本文首发于泊浮目的简书:https://www.jianshu.com/u/204...
| 版本 | 日期 | 备注 |
|---|---|---|
| 1.0 | 2020.4.8 | 文章首发 |
| 1.1 | 2020.4.18 | 优化小结部分描述，使其更加详细易懂 |
| 1.2 | 2020.8.10 | 删除部分不重要的代码，减少篇幅，优化描述方式 |
1. 前言
本文面向读者：有java基础并对zk源码实现感兴趣的同学。
用过zookeeper的同学都知道watch是一个非常好用的机制，今天我们就来看看它的实现原理。
在正文开始前，我们先来简单回忆一下watch是什么？
zk提供了分布式数据的发布/订阅功能——即典型的发布订阅模型，其定义了一种一对多的订阅关系，能够让多个订阅者同时监听某个主题对象，当这个主题对象自身状态变化时，则会通知所有订阅者。具体来说，则是zk允许一个客户端向服务端注册一个watch监听，当服务端的一些指定事件触发了这个watch，那么就会向该客户端发送事件通知。
2. 实现剖析
在剖析其实现前，我们不妨来想一想，如果自己动手实现一个watch机制，该怎么做呢？
最简单的方法是在client保存当前节点的版本，并去轮询这个节点的状态。如果发现版本变化，则client触发watch。不过比起轮询，不是有更多的好方法，不是吗？
轮询会给服务器带来不小的压力，或许我们可以考虑采用类似webhook的方式，让server保存和client约定好的地址，当watch的数据节点发生变化时，便通知client。
想到这儿，其实已经和zk自己的watch实现有点像了。沿着这个思路，我们开始剖析：
一般我们在使用client时，getData、getChildren、exist都可以用来向zk注册watcher。其原理都是一样的，我们以exist方法为例子进行剖析——即当关注的节点被创建or删除时，client会收到通知。
2.1 客户端的实现
先从客户端部分开始解析——即以我们调用的原生API为入口。
2.1.1 Zookeeper.exists
/**
* The asynchronous version of exists.
*
* @see #exists(String, Watcher)
*/
public void exists(final String path, Watcher watcher,
StatCallback cb, Object ctx)
{
final String clientPath = path;
PathUtils.validatePath(clientPath);
// the watch contains the un-chroot path
WatchRegistration wcb = null; //1.
if (watcher != null) {
wcb = new ExistsWatchRegistration(watcher, clientPath);
}
final String serverPath = prependChroot(clientPath);
RequestHeader h = new RequestHeader();
h.setType(ZooDefs.OpCode.exists);
ExistsRequest request = new ExistsRequest();
request.setPath(serverPath);
request.setWatch(watcher != null);//3.
SetDataResponse response = new SetDataResponse();
cnxn.queuePacket(h, new ReplyHeader(), request, response, cb,
clientPath, serverPath, ctx, wcb); //2.
}和watch相关的地方有两处，一个是其被转换成WatchRegistration中的一个属性（见注释标记1），最后变成一个Packet——即client与server的最小通信单元（见注释标记2）。
另外一处则是request.setWatch(watcher != null)，是个布尔变量。（见注释标记3）
2.1.2 ClientCnxn.queuePacket
在上文中的最后一段代码是 cnxn.queuePacket(h, new ReplyHeader(), request, response, cb,clientPath, serverPath, ctx, wcb); ，我们继续看下去。
public Packet queuePacket(RequestHeader h, ReplyHeader r, Record request,
Record response, AsyncCallback cb, String clientPath,
String serverPath, Object ctx, WatchRegistration watchRegistration) {
return queuePacket(h, r, request, response, cb, clientPath, serverPath,
ctx, watchRegistration, null);
}
public Packet queuePacket(RequestHeader h, ReplyHeader r, Record request,
Record response, AsyncCallback cb, String clientPath,
String serverPath, Object ctx, WatchRegistration watchRegistration,
WatchDeregistration watchDeregistration) {
Packet packet = null;
// Note that we do not generate the Xid for the packet yet. It is
// generated later at send-time, by an implementation of ClientCnxnSocket::doIO(),
// where the packet is actually sent.
packet = new Packet(h, r, request, response, watchRegistration);
packet.cb = cb;
packet.ctx = ctx;
packet.clientPath = clientPath;
packet.serverPath = serverPath;
packet.watchDeregistration = watchDeregistration;
// The synchronized block here is for two purpose:
// 1. synchronize with the final cleanup() in SendThread.run() to avoid race
// 2. synchronized against each packet. So if a closeSession packet is added,
// later packet will be notified.
synchronized (state) {
if (!state.isAlive() || closing) {
conLossPacket(packet);
} else {
// If the client is asking to close the session then
// mark as closing
if (h.getType() == OpCode.closeSession) {
closing = true;
}
outgoingQueue.add(packet);
}
}
sendThread.getClientCnxnSocket().packetAdded();
return packet;
}这段代码看起来很多，不过仅仅在做一件事——即拼装Packet，并将其加入发送队列。该队列由ClientCnxn中的一个SendThread消费（见SendThread.run）。该方法有较多的条件分支，且不够clean code，故在此不再贴代码，避免扰乱视听。
从下面的代码可以得知，在Zk的client中，会维护发送队列和等待回复的队列，里面都是一个个Packet。
/**
* These are the packets that have been sent and are waiting for a response.
*/
private final LinkedList<Packet> pendingQueue = new LinkedList<Packet>();
/**
* These are the packets that need to be sent.
*/
private final LinkedBlockingDeque<Packet> outgoingQueue = new LinkedBlockingDeque<Packet>();接下来，我们查看SendThread.readReponse，即消费client队列中Packet的逻辑：
void readResponse(ByteBuffer incomingBuffer) throws IOException {
ByteBufferInputStream bbis = new ByteBufferInputStream(
incomingBuffer);
BinaryInputArchive bbia = BinaryInputArchive.getArchive(bbis);
ReplyHeader replyHdr = new ReplyHeader();
replyHdr.deserialize(bbia, "header");
if (replyHdr.getXid() == -2) {
// -2 is the xid for pings
if (LOG.isDebugEnabled()) {
LOG.debug("Got ping response for sessionid: 0x"
+ Long.toHexString(sessionId)
+ " after "
+ ((System.nanoTime() - lastPingSentNs) / 1000000)
+ "ms");
}
return;
}
if (replyHdr.getXid() == -4) {
// -4 is the xid for AuthPacket
if(replyHdr.getErr() == KeeperException.Code.AUTHFAILED.intValue()) {
state = States.AUTH_FAILED;
eventThread.queueEvent( new WatchedEvent(Watcher.Event.EventType.None,
Watcher.Event.KeeperState.AuthFailed, null) );
eventThread.queueEventOfDeath();
}
if (LOG.isDebugEnabled()) {
LOG.debug("Got auth sessionid:0x"
+ Long.toHexString(sessionId));
}
return;
}
if (replyHdr.getXid() == -1) {
// -1 means notification
if (LOG.isDebugEnabled()) {
LOG.debug("Got notification sessionid:0x"
+ Long.toHexString(sessionId));
}
WatcherEvent event = new WatcherEvent();
event.deserialize(bbia, "response");
// convert from a server path to a client path
if (chrootPath != null) {
String serverPath = event.getPath();
if(serverPath.compareTo(chrootPath)==0)
event.setPath("/");
else if (serverPath.length() > chrootPath.length())
event.setPath(serverPath.substring(chrootPath.length()));
else {
LOG.warn("Got server path " + event.getPath()
+ " which is too short for chroot path "
+ chrootPath);
}
}
WatchedEvent we = new WatchedEvent(event);
if (LOG.isDebugEnabled()) {
LOG.debug("Got " + we + " for sessionid 0x"
+ Long.toHexString(sessionId));
}
eventThread.queueEvent( we );
return;
}
// If SASL authentication is currently in progress, construct and
// send a response packet immediately, rather than queuing a
// response as with other packets.
if (tunnelAuthInProgress()) {
GetSASLRequest request = new GetSASLRequest();
request.deserialize(bbia,"token");
zooKeeperSaslClient.respondToServer(request.getToken(),
ClientCnxn.this);
return;
}
Packet packet;
synchronized (pendingQueue) {
if (pendingQueue.size() == 0) {
throw new IOException("Nothing in the queue, but got "
+ replyHdr.getXid());
}
packet = pendingQueue.remove();
}
/*
* Since requests are processed in order, we better get a response
* to the first request!
*/
try {
if (packet.requestHeader.getXid() != replyHdr.getXid()) {
packet.replyHeader.setErr(
KeeperException.Code.CONNECTIONLOSS.intValue());
throw new IOException("Xid out of order. Got Xid "
+ replyHdr.getXid() + " with err " +
+ replyHdr.getErr() +
" expected Xid "
+ packet.requestHeader.getXid()
+ " for a packet with details: "
+ packet );
}
packet.replyHeader.setXid(replyHdr.getXid());
packet.replyHeader.setErr(replyHdr.getErr());
packet.replyHeader.setZxid(replyHdr.getZxid());
if (replyHdr.getZxid() > 0) {
lastZxid = replyHdr.getZxid();
}
if (packet.response != null && replyHdr.getErr() == 0) {
packet.response.deserialize(bbia, "response");
}
if (LOG.isDebugEnabled()) {
LOG.debug("Reading reply sessionid:0x"
+ Long.toHexString(sessionId) + ", packet:: " + packet);
}
} finally {
finishPacket(packet);
}
} synchronized (pendingQueue) 中，我们可以看到从队列中拿出了Packet，并最后将其丢入了finishPacket。
protected void finishPacket(Packet p) {
int err = p.replyHeader.getErr();
if (p.watchRegistration != null) {
p.watchRegistration.register(err);
}
// Add all the removed watch events to the event queue, so that the
// clients will be notified with 'Data/Child WatchRemoved' event type.
if (p.watchDeregistration != null) {
Map<EventType, Set<Watcher>> materializedWatchers = null;
try {
materializedWatchers = p.watchDeregistration.unregister(err);
for (Entry<EventType, Set<Watcher>> entry : materializedWatchers
.entrySet()) {
Set<Watcher> watchers = entry.getValue();
if (watchers.size() > 0) {
queueEvent(p.watchDeregistration.getClientPath(), err,
watchers, entry.getKey());
// ignore connectionloss when removing from local
// session
p.replyHeader.setErr(Code.OK.intValue());
}
}
} catch (KeeperException.NoWatcherException nwe) {
p.replyHeader.setErr(nwe.code().intValue());
} catch (KeeperException ke) {
p.replyHeader.setErr(ke.code().intValue());
}
}
if (p.cb == null) {
synchronized (p) {
p.finished = true;
p.notifyAll();
}
} else {
p.finished = true;
eventThread.queuePacket(p);
}
}这段方法中，我们会分析这两段逻辑：
- p.watchRegistration.register
- queueEvent
2.1.3 watchRegistration
需要注意的是，WatchRegistration在Packet发送前并不会被序列化发送过去，避免发送不必要的信息，毕竟已经在request中标记为watch了。那么这个WatchRegistration有什么用呢？还好register的逻辑很简单，我们来看一下：
/**
* Register the watcher with the set of watches on path.
* @param rc the result code of the operation that attempted to
* add the watch on the path.
*/
public void register(int rc) {
if (shouldAddWatch(rc)) {
Map<String, Set<Watcher>> watches = getWatches(rc);
synchronized(watches) {
Set<Watcher> watchers = watches.get(clientPath);
if (watchers == null) {
watchers = new HashSet<Watcher>();
watches.put(clientPath, watchers);
}
watchers.add(watcher);
}
}
}2.1.4 queueEvent
代码比较少，可以看到client维护了一个path-watchers的字典，到这里，相信大多数读者都能猜到实现了——即收到回复时根据相应的path去找对应wacher。接下来来看queueEvent：
void queueEvent(String clientPath, int err,
Set<Watcher> materializedWatchers, EventType eventType) {
KeeperState sessionState = KeeperState.SyncConnected;
if (KeeperException.Code.SESSIONEXPIRED.intValue() == err
|| KeeperException.Code.CONNECTIONLOSS.intValue() == err) {
sessionState = Event.KeeperState.Disconnected;
}
WatchedEvent event = new WatchedEvent(eventType, sessionState,
clientPath);
eventThread.queueEvent(event, materializedWatchers);
}逻辑很简单，判断状态，然后组装event，交给eventThread去做通知。
private void queueEvent(WatchedEvent event,
Set<Watcher> materializedWatchers) {
if (event.getType() == EventType.None
&& sessionState == event.getState()) {
return;
}
sessionState = event.getState();
final Set<Watcher> watchers;
if (materializedWatchers == null) {
// materialize the watchers based on the event
watchers = watcher.materialize(event.getState(),
event.getType(), event.getPath());
} else {
watchers = new HashSet<Watcher>();
watchers.addAll(materializedWatchers);
}
WatcherSetEventPair pair = new WatcherSetEventPair(watchers, event);
// queue the pair (watch set & event) for later processing
waitingEvents.add(pair);
}ClientWatchManager.materialize不再展示源码，我们只要知道，在这段逻辑中
，当watch被触发后，即会被移除，而状态正是保存在ZkWatchManager里：
static class ZKWatchManager implements ClientWatchManager {
private final Map<String, Set<Watcher>> dataWatches =
new HashMap<String, Set<Watcher>>();
private final Map<String, Set<Watcher>> existWatches =
new HashMap<String, Set<Watcher>>();
private final Map<String, Set<Watcher>> childWatches =
new HashMap<String, Set<Watcher>>();
//......
}再说回来eventThread.run最后做的事情——即入队。那么我们来看看这个线程的核心方法：
@Override
@SuppressFBWarnings("JLM_JSR166_UTILCONCURRENT_MONITORENTER")
public void run() {
try {
isRunning = true;
while (true) {
Object event = waitingEvents.take();
if (event == eventOfDeath) {
wasKilled = true;
} else {
processEvent(event);
}
if (wasKilled)
synchronized (waitingEvents) {
if (waitingEvents.isEmpty()) {
isRunning = false;
break;
}
}
}
} catch (InterruptedException e) {
LOG.error("Event thread exiting due to interruption", e);
}
LOG.info("EventThread shut down for session: 0x{}",
Long.toHexString(getSessionId()));
}又是熟悉的配方熟悉的味道——就是一个死循环去消费队列里的元素，然后我们来看看processEvent：
private void processEvent(Object event) {
try {
if (event instanceof WatcherSetEventPair) {
// each watcher will process the event
WatcherSetEventPair pair = (WatcherSetEventPair) event;
for (Watcher watcher : pair.watchers) {
try {
watcher.process(pair.event);
} catch (Throwable t) {
LOG.error("Error while calling watcher ", t);
}
}
} else if (event instanceof LocalCallback) {
//在本文中这些逻辑不重要，skip
}当process被调用后，我们自己编写的逻辑就会被触发。
看完客户端部分的代码，大家不妨可以思考思考，将一个上层的机制（watch）与底层的通信代码（如finishPacket）显示的写在一起真的好吗？如果让你来写，你会怎么做呢？
2.2 服务端实现
在上文，我们了解了client的watch相关实现，接下来，我们就来捋一捋服务端的watch实现。
我们直接到ZkServer handle request的地方——FinalRequestProcessor的processRequest中的相关部分：
case OpCode.exists: {
lastOp = "EXIS";
// TODO we need to figure out the security requirement for this!
ExistsRequest existsRequest = new ExistsRequest();
ByteBufferInputStream.byteBuffer2Record(request.request,
existsRequest);
String path = existsRequest.getPath();
if (path.indexOf('\0') != -1) {
throw new KeeperException.BadArgumentsException();
}
Stat stat = zks.getZKDatabase().statNode(path, existsRequest
.getWatch() ? cnxn : null);
rsp = new ExistsResponse(stat);
break;
}可以看到，如果request是要求watch的，那么会将ServerCnxn传递下去，ServerCnxn代表了客户端和服务器之间的连接。这样当数据事件发生时，可以通过连接触发client的watch。
跳转DataTree.statNode：
public Stat statNode(String path, Watcher watcher)
throws KeeperException.NoNodeException {
Stat stat = new Stat();
DataNode n = nodes.get(path);
if (watcher != null) {
dataWatches.addWatch(path, watcher);
}
if (n == null) {
throw new KeeperException.NoNodeException();
}
synchronized (n) {
n.copyStat(stat);
return stat;
}
}当watcher != null时，则会添加一个watcher当服务端的dataWatches中。接下来，我们来看一下服务端的watch核心类——WatchManager:
/**
* This class manages watches. It allows watches to be associated with a string
* and removes watchers and their watches in addition to managing triggers.
*/
class WatchManager {
private static final Logger LOG = LoggerFactory.getLogger(WatchManager.class);
private final HashMap<String, HashSet<Watcher>> watchTable =
new HashMap<String, HashSet<Watcher>>();
private final HashMap<Watcher, HashSet<String>> watch2Paths =
new HashMap<Watcher, HashSet<String>>();
synchronized int size(){
int result = 0;
for(Set<Watcher> watches : watchTable.values()) {
result += watches.size();
}
return result;
}
synchronized void addWatch(String path, Watcher watcher) {
HashSet<Watcher> list = watchTable.get(path);
if (list == null) {
// don't waste memory if there are few watches on a node
// rehash when the 4th entry is added, doubling size thereafter
// seems like a good compromise
list = new HashSet<Watcher>(4);
watchTable.put(path, list);
}
list.add(watcher);
HashSet<String> paths = watch2Paths.get(watcher);
if (paths == null) {
// cnxns typically have many watches, so use default cap here
paths = new HashSet<String>();
watch2Paths.put(watcher, paths);
}
paths.add(path);
}
synchronized void removeWatcher(Watcher watcher) {
HashSet<String> paths = watch2Paths.remove(watcher);
if (paths == null) {
return;
}
for (String p : paths) {
HashSet<Watcher> list = watchTable.get(p);
if (list != null) {
list.remove(watcher);
if (list.size() == 0) {
watchTable.remove(p);
}
}
}
}
Set<Watcher> triggerWatch(String path, EventType type) {
return triggerWatch(path, type, null);
}
Set<Watcher> triggerWatch(String path, EventType type, Set<Watcher> supress) {
WatchedEvent e = new WatchedEvent(type,
KeeperState.SyncConnected, path);
HashSet<Watcher> watchers;
synchronized (this) {
watchers = watchTable.remove(path);
if (watchers == null || watchers.isEmpty()) {
if (LOG.isTraceEnabled()) {
ZooTrace.logTraceMessage(LOG,
ZooTrace.EVENT_DELIVERY_TRACE_MASK,
"No watchers for " + path);
}
return null;
}
for (Watcher w : watchers) {
HashSet<String> paths = watch2Paths.get(w);
if (paths != null) {
paths.remove(path);
}
}
}
for (Watcher w : watchers) {
if (supress != null && supress.contains(w)) {
continue;
}
w.process(e);
}
return watchers;
}
/**
* Brief description of this object.
*/
@Override
public synchronized String toString() {
StringBuilder sb = new StringBuilder();
sb.append(watch2Paths.size()).append(" connections watching ")
.append(watchTable.size()).append(" paths\n");
int total = 0;
for (HashSet<String> paths : watch2Paths.values()) {
total += paths.size();
}
sb.append("Total watches:").append(total);
return sb.toString();
}
/**
* String representation of watches. Warning, may be large!
* @param byPath iff true output watches by paths, otw output
* watches by connection
* @return string representation of watches
*/
synchronized void dumpWatches(PrintWriter pwriter, boolean byPath) {
if (byPath) {
for (Entry<String, HashSet<Watcher>> e : watchTable.entrySet()) {
pwriter.println(e.getKey());
for (Watcher w : e.getValue()) {
pwriter.print("\t0x");
pwriter.print(Long.toHexString(((ServerCnxn)w).getSessionId()));
pwriter.print("\n");
}
}
} else {
for (Entry<Watcher, HashSet<String>> e : watch2Paths.entrySet()) {
pwriter.print("0x");
pwriter.println(Long.toHexString(((ServerCnxn)e.getKey()).getSessionId()));
for (String path : e.getValue()) {
pwriter.print("\t");
pwriter.println(path);
}
}
}
}
/**
* Checks the specified watcher exists for the given path
*
* @param path
* znode path
* @param watcher
* watcher object reference
* @return true if the watcher exists, false otherwise
*/
synchronized boolean containsWatcher(String path, Watcher watcher) {
HashSet<String> paths = watch2Paths.get(watcher);
if (paths == null || !paths.contains(path)) {
return false;
}
return true;
}
/**
* Removes the specified watcher for the given path
*
* @param path
* znode path
* @param watcher
* watcher object reference
* @return true if the watcher successfully removed, false otherwise
*/
synchronized boolean removeWatcher(String path, Watcher watcher) {
HashSet<String> paths = watch2Paths.get(watcher);
if (paths == null || !paths.remove(path)) {
return false;
}
HashSet<Watcher> list = watchTable.get(path);
if (list == null || !list.remove(watcher)) {
return false;
}
if (list.size() == 0) {
watchTable.remove(path);
}
return true;
}
/**
* Returns a watch report.
*
* @return watch report
* @see WatchesReport
*/
synchronized WatchesReport getWatches() {
Map<Long, Set<String>> id2paths = new HashMap<Long, Set<String>>();
for (Entry<Watcher, HashSet<String>> e: watch2Paths.entrySet()) {
Long id = ((ServerCnxn) e.getKey()).getSessionId();
HashSet<String> paths = new HashSet<String>(e.getValue());
id2paths.put(id, paths);
}
return new WatchesReport(id2paths);
}
/**
* Returns a watch report by path.
*
* @return watch report
* @see WatchesPathReport
*/
synchronized WatchesPathReport getWatchesByPath() {
Map<String, Set<Long>> path2ids = new HashMap<String, Set<Long>>();
for (Entry<String, HashSet<Watcher>> e : watchTable.entrySet()) {
Set<Long> ids = new HashSet<Long>(e.getValue().size());
path2ids.put(e.getKey(), ids);
for (Watcher watcher : e.getValue()) {
ids.add(((ServerCnxn) watcher).getSessionId());
}
}
return new WatchesPathReport(path2ids);
}
/**
* Returns a watch summary.
*
* @return watch summary
* @see WatchesSummary
*/
synchronized WatchesSummary getWatchesSummary() {
int totalWatches = 0;
for (HashSet<String> paths : watch2Paths.values()) {
totalWatches += paths.size();
}
return new WatchesSummary (watch2Paths.size(), watchTable.size(),
totalWatches);
}
}
整个类非常好理解，先看两个核心成员变量：
- watchTable:path-watchs
- watch2Paths:watcher-paths
addWatch就是往两个map中添加数据，而触发便是根据path遍历出那些watcher，并从内存中删除它们，然后调用它们的process——这时ServerCnxn就会发送一个Packet到client。
那么什么时候触发呢？也很简单。就在DataTree的代码里，对相应数据进行操作时，就会触发watcher。我们以DataTree.setData为例：
public Stat setData(String path, byte data[], int version, long zxid,
long time) throws KeeperException.NoNodeException {
Stat s = new Stat();
DataNode n = nodes.get(path);
if (n == null) {
throw new KeeperException.NoNodeException();
}
byte lastdata[] = null;
synchronized (n) {
lastdata = n.data;
n.data = data;
n.stat.setMtime(time);
n.stat.setMzxid(zxid);
n.stat.setVersion(version);
n.copyStat(s);
}
// now update if the path is in a quota subtree.
String lastPrefix = getMaxPrefixWithQuota(path);
if(lastPrefix != null) {
this.updateBytes(lastPrefix, (data == null ? 0 : data.length)
- (lastdata == null ? 0 : lastdata.length));
}
// 触发处
dataWatches.triggerWatch(path, EventType.NodeDataChanged);
return s;
}至此，我们就理清watch在Zk里到底是怎么一回事了。同时，我们也了解watcher的几个特性：
- 一次性：无论是client还是server，一旦watcher被触发，zk都会将其移除。这意味着开发者需要反复注册，但是好处也很明显——降低了服务器压力，避免频繁更新的节点一直触发watcher。
- 客户端串行执行：客户端回调是一个串行同步的过程，这使得回调是有序的。同样，开发者要注意不要因为一个watcher的逻辑影响整个client回调。
轻量：client的request是否要watch其实仅仅通过一个boolean来决定，同样的，server的response的watch回调——WatchedEvent也仅仅只有三个属性：
- 通知状态
- 事件类型
- 节点路径
这种轻量化的设计使得网络开销和服务端内存开销上都是很廉价的。
3. 小结
在本文中，我们一起了解了watch的实现机理。简单总结如下：
- client在发送请求时候，会将watch的具体状态保存在client中，即存在于等待回复队列中
- 标记watch的request到达服务端后，服务端会将这个watcher（包含client的连接属性）以字典的形式保存在内存中
- 当watch的数据发生相应变化时，去字典里找出注册的watch，并拿到对应client连接
- 根据连接，发送一个通知到client
- client从等待回复队列中取出元素，watch的回调被触发
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