddFunc: func(obj interface{}) {mObj := obj.(v1.Object)log.Printf("New pod added: %s", mObj.GetName())},UpdateFunc: func(oldObj, newObj interface{}) {oObj := oldObj.(v1.Object)nObj := newObj.(v1.Object)log.Printf("%s pod updated to %s", oObj.GetName(), nObj.GetName())},DeleteFunc: func(obj interface{}) {mObj := obj.(v1.Object)log.Printf("pod deleted from store: %s", mObj.GetName())},})

func (s *sharedIndexInformer) AddEventHandler(handler ResourceEventHandler) {s.AddEventHandlerWithResyncPeriod(handler, s.defaultEventHandlerResyncPeriod)
}

func (s *sharedIndexInformer) AddEventHandlerWithResyncPeriod(handler ResourceEventHandler, resyncPeriod time.Duration) {//省略代码//......//创建监听者listener := newProcessListener(handler, resyncPeriod, determineResyncPeriod(resyncPeriod, s.resyncCheckPeriod), s.clock.Now(), initialBufferSize)if !s.started {s.processor.addListener(listener)return}// in order to safely join, we have to// 1. stop sending add/update/delete notifications// 2. do a list against the store// 3. send synthetic "Add" events to the new handler// 4. unblocks.blockDeltas.Lock()defer s.blockDeltas.Unlock()//添加监听者s.processor.addListener(listener)for _, item := range s.indexer.List() {listener.add(addNotification{newObj: item})}
}

func (p *sharedProcessor) addListener(listener *processorListener) {p.listenersLock.Lock()defer p.listenersLock.Unlock()p.addListenerLocked(listener)if p.listenersStarted {//在不同的协程使监听者运行起来p.wg.Start(listener.run)p.wg.Start(listener.pop)}
}

func (p *sharedProcessor) addListenerLocked(listener *processorListener) {p.listeners = append(p.listeners, listener)p.syncingListeners = append(p.syncingListeners, listener)
}

func (p *processorListener) pop() {defer utilruntime.HandleCrash()defer close(p.nextCh) // Tell .run() to stopvar nextCh chan<- interface{}var notification interface{}for {select {case nextCh <- notification:// Notification dispatchedvar ok boolnotification, ok = p.pendingNotifications.ReadOne()if !ok { // Nothing to popnextCh = nil // Disable this select case}//联系前面distribute分发监听者的时候将notification发送到addChcase notificationToAdd, ok := <-p.addCh:if !ok {return}if notification == nil { // No notification to pop (and pendingNotifications is empty)// Optimize the case - skip adding to pendingNotificationsnotification = notificationToAddnextCh = p.nextCh} else { // There is already a notification waiting to be dispatchedp.pendingNotifications.WriteOne(notificationToAdd)}}}
}

func (p *processorListener) run() {// this call blocks until the channel is closed.  When a panic happens during the notification// we will catch it, **the offending item will be skipped!**, and after a short delay (one second)// the next notification will be attempted.  This is usually better than the alternative of never// delivering again.stopCh := make(chan struct{})wait.Until(func() {for next := range p.nextCh {//这里调用到用户定义的handler方法switch notification := next.(type) {case updateNotification:p.handler.OnUpdate(notification.oldObj, notification.newObj)case addNotification:p.handler.OnAdd(notification.newObj)case deleteNotification:p.handler.OnDelete(notification.oldObj)default:utilruntime.HandleError(fmt.Errorf("unrecognized notification: %T", next))}}// the only way to get here is if the p.nextCh is empty and closedclose(stopCh)}, 1*time.Second, stopCh)
}

最后看一下informer的详细全局设计