【Zookeeper】源码分析之服务器（二）之ZooKeeperServer

一、前言

　　前面阐述了服务器的总体框架，下面来分析服务器的所有父类ZooKeeperServer。

二、ZooKeeperServer源码分析

　　2.1 类的继承关系　

public class ZooKeeperServer implements SessionExpirer, ServerStats.Provider {}

　　说明：ZooKeeperServer是ZooKeeper中所有服务器的父类，其实现了Session.Expirer和ServerStats.Provider接口，SessionExpirer中定义了expire方法（表示会话过期）和getServerId方法（表示获取服务器ID），而Provider则主要定义了获取服务器某些数据的方法。

　　2.2 类的内部类

　　1. DataTreeBuilder类

    public interface DataTreeBuilder {
        // 构建DataTree
        public DataTree build();
    }

　　说明：其定义了构建树DataTree的接口。

　　2. BasicDataTreeBuilder类　　

    static public class BasicDataTreeBuilder implements DataTreeBuilder {
        public DataTree build() {
            return new DataTree();
        }
    }

　　说明：实现DataTreeBuilder接口，返回新创建的树DataTree。

　　3. MissingSessionException类　　

    public static class MissingSessionException extends IOException {
        private static final long serialVersionUID = 7467414635467261007L;

        public MissingSessionException(String msg) {
            super(msg);
        }
    }

　　说明：表示会话缺失异常。

　　4. ChangeRecord类　

    static class ChangeRecord {
        ChangeRecord(long zxid, String path, StatPersisted stat, int childCount,
                List<ACL> acl) {
            // 属性赋值
            this.zxid = zxid;
            this.path = path;
            this.stat = stat;
            this.childCount = childCount;
            this.acl = acl;
        }
        
        // zxid
        long zxid;

        // 路径
        String path;

        // 统计数据
        StatPersisted stat; /* Make sure to create a new object when changing */

        // 子节点个数
        int childCount;

        // ACL列表
        List<ACL> acl; /* Make sure to create a new object when changing */

        @SuppressWarnings("unchecked")
        // 拷贝
        ChangeRecord duplicate(long zxid) {
            StatPersisted stat = new StatPersisted();
            if (this.stat != null) {
                DataTree.copyStatPersisted(this.stat, stat);
            }
            return new ChangeRecord(zxid, path, stat, childCount,
                    acl == null ? new ArrayList<ACL>() : new ArrayList(acl));
        }
    }

　　说明：ChangeRecord数据结构是用于方便PrepRequestProcessor和FinalRequestProcessor之间进行信息共享，其包含了一个拷贝方法duplicate，用于返回属性相同的ChangeRecord实例。

　　2.3 类的属性　　

public class ZooKeeperServer implements SessionExpirer, ServerStats.Provider {
    // 日志器
    protected static final Logger LOG;
    
    static {
        // 初始化日志器
        LOG = LoggerFactory.getLogger(ZooKeeperServer.class);
        
        Environment.logEnv("Server environment:", LOG);
    }
    // JMX服务
    protected ZooKeeperServerBean jmxServerBean;
    protected DataTreeBean jmxDataTreeBean;

    
    // 默认心跳频率
    public static final int DEFAULT_TICK_TIME = 3000;
    protected int tickTime = DEFAULT_TICK_TIME;
    /** value of -1 indicates unset, use default */
    // 最小会话过期时间
    protected int minSessionTimeout = -1;
    /** value of -1 indicates unset, use default */
    // 最大会话过期时间
    protected int maxSessionTimeout = -1;
    // 会话跟踪器
    protected SessionTracker sessionTracker;
    // 事务日志快照
    private FileTxnSnapLog txnLogFactory = null;
    // Zookeeper内存数据库
    private ZKDatabase zkDb;
    // 
    protected long hzxid = 0;
    // 异常
    public final static Exception ok = new Exception("No prob");
    // 请求处理器
    protected RequestProcessor firstProcessor;
    // 运行标志
    protected volatile boolean running;

    /**
     * This is the secret that we use to generate passwords, for the moment it
     * is more of a sanity check.
     */
    // 生成密码的密钥
    static final private long superSecret = 0XB3415C00L;
    
    // 
    int requestsInProcess;
    
    // 未处理的ChangeRecord
    final List<ChangeRecord> outstandingChanges = new ArrayList<ChangeRecord>();
    
    // this data structure must be accessed under the outstandingChanges lock
    // 记录path对应的ChangeRecord
    final HashMap<String, ChangeRecord> outstandingChangesForPath =
        new HashMap<String, ChangeRecord>();
        
    // 连接工厂
    private ServerCnxnFactory serverCnxnFactory;
    
    // 服务器统计数据
    private final ServerStats serverStats;
}

　　说明：类中包含了心跳频率，会话跟踪器（处理会话）、事务日志快照、内存数据库、请求处理器、未处理的ChangeRecord、服务器统计信息等。

　　2.4 类的构造函数

　　1. ZooKeeperServer()型构造函数　　

    public ZooKeeperServer() {
        serverStats = new ServerStats(this);
    }

　　说明：其只初始化了服务器的统计信息。

　　2. ZooKeeperServer(FileTxnSnapLog, int, int, int, DataTreeBuilder, ZKDatabase)型构造函数　　

    public ZooKeeperServer(FileTxnSnapLog txnLogFactory, int tickTime,
            int minSessionTimeout, int maxSessionTimeout,
            DataTreeBuilder treeBuilder, ZKDatabase zkDb) {
        // 给属性赋值
        serverStats = new ServerStats(this);
        this.txnLogFactory = txnLogFactory;
        this.zkDb = zkDb;
        this.tickTime = tickTime;
        this.minSessionTimeout = minSessionTimeout;
        this.maxSessionTimeout = maxSessionTimeout;
        
        LOG.info("Created server with tickTime " + tickTime
                + " minSessionTimeout " + getMinSessionTimeout()
                + " maxSessionTimeout " + getMaxSessionTimeout()
                + " datadir " + txnLogFactory.getDataDir()
                + " snapdir " + txnLogFactory.getSnapDir());
    }

　　说明：该构造函数会初始化服务器统计数据、事务日志工厂、心跳时间、会话时间（最短超时时间和最长超时时间）。

　　3. ZooKeeperServer(FileTxnSnapLog, int, DataTreeBuilder)型构造函数　　

    public ZooKeeperServer(FileTxnSnapLog txnLogFactory, int tickTime,
            DataTreeBuilder treeBuilder) throws IOException {
        this(txnLogFactory, tickTime, -1, -1, treeBuilder,
                new ZKDatabase(txnLogFactory));
    }

　　说明：其首先会生成ZooKeeper内存数据库后，然后调用第二个构造函数进行初始化操作。

　　4. ZooKeeperServer(File, File, int)型构造函数　

    public ZooKeeperServer(File snapDir, File logDir, int tickTime)
            throws IOException {
        this( new FileTxnSnapLog(snapDir, logDir),
                tickTime, new BasicDataTreeBuilder());
    }

　　说明：其会调用同名构造函数进行初始化操作。

　　5. ZooKeeperServer(FileTxnSnapLog, DataTreeBuilder)型构造函数　　

    public ZooKeeperServer(FileTxnSnapLog txnLogFactory,
            DataTreeBuilder treeBuilder)
        throws IOException
    {
        this(txnLogFactory, DEFAULT_TICK_TIME, -1, -1, treeBuilder,
                new ZKDatabase(txnLogFactory));
    }

　　说明：其生成内存数据库之后再调用同名构造函数进行初始化操作。

　　2.5 核心函数分析

　　1. loadData函数　

    public void loadData() throws IOException, InterruptedException {
        /*
         * When a new leader starts executing Leader#lead, it 
         * invokes this method. The database, however, has been
         * initialized before running leader election so that
         * the server could pick its zxid for its initial vote.
         * It does it by invoking QuorumPeer#getLastLoggedZxid.
         * Consequently, we don't need to initialize it once more
         * and avoid the penalty of loading it a second time. Not 
         * reloading it is particularly important for applications
         * that host a large database.
         * 
         * The following if block checks whether the database has
         * been initialized or not. Note that this method is
         * invoked by at least one other method: 
         * ZooKeeperServer#startdata.
         *  
         * See ZOOKEEPER-1642 for more detail.
         */
        if(zkDb.isInitialized()){ // 内存数据库已被初始化
            // 设置为最后处理的Zxid
            setZxid(zkDb.getDataTreeLastProcessedZxid());
        }
        else { // 未被初始化，则加载数据库
            setZxid(zkDb.loadDataBase());
        }
        
        // Clean up dead sessions
        LinkedList<Long> deadSessions = new LinkedList<Long>();
        for (Long session : zkDb.getSessions()) { // 遍历所有的会话
            if (zkDb.getSessionWithTimeOuts().get(session) == null) { // 删除过期的会话
                deadSessions.add(session);
            }
        }
        // 完成DataTree的初始化
        zkDb.setDataTreeInit(true);
        for (long session : deadSessions) { // 遍历过期会话
            // XXX: Is lastProcessedZxid really the best thing to use?
            // 删除会话
            killSession(session, zkDb.getDataTreeLastProcessedZxid());
        }
    }

　　说明：该函数用于加载数据，其首先会判断内存库是否已经加载设置zxid，之后会调用killSession函数删除过期的会话，killSession会从sessionTracker中删除session，并且killSession最后会调用DataTree的killSession函数，其源码如下　

    void killSession(long session, long zxid) {
        // the list is already removed from the ephemerals
        // so we do not have to worry about synchronizing on
        // the list. This is only called from FinalRequestProcessor
        // so there is no need for synchronization. The list is not
        // changed here. Only create and delete change the list which
        // are again called from FinalRequestProcessor in sequence.
        // 移除session，并获取该session对应的所有临时节点
        HashSet<String> list = ephemerals.remove(session);
        if (list != null) {
            for (String path : list) { // 遍历所有临时节点
                try {
                    // 删除路径对应的节点
                    deleteNode(path, zxid);
                    if (LOG.isDebugEnabled()) {
                        LOG
                                .debug("Deleting ephemeral node " + path
                                        + " for session 0x"
                                        + Long.toHexString(session));
                    }
                } catch (NoNodeException e) {
                    LOG.warn("Ignoring NoNodeException for path " + path
                            + " while removing ephemeral for dead session 0x"
                            + Long.toHexString(session));
                }
            }
        }
    }

　　说明：DataTree的killSession函数的逻辑首先移除session，然后取得该session下的所有临时节点，然后逐一删除临时节点。

　　2. submit函数　

    public void submitRequest(Request si) {
        if (firstProcessor == null) { // 第一个处理器为空
            synchronized (this) {
                try {
                    while (!running) { // 直到running为true，否则继续等待
                        wait(1000);
                    }
                } catch (InterruptedException e) {
                    LOG.warn("Unexpected interruption", e);
                }
                if (firstProcessor == null) {
                    throw new RuntimeException("Not started");
                }
            }
        }
        try {
            touch(si.cnxn);
            // 是否为合法的packet
            boolean validpacket = Request.isValid(si.type);
            if (validpacket) { 
                // 处理请求
                firstProcessor.processRequest(si);
                if (si.cnxn != null) {
                    incInProcess();
                }
            } else {
                LOG.warn("Received packet at server of unknown type " + si.type);
                new UnimplementedRequestProcessor().processRequest(si);
            }
        } catch (MissingSessionException e) {
            if (LOG.isDebugEnabled()) {
                LOG.debug("Dropping request: " + e.getMessage());
            }
        } catch (RequestProcessorException e) {
            LOG.error("Unable to process request:" + e.getMessage(), e);
        }
    }

　　说明：当firstProcessor为空时，并且running标志为false时，其会一直等待，直到running标志为true，之后调用touch函数判断session是否存在或者已经超时，之后判断请求的类型是否合法，合法则使用请求处理器进行处理。

　　3. processConnectRequest函数　　

    public void processConnectRequest(ServerCnxn cnxn, ByteBuffer incomingBuffer) throws IOException {
        BinaryInputArchive bia = BinaryInputArchive.getArchive(new ByteBufferInputStream(incomingBuffer));
        ConnectRequest connReq = new ConnectRequest();
        // 反序列化
        connReq.deserialize(bia, "connect");
        if (LOG.isDebugEnabled()) {
            LOG.debug("Session establishment request from client "
                    + cnxn.getRemoteSocketAddress()
                    + " client's lastZxid is 0x"
                    + Long.toHexString(connReq.getLastZxidSeen()));
        }
        boolean readOnly = false;
        try {
            // 是否为只读
            readOnly = bia.readBool("readOnly");
            cnxn.isOldClient = false;
        } catch (IOException e) {
            // this is ok -- just a packet from an old client which
            // doesn't contain readOnly field
            LOG.warn("Connection request from old client "
                    + cnxn.getRemoteSocketAddress()
                    + "; will be dropped if server is in r-o mode");
        }
        if (readOnly == false && this instanceof ReadOnlyZooKeeperServer) { // 为只读模式但是该服务器是只读服务器，抛出异常
            String msg = "Refusing session request for not-read-only client "
                + cnxn.getRemoteSocketAddress();
            LOG.info(msg);
            throw new CloseRequestException(msg);
        }
        if (connReq.getLastZxidSeen() > zkDb.dataTree.lastProcessedZxid) { // 请求连接的zxid大于DataTree处理的最大的zxid，抛出异常
            String msg = "Refusing session request for client "
                + cnxn.getRemoteSocketAddress()
                + " as it has seen zxid 0x"
                + Long.toHexString(connReq.getLastZxidSeen())
                + " our last zxid is 0x"
                + Long.toHexString(getZKDatabase().getDataTreeLastProcessedZxid())
                + " client must try another server";

            LOG.info(msg);
            throw new CloseRequestException(msg);
        }
        // 获取超时时间
        int sessionTimeout = connReq.getTimeOut();
        // 获取密码
        byte passwd[] = connReq.getPasswd();
        // 获取最短超时时间
        int minSessionTimeout = getMinSessionTimeout();
        if (sessionTimeout < minSessionTimeout) { 
            sessionTimeout = minSessionTimeout;
        }
        // 获取最长超时时间
        int maxSessionTimeout = getMaxSessionTimeout();
        if (sessionTimeout > maxSessionTimeout) {
            sessionTimeout = maxSessionTimeout;
        }
        // 设置超时时间
        cnxn.setSessionTimeout(sessionTimeout);
        // We don't want to receive any packets until we are sure that the
        // session is setup
        // 不接收任何packet，直到会话创建成功
        cnxn.disableRecv();
        // 获取会话id
        long sessionId = connReq.getSessionId();
        if (sessionId != 0) { // 表示重新创建会话
            long clientSessionId = connReq.getSessionId();
            LOG.info("Client attempting to renew session 0x"
                    + Long.toHexString(clientSessionId)
                    + " at " + cnxn.getRemoteSocketAddress());
            // 关闭会话
            serverCnxnFactory.closeSession(sessionId);
            // 设置会话id
            cnxn.setSessionId(sessionId);
            // 重新打开会话
            reopenSession(cnxn, sessionId, passwd, sessionTimeout);
        } else {
            LOG.info("Client attempting to establish new session at "
                    + cnxn.getRemoteSocketAddress());
            // 创建会话
            createSession(cnxn, passwd, sessionTimeout);
        }
    }

　　说明：其首先将传递的ByteBuffer进行反序列化，转化为相应的ConnectRequest，之后进行一系列判断（可能抛出异常），然后获取并判断该ConnectRequest中会话id是否为0，若为0，则表示可以创建会话，否则，重新打开会话。

　　4. processPacket函数　

    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);
        // 创建请求头
        RequestHeader h = new RequestHeader();
        // 将头反序列化为RequestHeader
        h.deserialize(bia, "header");
        // Through the magic of byte buffers, txn will not be
        // pointing
        // to the start of the txn
        incomingBuffer = incomingBuffer.slice();
        if (h.getType() == OpCode.auth) { // 需要进行认证（有密码）
            LOG.info("got auth packet " + cnxn.getRemoteSocketAddress());
            AuthPacket authPacket = new AuthPacket();
            // 将ByteBuffer转化为AuthPacket
            ByteBufferInputStream.byteBuffer2Record(incomingBuffer, authPacket);
            // 获取AuthPacket的模式
            String scheme = authPacket.getScheme();
            AuthenticationProvider ap = ProviderRegistry.getProvider(scheme);
            Code authReturn = KeeperException.Code.AUTHFAILED;
            if(ap != null) {
                try {
                    // 进行认证
                    authReturn = ap.handleAuthentication(cnxn, authPacket.getAuth());
                } catch(RuntimeException e) {
                    LOG.warn("Caught runtime exception from AuthenticationProvider: " + scheme + " due to " + e);
                    authReturn = KeeperException.Code.AUTHFAILED;                   
                }
            }
            if (authReturn!= KeeperException.Code.OK) { // 认证失败
                if (ap == null) {
                    LOG.warn("No authentication provider for scheme: "
                            + scheme + " has "
                            + ProviderRegistry.listProviders());
                } else {
                    LOG.warn("Authentication failed for scheme: " + scheme);
                }
                // send a response...
                // 构造响应头
                ReplyHeader rh = new ReplyHeader(h.getXid(), 0,
                        KeeperException.Code.AUTHFAILED.intValue());
                // 发送响应
                cnxn.sendResponse(rh, null, null);
                // ... and close connection
                // 关闭连接的信息
                cnxn.sendBuffer(ServerCnxnFactory.closeConn);
                // 不接收任何packet
                cnxn.disableRecv();
            } else { // 认证成功
                if (LOG.isDebugEnabled()) {
                    LOG.debug("Authentication succeeded for scheme: "
                              + scheme);
                }
                LOG.info("auth success " + cnxn.getRemoteSocketAddress());
                // 构造响应头
                ReplyHeader rh = new ReplyHeader(h.getXid(), 0,
                        KeeperException.Code.OK.intValue());
                // 发送响应
                cnxn.sendResponse(rh, null, null);
            }
            return;
        } else {
            if (h.getType() == OpCode.sasl) { // 为SASL类型
                // 处理SASL
                Record rsp = processSasl(incomingBuffer,cnxn);
                // 构造响应头
                ReplyHeader rh = new ReplyHeader(h.getXid(), 0, KeeperException.Code.OK.intValue());
                // 发送响应
                cnxn.sendResponse(rh,rsp, "response"); // not sure about 3rd arg..what is it?
            }
            else { // 不为SASL类型
                // 创建请求
                Request si = new Request(cnxn, cnxn.getSessionId(), h.getXid(),
                  h.getType(), incomingBuffer, cnxn.getAuthInfo());
                // 设置请求所有者
                si.setOwner(ServerCnxn.me);
                // 提交请求
                submitRequest(si);
            }
        }
        // 
        cnxn.incrOutstandingRequests(h);
    }

　　说明：该函数首先将传递的ByteBuffer进行反序列，转化为相应的RequestHeader，然后根据该RequestHeader判断是否需要认证，若认证失败，则构造认证失败的响应并发送给客户端，然后关闭连接，并且再补接收任何packet。若认证成功，则构造认证成功的响应并发送给客户端。若不需要认证，则再判断其是否为SASL类型，若是，则进行处理，然后构造响应并发送给客户端，否则，构造请求并且提交请求。

三、总结

　　本篇分析了ZooKeeperServer的源码，了解了其对于请求和会话的处理，也谢谢各位园友的观看~