- 前言
- 介绍
- 总结
前言
此篇文章将继续叙述Region的定位过程
介绍
在分析locateRegion()方法前，我们先折回去看看使用缓存情况的处理，它也是调用的locateRegion()方法，只不过传入的使用缓存标志位useCache为true这一个区别而已，好了，殊途同归，这里我们就只研究locateRegion()方法就行了，代码如下：
@Override
public RegionLocations locateRegion(final TableName tableName, final byte[] row, boolean useCache,
boolean retry) throws IOException {
return locateRegion(tableName, row, useCache, retry, RegionReplicaUtil.DEFAULT_REPLICA_ID);
}
@Override
public RegionLocations locateRegion(final TableName tableName, final byte[] row, boolean useCache,
boolean retry, int replicaId) throws IOException {
checkClosed();
if (tableName == null || tableName.getName().length == 0) {
throw new IllegalArgumentException("table name cannot be null or zero length");
}
if (tableName.equals(TableName.meta_TABLE_NAME)) {
return locatemeta(tableName, useCache, replicaId);
} else {
// Region not in the cache - have to go to the meta RS
return locateRegionInmeta(tableName, row, useCache, retry, replicaId);
}
}
locateRegion()方法上来先做一些必要的检查：
1、判断连接是否已关闭的标志位closed，为true则直接抛出IOException异常；
2、判断表名tableName，表名为空的话直接抛出IllegalArgumentException异常。
然后，根据表是否为meta表，做以下处理：
1、如果是meta表，直接调用locatemeta()方法进行定位；
2、如果不是meta表，cache中没有，需要访问meta RS，调用locateRegionInmeta()方法进行定位；
我们今天先看非meta表，进入locateRegionInmeta()方法，代码如下：
private RegionLocations locateRegionInmeta(TableName tableName, byte[] row, boolean useCache,
boolean retry, int replicaId) throws IOException {
// If we are supposed to be using the cache, look in the cache to see if we already have the
// region.
if (useCache) {
RegionLocations locations = getCachedLocation(tableName, row);
if (locations != null && locations.getRegionLocation(replicaId) != null) {
return locations;
}
}
// build the key of the meta region we should be looking for.
// the extra 9’s on the end are necessary to allow “exact” matches
// without knowing the precise region names.
byte[] metaStartKey = RegionInfo.createRegionName(tableName, row, HConstants.NINES, false);
byte[] metaStopKey =
RegionInfo.createRegionName(tableName, HConstants.EMPTY_START_ROW, “”, false);
Scan s = new Scan().withStartRow(metaStartKey).withStopRow(metaStopKey, true)
.addFamily(HConstants.CATALOG_FAMILY).setReversed(true).setCaching(5)
.setReadType(ReadType.PREAD);
switch (this.metaReplicaMode) {
case LOAD_BALANCE:
int metaReplicaId = this.metaReplicaSelector.select(tableName, row,
RegionLocateType.CURRENT);
if (metaReplicaId != RegionInfo.DEFAULT_REPLICA_ID) {
// If the selector gives a non-primary meta replica region, then go with it.
// Otherwise, just go to primary in non-hedgedRead mode.
s.setConsistency(Consistency.TIMELINE);
s.setReplicaId(metaReplicaId);
}
break;
case HEDGED_READ:
s.setConsistency(Consistency.TIMELINE);
break;
default:
// do nothing
}
int maxAttempts = (retry ? numTries : 1);
boolean relocatemeta = false;
for (int tries = 0; ; tries++) {
if (tries >= maxAttempts) {
throw new NoServerForRegionException("Unable to find region for "
+ Bytes.toStringBinary(row) + " in " + tableName + " after " + tries + " tries.");
}
if (useCache) {
RegionLocations locations = getCachedLocation(tableName, row);
if (locations != null && locations.getRegionLocation(replicaId) != null) {
return locations;
}
} else {
// If we are not supposed to be using the cache, delete any existing cached location
// so it won't interfere.
// We are only supposed to clean the cache for the specific replicaId
metaCache.clearCache(tableName, row, replicaId);
}
// Query the meta region
long pausebase = this.pause;
takeUserRegionLock();
try {
// We don't need to check if useCache is enabled or not. Even if useCache is false
// we already cleared the cache for this row before acquiring userRegion lock so if this
// row is present in cache that means some other thread has populated it while we were
// waiting to acquire user region lock.
RegionLocations locations = getCachedLocation(tableName, row);
if (locations != null && locations.getRegionLocation(replicaId) != null) {
return locations;
}
if (relocatemeta) {
relocateRegion(TableName.meta_TABLE_NAME, HConstants.EMPTY_START_ROW,
RegionInfo.DEFAULT_REPLICA_ID);
}
s.resetMvccReadPoint();
try (ReversedClientScanner rcs =
new ReversedClientScanner(conf, s, TableName.meta_TABLE_NAME, this, rpcCallerFactory,
rpcControllerFactory, getmetaLookupPool(), metaReplicaCallTimeoutScanInMicroSecond)) {
boolean tableNotFound = true;
for (;;) {
Result regionInfoRow = rcs.next();
if (regionInfoRow == null) {
if (tableNotFound) {
throw new TableNotFoundException(tableName);
} else {
throw new IOException(
"Unable to find region for " + Bytes.toStringBinary(row) + " in " + tableName);
}
}
tableNotFound = false;
// convert the row result into the HRegionLocation we need!
locations = metaTableAccessor.getRegionLocations(regionInfoRow);
if (locations == null || locations.getRegionLocation(replicaId) == null) {
throw new IOException("RegionInfo null in " + tableName + ", row=" + regionInfoRow);
}
RegionInfo regionInfo = locations.getRegionLocation(replicaId).getRegion();
if (regionInfo == null) {
throw new IOException("RegionInfo null or empty in " + TableName.meta_TABLE_NAME +
", row=" + regionInfoRow);
}
// See Hbase-20182. It is possible that we locate to a split parent even after the
// children are online, so here we need to skip this region and go to the next one.
if (regionInfo.isSplitParent()) {
continue;
}
if (regionInfo.isOffline()) {
throw new RegionOfflineException("Region offline; disable table call? " +
regionInfo.getRegionNameAsString());
}
// It is possible that the split children have not been online yet and we have skipped
// the parent in the above condition, so we may have already reached a region which does
// not contains us.
if (!regionInfo.containsRow(row)) {
throw new IOException(
"Unable to find region for " + Bytes.toStringBinary(row) + " in " + tableName);
}
ServerName serverName = locations.getRegionLocation(replicaId).getServerName();
if (serverName == null) {
throw new NoServerForRegionException("No server address listed in " +
TableName.meta_TABLE_NAME + " for region " + regionInfo.getRegionNameAsString() +
" containing row " + Bytes.toStringBinary(row));
}
if (isDeadServer(serverName)) {
throw new RegionServerStoppedException(
"hbase:meta says the region " + regionInfo.getRegionNameAsString() +
" is managed by the server " + serverName + ", but it is dead.");
}
// Instantiate the location
cacheLocation(tableName, locations);
return locations;
}
}
} catch (TableNotFoundException e) {
// if we got this error, probably means the table just plain doesn't
// exist. rethrow the error immediately. this should always be coming
// from the HTable constructor.
throw e;
} catch (LocalConnectionClosedException cce) {
// LocalConnectionClosedException is specialized instance of DoNotRetryIOE.
// Thrown when we check if this connection is closed. If it is, don't retry.
throw cce;
} catch (IOException e) {
ExceptionUtil.rethrowIfInterrupt(e);
if (e instanceof RemoteException) {
e = ((RemoteException)e).unwrapRemoteException();
}
if (e instanceof CallQueueTooBigException) {
// Give a special check on CallQueueTooBigException, see #Hbase-17114
pausebase = this.pauseForCQTBE;
}
if (tries < maxAttempts - 1) {
LOG.debug("locateRegionInmeta parentTable='{}', attempt={} of {} failed; retrying " +
"after sleep of {}", TableName.meta_TABLE_NAME, tries, maxAttempts, maxAttempts, e);
} else {
throw e;
}
// only relocate the parent region if necessary
relocatemeta =
!(e instanceof RegionOfflineException || e instanceof NoServerForRegionException);
} finally {
userRegionLock.unlock();
}
try{
Thread.sleep(ConnectionUtils.getPauseTime(pausebase, tries));
} catch (InterruptedException e) {
throw new InterruptedIOException("Giving up trying to location region in " +
"meta: thread is interrupted.");
}
}
}
locateRegionInmeta()方法是对非meta表中特定行row所在Region位置信息的检索，它本质上是通过检索Hbase中meta表数据来获取对应非meta表中行row对应的Region位置信息的，其处理逻辑如下：
1、根据标志位useCache确定：如果我们支持在缓存中查找，先在缓存中看看是否我们已经有该Region，调用的是getCachedLocation()方法，传入tableName和row即可，存在即返回，否则继续；
2、缓存中没有，构造一个scan，先根据表名tableName、行row、字符串"99999999999999"，调用HRegionInfo的createRegionName()方法，创建一个Region Name：metaKey；
3、构造一个Scan，scan的起始行为上述metaKey，并且是一个反向小scan，即reversed small Scan；
4、确定重试上限次数localNumRetries：如果标志位retry为true的话，重试上限次数localNumRetries取numTries，即取参数hbase.client.retries.number，参数未配置的话默认为31；
5、在一个循环内，当重试次数tries未达到上限localNumRetries且未定位到对应Region位置信息时：
5.1、先判断重试次数tries是否达到上限localNumRetries，达到的话，直接抛出NoServerForRegionException异常；
.2、根据是否支持从缓存中取来判断：
5.2.1、如果支持使用缓存的话，每次再从缓存中取一遍，存在即返回，否则继续；
5.2.2、如果我们不支持使用缓存，删除任何存在的相关缓存，以确保它不会干扰我们的查询，调用metaCache的clearCache()方法，根据tableName和row来删除；
5.3、构造ClientSmallReversedScanner实例rcs，从meta表中查找，而meta表的表名固定为hbase:meta，它的namespace为"meta"，qualifier为"meta"，获取scanner，注意，这一步实际上是一个内嵌的scan，它也需要根据表和行进行Region的定位，而这个表就是Hbase中的meta表，既然从meta表中查找数据，那么就又折回到上面针对meta表和非meta标的的if…else…判断了，关于meta表的定位我们稍等再讲；
5.4、通过scanner的next()方法，获取唯一的结果regionInfoRow；
5.5、关闭ClientSmallReversedScanner；
5.6、如果regionInfoRow为空，直接抛出TableNotFoundException异常；
5.7、将Result转换为我们需要的RegionLocations，即regionInfoRow->locations；
5.8、从locations中获取Region信息HRegionInfo；
5.9、做一些必要的数据和状态校验，比如：
5.9.1、验证表名是否一致；
5.9.2、验证Rgion是否已分裂；
5.9.3、验证Rgion是否已下线；
5.9.4、从locations中根据replicaId获取ServerName，验证ServerName是否已死亡；
5.10、调用cacheLocation()方法缓存获得的位置信息locations，并返回；
5.11、如果中间出现异常，则当前线程休眠一段时间，再次重试，休眠的时间与pause和tries有关，越往后，停顿时间一般越长（波动时间除外）。
总结以上就是今天要讲的内容，本文介绍了将需要读写的行Row准确的定位到其所在Region和RegionServer上，Hbase是如何实现数据的检索的。
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