前一篇介绍了zookeeper的client和server之间session是如何建立的。在DataMonitor的例子中，DataMonitor通过exists异步接口和server端交互，本文将介绍exists操作是如何完成。
dataMonitor开始exist操作
public void exists(final String path, Watcher watcher,
StatCallback cb, Object ctx)
{
......
//exist请求头
RequestHeader h = new RequestHeader();
h.setType(ZooDefs.OpCode.exists);
//请求体
ExistsRequest request = new ExistsRequest();
request.setPath(serverPath);
request.setWatch(watcher != null);
SetDataResponse response = new SetDataResponse();
//添加到发送队列
cnxn.queuePacket(h, new ReplyHeader(), request, response, cb,
clientPath, serverPath, ctx, wcb);
}
添加过程
Packet queuePacket(RequestHeader h, ReplyHeader r, Record request,
Record response, AsyncCallback cb, String clientPath,
String serverPath, Object ctx, WatchRegistration watchRegistration)
{
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.
synchronized (outgoingQueue) {
//转换成Packet
packet = new Packet(h, r, request, response, watchRegistration);
packet.cb = cb;
packet.ctx = ctx;
packet.clientPath = clientPath;
packet.serverPath = serverPath;
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;
}
//添加到发送队列
outgoingQueue.add(packet);
}
}
//唤醒下Selector，快点处理
sendThread.getClientCnxnSocket().wakeupCnxn();
return packet;
}
接下来还是回到SendThread的发送过程，之前Session建立时已经分析过，这里有一点要注意下：
//重要的业务请求，需要设置事务id
if ((p.requestHeader != null) &&
(p.requestHeader.getType() != OpCode.ping) &&
(p.requestHeader.getType() != OpCode.auth)) {
p.requestHeader.setXid(cnxn.getXid());
}
接下来Server端IO线程拿到请求，处理，过程和之前session建立时一样，就不赘述了。变化的是后续的处理链。
PrepRequestProcessor预处理
//All the rest don't need to create a Txn - just verify session
//读请求，不需要创建事务，只是检查了下session是否还在，此时事务头和事务体都是null
case OpCode.sync:
case OpCode.exists:
case OpCode.getData:
case OpCode.getACL:
case OpCode.getChildren:
case OpCode.getChildren2:
case OpCode.ping:
case OpCode.setWatches:
zks.sessionTracker.checkSession(request.sessionId,
request.getOwner());
break;
SyncRequestProcessor处理逻辑之前已经分析过了，这里就挑重点说一下
//试图将其写log，由于ExistsRequest并不是一个事务型请求，所以这里直接返回false，也就是说ExistsRequest不会被记录到log文件中 zks.getZKDatabase().append(si)接下来FinalRequestProcessor处理，由于不是事务型请求，省了很多步骤，直接进入switch处理：
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case OpCode.exists: {
lastOp = "EXIS";
// TODO we need to figure out the security requirement for this!
ExistsRequest existsRequest = new ExistsRequest();
//反序列化
ByteBufferInputStream.byteBuffer2Record(request.request,
existsRequest);
String path = existsRequest.getPath();
if (path.indexOf('\0') != -1) {
throw new KeeperException.BadArgumentsException();
}
//拿对应node的状态，并设置是否watch
Stat stat = zks.getZKDatabase().statNode(path, existsRequest
.getWatch() ? cnxn : null);
//结果
rsp = new ExistsResponse(stat);
break;
}
......
//当前处理zxid
long lastZxid = zks.getZKDatabase().getDataTreeLastProcessedZxid();
//构造相应头
ReplyHeader hdr =
new ReplyHeader(request.cxid, lastZxid, err.intValue());
zks.serverStats().updateLatency(request.createTime);
cnxn.updateStatsForResponse(request.cxid, lastZxid, lastOp,
request.createTime, System.currentTimeMillis());
try {
//写回响应
cnxn.sendResponse(hdr, rsp, "response");
statNode过程
//此处的watcher就是对应ServerCnxn，代表是哪个client，server端需要notify的时候，直接往对应ServerCnxn写数据即可
public Stat statNode(String path, Watcher watcher)
throws KeeperException.NoNodeException {
Stat stat = new Stat();
DataNode n = nodes.get(path);
if (watcher != null) {
//对于exists请求，需要监听data变化事件，添加watcher
dataWatches.addWatch(path, watcher);
}
if (n == null) {
throw new KeeperException.NoNodeException();
}
synchronized (n) {
n.copyStat(stat);
return stat;
}
}
好了，以上server端就完成了ExistsRequest的处理了。接下来client端SendThread收到ExistsResponse进行处理
if (sockKey.isReadable()) {
int rc = sock.read(incomingBuffer);
......
else if (!initialized) {
readConnectResult();
enableRead();
if (findSendablePacket(outgoingQueue,
cnxn.sendThread.clientTunneledAuthenticationInProgress()) != null) {
// Since SASL authentication has completed (if client is configured to do so),
// outgoing packets waiting in the outgoingQueue can now be sent.
enableWrite();
}
lenBuffer.clear();
incomingBuffer = lenBuffer;
updateLastHeard();
initialized = true;
}
//此时client session已经建立，init完成
else {
sendThread.readResponse(incomingBuffer);
lenBuffer.clear();
incomingBuffer = lenBuffer;
updateLastHeard();
}
}
}
具体读取：
void readResponse(ByteBuffer incomingBuffer) throws IOException {
ByteBufferInputStream bbis = new ByteBufferInputStream(
incomingBuffer);
BinaryInputArchive bbia = BinaryInputArchive.getArchive(bbis);
ReplyHeader replyHdr = new ReplyHeader();
//先读响应头，先对特殊的xid进行处理
replyHdr.deserialize(bbia, "header");
......
//从头拿之前入队的ExistsRequest请求Packet，由于client和server都是单线程处理，多队列处理，所以认为全局有序
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 {
//检查一下是否一样
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());
//更新client端的最新zxid
if (replyHdr.getZxid() > 0) {
lastZxid = replyHdr.getZxid();
}
//反序列化响应
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 {
//处理packet，主要是注册watcher，回调，触发事件
finishPacket(packet);
}
}
finishPacket过程
private void finishPacket(Packet p) {
//注册watcher，如果是exists请求，则注册到dataWatches中
if (p.watchRegistration != null) {
p.watchRegistration.register(p.replyHeader.getErr());
}
//如果是同步接口，则唤醒等待的业务线程
if (p.cb == null) {
synchronized (p) {
p.finished = true;
p.notifyAll();
}
}
//如果是异步请求，则发送异步事件
else {
p.finished = true;
eventThread.queuePacket(p);
}
}
EventThread端
else if (p.response instanceof ExistsResponse
|| p.response instanceof SetDataResponse
|| p.response instanceof SetACLResponse) {
StatCallback cb = (StatCallback) p.cb;
//如果成功，增加node stat的回调参数
if (rc == 0) {
if (p.response instanceof ExistsResponse) {
cb.processResult(rc, clientPath, p.ctx,
((ExistsResponse) p.response)
.getStat());
} else if (p.response instanceof SetDataResponse) {
cb.processResult(rc, clientPath, p.ctx,
((SetDataResponse) p.response)
.getStat());
} else if (p.response instanceof SetACLResponse) {
cb.processResult(rc, clientPath, p.ctx,
((SetACLResponse) p.response)
.getStat());
}
}
//如果响应失败，stat为null
else {
cb.processResult(rc, clientPath, p.ctx, null);
}
}
在DataMonitor例子中，它本身就是一个StatCallback
public void processResult(int rc, String path, Object ctx, Stat stat) {
boolean exists;
switch (rc) {
case Code.Ok:
exists = true;
break;
case Code.NoNode:
exists = false;
break;
case Code.SessionExpired:
case Code.NoAuth:
dead = true;
listener.closing(rc);
return;
default:
// Retry errors
zk.exists(znode, true, this, null);
return;
}
Exists过程大致就是上面描述的，主要注意点：
1.客户端Request发送完之后会进入Pending队列，等待响应之后拿出来继续处理
2.同步接口是使用Packet.wait()实现的
3.server端exists操作不是事务型的操作，不会写入log
4.server端的watcher就是一个客户端连接ServerCxcn，代表一个客户端，notify的时候直接往连接里写数据即可