/*
 * Licensed to the Apache Software Foundation (ASF) under one
 * or more contributor license agreements.  See the NOTICE file
 * distributed with this work for additional information
 * regarding copyright ownership.  The ASF licenses this file
 * to you under the Apache License, Version 2.0 (the
 * "License"); you may not use this file except in compliance
 * with the License.  You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package org.apache.zookeeper.server.quorum;

import static org.apache.zookeeper.common.NetUtils.formatInetAddr;

import java.io.BufferedInputStream;
import java.io.BufferedOutputStream;
import java.io.Closeable;
import java.io.DataInputStream;
import java.io.DataOutputStream;
import java.io.IOException;
import java.net.InetAddress;
import java.net.InetSocketAddress;
import java.net.ServerSocket;
import java.net.Socket;
import java.net.SocketException;
import java.net.SocketTimeoutException;
import java.net.UnknownHostException;
import java.nio.BufferUnderflowException;
import java.nio.ByteBuffer;
import java.nio.channels.UnresolvedAddressException;
import java.time.Duration;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.Collection;
import java.util.Collections;
import java.util.Enumeration;
import java.util.HashSet;
import java.util.List;
import java.util.Map;
import java.util.Set;
import java.util.concurrent.BlockingQueue;
import java.util.concurrent.ConcurrentHashMap;
import java.util.concurrent.CountDownLatch;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.SynchronousQueue;
import java.util.concurrent.ThreadFactory;
import java.util.concurrent.ThreadPoolExecutor;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.atomic.AtomicBoolean;
import java.util.concurrent.atomic.AtomicInteger;
import java.util.concurrent.atomic.AtomicLong;
import java.util.function.Supplier;
import java.util.stream.Collectors;
import javax.net.ssl.SSLSocket;
import javax.swing.table.TableModel;

import org.apache.zookeeper.common.NetUtils;
import org.apache.zookeeper.common.X509Exception;
import org.apache.zookeeper.server.ExitCode;
import org.apache.zookeeper.server.ZooKeeperThread;
import org.apache.zookeeper.server.quorum.QuorumPeerConfig.ConfigException;
import org.apache.zookeeper.server.quorum.auth.QuorumAuthLearner;
import org.apache.zookeeper.server.quorum.auth.QuorumAuthServer;
import org.apache.zookeeper.server.quorum.flexible.QuorumVerifier;
import org.apache.zookeeper.server.util.ConfigUtils;
import org.apache.zookeeper.util.CircularBlockingQueue;
import org.apache.zookeeper.util.ServiceUtils;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;


/**
 * This class implements a connection manager for leader election using TCP. It
 * maintains one connection for every pair of servers. The tricky part is to
 * guarantee that there is exactly one connection for every pair of servers that
 * are operating correctly and that can communicate over the network.
 *
 * If two servers try to start a connection concurrently, then the connection
 * manager uses a very simple tie-breaking mechanism to decide which connection
 * to drop based on the IP addressed of the two parties.
 *
 * For every peer, the manager maintains a queue of messages to send. If the
 * connection to any particular peer drops, then the sender thread puts the
 * message back on the list. As this implementation currently uses a queue
 * implementation to maintain messages to send to another peer, we add the
 * message to the tail of the queue, thus changing the order of messages.
 * Although this is not a problem for the leader election, it could be a problem
 * when consolidating peer communication. This is to be verified, though.
 */

public class QuorumCnxManager {

    private static final Logger LOG = LoggerFactory.getLogger(QuorumCnxManager.class);

    /*
     * Maximum capacity of thread queues
     */
    static final int RECV_CAPACITY = 100;
    // Initialized to 1 to prevent sending
    // stale notifications to peers
    static final int SEND_CAPACITY = 1;

    // TODO_MA 注释： 数据包最大长度
    static final int PACKETMAXSIZE = 1024 * 512;

    /*
     * Negative counter for observer server ids.
     */

    private AtomicLong observerCounter = new AtomicLong(-1);

    /*
     * Protocol identifier used among peers (must be a negative number for backward compatibility reasons)
     */
    // the following protocol version was sent in every connection initiation message since ZOOKEEPER-107 released in 3.5.0
    public static final long PROTOCOL_VERSION_V1 = -65536L;
    // ZOOKEEPER-3188 introduced multiple addresses in the connection initiation message, released in 3.6.0
    public static final long PROTOCOL_VERSION_V2 = -65535L;

    /*
     * Max buffer size to be read from the network.
     */
    public static final int maxBuffer = 2048;

    /*
     * Connection time out value in milliseconds
     */

    private int cnxTO = 5000;

    final QuorumPeer self;

    /*
     * Local IP address
     */
    final long mySid;
    final int socketTimeout;
    final Map<Long, QuorumPeer.QuorumServer> view;
    final boolean listenOnAllIPs;
    private ThreadPoolExecutor connectionExecutor;
    private final Set<Long> inprogressConnections = Collections.synchronizedSet(new HashSet<>());
    private QuorumAuthServer authServer;
    private QuorumAuthLearner authLearner;
    private boolean quorumSaslAuthEnabled;
    /*
     * Counter to count connection processing threads.
     */
    private AtomicInteger connectionThreadCnt = new AtomicInteger(0);

    /*
     * Mapping from Peer to Thread number
     */
    final ConcurrentHashMap<Long, SendWorker> senderWorkerMap;
    final ConcurrentHashMap<Long, BlockingQueue<ByteBuffer>> queueSendMap;
    final ConcurrentHashMap<Long, ByteBuffer> lastMessageSent;

    /*
     * Reception queue
     */
    public final BlockingQueue<Message> recvQueue;

    /*
     * Shutdown flag
     */

    volatile boolean shutdown = false;

    /*
     * Listener thread
     */
    public final Listener listener;

    /*
     * Counter to count worker threads
     */
    private AtomicInteger threadCnt = new AtomicInteger(0);

    /*
     * Socket options for TCP keepalive
     */
    private final boolean tcpKeepAlive = Boolean.getBoolean("zookeeper.tcpKeepAlive");


    /*
     * Socket factory, allowing the injection of custom socket implementations for testing
     */
    static final Supplier<Socket> DEFAULT_SOCKET_FACTORY = () -> new Socket();
    private static Supplier<Socket> SOCKET_FACTORY = DEFAULT_SOCKET_FACTORY;

    static void setSocketFactory(Supplier<Socket> factory) {
        SOCKET_FACTORY = factory;
    }


    public static class Message {

        Message(ByteBuffer buffer, long sid) {
            this.buffer = buffer;
            this.sid = sid;
        }

        ByteBuffer buffer;
        long sid;

    }

    /*
     * This class parses the initial identification sent out by peers with their
     * sid & hostname.
     */
    public static class InitialMessage {

        public Long sid;
        public List<InetSocketAddress> electionAddr;

        InitialMessage(Long sid, List<InetSocketAddress> addresses) {
            this.sid = sid;
            this.electionAddr = addresses;
        }

        @SuppressWarnings("serial")
        public static class InitialMessageException extends Exception {

            InitialMessageException(String message, Object... args) {
                super(String.format(message, args));
            }

        }

        public static InitialMessage parse(Long protocolVersion,
                                           DataInputStream din) throws InitialMessageException, IOException {
            Long sid;

            if (protocolVersion != PROTOCOL_VERSION_V1 && protocolVersion != PROTOCOL_VERSION_V2) {
                throw new InitialMessageException("Got unrecognized protocol version %s", protocolVersion);
            }

            // TODO_MA 注释： 读取 myid
            sid = din.readLong();

            // TODO_MA 注释： 读取 address length
            int remaining = din.readInt();
            if (remaining <= 0 || remaining > maxBuffer) {
                throw new InitialMessageException("Unreasonable buffer length: %s", remaining);
            }


            // TODO_MA 注释： 读取 address
            byte[] b = new byte[remaining];
            int num_read = din.read(b);

            if (num_read != remaining) {
                throw new InitialMessageException("Read only %s bytes out of %s sent by server %s", num_read, remaining,
                        sid);
            }

            // in PROTOCOL_VERSION_V1 we expect to get a single address here represented as a 'host:port' string
            // in PROTOCOL_VERSION_V2 we expect to get multiple addresses like: 'host1:port1|host2:port2|...'
            String[] addressStrings = new String(b).split("\\|");
            List<InetSocketAddress> addresses = new ArrayList<>(addressStrings.length);
            for (String addr : addressStrings) {

                String[] host_port;
                try {
                    host_port = ConfigUtils.getHostAndPort(addr);
                } catch (ConfigException e) {
                    throw new InitialMessageException("Badly formed address: %s", addr);
                }

                if (host_port.length != 2) {
                    throw new InitialMessageException("Badly formed address: %s", addr);
                }

                int port;
                try {
                    port = Integer.parseInt(host_port[1]);
                } catch (NumberFormatException e) {
                    throw new InitialMessageException("Bad port number: %s", host_port[1]);
                } catch (ArrayIndexOutOfBoundsException e) {
                    throw new InitialMessageException("No port number in: %s", addr);
                }
                if (!isWildcardAddress(host_port[0])) {
                    addresses.add(new InetSocketAddress(host_port[0], port));
                }
            }

            return new InitialMessage(sid, addresses);
        }

        /**
         * Returns true if the specified hostname is a wildcard address,
         * like 0.0.0.0 for IPv4 or :: for IPv6
         *
         * (the function is package-private to be visible for testing)
         */
        static boolean isWildcardAddress(final String hostname) {
            try {
                return InetAddress.getByName(hostname).isAnyLocalAddress();
            } catch (UnknownHostException e) {
                // if we can not resolve, it can not be a wildcard address
                return false;
            }
        }

        @Override
        public String toString() {
            return "InitialMessage{sid=" + sid + ", electionAddr=" + electionAddr + '}';
        }
    }

    public QuorumCnxManager(QuorumPeer self, final long mySid, Map<Long, QuorumPeer.QuorumServer> view,
                            QuorumAuthServer authServer, QuorumAuthLearner authLearner, int socketTimeout, boolean listenOnAllIPs,
                            int quorumCnxnThreadsSize, boolean quorumSaslAuthEnabled) {

        /*************************************************
         * TODO_MA 马中华 https://blog.csdn.net/zhongqi2513
         *  注释：
         */
        this.recvQueue = new CircularBlockingQueue<>(RECV_CAPACITY);

        /*************************************************
         * TODO_MA 马中华 https://blog.csdn.net/zhongqi2513
         *  注释：
         */
        this.queueSendMap = new ConcurrentHashMap<>();

        /*************************************************
         * TODO_MA 马中华 https://blog.csdn.net/zhongqi2513
         *  注释：
         */
        this.senderWorkerMap = new ConcurrentHashMap<>();
        this.lastMessageSent = new ConcurrentHashMap<>();

        String cnxToValue = System.getProperty("zookeeper.cnxTimeout");
        if (cnxToValue != null) {
            this.cnxTO = Integer.parseInt(cnxToValue);
        }
        this.self = self;
        this.mySid = mySid;
        this.socketTimeout = socketTimeout;
        this.view = view;
        this.listenOnAllIPs = listenOnAllIPs;
        this.authServer = authServer;
        this.authLearner = authLearner;
        this.quorumSaslAuthEnabled = quorumSaslAuthEnabled;

        // TODO_MA 注释： 启动 3 个线程用来完成选票交换的事儿，相当于 BIO 客户端
        initializeConnectionExecutor(mySid, quorumCnxnThreadsSize);
        // TODO_MA 注释： 原来的实现：对方节点有多少个，则有多少个客户端
        // TODO_MA 注释： 此时做了优化：提供一个线程池来完成这个工作：初始化线程数量 = 3， 最大线程数量是 20

        /*************************************************
         * TODO_MA 马中华 https://blog.csdn.net/zhongqi2513
         *  注释： 启动监听，监听选举端口
         */
        // Starts listener thread that waits for connection requests
        listener = new Listener();
        listener.setName("QuorumPeerListener");
    }

    // we always use the Connection Executor during connection initiation (to handle connection
    // timeouts), and optionally use it during receiving connections (as the Quorum SASL authentication
    // can take extra time)
    private void initializeConnectionExecutor(final long mySid, final int quorumCnxnThreadsSize) {
        final AtomicInteger threadIndex = new AtomicInteger(1);
        SecurityManager s = System.getSecurityManager();
        final ThreadGroup group = (s != null) ? s.getThreadGroup() : Thread.currentThread().getThreadGroup();

        final ThreadFactory daemonThFactory = runnable -> new Thread(group, runnable,
                String.format("QuorumConnectionThread-[myid=%d]-%d", mySid, threadIndex.getAndIncrement()));

        /*************************************************
         * TODO_MA 马中华 https://blog.csdn.net/zhongqi2513
         *  注释： 初始化 3 个线程，用来监听 选举链接请求
         */
        this.connectionExecutor = new ThreadPoolExecutor(3, quorumCnxnThreadsSize, 60, TimeUnit.SECONDS,
                new SynchronousQueue<>(), daemonThFactory);
        this.connectionExecutor.allowCoreThreadTimeOut(true);
    }

    /**
     * Invokes initiateConnection for testing purposes
     *
     * @param sid
     */
    public void testInitiateConnection(long sid) {
        LOG.debug("Opening channel to server {}", sid);
        initiateConnection(self.getVotingView().get(sid).electionAddr, sid);
    }

    /**
     * First we create the socket, perform SSL handshake and authentication if needed.
     * Then we perform the initiation protocol.
     * If this server has initiated the connection, then it gives up on the
     * connection if it loses challenge. Otherwise, it keeps the connection.
     */
    public void initiateConnection(final MultipleAddresses electionAddr, final Long sid) {

        // TODO_MA 注释： 初始化一个 BIO 客户端实例 Socket，并且完成和对方的链接
        Socket sock = null;
        try {
            /*************************************************
             * TODO_MA 马中华 https://blog.csdn.net/zhongqi2513
             *  注释： 初始化 Socket 客户端
             */
            LOG.debug("Opening channel to server {}", sid);
            if (self.isSslQuorum()) {
                sock = self.getX509Util().createSSLSocket();
            } else {
                // TODO_MA 注释： 创建一个 Socket 对象
                sock = SOCKET_FACTORY.get();
            }
            setSockOpts(sock);

            // TODO_MA 注释： 连接对方服务器
            // TODO_MA 注释： 服务器 A 发起链接请求
            sock.connect(electionAddr.getReachableOrOne(), cnxTO);

            if (sock instanceof SSLSocket) {
                SSLSocket sslSock = (SSLSocket) sock;

                /*************************************************
                 * TODO_MA 马中华 https://blog.csdn.net/zhongqi2513
                 *  注释： 握手，建立链接
                 */
                sslSock.startHandshake();
                LOG.info("SSL handshake complete with {} - {} - {}", sslSock.getRemoteSocketAddress(),
                        sslSock.getSession().getProtocol(), sslSock.getSession().getCipherSuite());
            }

            LOG.debug("Connected to server {} using election address: {}:{}", sid, sock.getInetAddress(), sock.getPort());
        } catch (X509Exception e) {
            LOG.warn("Cannot open secure channel to {} at election address {}", sid, electionAddr, e);
            closeSocket(sock);
            return;
        } catch (UnresolvedAddressException | IOException e) {
            LOG.warn("Cannot open channel to {} at election address {}", sid, electionAddr, e);
            closeSocket(sock);
            return;
        }

        /*************************************************
         * TODO_MA 马中华 https://blog.csdn.net/zhongqi2513
         *  注释： 开始链接
         */
        try {
            startConnection(sock, sid);
        } catch (IOException e) {
            LOG.error("Exception while connecting, id: {}, addr: {}, closing learner connection", sid,
                    sock.getRemoteSocketAddress(), e);
            closeSocket(sock);
        }
    }

    /**
     * Server will initiate the connection request to its peer server
     * asynchronously via separate connection thread.
     */
    public boolean initiateConnectionAsync(final MultipleAddresses electionAddr, final Long sid) {
        if (!inprogressConnections.add(sid)) {
            // simply return as there is a connection request to
            // server 'sid' already in progress.
            LOG.debug("Connection request to server id: {} is already in progress, so skipping this request", sid);
            return true;
        }
        try {

            /*************************************************
             * TODO_MA 马中华 https://blog.csdn.net/zhongqi2513
             *  注释： 通过一个线程来完成和对方的链接
             */
            connectionExecutor.execute(new QuorumConnectionReqThread(electionAddr, sid));
            connectionThreadCnt.incrementAndGet();

        } catch (Throwable e) {
            // Imp: Safer side catching all type of exceptions and remove 'sid'
            // from inprogress connections. This is to avoid blocking further
            // connection requests from this 'sid' in case of errors.
            inprogressConnections.remove(sid);
            LOG.error("Exception while submitting quorum connection request", e);
            return false;
        }
        return true;
    }

    /**
     * Thread to send connection request to peer server.
     */
    private class QuorumConnectionReqThread extends ZooKeeperThread {
        final MultipleAddresses electionAddr;
        final Long sid;

        QuorumConnectionReqThread(final MultipleAddresses electionAddr, final Long sid) {
            super("QuorumConnectionReqThread-" + sid);
            this.electionAddr = electionAddr;
            this.sid = sid;
        }

        @Override
        public void run() {
            try {

                /*************************************************
                 * TODO_MA 马中华 https://blog.csdn.net/zhongqi2513
                 *  注释： 初始化链接
                 */
                initiateConnection(electionAddr, sid);
            } finally {
                inprogressConnections.remove(sid);
            }
        }

    }

    private boolean startConnection(Socket sock, Long sid) throws IOException {
        DataOutputStream dout = null;
        DataInputStream din = null;
        LOG.debug("startConnection (myId:{} --> sid:{})", self.getId(), sid);
        try {

            // TODO_MA 注释： 此时客户端中，初始化了饿一个 输出流
            // Use BufferedOutputStream to reduce the number of IP packets. This is
            // important for x-DC scenarios.
            BufferedOutputStream buf = new BufferedOutputStream(sock.getOutputStream());
            dout = new DataOutputStream(buf);

            // Sending id and challenge

            /*************************************************
             * TODO_MA 马中华 https://blog.csdn.net/zhongqi2513
             *  注释： 写自己的 信息给 给对方
             *  1、protocolVersion
             *  2、myid
             *  3、ddr_bytes.lengtah
             *  4、addr_bytes（hostname:port）
             */
            // First sending the protocol version (in other words - message type).
            // For backward compatibility reasons we stick to the old protocol version, unless the MultiAddress
            // feature is enabled. During rolling upgrade, we must make sure that all the servers can
            // understand the protocol version we use to avoid multiple partitions. see ZOOKEEPER-3720
            long protocolVersion = self.isMultiAddressEnabled() ? PROTOCOL_VERSION_V2 : PROTOCOL_VERSION_V1;

            // TODO_MA 注释： 写 protocolVersion
            dout.writeLong(protocolVersion);

            // TODO_MA 注释： 写 myid
            dout.writeLong(self.getId());

            // now we send our election address. For the new protocol version, we can send multiple addresses.
            Collection<InetSocketAddress> addressesToSend = protocolVersion == PROTOCOL_VERSION_V2 ? self
                    .getElectionAddress().getAllAddresses() : Arrays.asList(self.getElectionAddress().getOne());

            String addr = addressesToSend.stream().map(NetUtils::formatInetAddr).collect(Collectors.joining("|"));
            byte[] addr_bytes = addr.getBytes();

            // TODO_MA 注释： 写 addr_bytes.length
            dout.writeInt(addr_bytes.length);

            // TODO_MA 注释： 写具体的值： addr_bytes
            dout.write(addr_bytes);
            dout.flush();

            din = new DataInputStream(new BufferedInputStream(sock.getInputStream()));
        } catch (IOException e) {
            LOG.warn("Ignoring exception reading or writing challenge: ", e);
            closeSocket(sock);
            return false;
        }

        // authenticate learner
        QuorumPeer.QuorumServer qps = self.getVotingView().get(sid);
        if (qps != null) {
            // TODO - investigate why reconfig makes qps null.
            authLearner.authenticate(sock, qps.hostname);
        }

        /*************************************************
         * TODO_MA 马中华 https://blog.csdn.net/zhongqi2513
         *  注释： 进行 myid 比对，如果对方的比我大，关闭连接
         *  只有 myid 大的才有资格跟 myid 小的发起链接请求，建立链接。否则就关闭链接
         *  如果关闭了链接，则对方会发起链接请求过来的
         */
        // If lost the challenge, then drop the new connection
        if (sid > self.getId()) {
            LOG.info("Have smaller server identifier, so dropping the connection: (myId:{} --> sid:{})", self.getId(),
                    sid);
            closeSocket(sock);
            // Otherwise proceed with the connection
        }

        // TODO_MA 注释： 如果代码走这儿，证明，我的 myid 比对方大。
        // TODO_MA 注释： 意味着，我并没有关闭链接，则初始化两个线程和一个队列
        else {
            LOG.debug("Have larger server identifier, so keeping the connection: (myId:{} --> sid:{})", self.getId(),
                    sid);
            SendWorker sw = new SendWorker(sock, sid);
            RecvWorker rw = new RecvWorker(sock, din, sid, sw);
            sw.setRecv(rw);
            SendWorker vsw = senderWorkerMap.get(sid);
            if (vsw != null) {
                vsw.finish();
            }
            senderWorkerMap.put(sid, sw);
            queueSendMap.putIfAbsent(sid, new CircularBlockingQueue<>(SEND_CAPACITY));
            sw.start();
            rw.start();
            return true;
        }
        return false;
    }

    /**
     * If this server receives a connection request, then it gives up on the new
     * connection if it wins. Notice that it checks whether it has a connection
     * to this server already or not. If it does, then it sends the smallest
     * possible long value to lose the challenge.
     */
    public void receiveConnection(final Socket sock) {
        DataInputStream din = null;
        try {

            // TODO_MA 注释： 获取一个输入流用于读取数据
            din = new DataInputStream(new BufferedInputStream(sock.getInputStream()));

            LOG.debug("Sync handling of connection request received from: {}", sock.getRemoteSocketAddress());

            /*************************************************
             * TODO_MA 马中华 https://blog.csdn.net/zhongqi2513
             *  注释： 处理链接
             */
            handleConnection(sock, din);
        } catch (IOException e) {
            LOG.error("Exception handling connection, addr: {}, closing server connection",
                    sock.getRemoteSocketAddress());
            LOG.debug("Exception details: ", e);
            closeSocket(sock);
        }
    }

    /**
     * Server receives a connection request and handles it asynchronously via
     * separate thread.
     */
    public void receiveConnectionAsync(final Socket sock) {
        try {
            LOG.debug("Async handling of connection request received from: {}", sock.getRemoteSocketAddress());
            connectionExecutor.execute(new QuorumConnectionReceiverThread(sock));
            connectionThreadCnt.incrementAndGet();
        } catch (Throwable e) {
            LOG.error("Exception handling connection, addr: {}, closing server connection",
                    sock.getRemoteSocketAddress());
            LOG.debug("Exception details: ", e);
            closeSocket(sock);
        }
    }

    /**
     * Thread to receive connection request from peer server.
     */
    private class QuorumConnectionReceiverThread extends ZooKeeperThread {

        private final Socket sock;

        QuorumConnectionReceiverThread(final Socket sock) {
            super("QuorumConnectionReceiverThread-" + sock.getRemoteSocketAddress());
            this.sock = sock;
        }

        @Override
        public void run() {
            receiveConnection(sock);
        }

    }

    /*************************************************
     * TODO_MA 马中华 https://blog.csdn.net/zhongqi2513
     *  注释：  这是指 ServerSocket 接收到一个 Socket 的链接的时候
     */
    private void handleConnection(Socket sock, DataInputStream din) throws IOException {
        Long sid = null, protocolVersion = null;
        MultipleAddresses electionAddr = null;

        /*************************************************
         * TODO_MA 马中华 https://blog.csdn.net/zhongqi2513
         *  注释： 客户端其实会写过来四个东西：
         *  1、protocolVersion
         *  2、myid
         *  3、address length
         *  4、address
         */
        try {

            /*************************************************
             * TODO_MA 马中华 https://blog.csdn.net/zhongqi2513
             *  注释： 读取 protocolVersion
             */
            protocolVersion = din.readLong();

            if (protocolVersion >= 0) { // this is a server id and not a protocol version
                sid = protocolVersion;
            } else {
                try {

                    /*************************************************
                     * TODO_MA 马中华 https://blog.csdn.net/zhongqi2513
                     *  注释： 读取 myid, address length, address 信息
                     */
                    InitialMessage init = InitialMessage.parse(protocolVersion, din);

                    // TODO_MA 注释： 读取到了对方服务器的 myid
                    sid = init.sid;

                    if (!init.electionAddr.isEmpty()) {
                        electionAddr = new MultipleAddresses(init.electionAddr,
                                Duration.ofMillis(self.getMultiAddressReachabilityCheckTimeoutMs()));
                    }
                    LOG.debug("Initial message parsed by {}: {}", self.getId(), init.toString());
                } catch (InitialMessage.InitialMessageException ex) {
                    LOG.error("Initial message parsing error!", ex);
                    closeSocket(sock);
                    return;
                }
            }

            if (sid == QuorumPeer.OBSERVER_ID) {
                /*
                 * Choose identifier at random. We need a value to identify the connection.
                 */
                sid = observerCounter.getAndDecrement();
                LOG.info("Setting arbitrary identifier to observer: {}", sid);
            }
        } catch (IOException e) {
            LOG.warn("Exception reading or writing challenge", e);
            closeSocket(sock);
            return;
        }

        // do authenticating learner
        authServer.authenticate(sock, din);
        //If wins the challenge, then close the new connection.

        /*************************************************
         * TODO_MA 马中华 https://blog.csdn.net/zhongqi2513
         *  注释： 如果对方 myid 小于 我的， 则关闭链接
         *  1、sid 就是 对方 myid
         *  2、self.getId() 就是自己的 myid
         */
        if (sid < self.getId()) {
            /*
             * This replica might still believe that the connection to sid is
             * up, so we have to shut down the workers before trying to open a
             * new connection.
             */
            SendWorker sw = senderWorkerMap.get(sid);
            if (sw != null) {
                sw.finish();
            }

            /*
             * Now we start a new connection
             * // TODO_MA 注释： 关闭链接！
             */
            LOG.debug("Create new connection to server: {}", sid);
            closeSocket(sock);

            /*************************************************
             * TODO_MA 马中华 https://blog.csdn.net/zhongqi2513
             *  注释： 重新建立链接
             */
            if (electionAddr != null) {
                connectOne(sid, electionAddr);
            } else {
                connectOne(sid);
            }
        }

        // TODO_MA 注释： 相等不太可能
        else if (sid == self.getId()) {
            // we saw this case in ZOOKEEPER-2164
            LOG.warn(
                    "We got a connection request from a server with our own ID. " + "This should be either a configuration error, or a bug.");
        }


        // TODO_MA 注释： 到底为止，两个 ZK Server 的 选举链接一定是建立好了
        // TODO_MA 注释： 两个 ZK Server 都要去创建一组工作组件


        /*************************************************
         * TODO_MA 马中华 https://blog.csdn.net/zhongqi2513
         *  注释： 当对方的 myid 大于我的！！！！！！
         *  相当于 客户端的 myid 要大！！ 我靠！！！！
         *  if(sid > self.getId())
         *  1、sid 是客户端的 id
         *  2、self.getId() 是服务端的 ID
         */
        else { // Otherwise start worker threads to receive data.
            SendWorker sw = new SendWorker(sock, sid);
            RecvWorker rw = new RecvWorker(sock, din, sid, sw);
            sw.setRecv(rw);
            SendWorker vsw = senderWorkerMap.get(sid);
            if (vsw != null) {
                vsw.finish();
            }
            senderWorkerMap.put(sid, sw);
            queueSendMap.putIfAbsent(sid, new CircularBlockingQueue<>(SEND_CAPACITY));
            sw.start();
            rw.start();
        }
    }

    /**
     * Processes invoke this message to queue a message to send. Currently,
     * only leader election uses it.
     * // TODO_MA 注释： sid 是对方服务器的 myid
     */
    public void toSend(Long sid, ByteBuffer b) {
        /*
         * If sending message to myself, then simply enqueue it (loopback).
         * // TODO_MA 注释： 发给自己
         */
        if (this.mySid == sid) {
            b.position(0);
            addToRecvQueue(new Message(b.duplicate(), sid));
            /*
             * Otherwise send to the corresponding thread to send.
             */
        }

        /*************************************************
         * TODO_MA 马中华 https://blog.csdn.net/zhongqi2513
         *  注释： 发给对方
         */
        else {
            /*
             * Start a new connection if doesn't have one already.
             * // TODO_MA 注释： 每一个对方服务器，都有一个专门的发送队列，和发送线程
             */
            BlockingQueue<ByteBuffer> bq = queueSendMap
                    .computeIfAbsent(sid, serverId -> new CircularBlockingQueue<>(SEND_CAPACITY));
            addToSendQueue(bq, b);

            /*************************************************
             * TODO_MA 马中华 https://blog.csdn.net/zhongqi2513
             *  注释： 将选票放入对应的发送队列中
             */
            connectOne(sid);
        }
    }

    /**
     * Try to establish a connection to server with id sid using its electionAddr.
     * The function will return quickly and the connection will be established asynchronously.
     *
     * VisibleForTesting.
     *
     * @param sid server id
     * @return boolean success indication
     */
    synchronized boolean connectOne(long sid, MultipleAddresses electionAddr) {
        if (senderWorkerMap.get(sid) != null) {
            LOG.debug("There is a connection already for server {}", sid);
            if (self.isMultiAddressEnabled() && electionAddr.size() > 1 && self.isMultiAddressReachabilityCheckEnabled()) {
                // since ZOOKEEPER-3188 we can use multiple election addresses to reach a server. It is possible, that the
                // one we are using is already dead and we need to clean-up, so when we will create a new connection
                // then we will choose an other one, which is actually reachable
                senderWorkerMap.get(sid).asyncValidateIfSocketIsStillReachable();
            }
            return true;
        }

        /*************************************************
         * TODO_MA 马中华 https://blog.csdn.net/zhongqi2513
         *  注释： 初始化链接并且完成选票发送
         */
        // we are doing connection initiation always asynchronously, since it is possible that
        // the socket connection timeouts or the SSL handshake takes too long and don't want
        // to keep the rest of the connections to wait
        return initiateConnectionAsync(electionAddr, sid);
    }

    /**
     * Try to establish a connection to server with id sid.
     * The function will return quickly and the connection will be established asynchronously.
     *
     * @param sid server id
     */
    synchronized void connectOne(long sid) {
        if (senderWorkerMap.get(sid) != null) {
            LOG.debug("There is a connection already for server {}", sid);
            if (self.isMultiAddressEnabled() && self.isMultiAddressReachabilityCheckEnabled()) {
                // since ZOOKEEPER-3188 we can use multiple election addresses to reach a server. It is possible, that the
                // one we are using is already dead and we need to clean-up, so when we will create a new connection
                // then we will choose an other one, which is actually reachable
                senderWorkerMap.get(sid).asyncValidateIfSocketIsStillReachable();
            }
            return;
        }
        synchronized (self.QV_LOCK) {
            boolean knownId = false;
            // Resolve hostname for the remote server before attempting to
            // connect in case the underlying ip address has changed.
            self.recreateSocketAddresses(sid);
            Map<Long, QuorumPeer.QuorumServer> lastCommittedView = self.getView();
            QuorumVerifier lastSeenQV = self.getLastSeenQuorumVerifier();
            Map<Long, QuorumPeer.QuorumServer> lastProposedView = lastSeenQV.getAllMembers();

            if (lastCommittedView.containsKey(sid)) {
                knownId = true;
                LOG.debug("Server {} knows {} already, it is in the lastCommittedView", self.getId(), sid);

                /*************************************************
                 * TODO_MA 马中华 https://blog.csdn.net/zhongqi2513
                 *  注释： 联系对方服务器，执行选票发送
                 */
                if (connectOne(sid, lastCommittedView.get(sid).electionAddr)) {
                    return;
                }
            }
            if (lastSeenQV != null && lastProposedView.containsKey(sid) && (!knownId || !lastProposedView
                    .get(sid).electionAddr.equals(lastCommittedView.get(sid).electionAddr))) {
                knownId = true;
                LOG.debug("Server {} knows {} already, it is in the lastProposedView", self.getId(), sid);

                if (connectOne(sid, lastProposedView.get(sid).electionAddr)) {
                    return;
                }
            }
            if (!knownId) {
                LOG.warn("Invalid server id: {} ", sid);
            }
        }
    }

    /**
     * Try to establish a connection with each server if one
     * doesn't exist.
     */

    public void connectAll() {
        long sid;
        for (Enumeration<Long> en = queueSendMap.keys(); en.hasMoreElements(); ) {
            sid = en.nextElement();
            connectOne(sid);
        }
    }

    /**
     * Check if all queues are empty, indicating that all messages have been delivered.
     */
    boolean haveDelivered() {
        for (BlockingQueue<ByteBuffer> queue : queueSendMap.values()) {
            final int queueSize = queue.size();
            LOG.debug("Queue size: {}", queueSize);
            if (queueSize == 0) {
                return true;
            }
        }

        return false;
    }

    /**
     * Flag that it is time to wrap up all activities and interrupt the listener.
     */
    public void halt() {
        shutdown = true;
        LOG.debug("Halting listener");
        listener.halt();

        // Wait for the listener to terminate.
        try {
            listener.join();
        } catch (InterruptedException ex) {
            LOG.warn("Got interrupted before joining the listener", ex);
        }
        softHalt();

        // clear data structures used for auth
        if (connectionExecutor != null) {
            connectionExecutor.shutdown();
        }
        inprogressConnections.clear();
        resetConnectionThreadCount();
    }

    /**
     * A soft halt simply finishes workers.
     */
    public void softHalt() {
        for (SendWorker sw : senderWorkerMap.values()) {
            LOG.debug("Server {} is soft-halting sender towards: {}", self.getId(), sw);
            sw.finish();
        }
    }

    /**
     * Helper method to set socket options.
     *
     * @param sock Reference to socket
     */
    private void setSockOpts(Socket sock) throws SocketException {
        sock.setTcpNoDelay(true);
        sock.setKeepAlive(tcpKeepAlive);
        sock.setSoTimeout(this.socketTimeout);
    }

    /**
     * Helper method to close a socket.
     *
     * @param sock Reference to socket
     */
    private void closeSocket(Socket sock) {
        if (sock == null) {
            return;
        }

        try {
            sock.close();
        } catch (IOException ie) {
            LOG.error("Exception while closing", ie);
        }
    }

    /**
     * Return number of worker threads
     */
    public long getThreadCount() {
        return threadCnt.get();
    }

    /**
     * Return number of connection processing threads.
     */
    public long getConnectionThreadCount() {
        return connectionThreadCnt.get();
    }

    /**
     * Reset the value of connection processing threads count to zero.
     */
    private void resetConnectionThreadCount() {
        connectionThreadCnt.set(0);
    }

    /**
     * Thread to listen on some ports
     */
    public class Listener extends ZooKeeperThread {

        private static final String ELECTION_PORT_BIND_RETRY = "zookeeper.electionPortBindRetry";
        private static final int DEFAULT_PORT_BIND_MAX_RETRY = 3;

        private final int portBindMaxRetry;
        private Runnable socketBindErrorHandler = () -> ServiceUtils
                .requestSystemExit(ExitCode.UNABLE_TO_BIND_QUORUM_PORT.getValue());
        private List<ListenerHandler> listenerHandlers;
        private final AtomicBoolean socketException;

        public Listener() {
            // During startup of thread, thread name will be overridden to
            // specific election address
            super("ListenerThread");

            socketException = new AtomicBoolean(false);

            // maximum retry count while trying to bind to election port
            // see ZOOKEEPER-3320 for more details
            final Integer maxRetry = Integer.getInteger(ELECTION_PORT_BIND_RETRY, DEFAULT_PORT_BIND_MAX_RETRY);
            if (maxRetry >= 0) {
                LOG.info("Election port bind maximum retries is {}", maxRetry == 0 ? "infinite" : maxRetry);
                portBindMaxRetry = maxRetry;
            } else {
                LOG.info("'{}' contains invalid value: {}(must be >= 0). Use default value of {} instead.",
                        ELECTION_PORT_BIND_RETRY, maxRetry, DEFAULT_PORT_BIND_MAX_RETRY);
                portBindMaxRetry = DEFAULT_PORT_BIND_MAX_RETRY;
            }
        }

        /**
         * Change socket bind error handler. Used for testing.
         */
        void setSocketBindErrorHandler(Runnable errorHandler) {
            this.socketBindErrorHandler = errorHandler;
        }

        @Override
        public void run() {
            if (!shutdown) {
                // TODO_MA 马中华 注释： 获取地址
                LOG.debug("Listener thread started, myId: {}", self.getId());
                Set<InetSocketAddress> addresses;
                if (self.getQuorumListenOnAllIPs()) {
                    addresses = self.getElectionAddress().getWildcardAddresses();
                } else {
                    addresses = self.getElectionAddress().getAllAddresses();
                }

                CountDownLatch latch = new CountDownLatch(addresses.size());

                /*************************************************
                 * TODO_MA 马中华 https://blog.csdn.net/zhongqi2513
                 *  注释： 初始化 ListenerHandler
                 *  addresses 你就认为是一个地址
                 *  一个地址，就被响应的构建了一个 ListenerHandler 的一个线程
                 */
                listenerHandlers = addresses.stream()
                        .map(address -> new ListenerHandler(address, self.shouldUsePortUnification(), self.isSslQuorum(),
                                latch)).collect(Collectors.toList());

                /*************************************************
                 * TODO_MA 马中华 https://blog.csdn.net/zhongqi2513
                 *  注释： 初始化一个线程池运行这些 ListenerHandler
                 */
                ExecutorService executor = Executors.newFixedThreadPool(addresses.size());
                listenerHandlers.forEach(executor::submit);

                try {
                    latch.await();
                } catch (InterruptedException ie) {
                    LOG.error("Interrupted while sleeping. Ignoring exception", ie);
                } finally {
                    // Clean up for shutdown.
                    for (ListenerHandler handler : listenerHandlers) {
                        try {
                            handler.close();
                        } catch (IOException ie) {
                            // Don't log an error for shutdown.
                            LOG.debug("Error closing server socket", ie);
                        }
                    }
                }
            }

            LOG.info("Leaving listener");
            if (!shutdown) {
                LOG.error(
                        "As I'm leaving the listener thread, I won't be able to participate in leader election any longer: {}",
                        self.getElectionAddress().getAllAddresses().stream().map(NetUtils::formatInetAddr)
                                .collect(Collectors.joining("|")));
                if (socketException.get()) {
                    // After leaving listener thread, the host cannot join the quorum anymore,
                    // this is a severe error that we cannot recover from, so we need to exit
                    socketBindErrorHandler.run();
                }
            }
        }

        /**
         * Halts this listener thread.
         */
        void halt() {
            LOG.debug("Halt called: Trying to close listeners");
            if (listenerHandlers != null) {
                LOG.debug("Closing listener: {}", QuorumCnxManager.this.mySid);
                for (ListenerHandler handler : listenerHandlers) {
                    try {
                        handler.close();
                    } catch (IOException e) {
                        LOG.warn("Exception when shutting down listener: ", e);
                    }
                }
            }
        }

        class ListenerHandler implements Runnable, Closeable {

            // TODO_MA 注释： 这个类就是 BIO 服务端
            private ServerSocket serverSocket;
            private InetSocketAddress address;
            private boolean portUnification;
            private boolean sslQuorum;
            private CountDownLatch latch;

            ListenerHandler(InetSocketAddress address, boolean portUnification, boolean sslQuorum, CountDownLatch latch) {
                this.address = address;
                this.portUnification = portUnification;
                this.sslQuorum = sslQuorum;
                this.latch = latch;
            }

            /**
             * Sleeps on acceptConnections().
             */
            @Override
            public void run() {
                try {
                    Thread.currentThread().setName("ListenerHandler-" + address);

                    /*************************************************
                     * TODO_MA 马中华 https://blog.csdn.net/zhongqi2513
                     *  注释： 接受链接
                     */
                    acceptConnections();

                    try {
                        close();
                    } catch (IOException e) {
                        LOG.warn("Exception when shutting down listener: ", e);
                    }
                } catch (Exception e) {
                    // Output of unexpected exception, should never happen
                    LOG.error("Unexpected error ", e);
                } finally {
                    latch.countDown();
                }
            }

            @Override
            public synchronized void close() throws IOException {
                if (serverSocket != null && !serverSocket.isClosed()) {
                    LOG.debug("Trying to close listeners: {}", serverSocket);
                    serverSocket.close();
                }
            }

            /**
             * Sleeps on accept().
             */
            private void acceptConnections() {
                int numRetries = 0;
                Socket client = null;

                while ((!shutdown) && (portBindMaxRetry == 0 || numRetries < portBindMaxRetry)) {
                    try {

                        /*************************************************
                         * TODO_MA 马中华 https://blog.csdn.net/zhongqi2513
                         *  注释： 创建一个新的 ServerSocket
                         */
                        serverSocket = createNewServerSocket();
                        LOG.info("{} is accepting connections now, my election bind port: {}",
                                QuorumCnxManager.this.mySid, address.toString());

                        /*************************************************
                         * TODO_MA 马中华 https://blog.csdn.net/zhongqi2513
                         *  注释： 开始运行
                         */
                        while (!shutdown) {
                            try {

                                // TODO_MA 注释： 接受一个链接
                                // TODO_MA 注释： 等待有 其他节点的 BIO 客户端发起链接请求过来
                                client = serverSocket.accept();
                                setSockOpts(client);
                                LOG.info("Received connection request from {}", client.getRemoteSocketAddress());
                                // Receive and handle the connection request
                                // asynchronously if the quorum sasl authentication is
                                // enabled. This is required because sasl server
                                // authentication process may take few seconds to finish,
                                // this may delay next peer connection requests.
                                if (quorumSaslAuthEnabled) {
                                    receiveConnectionAsync(client);
                                }

                                /*************************************************
                                 * TODO_MA 马中华 https://blog.csdn.net/zhongqi2513
                                 *  注释： 完成选举链接建立
                                 */
                                else {
                                    receiveConnection(client);
                                }
                                numRetries = 0;
                            } catch (SocketTimeoutException e) {
                                LOG.warn(
                                        "The socket is listening for the election accepted " + "and it timed out unexpectedly, but will retry." + "see ZOOKEEPER-2836");
                            }
                        }
                    } catch (IOException e) {
                        if (shutdown) {
                            break;
                        }

                        LOG.error("Exception while listening", e);

                        if (e instanceof SocketException) {
                            socketException.set(true);
                        }

                        numRetries++;
                        try {
                            close();
                            Thread.sleep(1000);
                        } catch (IOException ie) {
                            LOG.error("Error closing server socket", ie);
                        } catch (InterruptedException ie) {
                            LOG.error("Interrupted while sleeping. Ignoring exception", ie);
                        }
                        closeSocket(client);
                    }
                }
                if (!shutdown) {
                    LOG.error(
                            "Leaving listener thread for address {} after {} errors. Use {} property to increase retry count.",
                            formatInetAddr(address), numRetries, ELECTION_PORT_BIND_RETRY);
                }
            }

            private ServerSocket createNewServerSocket() throws IOException {
                ServerSocket socket;

                if (portUnification) {
                    LOG.info("Creating TLS-enabled quorum server socket");
                    socket = new UnifiedServerSocket(self.getX509Util(), true);
                } else if (sslQuorum) {
                    LOG.info("Creating TLS-only quorum server socket");
                    socket = new UnifiedServerSocket(self.getX509Util(), false);
                } else {

                    /*************************************************
                     * TODO_MA 马中华 https://blog.csdn.net/zhongqi2513
                     *  注释： ServerSocket
                     */
                    socket = new ServerSocket();
                }

                socket.setReuseAddress(true);

                /*************************************************
                 * TODO_MA 马中华 https://blog.csdn.net/zhongqi2513
                 *  注释： 绑定端口号
                 */
                address = new InetSocketAddress(address.getHostString(), address.getPort());
                socket.bind(address);

                return socket;
            }
        }

    }

    /**
     * Thread to send messages. Instance waits on a queue, and send a message as
     * soon as there is one available. If connection breaks, then opens a new
     * one.
     */
    class SendWorker extends ZooKeeperThread {

        Long sid;
        Socket sock;
        RecvWorker recvWorker;
        volatile boolean running = true;
        DataOutputStream dout;
        AtomicBoolean ongoingAsyncValidation = new AtomicBoolean(false);

        /**
         * An instance of this thread receives messages to send
         * through a queue and sends them to the server sid.
         *
         * @param sock Socket to remote peer
         * @param sid  Server identifier of remote peer
         */
        SendWorker(Socket sock, Long sid) {
            super("SendWorker:" + sid);
            this.sid = sid;
            this.sock = sock;
            recvWorker = null;
            try {
                dout = new DataOutputStream(sock.getOutputStream());
            } catch (IOException e) {
                LOG.error("Unable to access socket output stream", e);
                closeSocket(sock);
                running = false;
            }
            LOG.debug("Address of remote peer: {}", this.sid);
        }

        synchronized void setRecv(RecvWorker recvWorker) {
            this.recvWorker = recvWorker;
        }

        /**
         * Returns RecvWorker that pairs up with this SendWorker.
         *
         * @return RecvWorker
         */
        synchronized RecvWorker getRecvWorker() {
            return recvWorker;
        }

        synchronized boolean finish() {
            LOG.debug("Calling SendWorker.finish for {}", sid);

            if (!running) {
                /*
                 * Avoids running finish() twice.
                 */
                return running;
            }

            running = false;
            closeSocket(sock);

            this.interrupt();
            if (recvWorker != null) {
                recvWorker.finish();
            }

            LOG.debug("Removing entry from senderWorkerMap sid={}", sid);

            senderWorkerMap.remove(sid, this);
            threadCnt.decrementAndGet();
            return running;
        }

        synchronized void send(ByteBuffer b) throws IOException {
            byte[] msgBytes = new byte[b.capacity()];
            try {
                b.position(0);
                b.get(msgBytes);
            } catch (BufferUnderflowException be) {
                LOG.error("BufferUnderflowException ", be);
                return;
            }

            // TODO_MA 注释： 客户端把 选票的 长度 和 具体数据发送给了 服务端
            dout.writeInt(b.capacity());
            dout.write(b.array());

            /*************************************************
             * TODO_MA 马中华 https://blog.csdn.net/zhongqi2513
             *  注释： 真实的完成了  选票的一个发送
             */
            dout.flush();
        }

        @Override
        public void run() {
            threadCnt.incrementAndGet();
            try {
                /**
                 * If there is nothing in the queue to send, then we
                 * send the lastMessage to ensure that the last message
                 * was received by the peer. The message could be dropped
                 * in case self or the peer shutdown their connection
                 * (and exit the thread) prior to reading/processing
                 * the last message. Duplicate messages are handled correctly
                 * by the peer.
                 *
                 * If the send queue is non-empty, then we have a recent
                 * message than that stored in lastMessage. To avoid sending
                 * stale message, we should send the message in the send queue.
                 */
                BlockingQueue<ByteBuffer> bq = queueSendMap.get(sid);
                if (bq == null || isSendQueueEmpty(bq)) {
                    ByteBuffer b = lastMessageSent.get(sid);
                    if (b != null) {
                        LOG.debug("Attempting to send lastMessage to sid={}", sid);
                        send(b);
                    }
                }
            } catch (IOException e) {
                LOG.error("Failed to send last message. Shutting down thread.", e);
                this.finish();
            }
            LOG.debug("SendWorker thread started towards {}. myId: {}", sid, QuorumCnxManager.this.mySid);

            try {
                while (running && !shutdown && sock != null) {

                    ByteBuffer b = null;
                    try {

                        // TODO_MA 注释： 根据对方 Server 的 myid 找到对应的 选票发送队列
                        BlockingQueue<ByteBuffer> bq = queueSendMap.get(sid);

                        /*************************************************
                         * TODO_MA 马中华 https://blog.csdn.net/zhongqi2513
                         *  注释： 阻塞获取对应发送队列中的 ByteBuffer
                         */
                        if (bq != null) {
                            b = pollSendQueue(bq, 1000, TimeUnit.MILLISECONDS);
                        } else {
                            LOG.error("No queue of incoming messages for server {}", sid);
                            break;
                        }

                        if (b != null) {

                            // TODO_MA 注释： lastMessageSent 用来记录，当前节点，给所有的其他服务器发送的最近一张票是什么
                            lastMessageSent.put(sid, b);

                            /*************************************************
                             * TODO_MA 马中华 https://blog.csdn.net/zhongqi2513
                             *  注释： 真正执行发送
                             */
                            send(b);
                        }
                    } catch (InterruptedException e) {
                        LOG.warn("Interrupted while waiting for message on queue", e);
                    }
                }
            } catch (Exception e) {
                LOG.warn("Exception when using channel: for id {} my id = {}", sid, QuorumCnxManager.this.mySid, e);
            }
            this.finish();

            LOG.warn("Send worker leaving thread id {} my id = {}", sid, self.getId());
        }


        public void asyncValidateIfSocketIsStillReachable() {
            if (ongoingAsyncValidation.compareAndSet(false, true)) {
                new Thread(() -> {
                    LOG.debug("validate if destination address is reachable for sid {}", sid);
                    if (sock != null) {
                        InetAddress address = sock.getInetAddress();
                        try {
                            if (address.isReachable(500)) {
                                LOG.debug("destination address {} is reachable for sid {}", address.toString(), sid);
                                ongoingAsyncValidation.set(false);
                                return;
                            }
                        } catch (NullPointerException | IOException ignored) {
                        }
                        LOG.warn("destination address {} not reachable anymore, shutting down the SendWorker for sid {}",
                                address.toString(), sid);
                        this.finish();
                    }
                }).start();
            } else {
                LOG.debug("validation of destination address for sid {} is skipped (it is already running)", sid);
            }
        }

    }

    /**
     * Thread to receive messages. Instance waits on a socket read. If the
     * channel breaks, then removes itself from the pool of receivers.
     */
    class RecvWorker extends ZooKeeperThread {

        Long sid;
        Socket sock;
        volatile boolean running = true;
        final DataInputStream din;
        final SendWorker sw;

        RecvWorker(Socket sock, DataInputStream din, Long sid, SendWorker sw) {
            super("RecvWorker:" + sid);
            this.sid = sid;
            this.sock = sock;
            this.sw = sw;
            this.din = din;
            try {
                // OK to wait until socket disconnects while reading.
                sock.setSoTimeout(0);
            } catch (IOException e) {
                LOG.error("Error while accessing socket for {}", sid, e);
                closeSocket(sock);
                running = false;
            }
        }

        /**
         * Shuts down this worker
         *
         * @return boolean  Value of variable running
         */
        synchronized boolean finish() {
            LOG.debug("RecvWorker.finish called. sid: {}. myId: {}", sid, QuorumCnxManager.this.mySid);
            if (!running) {
                /*
                 * Avoids running finish() twice.
                 */
                return running;
            }
            running = false;

            this.interrupt();
            threadCnt.decrementAndGet();
            return running;
        }

        @Override
        public void run() {
            threadCnt.incrementAndGet();
            try {

                // TODO_MA 注释： 进入循环，不停的去读取其他服务发送过来的票
                LOG.debug("RecvWorker thread towards {} started. myId: {}", sid, QuorumCnxManager.this.mySid);
                while (running && !shutdown && sock != null) {
                    /**
                     * Reads the first int to determine the length of the message
                     */
                    // TODO_MA 注释： 读取到数据包的长度
                    int length = din.readInt();
                    if (length <= 0 || length > PACKETMAXSIZE) {
                        throw new IOException("Received packet with invalid packet: " + length);
                    }
                    /**
                     * Allocates a new ByteBuffer to receive the message
                     */
                    final byte[] msgArray = new byte[length];

                    // TODO_MA 注释： 读取到整个数据
                    din.readFully(msgArray, 0, length);

                    /*************************************************
                     * TODO_MA 马中华 https://blog.csdn.net/zhongqi2513
                     *  注释： 把 msgArray 编程 Message 放入到 recevQueue 中
                     */
                    addToRecvQueue(new Message(ByteBuffer.wrap(msgArray), sid));
                }
            } catch (Exception e) {
                LOG.warn("Connection broken for id {}, my id = {}", sid, QuorumCnxManager.this.mySid, e);
            } finally {
                LOG.warn("Interrupting SendWorker thread from RecvWorker. sid: {}. myId: {}", sid,
                        QuorumCnxManager.this.mySid);
                sw.finish();
                closeSocket(sock);
            }
        }

    }

    /**
     * Inserts an element in the provided {@link BlockingQueue}. This method
     * assumes that if the Queue is full, an element from the head of the Queue is
     * removed and the new item is inserted at the tail of the queue. This is done
     * to prevent a thread from blocking while inserting an element in the queue.
     *
     * @param queue  Reference to the Queue
     * @param buffer Reference to the buffer to be inserted in the queue
     */
    private void addToSendQueue(final BlockingQueue<ByteBuffer> queue, final ByteBuffer buffer) {

        // TODO_MA 注释： 将选票放入队列
        final boolean success = queue.offer(buffer);
        if (!success) {
            throw new RuntimeException("Could not insert into receive queue");
        }
    }

    /**
     * Returns true if queue is empty.
     *
     * @param queue Reference to the queue
     * @return true if the specified queue is empty
     */
    private boolean isSendQueueEmpty(final BlockingQueue<ByteBuffer> queue) {
        return queue.isEmpty();
    }

    /**
     * Retrieves and removes buffer at the head of this queue,
     * waiting up to the specified wait time if necessary for an element to
     * become available.
     *
     * {@link BlockingQueue#poll(long, java.util.concurrent.TimeUnit)}
     */
    private ByteBuffer pollSendQueue(final BlockingQueue<ByteBuffer> queue, final long timeout,
                                     final TimeUnit unit) throws InterruptedException {
        return queue.poll(timeout, unit);
    }

    /**
     * Inserts an element in the {@link #recvQueue}. If the Queue is full, this
     * methods removes an element from the head of the Queue and then inserts the
     * element at the tail of the queue.
     *
     * @param msg Reference to the message to be inserted in the queue
     */
    public void addToRecvQueue(final Message msg) {

        /*************************************************
         * TODO_MA 马中华 https://blog.csdn.net/zhongqi2513
         *  注释：  发给自己
         */
        final boolean success = this.recvQueue.offer(msg);
        if (!success) {
            throw new RuntimeException("Could not insert into receive queue");
        }
    }

    /**
     * Retrieves and removes a message at the head of this queue,
     * waiting up to the specified wait time if necessary for an element to
     * become available.
     *
     * {@link BlockingQueue#poll(long, java.util.concurrent.TimeUnit)}
     */
    public Message pollRecvQueue(final long timeout, final TimeUnit unit) throws InterruptedException {

        /*************************************************
         * TODO_MA 马中华 https://blog.csdn.net/zhongqi2513
         *  注释：
         */
        return this.recvQueue.poll(timeout, unit);
    }

    public boolean connectedToPeer(long peerSid) {
        return senderWorkerMap.get(peerSid) != null;
    }

    public boolean isReconfigEnabled() {
        return self.isReconfigEnabled();
    }

}
