關於zookeeper數據 set與get 一致性問題

 public void run() {
        try {
            while (true) {
                Request request = submittedRequests.take();  // 這裏 使用隊列來存儲來自
                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!");
    }

上面的代碼自zk的源碼 org.apache.zookeeper.server.PrepRequestProcessor類 這裏個類負責處理 zk客戶端發送的請求

 

好了從方法裏面可以看出 zk服務端處理消息使用的隊列，並且是一個單線程，所以會一次只能一條消息 接着看下來的代碼

  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;
.........}

這個是pRequest的具體處理邏輯 我們在看下 當請求是set的時候處理邏輯

   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 真正的執行方法應該是這個 addChangeRecord(nodeRecord);  讓我們在往下看

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

結果很明顯了 這裏還使用了syn關鍵字  

同時我還在本地試了一下，開兩個cmd窗口 先用一個窗口進行 set操作  然後讓其在對應的代碼這個方法裏面 停下來

然後在另一個窗口裏面get 相同的節點， 結果兩個cmd窗口都卡住了，過一會都自動關閉了。

 

所以綜上所述：

 當請求的順序合理的時候是不會發生後數據不一致的現象的，即不會再某一刻 某一個節點 出現兩個不一樣的值。

最後附上zk的源碼  這個是我整理過的 裏面只有對應的java代碼 沒有其他與源碼的無關 的文件夾 在一些代碼中添加了一些註釋

如果還有懷疑可以在本地試一下

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