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// Copyright 2015 The etcd Authors
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// See the License for the specific language governing permissions and
// limitations under the License.
package store
type Watcher interface {
EventChan() chan *Event
StartIndex() uint64 // The EtcdIndex at which the Watcher was created
type watcher struct {
eventChan chan *Event
stream bool
recursive bool
sinceIndex uint64
startIndex uint64
hub *watcherHub
removed bool
remove func()
func (w *watcher) EventChan() chan *Event {
return w.eventChan
func (w *watcher) StartIndex() uint64 {
return w.startIndex
// notify function notifies the watcher. If the watcher interests in the given path,
// the function will return true.
func (w *watcher) notify(e *Event, originalPath bool, deleted bool) bool {
// watcher is interested the path in three cases and under one condition
// the condition is that the event happens after the watcher's sinceIndex
// 1. the path at which the event happens is the path the watcher is watching at.
// For example if the watcher is watching at "/foo" and the event happens at "/foo",
// the watcher must be interested in that event.
// 2. the watcher is a recursive watcher, it interests in the event happens after
// its watching path. For example if watcher A watches at "/foo" and it is a recursive
// one, it will interest in the event happens at "/foo/bar".
// 3. when we delete a directory, we need to force notify all the watchers who watches
// at the file we need to delete.
// For example a watcher is watching at "/foo/bar". And we deletes "/foo". The watcher
// should get notified even if "/foo" is not the path it is watching.
if (w.recursive || originalPath || deleted) && e.Index() >= w.sinceIndex {
// We cannot block here if the eventChan capacity is full, otherwise
// etcd will hang. eventChan capacity is full when the rate of
// notifications are higher than our send rate.
// If this happens, we close the channel.
select {
case w.eventChan <- e:
// We have missed a notification. Remove the watcher.
// Removing the watcher also closes the eventChan.
return true
return false
// Remove removes the watcher from watcherHub
// The actual remove function is guaranteed to only be executed once
func (w *watcher) Remove() {
defer w.hub.mutex.Unlock()
if w.remove != nil {
// nopWatcher is a watcher that receives nothing, always blocking.
type nopWatcher struct{}
func NewNopWatcher() Watcher { return &nopWatcher{} }
func (w *nopWatcher) EventChan() chan *Event { return nil }
func (w *nopWatcher) StartIndex() uint64 { return 0 }
func (w *nopWatcher) Remove() {}