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();
}