最近正在研究zookeeper，一些心得记录一下，如有错误，还请大神指正。
zookeeper下载地址：http://zookeeper.apache.org/releases.html，百度一下就能找到，不过还是在这里列一下。
我认为学习一个东西，首先要理出一个头绪，否则感觉无从下手，这里我从启动开始研究，即从zkSever.sh入手。
if [ "x$JMXDISABLE" = "x" ]
then
echo "JMX enabled by default" >&2
# for some reason these two options are necessary on jdk6 on Ubuntu
# accord to the docs they are not necessary, but otw jconsole cannot
# do a local attach
ZOOMAIN="-Dcom.sun.management.jmxremote -Dcom.sun.management.jmxremote.local.only=$JMXLOCALONLY org.apache.zookeeper.server.quorum.QuorumPeerMain"
else
echo "JMX disabled by user request" >&2
ZOOMAIN="org.apache.zookeeper.server.quorum.QuorumPeerMain"
fi
从zkSever.sh可以看出，启动入口在QuorumPeerMain中，源码如下：
// 入口函数
public static void main(String[] args)
{
QuorumPeerMain main = new QuorumPeerMain();
//...1、启动初始化
main.initializeAndRun(args);
// ...
} protected void initializeAndRun(String[] args)
throws QuorumPeerConfig.ConfigException, IOException
{
// 2、加载配置文件
QuorumPeerConfig config = new QuorumPeerConfig();
if (args.length == 1) {
// 解析配置文件
config.parse(args[0]);
} if ((args.length == 1) && (config.servers.size() > 0)) {
// 配置文件的信息加载至QuorumPeer
runFromConfig(config);
} else {
LOG.warn("Either no config or no quorum defined in config, running in standalone mode"); ZooKeeperServerMain.main(args);
}
} public void runFromConfig(QuorumPeerConfig config) throws IOException {
try {
ManagedUtil.registerLog4jMBeans();
} catch (JMException e) {
LOG.warn("Unable to register log4j JMX control", e);
} LOG.info("Starting quorum peer");
try {
NIOServerCnxn.Factory cnxnFactory = new NIOServerCnxn.Factory(config.getClientPortAddress(), config.getMaxClientCnxns()); // 3、启动QuorumPeer
this.quorumPeer = new QuorumPeer();
this.quorumPeer.setClientPortAddress(config.getClientPortAddress());
//...加载各种配置信息 this.quorumPeer.start();
this.quorumPeer.join();
}
catch (InterruptedException e) {
LOG.warn("Quorum Peer interrupted", e);
}
}
可以看出，配置文件的解析由QuorumPeerConfig 类完成，其部分源码如下：
public void parse(String path)
throws QuorumPeerConfig.ConfigException
{
File configFile = new File(path); LOG.info("Reading configuration from: " + configFile);
try
{
if (!configFile.exists()) {
throw new IllegalArgumentException(configFile.toString() + " file is missing");
}
// 将配置信息加载如property文件
Properties cfg = new Properties();
FileInputStream in = new FileInputStream(configFile);
try {
cfg.load(in);
} finally {
in.close();
} parseProperties(cfg);
} catch (IOException e) {
throw new ConfigException("Error processing " + path, e);
} catch (IllegalArgumentException e) {
throw new ConfigException("Error processing " + path, e);
}
} public void parseProperties(Properties zkProp)
throws IOException, QuorumPeerConfig.ConfigException
{
int clientPort = 0;
String clientPortAddress = null;
// 循环解析配置文件
for (Map.Entry entry : zkProp.entrySet()) {
String key = entry.getKey().toString().trim();
String value = entry.getValue().toString().trim();
if (key.equals("dataDir")) {
this.dataDir = value;
} else if (key.equals("dataLogDir")) {
this.dataLogDir = value;
} else if (key.equals("clientPort")) {
// 客户端连接的端口号
clientPort = Integer.parseInt(value);
} else if (key.equals("clientPortAddress")) {
clientPortAddress = value.trim();
} else if (key.equals("tickTime")) {
// 心跳时间
this.tickTime = Integer.parseInt(value);
} else if (key.equals("maxClientCnxns")) {
this.maxClientCnxns = Integer.parseInt(value);
} else if (key.equals("minSessionTimeout")) {
this.minSessionTimeout = Integer.parseInt(value);
} else if (key.equals("maxSessionTimeout")) {
this.maxSessionTimeout = Integer.parseInt(value);
} else if (key.equals("initLimit")) {
this.initLimit = Integer.parseInt(value);
} else if (key.equals("syncLimit")) {
this.syncLimit = Integer.parseInt(value);
} else if (key.equals("electionAlg")) {
// 选举算法的类型，默认算法为FastLeaderElection
this.electionAlg = Integer.parseInt(value);
} else if (key.equals("peerType")) {
if (value.toLowerCase().equals("observer"))
this.peerType = QuorumPeer.LearnerType.OBSERVER;
else if (value.toLowerCase().equals("participant")) {
this.peerType = QuorumPeer.LearnerType.PARTICIPANT;
}
else
throw new ConfigException("Unrecognised peertype: " + value);
}
//...
回到QuorumPeerMain类的runFromConfig方法。此方法中，会将配置信息加载至QuorumPeer，并调用其start方法：
public synchronized void start()
{
try {
this.zkDb.loadDataBase();
} catch (IOException ie) {
LOG.fatal("Unable to load database on disk", ie);
throw new RuntimeException("Unable to run quorum server ", ie);
}
this.cnxnFactory.start();
startLeaderElection();
super.start();
}
在start方法中，会现价在硬盘中的数据，
this.zkDb.loadDataBase();即ZKDatabase中
public long loadDataBase()
throws IOException
{
FileTxnSnapLog.PlayBackListener listener = new FileTxnSnapLog.PlayBackListener() {
public void onTxnLoaded(TxnHeader hdr, Record txn) {
Request r = new Request(null, 0L, hdr.getCxid(), hdr.getType(), null, null); r.txn = txn;
r.hdr = hdr;
r.zxid = hdr.getZxid();
ZKDatabase.this.addCommittedProposal(r);
}
};
long zxid = this.snapLog.restore(this.dataTree, this.sessionsWithTimeouts, listener);
this.initialized = true;
return zxid;
}
然后开确定选类型，startLeaderElection
public synchronized void startLeaderElection() {
this.currentVote = new Vote(this.myid, getLastLoggedZxid());
for (QuorumServer p : getView().values()) {
if (p.id == this.myid) {
this.myQuorumAddr = p.addr;
break;
}
}
if (this.myQuorumAddr == null) {
throw new RuntimeException("My id " + this.myid + " not in the peer list");
}
if (this.electionType == 0) {
try {
this.udpSocket = new DatagramSocket(this.myQuorumAddr.getPort());
this.responder = new ResponderThread();
this.responder.start();
} catch (SocketException e) {
throw new RuntimeException(e);
}
}
this.electionAlg = createElectionAlgorithm(this.electionType);//加载选举类型
}
然后启动run方法