深入剖析SolrCloud（三）

在上一篇中介绍了SolrCloud的第一个模块---构建管理solr集群状态信息的zookeeper集群。当我们在solr服务器启动时拥有了这样一个Zookeeper集群后，显然我们需要连接到Zookeeper集群的方便手段，在这一篇中我将对Zookeeper客户端相关的各个封装类进行分析。

SolrZkClient类是Solr服务器用来与Zookeeper集群进行通信的接口类，它包含的主要组件有：

  private ConnectionManager connManager;
  private volatile SolrZooKeeper keeper;
  private ZkCmdExecutor zkCmdExecutor = new ZkCmdExecutor();

    其中ConnectionManager是Watcher的实现类，主要负责对客户端与Zookeeper集群之间连接的状态变化信息进行响应，关于Watcher的详细介绍，可以参考http://zookeeper.apache.org/doc/trunk/zookeeperProgrammers.html#ch_zkWatches，

SolrZooKeeper类是一个包装类，没有实际意义，ZkCmdExecutor类是负责在连接失败的情况下，重试某种操作特定次数，具体的操作是ZkOperation这个抽象类的具体实现子类，其execute方法中包含了具体操作步骤，这些操作包括新建一个Znode节点，读取Znode节点数据，创建Znode路径，删除Znode节点等Zookeeper操作。

首先来看它的构造函数，先创建ConnectionManager对象来响应两端之间的状态变化信息，然后ZkClientConnectionStrategy类是一个连接策略抽象类，它包含连接和重连两种策略，并且采用模板方法模式，具体的实现是通过静态累不类ZkUpdate来实现的，DefaultConnectionStrategy是它的一个实现子类，它覆写了connect和reconnect两个连接策略方法。

  public SolrZkClient(String zkServerAddress, int zkClientTimeout,
      ZkClientConnectionStrategy strat, final OnReconnect onReconnect, int clientConnectTimeout) throws InterruptedException,
      TimeoutException, IOException {
    connManager = new ConnectionManager("ZooKeeperConnection Watcher:"
        + zkServerAddress, this, zkServerAddress, zkClientTimeout, strat, onReconnect);
    strat.connect(zkServerAddress, zkClientTimeout, connManager,
        new ZkUpdate() {
          @Override
          public void update(SolrZooKeeper zooKeeper) {
            SolrZooKeeper oldKeeper = keeper;
            keeper = zooKeeper;
            if (oldKeeper != null) {
              try {
                oldKeeper.close();
              } catch (InterruptedException e) {
                // Restore the interrupted status
                Thread.currentThread().interrupt();
                log.error("", e);
                throw new ZooKeeperException(SolrException.ErrorCode.SERVER_ERROR,
                    "", e);
              }
            }
          }
        });
    connManager.waitForConnected(clientConnectTimeout);
    numOpens.incrementAndGet();
  }

值得注意的是，构造函数中生成的ZkUpdate匿名类对象，它的update方法会被调用，

在这个方法里，会首先将已有的老的SolrZooKeeperg关闭掉，然后放置上一个新的SolrZooKeeper。做好这些准备工作以后，就会去连接Zookeeper服务器集群，

connManager.waitForConnected(clientConnectTimeout);//连接zk服务器集群，默认30秒超时时间

其实具体的连接动作是new SolrZooKeeper(serverAddress, timeout, watcher)引发的，上面那句代码只是在等待指定时间，看是否已经连接上。

如果连接Zookeeper服务器集群成功，那么就可以进行Zookeeper的常规操作了：

1） 是否已经连接

  public boolean isConnected() {
    return keeper != null && keeper.getState() == ZooKeeper.States.CONNECTED;
  }

2）  是否存在某个路径的Znode

  public Stat exists(final String path, final Watcher watcher, boolean retryOnConnLoss) throws KeeperException, InterruptedException {
    if (retryOnConnLoss) {
      return zkCmdExecutor.retryOperation(new ZkOperation() {
        @Override
        public Stat execute() throws KeeperException, InterruptedException {
          return keeper.exists(path, watcher);
        }
      });
    } else {
      return keeper.exists(path, watcher);
    }
  }

3） 创建一个Znode节点

  public String create(final String path, final byte data[], final List<ACL> acl, final CreateMode createMode, boolean retryOnConnLoss) throws KeeperException, InterruptedException {
    if (retryOnConnLoss) {
      return zkCmdExecutor.retryOperation(new ZkOperation() {
        @Override
        public String execute() throws KeeperException, InterruptedException {
          return keeper.create(path, data, acl, createMode);
        }
      });
    } else {
      return keeper.create(path, data, acl, createMode);
    }
  }

4）  获取指定路径下的孩子Znode节点

  public List<String> getChildren(final String path, final Watcher watcher, boolean retryOnConnLoss) throws KeeperException, InterruptedException {
    if (retryOnConnLoss) {
      return zkCmdExecutor.retryOperation(new ZkOperation() {
        @Override
        public List<String> execute() throws KeeperException, InterruptedException {
          return keeper.getChildren(path, watcher);
        }
      });
    } else {
      return keeper.getChildren(path, watcher);
    }
  }

5） 获取指定Znode上附加的数据

  public byte[] getData(final String path, final Watcher watcher, final Stat stat, boolean retryOnConnLoss) throws KeeperException, InterruptedException {
    if (retryOnConnLoss) {
      return zkCmdExecutor.retryOperation(new ZkOperation() {
        @Override
        public byte[] execute() throws KeeperException, InterruptedException {
          return keeper.getData(path, watcher, stat);
        }
      });
    } else {
      return keeper.getData(path, watcher, stat);
    }
  }

6）  在指定Znode上设置数据

  public Stat setData(final String path, final byte data[], final int version, boolean retryOnConnLoss) throws KeeperException, InterruptedException {
    if (retryOnConnLoss) {
      return zkCmdExecutor.retryOperation(new ZkOperation() {
        @Override
        public Stat execute() throws KeeperException, InterruptedException {
          return keeper.setData(path, data, version);
        }
      });
    } else {
      return keeper.setData(path, data, version);
    }
  }

7） 创建路径

  public void makePath(String path, byte[] data, CreateMode createMode, Watcher watcher, boolean failOnExists, boolean retryOnConnLoss) throws KeeperException, InterruptedException {
    if (log.isInfoEnabled()) {
      log.info("makePath: " + path);
    }
    boolean retry = true;
    
    if (path.startsWith("/")) {
      path = path.substring(1, path.length());
    }
    String[] paths = path.split("/");
    StringBuilder sbPath = new StringBuilder();
    for (int i = 0; i < paths.length; i++) {
      byte[] bytes = null;
      String pathPiece = paths[i];
      sbPath.append("/" + pathPiece);
      final String currentPath = sbPath.toString();
      Object exists = exists(currentPath, watcher, retryOnConnLoss);
      if (exists == null || ((i == paths.length -1) && failOnExists)) {
        CreateMode mode = CreateMode.PERSISTENT;
        if (i == paths.length - 1) {
          mode = createMode;
          bytes = data;
          if (!retryOnConnLoss) retry = false;
        }
        try {
          if (retry) {
            final CreateMode finalMode = mode;
            final byte[] finalBytes = bytes;
            zkCmdExecutor.retryOperation(new ZkOperation() {
              @Override
              public Object execute() throws KeeperException, InterruptedException {
                keeper.create(currentPath, finalBytes, ZooDefs.Ids.OPEN_ACL_UNSAFE, finalMode);
                return null;
              }
            });
          } else {
            keeper.create(currentPath, bytes, ZooDefs.Ids.OPEN_ACL_UNSAFE, mode);
          }
        } catch (NodeExistsException e) {
          
          if (!failOnExists) {
            // TODO: version ? for now, don't worry about race
            setData(currentPath, data, -1, retryOnConnLoss);
            // set new watch
            exists(currentPath, watcher, retryOnConnLoss);
            return;
          }
          
          // ignore unless it's the last node in the path
          if (i == paths.length - 1) {
            throw e;
          }
        }
        if(i == paths.length -1) {
          // set new watch
          exists(currentPath, watcher, retryOnConnLoss);
        }
      } else if (i == paths.length - 1) {
        // TODO: version ? for now, don't worry about race
        setData(currentPath, data, -1, retryOnConnLoss);
        // set new watch
        exists(currentPath, watcher, retryOnConnLoss);
      }
    }
  }

8） 删除指定Znode

  public void delete(final String path, final int version, boolean retryOnConnLoss) throws InterruptedException, KeeperException {
    if (retryOnConnLoss) {
      zkCmdExecutor.retryOperation(new ZkOperation() {
        @Override
        public Stat execute() throws KeeperException, InterruptedException {
          keeper.delete(path, version);
          return null;
        }
      });
    } else {
      keeper.delete(path, version);
    }
  }

         我们再回过头来看看ConnectionManager类是如何响应两端的连接状态信息的变化的，它最重要的方法是process方法，当它被触发回调时，会从WatchedEvent参数中得到事件的各种状态信息，比如连接成功，会话过期（此时需要进行重连），连接断开等。

  public synchronized void process(WatchedEvent event) {
    if (log.isInfoEnabled()) {
      log.info("Watcher " + this + " name:" + name + " got event " + event + " path:" + event.getPath() + " type:" + event.getType());
    }

    state = event.getState();
    if (state == KeeperState.SyncConnected) {
      connected = true;
      clientConnected.countDown();
    } else if (state == KeeperState.Expired) {
      connected = false;
      log.info("Attempting to reconnect to recover relationship with ZooKeeper...");
      //尝试重新连接zk服务器
      try {
        connectionStrategy.reconnect(zkServerAddress, zkClientTimeout, this,
            new ZkClientConnectionStrategy.ZkUpdate() {
              @Override
              public void update(SolrZooKeeper keeper) throws InterruptedException, TimeoutException, IOException {
                synchronized (connectionStrategy) {
                  waitForConnected(SolrZkClient.DEFAULT_CLIENT_CONNECT_TIMEOUT);
                  client.updateKeeper(keeper);
                  if (onReconnect != null) {
                    onReconnect.command();
                  }
                  synchronized (ConnectionManager.this) {
                    ConnectionManager.this.connected = true;
                  }
                }
                
              }
            });
      } catch (Exception e) {
        SolrException.log(log, "", e);
      }
      log.info("Connected:" + connected);
    } else if (state == KeeperState.Disconnected) {
      connected = false;
    } else {
      connected = false;
    }
    notifyAll();
  }