背景:
10月29号，系统切换的时候，发生了master信息的变更，有个监听master信息的系统未收到zookeeper的通知。
分析：
原代码如下所示, 监听系统中用以下代码来监听path中数据的变更。当数据发生变更的时候，会回调process方法，然后处理相应的业务。（我们使用的是curator的jar包）
client = CuratorFrameworkFactory.newClient("*:2181", new ExponentialBackoffRetry(1000, 3));
client.start();
try {
client.getData().usingWatcher(this).inBackground().forPath("/id/master");
} catch (Exception e) {
throw new RuntimeException(e);
}问题点：
以上代码再第一次发生变更的时候，可以接收到zookeeper的变更通知，但是在zookeeper server会将watcher删除，在以后的变更中将不会继续通知。
源码分析：
当调用forPath函数时将会调用GetDataBuilderImpl的forPath方法，这个方法中会异步处理操作
@Override
public byte[] forPath(String path) throws Exception
{
path = client.fixForNamespace(path);
byte[] responseData = null;
if ( backgrounding.inBackground() )
{
client.processBackgroundOperation(new OperationAndData(this, path, backgrounding.getCallback(), null, backgrounding.getContext()), null);
}
else
{
responseData = pathInForeground(path);
}
return responseData;
}
异步调用GetDataBuilderImpl的performBackgroundOperation方法。然后会调用appcahe原生zookeeper的jar包。
@Override
public void performBackgroundOperation(final OperationAndData operationAndData) throws Exception
{
if ( watching.isWatched() )
{
client.getZooKeeper().getData(operationAndData.getData(), true, callback, backgrounding.getContext());
}
else
{
client.getZooKeeper().getData(operationAndData.getData(), watching.getWatcher(), callback, backgrounding.getContext());
}
}
然后会调用zookeeper.java中的getData函数，发送请求到zookeeper server. setWatch为true
public void getData(final String path, Watcher watcher,
DataCallback cb, Object ctx)
{
final String serverPath = prependChroot(clientPath);
RequestHeader h = new RequestHeader();
h.setType(ZooDefs.OpCode.getData);
GetDataRequest request = new GetDataRequest();
request.setPath(serverPath);
request.setWatch(watcher != null);
GetDataResponse response = new GetDataResponse();
cnxn.queuePacket(h, new ReplyHeader(), request, response, cb,
clientPath, serverPath, ctx, wcb);
}zookeeper server接收到客户端的getData请求时，会调用FinalRequestProcessor类中的以下代码处理
case OpCode.getData: {
lastOp = "GETD";
GetDataRequest getDataRequest = new GetDataRequest();
ByteBufferInputStream.byteBuffer2Record(request.request,
getDataRequest);
Stat stat = new Stat();
byte b[] = zks.getZKDatabase().getData(getDataRequest.getPath(), stat,
getDataRequest.getWatch() ? cnxn : null);
rsp = new GetDataResponse(b, stat);
break;
}接下去会调用DataTree中的getData函数，这个函数中会将监听的节点加入到WatcherManager中，这样我们就添加了对某个路径的监听
public byte[] getData(String path, Stat stat, Watcher watcher)
throws KeeperException.NoNodeException {
DataNode n = nodes.get(path);
synchronized (n) {
n.copyStat(stat);
if (watcher != null) {
dataWatches.addWatch(path, watcher);
}
return n.data;
}
}
当另一个客户端调用setData方法时，Zookeeper客户端将会调用DataTree类中的setData方法。这个方法将会变更数据，然后调用triggerWatch方法，通知变更。
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);
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;
if((lastPrefix = getMaxPrefixWithQuota(path)) != null) {
this.updateBytes(lastPrefix, (data == null ? 0 : data.length)
- (lastdata == null ? 0 : lastdata.length));
}
dataWatches.triggerWatch(path, EventType.NodeDataChanged);
return s;
}然后调用WatcherManager中的triggerWatch方法，方法中将会删除监听。然后调用watcher的process方法
public Set triggerWatch(String path, EventType type, Set supress) {
WatchedEvent e = new WatchedEvent(type,
KeeperState.SyncConnected, path);
HashSet watchers;
synchronized (this) {
watchers = watchTable.remove(path);
if (watchers == null || watchers.isEmpty()) {
return null;
}
for (Watcher w : watchers) {
HashSet 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;
} 调用NIOServerCnxn类中的process方法，发送通知到客户端。
synchronized public void process(WatchedEvent event) {
ReplyHeader h = new ReplyHeader(-1, -1L, 0);
if (LOG.isTraceEnabled()) {
ZooTrace.logTraceMessage(LOG, ZooTrace.EVENT_DELIVERY_TRACE_MASK,
"Deliver event " + event + " to 0x"
+ Long.toHexString(this.sessionId)
+ " through " + this);
}
// Convert WatchedEvent to a type that can be sent over the wire
WatcherEvent e = event.getWrapper();
sendResponse(h, e, "notification");
}
客户端接收到notification时，会调用ClientCnxn类的下面代码处理。queueEvent方法中会调用watcher.materialize方法确定watcherEvent的处理类
if (replyHdr.getXid() == -1) {
WatcherEvent event = new WatcherEvent();
event.deserialize(bbia, "response");
WatchedEvent we = new WatchedEvent(event);
eventThread.queueEvent( we );
return;
}
public void queueEvent(WatchedEvent event) {
// materialize the watchers based on the event
WatcherSetEventPair pair = new WatcherSetEventPair(
watcher.materialize(event.getState(), event.getType(),
event.getPath()),
event);
// queue the pair (watch set & event) for later processing
waitingEvents.add(pair);
}调用zookeeper的materialize方法获取watcher对象，并且重watcher列表中删除。
@Override
public Set materialize(Watcher.Event.KeeperState state,
Watcher.Event.EventType type,
String clientPath)
{
Set result = new HashSet();
switch (type) {
case None:
result.add(defaultWatcher);
boolean clear = ClientCnxn.getDisableAutoResetWatch() &&
state != Watcher.Event.KeeperState.SyncConnected;
synchronized(dataWatches) {
for(Set ws: dataWatches.values()) {
result.addAll(ws);
}
if (clear) {
dataWatches.clear();
}
}
synchronized(existWatches) {
for(Set ws: existWatches.values()) {
result.addAll(ws);
}
if (clear) {
existWatches.clear();
}
}
synchronized(childWatches) {
for(Set ws: childWatches.values()) {
result.addAll(ws);
}
if (clear) {
childWatches.clear();
}
}
return result;
} 总结：
从上面源码，我们可以看出，zookeeper的每次只能监听一次变更。
解决：
curator为我们封装了重复监听的类NodeCache.java. 它是怎么实现的呢？
NodeCache nc = new NodeCache(zkclient.getClient(), path);
nc.start(true);start方法会获取初始值，然后调用reset方法
public void start(boolean buildInitial) throws Exception
{
Preconditions.checkState(state.compareAndSet(State.LATENT, State.STARTED), "Cannot be started more than once");
ensurePath.ensure(client.getZookeeperClient());
client.getConnectionStateListenable().addListener(connectionStateListener);
if ( buildInitial )
{
internalRebuild();
}
reset();
}reset方法会调用zookeeper的forpath方法，并用包装的watcher类
private void reset() throws Exception
{
if ( (state.get() == State.STARTED) && isConnected.get() )
{
client.checkExists().usingWatcher(watcher).inBackground(backgroundCallback).forPath(path);
}
}当配置发生变化时，watcher类中会重新调用reset方法，再次监听
private final CuratorWatcher watcher = new CuratorWatcher()
{
@Override
public void process(WatchedEvent event) throws Exception
{
reset();
}
};