概述
ZK提供了分布式数据的发布/订阅功能，一个典型的发布/订阅模型系统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发生变化时，会通知所有的订阅者。在ZK中引入了 Watcher 机制来实现这种分布式的通知功能。
ZK允许客户端向服务器端注册一个 Watcher 监听，当服务端的一些指定事件触发了这个 Watcher ，那么就会向指定客户端发送一个事件通知来实现分布式通知功能。
大致流程就是 Client 向ZK中注册 Watcher，如果注册成功的话，会将对应的 Watcher 存储在本地。当ZK服务器端触发 Watcher 事件之后，会向客户端发送通知，客户端会从 ClientWatchManager 中取出对应的 Watcher 进行回调。
Watcher 接口
说了那么久、Watcher 究竟是啥？有什么用处？
/**
* This interface specifies the public interface an event handler class must
* implement. A ZooKeeper client will get various events from the ZooKeeper
* server it connects to. An application using such a client handles these
* events by registering a callback object with the client. The callback object
* is expected to be an instance of a class that implements Watcher interface.
*/
@InterfaceAudience.Public
public interface Watcher {
void process(WatchedEvent event);
}
只要你通过这个接口的实现类对象去向ZK服务端注册监听，那么当有ZK服务端有事件通知到Client，那么就会回调这个 process 方法。
WatchedEvent
那么 WatchedEvent 又有什么玄机呢？
public class WatchedEvent {
/**
* Enumeration of states the ZooKeeper may be at the event
*/
private final KeeperState keeperState;
/**
* Enumeration of types of events that may occur on the ZooKeeper
*/
private final EventType eventType;
private String path;
}
KeeperState 和 EventType 是两个枚举类，分别代表通知状态和事件类型。path 就是 client 监听到路径。
常见的 KeeperState 和 EventType 组合
| KeeperState | EventType | 触发条件 | 说明 |
|---|---|---|---|
| SyncConnected | None(-1) | 客户端与服务端成功建立会话 | 客户端和服务端处于连接状态 |
| SyncConnected | NodeCreated(1) | Watcher 监听对应的数据节点被创建 | 客户端和服务端处于连接状态 |
| SyncConnected | NodeDeleted(2) | Watcher 监听对应的数据节点被删除 | 客户端和服务端处于连接状态 |
| SyncConnected | NodeDataChanged(3) | Watcher 监听对应的数据节点的内容发生变更(数据内容和数据版本号) | 客户端和服务端处于连接状态 |
| SyncConnected | NodeChildrenChanged(4) | Watcher 监听对应的数据节点的子节点列表发生改变 | 客户端和服务端处于连接状态 |
关于 NodeDataChanged 事件类型，这里的变更包括节点的数据内容发生变更，也包括数据的版本号(dataVersion) 变更，所以只要有客户端调用了数据更新接口，不管数据内容是否发生改变、都会导致 dataVersion 发生改变，从而触发对应 Watcher 的监听。这样子就能避免典型乐观锁 ABA 的问题。
WatcherEvent
我们可以在 WatchedEvent 中发现有这么一个方法
/**
* Convert WatchedEvent to type that can be sent over network
*/
public WatcherEvent getWrapper() {
return new WatcherEvent(eventType.getIntValue(), keeperState.getIntValue(), path);
}
笼统的说，WatcherEvent 和 WatchedEvent 表示的是同一个事物，都是对服务端事件的封装。WatchedEvent 是一个用于逻辑处理的对象、而WatcherEvent 是用于传输的实体对象。从上面的代码我们可以看到，创建 WatcherEvent 的参数就是 WatchedEvent 中各个属性的值。
http://people.apache.org/~larsgeorge/zookeeper-1215258/build/docs/dev-api/org/apache/zookeeper/proto/WatcherEvent.html 中可以看到它实现了 Record 接口
public class WatcherEvent
extends Object
implements org.apache.jute.Record
而在 Record 接口中定义了序列化和反序列的方法
@InterfaceAudience.Public
public interface Record {
void serialize(OutputArchive archive, String tag) throws IOException;
void deserialize(InputArchive archive, String tag) throws IOException;
}
相关组件
相关过程
概括可以分为三个过程
- 客户端注册 Watcher
- 服务端处理 Watcher
- 客户端回调 Watcher
客户端注册 Watcher
我们在创建一个ZK 客户端实例对象的时候、可以向构造方法中传入一个默认的 Watcher
public ZooKeeper(String connectString, int sessionTimeout, Watcher watcher)
参数中的这个 Watcher 将会被保存在 ZKWatchManager 中,作为整个会话期间的默认的 Watcher
watchManager.defaultWatcher = watcher;
除此之外、ZK 客户端也可以通过 getData,getChildren,exist三个接口向ZK服务端注册 Watcher
我们以 getData接口来分析
public byte[] getData(final String path, Watcher watcher, Stat stat){
.....
}
public byte[] getData(String path, boolean watch, Stat stat) throws KeeperException, InterruptedException {
return getData(path, getDefaultWatcher(watch), stat);
}
如果我们的参数 watch为 true , 那么 getDefaultWatcher就是去拿我们创建Zookeeper 时传入的默认的 Watcher
private Watcher getDefaultWatcher(boolean required) {
if (required) {
if (watchManager.defaultWatcher != null) {
return watchManager.defaultWatcher;
} else {
throw new IllegalStateException("Default watcher is required, but it is null.");
}
}
return null;
}
下面是 完整的 getData代码
public byte[] getData(final String path, Watcher watcher, Stat stat) throws KeeperException, InterruptedException {
final String clientPath = path;
PathUtils.validatePath(clientPath);
// the watch contains the un-chroot path
// 创建 数据类型 的 watch registration
WatchRegistration wcb = null;
if (watcher != null) {
wcb = new DataWatchRegistration(watcher, clientPath);
}
// 将客户端change root directory 的路径加上、变回服务端那边正常的路径
final String serverPath = prependChroot(clientPath);
RequestHeader h = new RequestHeader();
h.setType(ZooDefs.OpCode.getData);
GetDataRequest request = new GetDataRequest();
request.setPath(serverPath);
// 标记是否有 watcher
request.setWatch(watcher != null);
GetDataResponse response = new GetDataResponse();
ReplyHeader r = cnxn.submitRequest(h, request, response, wcb);
if (r.getErr() != 0) {
throw KeeperException.create(KeeperException.Code.get(r.getErr()), clientPath);
}
if (stat != null) {
DataTree.copyStat(response.getStat(), stat);
}
return response.getData();
}
- 创建一个 DataWatchRegistration
- 转换 path (客户端这边可能 change root directory,发送请求前要将其转为为服务端那边的路径)
- 使用 ClientCnxn 提交这个请求
public ReplyHeader submitRequest(
RequestHeader h,
Record request,
Record response,
WatchRegistration watchRegistration,
WatchDeregistration watchDeregistration) throws InterruptedException {
ReplyHeader r = new ReplyHeader();
Packet packet = queuePacket(
h,
r,
request,
response,
null,
null,
null,
null,
watchRegistration,
watchDeregistration);
....
....
return r;
}
最终这个 Request 被加入到 outgoingQueue中
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;
packet = new Packet(h, r, request, response, watchRegistration);
synchronized (state) {
...
....
outgoingQueue.add(packet);
}
}
最终发送请求到服务端，在 SendThread#readResponse 中处理返回结果
void readResponse(ByteBuffer incomingBuffer) throws IOException {
ByteBufferInputStream bbis = new ByteBufferInputStream(incomingBuffer);
BinaryInputArchive bbia = BinaryInputArchive.getArchive(bbis);
ReplyHeader replyHdr = new ReplyHeader();
replyHdr.deserialize(bbia, "header");
switch (replyHdr.getXid()) {
case PING_XID:
....
....
return;
case AUTHPACKET_XID:
...
...
return;
// 处理服务端到通知
case NOTIFICATION_XID:
LOG.debug("Got notification session id: 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 {} which is too short for chroot path {}.",
event.getPath(), chrootPath);
}
}
WatchedEvent we = new WatchedEvent(event);
LOG.debug("Got {} for session id 0x{}", we, Long.toHexString(sessionId));
// 加入到事件队列中、由EventThread处理
eventThread.queueEvent(we);
return;
default:
break;
}
// 移除这个Pacjet
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 {
....
.....
} finally {
// 将Watcher 保存在 ClientWatchManager
finishPacket(packet);
}
}
主要做了啥事情
- 反序列化，获取请求头中的
XID判断是否是服务端到通知、如果是的话、加入到事件队列中、由EventThread去处理 - 从 outgoingQueue中移除 Packet。
- 调用 finishPacket 函数、进行一些后续处理
protected void finishPacket(Packet p) {
int err = p.replyHeader.getErr();
if (p.watchRegistration != null) {
p.watchRegistration.register(err);
}
...
...
}
最后回到 WatchRegistration将对应的 Watcher 注册到对应的 Map<String, Set<Watcher>>中。
服务端处理 Watcher
先来认识几个主要的组件类
WatchManager 是 ZK 服务端 Watcher 的管理者，其内部管理的 watchTable 和 watch2Paths 两个存储结构，分别用两个维度对 Watcher 进行存储。
- watchTable 从数据节点路径的粒度来托管 Watcher。
- watch2Paths 从 Watcher 的粒度来控制事件触发需要触发的数据节点。
ServerCnxn是一个 Zookeeper 客户端和负担之间的连接接口、代表了一个客户端和服务端的连接，其默认实现是 NIOServerCnxn，从 3.4.0 开始引入了基于Netty 的实现 NettyServerCnxn。
ServerCnxn同时实现了 Watcher接口，因此我们可以将其看作是一个 Watcher对象.
数据节点的路径和 ServerCnxn都会被存储在 WatchManager中
服务端收到客户端的请求后会在 FinalRequestProcessor#processRequest中判断当前请求是否需要注册 Watcher。
case OpCode.getData: {
lastOp = "GETD";
GetDataRequest getDataRequest = new GetDataRequest();
ByteBufferInputStream.byteBuffer2Record(request.request, getDataRequest);
path = getDataRequest.getPath();
// 调用处理 getData 请求的方法
rsp = handleGetDataRequest(getDataRequest, cnxn, request.authInfo);
requestPathMetricsCollector.registerRequest(request.type, path);
break;
}
private Record handleGetDataRequest(Record request, ServerCnxn cnxn, List<Id> authInfo) throws KeeperException, IOException {
....
....
// 这注意、客户端是否需要注册 Watcher、请求中只是有一个 boolean 字段来表示
// 从请求中获取是否需要注册 Watcher
byte[] b = zks.getZKDatabase().getData(path, stat, getDataRequest.getWatch() ? cnxn : null);
return new GetDataResponse(b, stat);
}
public byte[] getData(String path, Stat stat, Watcher watcher) {
return dataTree.getData(path, stat, watcher);
}
public byte[] getData(String path, Stat stat, Watcher watcher) {
synchronized (n) {
n.copyStat(stat);
if (watcher != null) {
// 这里的 dataWatches 就是 IWatchManager 接口对应的实例
dataWatches.addWatch(path, watcher);
}
data = n.data;
}
updateReadStat(path, data == null ? 0 : data.length);
return data;
}
最终会被放置到 watchTable和 watch2Paths中存储
@Override
public boolean addWatch(String path, Watcher watcher) {
return addWatch(path, watcher, WatcherMode.DEFAULT_WATCHER_MODE);
}
@Override
public synchronized boolean addWatch(String path, Watcher watcher, WatcherMode watcherMode) {
if (isDeadWatcher(watcher)) {
return false;
}
// 从中拿出 Set
Set<Watcher> list = watchTable.get(path);
if (list == null) {
list = new HashSet<>(4);
watchTable.put(path, list);
}
list.add(watcher);
//
Set<String> paths = watch2Paths.get(watcher);
if (paths == null) {
paths = new HashSet<>();
watch2Paths.put(watcher, paths);
}
watcherModeManager.setWatcherMode(watcher, path, watcherMode);
return paths.add(path);
}
Watcher 的触发
NodeDataChange的触发是我们节点的数据内容或者节点的 dataVersion发生改变。
那么我们可以来看看 org.apache.zookeeper.server.DataTree#setData方法
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;
nodes.preChange(path, n);
n.data = data;
n.stat.setMtime(time);
n.stat.setMzxid(zxid);
n.stat.setVersion(version);
n.copyStat(s);
nodes.postChange(path, n);
}
....
....
updateWriteStat(path, dataBytes);
// 调用IWatchManager 的方法
dataWatches.triggerWatch(path, EventType.NodeDataChanged);
return s;
}
@Override
public WatcherOrBitSet triggerWatch(String path, EventType type) {
return triggerWatch(path, type, null);
}
@Override
public WatcherOrBitSet triggerWatch(String path, EventType type, WatcherOrBitSet supress) {
// 封装成 WatchedEvent
WatchedEvent e = new WatchedEvent(type, KeeperState.SyncConnected, path);
Set<Watcher> watchers = new HashSet<>();
PathParentIterator pathParentIterator = getPathParentIterator(path);
synchronized (this) {
for (String localPath : pathParentIterator.asIterable()) {
Set<Watcher> thisWatchers = watchTable.get(localPath);
// 无监听
if (thisWatchers == null || thisWatchers.isEmpty()) {
continue;
}
Iterator<Watcher> iterator = thisWatchers.iterator();
while (iterator.hasNext()) {
Watcher watcher = iterator.next();
WatcherMode watcherMode = watcherModeManager.getWatcherMode(watcher, localPath);
if (watcherMode.isRecursive()) {
} else if (!pathParentIterator.atParentPath()) {
watchers.add(watcher);
if (!watcherMode.isPersistent()) {
// 移除掉
iterator.remove();
Set<String> paths = watch2Paths.get(watcher);
if (paths != null) {
// 从 watch2Paths 中移除掉
paths.remove(localPath);
}
}
}
}
}
}
for (Watcher w : watchers) {
if (supress != null && supress.contains(w)) {
continue;
}
// 调用 process 方法
w.process(e);
}
.....
.....
return new WatcherOrBitSet(watchers);
}
上面已经提及、ServerCnxn实现了 Watcher接口,我们看看 org.apache.zookeeper.server.NIOServerCnxn#process
@Override
public void process(WatchedEvent event) {
// 请求头中的 XID 设置为 -1，上面分析 SendThread.readResponse 的时候提及过
ReplyHeader h = new ReplyHeader(ClientCnxn.NOTIFICATION_XID, -1L, 0);
// WatchedEvent 变为 WatcherEvent
WatcherEvent e = event.getWrapper();
// 给客户端发送通知
sendResponse(h, e, "notification", null, null, ZooDefs.OpCode.error);
}
基本流程
- 封装 WatchedEvent
- 从
watchTable中找到对应的 Watcher，并将watchTable和watch2Paths中相关的 Watcher 和路径清除掉(只能触发一次喔) - 调用
process方法。
客户端回调 Watcher
我们先来认识下 EventThread这个类
继承自 Thread,使用 LinkedBlockingQueue<Object> waitingEvents保存将要处理的事件，然后 ```run`` 方法不断的从队列中获取进行处理。
我们已经知道客户端中由 SendThread#readResponse处理(这段代码也出现在上面的客户端注册 Watcher 的时候)
case NOTIFICATION_XID:
LOG.debug("Got notification session id: 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 {} which is too short for chroot path {}.",
event.getPath(), chrootPath);
}
}
WatchedEvent we = new WatchedEvent(event);
LOG.debug("Got {} for session id 0x{}", we, Long.toHexString(sessionId));
// 加入到事件队列中、由EventThread处理
eventThread.queueEvent(we);
return;
加入到 ```waitingEvents`` 队列中
public void queueEvent(WatchedEvent event) {
queueEvent(event, null);
}
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) {
// 从 clientWatchManager 中获取对应的 Watcher，也会从对应的 Map中移除 Watcher
// 一样是一次性的
watchers = watcher.materialize(event.getState(), event.getType(), event.getPath());
} else {
watchers = new HashSet<Watcher>();
watchers.addAll(materializedWatchers);
}
WatcherSetEventPair pair = new WatcherSetEventPair(watchers, event);
// 加入到 waitingEvents 中、等待 run 方法 拿出来处理
waitingEvents.add(pair);
}
run 方法
public void run() {
try {
isRunning = true;
while (true) {
Object event = waitingEvents.take();
if (event == eventOfDeath) {
wasKilled = true;
} else {
processEvent(event);
}
......
......
}}
}
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 {
// 调用 process 方法，串行同步处理
watcher.process(pair.event);
} catch (Throwable t) {
LOG.error("Error while calling watcher.", t);
}
}
} }
.......
.......
}
总结
Watcher 的特性
- 一次性:无论是客户端还是服务端、一旦 Watcher 触发、都会将其从存储中移除。
- 客户端串行执行: 串行同步执行的过程、千万不要因为一个 Watcher 而影响整个客户端回调 Watcher
- 轻量: WatchedEvent 是通知机制中最小的通知单元，只包含了三部分的内容: 通知状态、事件类型、节点路径。而不会将节点的内容以通知的方式告知客户端、而是需要客户端收到通知之后、主动去服务端获取数据。
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