关于zookeeper数据 set与get 一致性问题

 public void run() {
        try {
            while (true) {
                Request request = submittedRequests.take();  // 这里 使用队列来存储来自
                long traceMask = ZooTrace.CLIENT_REQUEST_TRACE_MASK;
                if (request.type == OpCode.ping) {
                    traceMask = ZooTrace.CLIENT_PING_TRACE_MASK;
                }
                if (LOG.isTraceEnabled()) {
                    ZooTrace.logRequest(LOG, traceMask, 'P', request, "");
                }
                if (Request.requestOfDeath == request) {
                    break;
                }
                pRequest(request);
            }
        } catch (RequestProcessorException e) {
            if (e.getCause() instanceof XidRolloverException) {
                LOG.info(e.getCause().getMessage());
            }
            handleException(this.getName(), e);
        } catch (Exception e) {
            handleException(this.getName(), e);
        }
        LOG.info("PrepRequestProcessor exited loop!");
    }

上面的代码自zk的源码 org.apache.zookeeper.server.PrepRequestProcessor类 这里个类负责处理 zk客户端发送的请求

 

好了从方法里面可以看出 zk服务端处理消息使用的队列，并且是一个单线程，所以会一次只能一条消息 接着看下来的代码

  protected void pRequest(Request request) throws RequestProcessorException {
        // LOG.info("Prep>>> cxid = " + request.cxid + " type = " +
        // request.type + " id = 0x" + Long.toHexString(request.sessionId));
        request.hdr = null;
        request.txn = null;
        
        try {
            switch (request.type) {
                case OpCode.create:
                CreateRequest createRequest = new CreateRequest();
                pRequest2Txn(request.type, zks.getNextZxid(), request, createRequest, true);
                break;
            case OpCode.delete:
                DeleteRequest deleteRequest = new DeleteRequest();               
                pRequest2Txn(request.type, zks.getNextZxid(), request, deleteRequest, true);
                break;
            case OpCode.setData:
                SetDataRequest setDataRequest = new SetDataRequest();                
                pRequest2Txn(request.type, zks.getNextZxid(), request, setDataRequest, true);
                break;
            case OpCode.setACL:
                SetACLRequest setAclRequest = new SetACLRequest();                
                pRequest2Txn(request.type, zks.getNextZxid(), request, setAclRequest, true);
                break;
            case OpCode.check:
                CheckVersionRequest checkRequest = new CheckVersionRequest();              
                pRequest2Txn(request.type, zks.getNextZxid(), request, checkRequest, true);
                break;
            case OpCode.multi:
                MultiTransactionRecord multiRequest = new MultiTransactionRecord();
                try {
                    ByteBufferInputStream.byteBuffer2Record(request.request, multiRequest);
                } catch(IOException e) {
                    request.hdr =  new TxnHeader(request.sessionId, request.cxid, zks.getNextZxid(),
                            Time.currentWallTime(), OpCode.multi);
                    throw e;
.........}

这个是pRequest的具体处理逻辑 我们在看下 当请求是set的时候处理逻辑

   protected void pRequest2Txn(int type, long zxid, Request request, Record record, boolean deserialize)
        throws KeeperException, IOException, RequestProcessorException
    {
 ........
 case OpCode.setData:
                zks.sessionTracker.checkSession(request.sessionId, request.getOwner());
                SetDataRequest setDataRequest = (SetDataRequest)record;
                if(deserialize)
                    ByteBufferInputStream.byteBuffer2Record(request.request, setDataRequest);
                path = setDataRequest.getPath();
                validatePath(path, request.sessionId);
                nodeRecord = getRecordForPath(path);
                checkACL(zks, nodeRecord.acl, ZooDefs.Perms.WRITE,
                        request.authInfo);
                version = setDataRequest.getVersion();
                int currentVersion = nodeRecord.stat.getVersion();
                if (version != -1 && version != currentVersion) {
                    throw new KeeperException.BadVersionException(path);
                }
                version = currentVersion + 1;
                request.txn = new SetDataTxn(path, setDataRequest.getData(), version);
                nodeRecord = nodeRecord.duplicate(request.hdr.getZxid());
                nodeRecord.stat.setVersion(version);
                addChangeRecord(nodeRecord);
                break;
...........
}

好了 真正的执行方法应该是这个 addChangeRecord(nodeRecord);  让我们在往下看

   void addChangeRecord(ChangeRecord c) {
        synchronized (zks.outstandingChanges) {
            zks.outstandingChanges.add(c);
            zks.outstandingChangesForPath.put(c.path, c);
        }
    }

结果很明显了 这里还使用了syn关键字  

同时我还在本地试了一下，开两个cmd窗口 先用一个窗口进行 set操作  然后让其在对应的代码这个方法里面 停下来

然后在另一个窗口里面get 相同的节点， 结果两个cmd窗口都卡住了，过一会都自动关闭了。

 

所以综上所述：

 当请求的顺序合理的时候是不会发生后数据不一致的现象的，即不会再某一刻 某一个节点 出现两个不一样的值。

最后附上zk的源码  这个是我整理过的 里面只有对应的java代码 没有其他与源码的无关 的文件夹 在一些代码中添加了一些注释

如果还有怀疑可以在本地试一下

https://gitee.com/zhaoyu123/zookeeper-java-source-real.git