最近在測試HCatalog，由于Hcatalog本身就是一個獨立JAR包，雖然它也可以運行service，但是其實這個service就是metastore thrift server，我們在寫基于Hcatalog的mapreduce job時候只要把hcatalog JAR包和對應的hive-site.xml文件加入libjars和HADOOP_CLASSPATH中就可以了。 不過在測試的時候還是遇到了一些問題，hive metastore server在運行了一段時間后會拋如下錯誤
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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)
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其中PersistenceManager負責控制一組持久化對象包括創建持久化對象和查詢對象，它是ObjectStore的一個實例變量，每個ObjectStore擁有一個pm，RawStore是metastore邏輯層和物理底層元數據庫（比如derby）交互的接口類，ObjectStore是RawStore的默認實現類。Hive Metastore Server啟動的時候會指定一個TProcessor，包裝了一個HMSHandler，內部有一個ThreadLocal<RawStore> threadLocalMS實例變量，每個thread維護一個RawStore
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private final ThreadLocal<RawStore> threadLocalMS =
new ThreadLocal<RawStore>() {
@Override
protected synchronized RawStore initialValue() {
return null;
}
};
每一個從hive metastore client過來的請求都會從線程池中分配一個
WorkerProcess來處理，在HMSHandler中每一個方法都會通過getMS()獲取rawstore instance來做具體操作
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public RawStore getMS() throws MetaException {
RawStore ms = threadLocalMS.get();
if (ms == null) {
ms = newRawStore();
threadLocalMS.set(ms);
ms = threadLocalMS.get();
}
return ms;
}
看得出來RawStore是延遲加載，初始化后綁定到threadlocal變量中可以為以后復用
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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使用了動態代理模式(繼承
InvocationHandler接口
)，內部實現了invoke函數，通過method.invoke()執行真正的邏輯，這樣的好處是可以在
method.invoke()上下文中添加自己其他的邏輯，RetryingRawStore就是在通過捕捉invoke函數拋出的異常，來達到重試的效果。由于使用reflection機制，異常是wrap在
InvocationTargetException中的，
不過在hive 0.9中竟然在捕捉到
此異常后直接throw出來了，而不是retry，明顯不對啊。我對它修改了下，拿出wrap的target exception，判斷是不是instance of jdoexception的，再做相應的處理
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@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有兩種方式，一種是在
RetryingRawStore的構造函數中調用"
this.base = (RawStore) ReflectionUtils.newInstance(rawStoreClass, conf);
" ?因為ObjectStore實現了Configurable，在newInstance方法中主動調用里面的setConf(conf)方法初始化RawStore，還有一種情況是在捕捉到異常后retry，也會調用
base.setConf(getConf());
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private void initMS() {
base.setConf(getConf());
}
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ObjectStore的setConf方法中，先將PersistenceManagerFactory鎖住，pm close掉，設置成NULL，再初始化pm
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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();
}
}
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private void initialize(Properties dsProps) {
LOG.info("ObjectStore, initialize called");
prop = dsProps;
pm = getPersistenceManager();
isInitialized = pm != null;
return;
}
回到一開始報錯的那段信息，怎么會Persistence Manager會被關閉呢，仔細排查后才發現是由于HCatalog使用HiveMetastoreClient用完后主動調用了close方法，而一般Hive里面內部不會調這個方法.
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HiveMetaStoreClient.java
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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();
}
}
對應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
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public void shutdown() {
if (pm != null) {
pm.close();
}
}
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我們看到shutdown方法里面只是把當前thread的ObjectStore拿出來后，做了一個ObjectStore shutdown方法，把pm關閉了。但是并沒有把ObjectStore銷毀掉，它還是存在于threadLocalMS中，下次還是會被拿出來，下一次這個thread服務于另外一個請求的時候又會被get出ObjectSture來，但是由于里面的pm已經close掉了所以肯定拋異常。正確的做法是應該加上threadLocalMS.remove()或者threadLocalMS.set(null)，主動將其從ThreadLocalMap中刪除。
修改后的 shutdown方法
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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.");
}
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