最近在测试HCatalog，由于Hcatalog本身就是一个独立JAR包，虽然它也可以运行service，但是其实这个service就是metastore thrift server，我们在写基于Hcatalog的mapreduce job时候只要把hcatalog JAR包和对应的hive-site.xml文件加入libjars和HADOOP_CLASSPATH中就可以了。 不过在测试的时候还是遇到了一些问题，hive metastore server在运行了一段时间后会抛如下错误

2013-06-19 10:35:51,718 ERROR server.TThreadPoolServer (TThreadPoolServer.java:run(182)) - Error occurred during processing of message.
javax.jdo.JDOFatalUserException: Persistence Manager has been closed
        at org.datanucleus.jdo.JDOPersistenceManager.assertIsOpen(JDOPersistenceManager.java:2124)
        at org.datanucleus.jdo.JDOPersistenceManager.currentTransaction(JDOPersistenceManager.java:315)
        at org.apache.hadoop.hive.metastore.ObjectStore.openTransaction(ObjectStore.java:294)
        at org.apache.hadoop.hive.metastore.ObjectStore.getTable(ObjectStore.java:732)
        at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
        at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
        at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
        at java.lang.reflect.Method.invoke(Method.java:597)
        at org.apache.hadoop.hive.metastore.RetryingRawStore.invoke(RetryingRawStore.java:111)
        at com.sun.proxy.$Proxy5.getTable(Unknown Source)
        at org.apache.hadoop.hive.metastore.HiveMetaStore$HMSHandler.get_table(HiveMetaStore.java:982)
        at org.apache.hadoop.hive.metastore.api.ThriftHiveMetastore$Processor$get_table.getResult(ThriftHiveMetastore.java:5017)
        at org.apache.hadoop.hive.metastore.api.ThriftHiveMetastore$Processor$get_table.getResult(ThriftHiveMetastore.java:5005)
        at org.apache.thrift.ProcessFunction.process(ProcessFunction.java:32)
        at org.apache.thrift.TBaseProcessor.process(TBaseProcessor.java:34)

其中PersistenceManager负责控制一组持久化对象包括创建持久化对象和查询对象，它是ObjectStore的一个实例变量，每个ObjectStore拥有一个pm，RawStore是metastore逻辑层和物理底层元数据库（比如derby）交互的接口类，ObjectStore是RawStore的默认实现类。Hive Metastore Server启动的时候会指定一个TProcessor，包装了一个HMSHandler，内部有一个ThreadLocal<RawStore> threadLocalMS实例变量，每个thread维护一个RawStore

    private final ThreadLocal<RawStore> threadLocalMS =
      new ThreadLocal<RawStore>() {
        @Override
        protected synchronized RawStore initialValue() {
          return null;
 每一个从hive metastore client过来的请求都会从线程池中分配一个
 WorkerProcess来处理，在HMSHandler中每一个方法都会通过getMS()获取rawstore instance来做具体操作
    public RawStore getMS() throws MetaException {
      RawStore ms = threadLocalMS.get();
      if (ms == null) {
        ms = newRawStore();
        threadLocalMS.set(ms);
        ms = threadLocalMS.get();
      return ms;
看得出来RawStore是延迟加载，初始化后绑定到threadlocal变量中可以为以后复用
    private RawStore newRawStore() throws MetaException {
      LOG.info(addPrefix("Opening raw store with implemenation class:"
          + rawStoreClassName));
      Configuration conf = getConf();
      return RetryingRawStore.getProxy(hiveConf, conf, rawStoreClassName, threadLocalId.get());
 RawStore使用了动态代理模式(继承
 InvocationHandler接口
 )，内部实现了invoke函数，通过method.invoke()执行真正的逻辑，这样的好处是可以在
 method.invoke()上下文中添加自己其他的逻辑，RetryingRawStore就是在通过捕捉invoke函数抛出的异常，来达到重试的效果。由于使用reflection机制，异常是wrap在
 InvocationTargetException中的，
 不过在hive 0.9中竟然在捕捉到
 此异常后直接throw出来了，而不是retry，明显不对啊。我对它修改了下，拿出wrap的target exception，判断是不是instance of jdoexception的，再做相应的处理

  @Override
  public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
    Object ret = null;
    boolean gotNewConnectUrl = false;
    boolean reloadConf = HiveConf.getBoolVar(hiveConf,
        HiveConf.ConfVars.METASTOREFORCERELOADCONF);
    boolean reloadConfOnJdoException = false;
    if (reloadConf) {
      updateConnectionURL(getConf(), null);
    int retryCount = 0;
    Exception caughtException = null;
    while (true) {
      try {
        if (reloadConf || gotNewConnectUrl || reloadConfOnJdoException) {
          initMS();
        ret = method.invoke(base, args);
        break;
      } catch (javax.jdo.JDOException e) {
        caughtException = (javax.jdo.JDOException) e.getCause();
      } catch (UndeclaredThrowableException e) {
        throw e.getCause();
      } catch (InvocationTargetException e) {
        Throwable t = e.getTargetException();
        if (t instanceof JDOException){
          caughtException = (JDOException) e.getTargetException();
          reloadConfOnJdoException = true;
          LOG.error("rawstore jdoexception:" + caughtException.toString());
        }else {
            throw e.getCause();
      if (retryCount >= retryLimit) {
        throw caughtException;
      assert (retryInterval >= 0);
      retryCount++;
      LOG.error(
          String.format(
              "JDO datastore error. Retrying metastore command " +
                  "after %d ms (attempt %d of %d)", retryInterval, retryCount, retryLimit));
      Thread.sleep(retryInterval);
      // If we have a connection error, the JDO connection URL hook might
      // provide us with a new URL to access the datastore.
      String lastUrl = getConnectionURL(getConf());
      gotNewConnectUrl = updateConnectionURL(getConf(), lastUrl);
    return ret;
 初始化RawStore有两种方式，一种是在
 RetryingRawStore的构造函数中调用"
 this.base = (RawStore) ReflectionUtils.newInstance(rawStoreClass, conf);
 "  因为ObjectStore实现了Configurable，在newInstance方法中主动调用里面的setConf(conf)方法初始化RawStore，还有一种情况是在捕捉到异常后retry，也会调用
 base.setConf(getConf());

private void initMS() {
    base.setConf(getConf());
 ObjectStore的setConf方法中，先将PersistenceManagerFactory锁住，pm close掉，设置成NULL，再初始化pm

public void setConf(Configuration conf) {
    // Although an instance of ObjectStore is accessed by one thread, there may
    // be many threads with ObjectStore instances. So the static variables
    // pmf and prop need to be protected with locks.
    pmfPropLock.lock();
    try {
      isInitialized = false;
      hiveConf = conf;
      Properties propsFromConf = getDataSourceProps(conf);
      boolean propsChanged = !propsFromConf.equals(prop);
      if (propsChanged) {
        pmf = null;
        prop = null;
      assert(!isActiveTransaction());
      shutdown();
      // Always want to re-create pm as we don't know if it were created by the
      // most recent instance of the pmf
      pm = null;
      openTrasactionCalls = 0;
      currentTransaction = null;
      transactionStatus = TXN_STATUS.NO_STATE;
      initialize(propsFromConf);
      if (!isInitialized) {
        throw new RuntimeException(
        "Unable to create persistence manager. Check dss.log for details");
      } else {
        LOG.info("Initialized ObjectStore");
    } finally {
      pmfPropLock.unlock();
private void initialize(Properties dsProps) {
    LOG.info("ObjectStore, initialize called");
    prop = dsProps;
    pm = getPersistenceManager();
    isInitialized = pm != null;
    return;
 回到一开始报错的那段信息，怎么会Persistence Manager会被关闭呢，仔细排查后才发现是由于HCatalog使用HiveMetastoreClient用完后主动调用了close方法，而一般Hive里面内部不会调这个方法.

 HiveMetaStoreClient.java
public void close() {
    isConnected = false;
    try {
      if (null != client) {
        client.shutdown();
    } catch (TException e) {
      LOG.error("Unable to shutdown local metastore client", e);
    // Transport would have got closed via client.shutdown(), so we dont need this, but
    // just in case, we make this call.
    if ((transport != null) && transport.isOpen()) {
      transport.close();
 对应server端HMSHandler中的shutdown方法
@Override
    public void shutdown() {
      logInfo("Shutting down the object store...");
      RawStore ms = threadLocalMS.get();
      if (ms != null) {
        ms.shutdown();
        ms = null;
      logInfo("Metastore shutdown complete.");
 ObjectStore的shutdown方法
public void shutdown() {
    if (pm != null) {
      pm.close();
 我们看到shutdown方法里面只是把当前thread的ObjectStore拿出来后，做了一个ObjectStore shutdown方法，把pm关闭了。但是并没有把ObjectStore销毁掉，它还是存在于threadLocalMS中，下次还是会被拿出来，下一次这个thread服务于另外一个请求的时候又会被get出ObjectSture来，但是由于里面的pm已经close掉了所以肯定抛异常。正确的做法是应该加上threadLocalMS.remove()或者threadLocalMS.set(null)，主动将其从ThreadLocalMap中删除。
 shutdown方法
public void shutdown() {
      logInfo("Shutting down the object store...");
      RawStore ms = threadLocalMS.get();
      if (ms != null) {
        ms.shutdown();
        ms = null;
        threadLocalMS.remove();
      logInfo("Metastore shutdown complete.");