ZooKeeper（四）：从TCP数据流到zk内部处理包的转换

　　通过前面几篇文章，我们可以从整体上看到zk是如何处理网络数据的宏观架构。

　　本文我们从细节着手，看一下一个tcp的包是如何转换到内部的数据流处理的。

一、监听用户请求socket

　　基于NIO的端口监听，获取tcp数据流。

        // org.apache.zookeeper.server.NIOServerCnxnFactory.AcceptThread#AcceptThread
        public AcceptThread(ServerSocketChannel ss, InetSocketAddress addr, Set<SelectorThread> selectorThreads) throws IOException {
            super("NIOServerCxnFactory.AcceptThread:" + addr);
            this.acceptSocket = ss;
            // 只监听 OP_ACCEPT 事件
            this.acceptKey = acceptSocket.register(selector, SelectionKey.OP_ACCEPT);
            this.selectorThreads = Collections.unmodifiableList(new ArrayList<SelectorThread>(selectorThreads));
            selectorIterator = this.selectorThreads.iterator();
        }
        // org.apache.zookeeper.server.NIOServerCnxnFactory.AcceptThread#run
        public void run() {
            try {
                // 死循环一直监听
                while (!stopped && !acceptSocket.socket().isClosed()) {
                    try {
                        select();
                    } catch (RuntimeException e) {
                        LOG.warn("Ignoring unexpected runtime exception", e);
                    } catch (Exception e) {
                        LOG.warn("Ignoring unexpected exception", e);
                    }
                }
            } finally {
                closeSelector();
                // This will wake up the selector threads, and tell the
                // worker thread pool to begin shutdown.
                if (!reconfiguring) {
                    NIOServerCnxnFactory.this.stop();
                }
                LOG.info("accept thread exitted run method");
            }
        }
        // org.apache.zookeeper.server.NIOServerCnxnFactory.AcceptThread#select
        private void select() {
            try {
                // nio select
                selector.select();

                Iterator<SelectionKey> selectedKeys = selector.selectedKeys().iterator();
                while (!stopped && selectedKeys.hasNext()) {
                    SelectionKey key = selectedKeys.next();
                    selectedKeys.remove();

                    if (!key.isValid()) {
                        continue;
                    }
                    if (key.isAcceptable()) {
                        // 有可用数据，接收数据
                        if (!doAccept()) {
                            // If unable to pull a new connection off the accept
                            // queue, pause accepting to give us time to free
                            // up file descriptors and so the accept thread
                            // doesn't spin in a tight loop.
                            pauseAccept(10);
                        }
                    } else {
                        LOG.warn("Unexpected ops in accept select {}", key.readyOps());
                    }
                }
            } catch (IOException e) {
                LOG.warn("Ignoring IOException while selecting", e);
            }
        }

        // org.apache.zookeeper.server.NIOServerCnxnFactory.AcceptThread#doAccept
        /**
         * Accept new socket connections. Enforces maximum number of connections
         * per client IP address. Round-robin assigns to selector thread for
         * handling. Returns whether pulled a connection off the accept queue
         * or not. If encounters an error attempts to fast close the socket.
         *
         * @return whether was able to accept a connection or not
         */
        private boolean doAccept() {
            boolean accepted = false;
            SocketChannel sc = null;
            try {
                sc = acceptSocket.accept();
                accepted = true;
                InetAddress ia = sc.socket().getInetAddress();
                int cnxncount = getClientCnxnCount(ia);

                if (maxClientCnxns > 0 && cnxncount >= maxClientCnxns) {
                    throw new IOException("Too many connections from " + ia + " - max is " + maxClientCnxns);
                }

                LOG.debug("Accepted socket connection from {}", sc.socket().getRemoteSocketAddress());

                sc.configureBlocking(false);

                // Round-robin assign this connection to a selector thread
                // 选择一个 selector线程出来进行接收连接请求，然后由该线程负责后续处理
                if (!selectorIterator.hasNext()) {
                    selectorIterator = selectorThreads.iterator();
                }
                SelectorThread selectorThread = selectorIterator.next();
                if (!selectorThread.addAcceptedConnection(sc)) {
                    throw new IOException("Unable to add connection to selector queue"
                                          + (stopped ? " (shutdown in progress)" : ""));
                }
                acceptErrorLogger.flush();
            } catch (IOException e) {
                // accept, maxClientCnxns, configureBlocking
                ServerMetrics.getMetrics().CONNECTION_REJECTED.add(1);
                acceptErrorLogger.rateLimitLog("Error accepting new connection: " + e.getMessage());
                fastCloseSock(sc);
            }
            return accepted;
        }

    }
        // org.apache.zookeeper.server.NIOServerCnxnFactory.SelectorThread#addAcceptedConnection
        /**
         * Place new accepted connection onto a queue for adding. Do this
         * so only the selector thread modifies what keys are registered
         * with the selector.
         */
        public boolean addAcceptedConnection(SocketChannel accepted) {
            // 添加到 SelectorThread 的队列中，然后即返回继续监听
            if (stopped || !acceptedQueue.offer(accepted)) {
                return false;
            }
            // 唤醒 selector, 使其开始工作
            wakeupSelector();
            return true;
        }

二、从连接中解析数据

　　接上一个nio提交过来的连接后，由 SelectorThread 进行数据的读写。

        // org.apache.zookeeper.server.NIOServerCnxnFactory.SelectorThread
        /**
         * The main loop for the thread selects() on the connections and
         * dispatches ready I/O work requests, then registers all pending
         * newly accepted connections and updates any interest ops on the
         * queue.
         */
        public void run() {
            try {
                // 死循环一直处理
                while (!stopped) {
                    try {
                        select();
                        processAcceptedConnections();
                        processInterestOpsUpdateRequests();
                    } catch (RuntimeException e) {
                        LOG.warn("Ignoring unexpected runtime exception", e);
                    } catch (Exception e) {
                        LOG.warn("Ignoring unexpected exception", e);
                    }
                }

                // Close connections still pending on the selector. Any others
                // with in-flight work, let drain out of the work queue.
                for (SelectionKey key : selector.keys()) {
                    NIOServerCnxn cnxn = (NIOServerCnxn) key.attachment();
                    if (cnxn.isSelectable()) {
                        cnxn.close(ServerCnxn.DisconnectReason.SERVER_SHUTDOWN);
                    }
                    cleanupSelectionKey(key);
                }
                SocketChannel accepted;
                while ((accepted = acceptedQueue.poll()) != null) {
                    fastCloseSock(accepted);
                }
                updateQueue.clear();
            } finally {
                closeSelector();
                // This will wake up the accept thread and the other selector
                // threads, and tell the worker thread pool to begin shutdown.
                NIOServerCnxnFactory.this.stop();
                LOG.info("selector thread exitted run method");
            }
        }

        private void select() {
            try {
                // 该selector为本线程私有，所以其操作将是线程安全的
                // 都是由 this.selector = Selector.open(); 获得
                // 由 AcceptThread 线程调用 wakeupSelector, selector.wakeup(); 唤醒处理
                selector.select();

                Set<SelectionKey> selected = selector.selectedKeys();
                ArrayList<SelectionKey> selectedList = new ArrayList<SelectionKey>(selected);
                Collections.shuffle(selectedList);
                Iterator<SelectionKey> selectedKeys = selectedList.iterator();
                while (!stopped && selectedKeys.hasNext()) {
                    SelectionKey key = selectedKeys.next();
                    selected.remove(key);

                    if (!key.isValid()) {
                        cleanupSelectionKey(key);
                        continue;
                    }
                    if (key.isReadable() || key.isWritable()) {
                        // 确认进入读写事件的处理流程
                        handleIO(key);
                    } else {
                        LOG.warn("Unexpected ops in select {}", key.readyOps());
                    }
                }
            } catch (IOException e) {
                LOG.warn("Ignoring IOException while selecting", e);
            }
        }

        /**
         * Schedule I/O for processing on the connection associated with
         * the given SelectionKey. If a worker thread pool is not being used,
         * I/O is run directly by this thread.
         */
        private void handleIO(SelectionKey key) {
            IOWorkRequest workRequest = new IOWorkRequest(this, key);
            NIOServerCnxn cnxn = (NIOServerCnxn) key.attachment();

            // Stop selecting this key while processing on its
            // connection
            cnxn.disableSelectable();
            key.interestOps(0);
            touchCnxn(cnxn);
            // 封装IOWorkRequest 添加到工作线程池中去
            workerPool.schedule(workRequest);
        }
    // org.apache.zookeeper.server.WorkerService#schedule    
    /**
     * Schedule work to be done.  If a worker thread pool is not being
     * used, work is done directly by this thread. This schedule API is
     * for use with non-assignable WorkerServices. For assignable
     * WorkerServices, will always run on the first thread.
     */
    public void schedule(WorkRequest workRequest) {
        schedule(workRequest, 0);
    }
    /**
     * Schedule work to be done by the thread assigned to this id. Thread
     * assignment is a single mod operation on the number of threads.  If a
     * worker thread pool is not being used, work is done directly by
     * this thread.
     */
    public void schedule(WorkRequest workRequest, long id) {
        if (stopped) {
            workRequest.cleanup();
            return;
        }

        ScheduledWorkRequest scheduledWorkRequest = new ScheduledWorkRequest(workRequest);

        // If we have a worker thread pool, use that; otherwise, do the work
        // directly.
        int size = workers.size();
        if (size > 0) {
            try {
                // make sure to map negative ids as well to [0, size-1]
                // 此处的 id=0, 所以只会有一个线程池使用，提交到 scheduledWorkRequest 去处理
                int workerNum = ((int) (id % size) + size) % size;
                ExecutorService worker = workers.get(workerNum);
                worker.execute(scheduledWorkRequest);
            } catch (RejectedExecutionException e) {
                LOG.warn("ExecutorService rejected execution", e);
                workRequest.cleanup();
            }
        } else {
            // When there is no worker thread pool, do the work directly
            // and wait for its completion
            scheduledWorkRequest.run();
        }
    }

三、获取指定通道的具体数据 WorkerService

　　接上一个线程submit过来的数据!

    // org.apache.zookeeper.server.WorkerService#schedule
    /**
     * Schedule work to be done by the thread assigned to this id. Thread
     * assignment is a single mod operation on the number of threads.  If a
     * worker thread pool is not being used, work is done directly by
     * this thread.
     */
    public void schedule(WorkRequest workRequest, long id) {
        if (stopped) {
            workRequest.cleanup();
            return;
        }

        ScheduledWorkRequest scheduledWorkRequest = new ScheduledWorkRequest(workRequest);

        // If we have a worker thread pool, use that; otherwise, do the work
        // directly.
        int size = workers.size();
        if (size > 0) {
            try {
                // make sure to map negative ids as well to [0, size-1]
                int workerNum = ((int) (id % size) + size) % size;
                ExecutorService worker = workers.get(workerNum);
                worker.execute(scheduledWorkRequest);
            } catch (RejectedExecutionException e) {
                LOG.warn("ExecutorService rejected execution", e);
                workRequest.cleanup();
            }
        } else {
            // When there is no worker thread pool, do the work directly
            // and wait for its completion
            scheduledWorkRequest.run();
        }
    }

    // org.apache.zookeeper.server.WorkerService.ScheduledWorkRequest
    private class ScheduledWorkRequest implements Runnable {

        private final WorkRequest workRequest;

        ScheduledWorkRequest(WorkRequest workRequest) {
            this.workRequest = workRequest;
        }

        @Override
        public void run() {
            try {
                // Check if stopped while request was on queue
                if (stopped) {
                    workRequest.cleanup();
                    return;
                }
                // 调用 WorkRequest
                workRequest.doWork();
            } catch (Exception e) {
                LOG.warn("Unexpected exception", e);
                workRequest.cleanup();
            }
        }

    }
        // org.apache.zookeeper.server.NIOServerCnxnFactory.IOWorkRequest#doWork
        public void doWork() throws InterruptedException {
            if (!key.isValid()) {
                selectorThread.cleanupSelectionKey(key);
                return;
            }

            // 针对客户端请求，进行读写处理
            if (key.isReadable() || key.isWritable()) {
                // 将key交给 对应的 事务处理服务处理
                cnxn.doIO(key);

                // Check if we shutdown or doIO() closed this connection
                if (stopped) {
                    cnxn.close(ServerCnxn.DisconnectReason.SERVER_SHUTDOWN);
                    return;
                }
                if (!key.isValid()) {
                    selectorThread.cleanupSelectionKey(key);
                    return;
                }
                // 关联到对应的channnel, 以便可以进行响应
                touchCnxn(cnxn);
            }

            // Mark this connection as once again ready for selection
            cnxn.enableSelectable();
            // Push an update request on the queue to resume selecting
            // on the current set of interest ops, which may have changed
            // as a result of the I/O operations we just performed.
            if (!selectorThread.addInterestOpsUpdateRequest(key)) {
                cnxn.close(ServerCnxn.DisconnectReason.CONNECTION_MODE_CHANGED);
            }
        }

    
    // 处理读写数据
    // org.apache.zookeeper.server.NIOServerCnxn#doIO
    void doIO(SelectionKey k) throws InterruptedException {
        try {
            if (!isSocketOpen()) {
                LOG.warn("trying to do i/o on a null socket for session: 0x{}", Long.toHexString(sessionId));

                return;
            }
            if (k.isReadable()) {
                int rc = sock.read(incomingBuffer);
                if (rc < 0) {
                    handleFailedRead();
                }
                if (incomingBuffer.remaining() == 0) {
                    boolean isPayload;
                    if (incomingBuffer == lenBuffer) { // start of next request
                        incomingBuffer.flip();
                        isPayload = readLength(k);
                        incomingBuffer.clear();
                    } else {
                        // continuation
                        isPayload = true;
                    }
                    if (isPayload) { // not the case for 4letterword
                        // 数据准备完成，读取消息体，顺便进行数据传递到下游
                        readPayload();
                    } else {
                        // four letter words take care
                        // need not do anything else
                        return;
                    }
                }
            }
            if (k.isWritable()) {
                handleWrite(k);

                if (!initialized && !getReadInterest() && !getWriteInterest()) {
                    throw new CloseRequestException("responded to info probe", DisconnectReason.INFO_PROBE);
                }
            }
        } catch (CancelledKeyException e) {
            LOG.warn("CancelledKeyException causing close of session: 0x{}", Long.toHexString(sessionId));

            LOG.debug("CancelledKeyException stack trace", e);

            close(DisconnectReason.CANCELLED_KEY_EXCEPTION);
        } catch (CloseRequestException e) {
            // expecting close to log session closure
            close();
        } catch (EndOfStreamException e) {
            LOG.warn("Unexpected exception", e);
            // expecting close to log session closure
            close(e.getReason());
        } catch (ClientCnxnLimitException e) {
            // Common case exception, print at debug level
            ServerMetrics.getMetrics().CONNECTION_REJECTED.add(1);
            LOG.warn("Closing session 0x{}", Long.toHexString(sessionId), e);
            close(DisconnectReason.CLIENT_CNX_LIMIT);
        } catch (IOException e) {
            LOG.warn("Close of session 0x{}", Long.toHexString(sessionId), e);
            close(DisconnectReason.IO_EXCEPTION);
        }
    }

    // org.apache.zookeeper.server.NIOServerCnxn#readPayload
    /** Read the request payload (everything following the length prefix) */
    private void readPayload() throws IOException, InterruptedException, ClientCnxnLimitException {
        if (incomingBuffer.remaining() != 0) { // have we read length bytes?
            int rc = sock.read(incomingBuffer); // sock is non-blocking, so ok
            if (rc < 0) {
                handleFailedRead();
            }
        }

        // 多次确认数据是否已到齐
        if (incomingBuffer.remaining() == 0) { // have we read length bytes?
            incomingBuffer.flip();
            packetReceived(4 + incomingBuffer.remaining());
            if (!initialized) {
                readConnectRequest();
            } else {
                // 对初始化后的数据处理，直接读取 buffers
                readRequest();
            }
            lenBuffer.clear();
            incomingBuffer = lenBuffer;
        }
    }
    
    // org.apache.zookeeper.server.NIOServerCnxn#readRequest
    private void readRequest() throws IOException {
        // 交给 zkServer 处理，其中包含了各种准备好的处理链配置
        zkServer.processPacket(this, incomingBuffer);
    }
    
    // org.apache.zookeeper.server.ZooKeeperServer#processPacket
    public void processPacket(ServerCnxn cnxn, ByteBuffer incomingBuffer) throws IOException {
        // We have the request, now process and setup for next
        InputStream bais = new ByteBufferInputStream(incomingBuffer);
        BinaryInputArchive bia = BinaryInputArchive.getArchive(bais);
        RequestHeader h = new RequestHeader();
        h.deserialize(bia, "header");

        // Need to increase the outstanding request count first, otherwise
        // there might be a race condition that it enabled recv after
        // processing request and then disabled when check throttling.
        //
        // Be aware that we're actually checking the global outstanding
        // request before this request.
        //
        // It's fine if the IOException thrown before we decrease the count
        // in cnxn, since it will close the cnxn anyway.
        cnxn.incrOutstandingAndCheckThrottle(h);

        // Through the magic of byte buffers, txn will not be
        // pointing
        // to the start of the txn
        incomingBuffer = incomingBuffer.slice();
        // 根据请求类型，简单分类处理
        if (h.getType() == OpCode.auth) {
            LOG.info("got auth packet {}", cnxn.getRemoteSocketAddress());
            AuthPacket authPacket = new AuthPacket();
            ByteBufferInputStream.byteBuffer2Record(incomingBuffer, authPacket);
            String scheme = authPacket.getScheme();
            ServerAuthenticationProvider ap = ProviderRegistry.getServerProvider(scheme);
            Code authReturn = KeeperException.Code.AUTHFAILED;
            if (ap != null) {
                try {
                    // handleAuthentication may close the connection, to allow the client to choose
                    // a different server to connect to.
                    authReturn = ap.handleAuthentication(
                        new ServerAuthenticationProvider.ServerObjs(this, cnxn),
                        authPacket.getAuth());
                } catch (RuntimeException e) {
                    LOG.warn("Caught runtime exception from AuthenticationProvider: {}", scheme, e);
                    authReturn = KeeperException.Code.AUTHFAILED;
                }
            }
            if (authReturn == KeeperException.Code.OK) {
                LOG.debug("Authentication succeeded for scheme: {}", scheme);
                LOG.info("auth success {}", cnxn.getRemoteSocketAddress());
                ReplyHeader rh = new ReplyHeader(h.getXid(), 0, KeeperException.Code.OK.intValue());
                cnxn.sendResponse(rh, null, null);
            } else {
                if (ap == null) {
                    LOG.warn(
                        "No authentication provider for scheme: {} has {}",
                        scheme,
                        ProviderRegistry.listProviders());
                } else {
                    LOG.warn("Authentication failed for scheme: {}", scheme);
                }
                // send a response...
                ReplyHeader rh = new ReplyHeader(h.getXid(), 0, KeeperException.Code.AUTHFAILED.intValue());
                cnxn.sendResponse(rh, null, null);
                // ... and close connection
                cnxn.sendBuffer(ServerCnxnFactory.closeConn);
                cnxn.disableRecv();
            }
            return;
        } else if (h.getType() == OpCode.sasl) {
            processSasl(incomingBuffer, cnxn, h);
        } else {
            // 通用请求处理
            if (shouldRequireClientSaslAuth() && !hasCnxSASLAuthenticated(cnxn)) {
                ReplyHeader replyHeader = new ReplyHeader(h.getXid(), 0, Code.SESSIONCLOSEDREQUIRESASLAUTH.intValue());
                cnxn.sendResponse(replyHeader, null, "response");
                cnxn.sendCloseSession();
                cnxn.disableRecv();
            } else {
                // 封装 Request, 至此, 内部数据模型已生成, sessionId, xid, buffers...
                Request si = new Request(cnxn, cnxn.getSessionId(), h.getXid(), h.getType(), incomingBuffer, cnxn.getAuthInfo());
                int length = incomingBuffer.limit();
                if (isLargeRequest(length)) {
                    // checkRequestSize will throw IOException if request is rejected
                    checkRequestSizeWhenMessageReceived(length);
                    si.setLargeRequestSize(length);
                }
                si.setOwner(ServerCnxn.me);
                // 提交到请求队列中
                submitRequest(si);
            }
        }
    }
    
    public void submitRequest(Request si) {
        enqueueRequest(si);
    }
    
    public void enqueueRequest(Request si) {
        if (requestThrottler == null) {
            synchronized (this) {
                try {
                    // Since all requests are passed to the request
                    // processor it should wait for setting up the request
                    // processor chain. The state will be updated to RUNNING
                    // after the setup.
                    while (state == State.INITIAL) {
                        wait(1000);
                    }
                } catch (InterruptedException e) {
                    LOG.warn("Unexpected interruption", e);
                }
                if (requestThrottler == null) {
                    throw new RuntimeException("Not started");
                }
            }
        }
        // 提交请求给限流器
        // requestThrottler = new RequestThrottler(this); 节流器处理完成后，最终仍会提交给当前 zkServer 处理
        requestThrottler.submitRequest(si);
    }

    // org.apache.zookeeper.server.RequestThrottler#submitRequest
    public void submitRequest(Request request) {
        // 如果已停止，则删除队列
        if (stopping) {
            LOG.debug("Shutdown in progress. Request cannot be processed");
            dropRequest(request);
        } else {
            // LinkedBlockingQueue 入队，最终由该线程去异步处理
            submittedRequests.add(request);
        }
    }

四、第一个队列处理请求 - 限流器 RequestThrottler

　　通过上面的tcp数据转换，将其转换为了 Request 实例，提交到队列了。接下来这个队列将被 RequestThrottler 处理，它的作用是判定是否超出了设置的最大请求数，如果超出，则作等待处理，防止下游无法应对。

    // org.apache.zookeeper.server.RequestThrottler#run
    @Override
    public void run() {
        try {
            // 死循环一直处理
            while (true) {
                if (killed) {
                    break;
                }
                // 阻塞式获取，即只要数据被提交，就会被立即处理
                Request request = submittedRequests.take();
                if (Request.requestOfDeath == request) {
                    break;
                }

                if (request.mustDrop()) {
                    continue;
                }

                // Throttling is disabled when maxRequests = 0
                if (maxRequests > 0) {
                    while (!killed) {
                        if (dropStaleRequests && request.isStale()) {
                            // Note: this will close the connection
                            dropRequest(request);
                            ServerMetrics.getMetrics().STALE_REQUESTS_DROPPED.add(1);
                            request = null;
                            break;
                        }
                        // 只要没达到最大限制，直接通过
                        if (zks.getInProcess() < maxRequests) {
                            break;
                        }
                        throttleSleep(stallTime);
                    }
                }

                if (killed) {
                    break;
                }

                // A dropped stale request will be null
                if (request != null) {
                    if (request.isStale()) {
                        ServerMetrics.getMetrics().STALE_REQUESTS.add(1);
                    }
                    // 验证通过后，提交给 zkServer 处理
                    zks.submitRequestNow(request);
                }
            }
        } catch (InterruptedException e) {
            LOG.error("Unexpected interruption", e);
        }
        int dropped = drainQueue();
        LOG.info("RequestThrottler shutdown. Dropped {} requests", dropped);
    }

    // org.apache.zookeeper.server.ZooKeeperServer#submitRequestNow
    public void submitRequestNow(Request si) {
        // 确保处理器链路已生成
        if (firstProcessor == null) {
            synchronized (this) {
                try {
                    // Since all requests are passed to the request
                    // processor it should wait for setting up the request
                    // processor chain. The state will be updated to RUNNING
                    // after the setup.
                    while (state == State.INITIAL) {
                        wait(1000);
                    }
                } catch (InterruptedException e) {
                    LOG.warn("Unexpected interruption", e);
                }
                if (firstProcessor == null || state != State.RUNNING) {
                    throw new RuntimeException("Not started");
                }
            }
        }
        try {
            touch(si.cnxn);
            // 验证数据类型是否支持
            boolean validpacket = Request.isValid(si.type);
            if (validpacket) {
                setLocalSessionFlag(si);
                // 调用第一个处理器进行处理了，进入正式流程，如 LeaderRequestProcessor
                firstProcessor.processRequest(si);
                if (si.cnxn != null) {
                    incInProcess();
                }
            } else {
                LOG.warn("Received packet at server of unknown type {}", si.type);
                // Update request accounting/throttling limits
                requestFinished(si);
                new UnimplementedRequestProcessor().processRequest(si);
            }
        } catch (MissingSessionException e) {
            LOG.debug("Dropping request.", e);
            // Update request accounting/throttling limits
            requestFinished(si);
        } catch (RequestProcessorException e) {
            LOG.error("Unable to process request", e);
            // Update request accounting/throttling limits
            requestFinished(si);
        }
    }

　　至此，请求数据初始化就完成了，就可以好好去分析处理器的处理过程了。

五、 firstProcessor 示例一号传球手 --LeaderRequestProcessor

    // org.apache.zookeeper.server.quorum.LeaderRequestProcessor#processRequest
    @Override
    public void processRequest(Request request) throws RequestProcessorException {
        // Screen quorum requests against ACLs first
        if (!lzks.authWriteRequest(request)) {
            return;
        }

        // Check if this is a local session and we are trying to create
        // an ephemeral node, in which case we upgrade the session
        Request upgradeRequest = null;
        try {
            upgradeRequest = lzks.checkUpgradeSession(request);
        } catch (KeeperException ke) {
            if (request.getHdr() != null) {
                LOG.debug("Updating header");
                request.getHdr().setType(OpCode.error);
                request.setTxn(new ErrorTxn(ke.code().intValue()));
            }
            request.setException(ke);
            LOG.warn("Error creating upgrade request", ke);
        } catch (IOException ie) {
            LOG.error("Unexpected error in upgrade", ie);
        }
        if (upgradeRequest != null) {
            nextProcessor.processRequest(upgradeRequest);
        }
        // 直接转给下一处理器方法，本身未做过多逻辑
        nextProcessor.processRequest(request);
    }
    // org.apache.zookeeper.server.PrepRequestProcessor#processRequest
    public void processRequest(Request request) {
        request.prepQueueStartTime = Time.currentElapsedTime();
        // LinkedBlockingQueue, 添加到 PrepRequestProcessor 的队列中，进入真正的处理链逻辑
        submittedRequests.add(request);
        // 添加监控信息统计
        ServerMetrics.getMetrics().PREP_PROCESSOR_QUEUED.add(1);
    }
    

六、firstProcessor 示例一号传球手 --FollowerRequestProcessor

    // org.apache.zookeeper.server.quorum.FollowerRequestProcessor#processRequest
    public void processRequest(Request request) {
        processRequest(request, true);
    }

    void processRequest(Request request, boolean checkForUpgrade) {
        if (!finished) {
            if (checkForUpgrade) {
                // Before sending the request, check if the request requires a
                // global session and what we have is a local session. If so do
                // an upgrade.
                Request upgradeRequest = null;
                try {
                    upgradeRequest = zks.checkUpgradeSession(request);
                } catch (KeeperException ke) {
                    if (request.getHdr() != null) {
                        request.getHdr().setType(OpCode.error);
                        request.setTxn(new ErrorTxn(ke.code().intValue()));
                    }
                    request.setException(ke);
                    LOG.warn("Error creating upgrade request", ke);
                } catch (IOException ie) {
                    LOG.error("Unexpected error in upgrade", ie);
                }
                if (upgradeRequest != null) {
                    queuedRequests.add(upgradeRequest);
                }
            }
            // FollowerRequestProcessor 有一定的处理逻辑，将其添加到自有队列 queuedRequests 中
            queuedRequests.add(request);
        }
    }
    // org.apache.zookeeper.server.quorum.FollowerRequestProcessor#run
    @Override
    public void run() {
        try {
            while (!finished) {
                Request request = queuedRequests.take();
                if (LOG.isTraceEnabled()) {
                    ZooTrace.logRequest(LOG, ZooTrace.CLIENT_REQUEST_TRACE_MASK, 'F', request, "");
                }
                if (request == Request.requestOfDeath) {
                    break;
                }

                // Screen quorum requests against ACLs first
                if (!zks.authWriteRequest(request)) {
                    continue;
                }

                // We want to queue the request to be processed before we submit
                // the request to the leader so that we are ready to receive
                // the response
                // 此处将会提交给 CommitProcessor, 进入 Follower 本地处理
                nextProcessor.processRequest(request);

                // We now ship the request to the leader. As with all
                // other quorum operations, sync also follows this code
                // path, but different from others, we need to keep track
                // of the sync operations this follower has pending, so we
                // add it to pendingSyncs.
                // 针对写请求，将其转发给 Leader 处理
                switch (request.type) {
                case OpCode.sync:
                    zks.pendingSyncs.add(request);
                    zks.getFollower().request(request);
                    break;
                case OpCode.create:
                case OpCode.create2:
                case OpCode.createTTL:
                case OpCode.createContainer:
                case OpCode.delete:
                case OpCode.deleteContainer:
                case OpCode.setData:
                case OpCode.reconfig:
                case OpCode.setACL:
                case OpCode.multi:
                case OpCode.check:
                    zks.getFollower().request(request);
                    break;
                case OpCode.createSession:
                case OpCode.closeSession:
                    // Don't forward local sessions to the leader.
                    if (!request.isLocalSession()) {
                        zks.getFollower().request(request);
                    }
                    break;
                }
            }
        } catch (Exception e) {
            handleException(this.getName(), e);
        }
        LOG.info("FollowerRequestProcessor exited loop!");
    }
    // org.apache.zookeeper.server.quorum.Learner#request
    /**
     * send a request packet to the leader
     *
     * @param request
     *                the request from the client
     * @throws IOException
     */
    void request(Request request) throws IOException {
        ByteArrayOutputStream baos = new ByteArrayOutputStream();
        DataOutputStream oa = new DataOutputStream(baos);
        oa.writeLong(request.sessionId);
        oa.writeInt(request.cxid);
        oa.writeInt(request.type);
        if (request.request != null) {
            request.request.rewind();
            int len = request.request.remaining();
            byte[] b = new byte[len];
            request.request.get(b);
            request.request.rewind();
            oa.write(b);
        }
        oa.close();
        // 转发请求给 Leader, 详情待续
        QuorumPacket qp = new QuorumPacket(Leader.REQUEST, -1, baos.toByteArray(), request.authInfo);
        writePacket(qp, true);
    }

七、firstProcessor 示例一号传球手 --ObserverRequestProcessor

　　ObserverRequestProcessor 与 Follower 类似.

    // org.apache.zookeeper.server.quorum.ObserverRequestProcessor#processRequest
    /**
     * Simply queue the request, which will be processed in FIFO order.
     */
    public void processRequest(Request request) {
        if (!finished) {
            Request upgradeRequest = null;
            try {
                upgradeRequest = zks.checkUpgradeSession(request);
            } catch (KeeperException ke) {
                if (request.getHdr() != null) {
                    request.getHdr().setType(OpCode.error);
                    request.setTxn(new ErrorTxn(ke.code().intValue()));
                }
                request.setException(ke);
                LOG.info("Error creating upgrade request", ke);
            } catch (IOException ie) {
                LOG.error("Unexpected error in upgrade", ie);
            }
            if (upgradeRequest != null) {
                queuedRequests.add(upgradeRequest);
            }
            // 添加到自身的工作队列中，稍后处理
            queuedRequests.add(request);
        }
    }
    // org.apache.zookeeper.server.quorum.ObserverRequestProcessor#run
    @Override
    public void run() {
        try {
            while (!finished) {
                Request request = queuedRequests.take();
                if (LOG.isTraceEnabled()) {
                    ZooTrace.logRequest(LOG, ZooTrace.CLIENT_REQUEST_TRACE_MASK, 'F', request, "");
                }
                if (request == Request.requestOfDeath) {
                    break;
                }

                // Screen quorum requests against ACLs first
                if (!zks.authWriteRequest(request)) {
                    continue;
                }

                // We want to queue the request to be processed before we submit
                // the request to the leader so that we are ready to receive
                // the response
                nextProcessor.processRequest(request);

                // We now ship the request to the leader. As with all
                // other quorum operations, sync also follows this code
                // path, but different from others, we need to keep track
                // of the sync operations this Observer has pending, so we
                // add it to pendingSyncs.
                switch (request.type) {
                case OpCode.sync:
                    zks.pendingSyncs.add(request);
                    zks.getObserver().request(request);
                    break;
                case OpCode.create:
                case OpCode.create2:
                case OpCode.createTTL:
                case OpCode.createContainer:
                case OpCode.delete:
                case OpCode.deleteContainer:
                case OpCode.setData:
                case OpCode.reconfig:
                case OpCode.setACL:
                case OpCode.multi:
                case OpCode.check:
                    zks.getObserver().request(request);
                    break;
                case OpCode.createSession:
                case OpCode.closeSession:
                    // Don't forward local sessions to the leader.
                    if (!request.isLocalSession()) {
                        zks.getObserver().request(request);
                    }
                    break;
                }
            }
        } catch (Exception e) {
            handleException(this.getName(), e);
        }
        LOG.info("ObserverRequestProcessor exited loop!");
    }

　　OK！至此，我们已经完整地看到了一个 tcp 数据流如何转变为 zk 的数据包，并准备进入 正式的业务处理流程。

整个过程总结下：

　　1. 一个 Accept 线程接入客户端请求;
　　2. 一组 Selector 线程轮询处理读写请求;
　　3. 线程池处理, 一组 WorkerService 进行提交读写数据;
　　4. 由 ZooKeeperServer 进行 Request包的封装;
　　5. 经限流器把关 RequestThrottler, hold过多的请求, 然后再回高 ZooKeeperServer 入队;
　　6. ZooKeeperServer 找到 firstProcessor, 将请求 Request 传递到处理链中, 准备过程完毕;
　　7. 各个 firstProcessor 的处理逻辑不一，其中 Leader 仅是将 Request 传递到处理链中, 而 Follower/Observer 则要自己做一些额外的工作;

　　整个过程，是在N个线程之间不停地传递处理，各自负责各自的领域点。而理清zk的工作方式，难点就在于其多线程的配合过程。