[HBase]Region location

HBase的table是该region切分的，client操作一个row的时候，如何知道这个row对应的region是在哪台Region server上呢？这里有个region location过程。主要涉及到2张系统表，-ROOT-，.META.。其结构见图

 

在zookeeper的/hbase/root-region-server节点中存着-ROOT-表所在的Region Server地址。

-ROOT-表的一个row代表着META的一个region信息，其key的结构是META表名,META表Region的startkey，RegionId。其value的主要保存regioninfo和server信息。ROOT表不能split

.META.表的一个row代表着用户表的一个region信息，其key的结构是其实就是用户表的regionName，用户表名，startKey，RegionId。其value同样保存着regioninfo和server信息。META表可以split，但是一个region默认有128M，可以存上亿个用户表的region信息，所以一般不会split。

其查找过程如下：

1.通过zk getData拿-ROOT-表的location

2.RPC -ROOT-表的rs，getClosestRowBefore，拿row对应的meta表的region location

3.RPC .META.表的某一个region，拿该row在真实table所在的region location

4.RPC对应region

 

region location需要3次网络IO，为了提升性能，client会cache数据。

LocationCache是一个2级Map，第一级的key是tableName的hash值，第二级的key是starRow，用SoftValueSortedMap包装了TreeMap实现，用软引用实现cache，内存不够时才会回收。Cache里存在META表和用户表的region location信息。

其代码实现如下，0.94版本：

HConnectionManager locateRegion入口

private HRegionLocation locateRegion(final byte [] tableName,
      final byte [] row, boolean useCache)
    throws IOException {
	.......
	//检查下都应的zkTracker是否启动
      ensureZookeeperTrackers();
	//如果是-ROOT-表，则通过zk节点/hbase/root-region-server获取-ROOT-表所在的Location
      if (Bytes.equals(tableName, HConstants.ROOT_TABLE_NAME)) {
        try {
		//通过zk的getData接口拿节点数据，此处会等待节点数据就位或者超时
          ServerName servername = this.rootRegionTracker.waitRootRegionLocation(this.rpcTimeout);
          LOG.debug("Looked up root region location, connection=" + this +
            "; serverName=" + ((servername == null)? "": servername.toString()));
          if (servername == null) return null;
	//返回一个拼装的HRegionLocation，因为-ROOT-表只有一个region，而且不会split
          return new HRegionLocation(HRegionInfo.ROOT_REGIONINFO,
            servername.getHostname(), servername.getPort());
        } catch (InterruptedException e) {
          Thread.currentThread().interrupt();
          return null;
        }
      } 
	//如果是.META.表，则请求.META.表，这里的row其实就是请求row拼装的regionName，类似test,key1,99999999999999
	//如果没命中cache，则继续请求-ROOT-表，拿到这个row对应的.META.表的region location
      else if (Bytes.equals(tableName, HConstants.META_TABLE_NAME)) {
        return locateRegionInMeta(HConstants.ROOT_TABLE_NAME, tableName, row,
            useCache, metaRegionLock);
      } 
	//如果是用户表，则请求用户表，这里的row就是key1
	//如果没命中cache，则请求.META.表，获取该row对应的region location
      else {
        // Region not in the cache - have to go to the meta RS
        return locateRegionInMeta(HConstants.META_TABLE_NAME, tableName, row,
            useCache, userRegionLock);
      }
    }

 locateRegionInMeta方法

private HRegionLocation locateRegionInMeta(final byte [] parentTable,
      final byte [] tableName, final byte [] row, boolean useCache,
      Object regionLockObject)
    throws IOException {
      HRegionLocation location;
      // If we are supposed to be using the cache, look in the cache to see if
      // we already have the region.
	//先读cache，cache没有再往上找
	//注意如果rowkey的region locatin变化了，RPC的时候会失败，客户端做重试的时候useCache是false
      if (useCache) {
        location = getCachedLocation(tableName, row);
        if (location != null) {
          return location;
        }
      }

      // 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.
	//先拼一个想查找的key，类似于test,key1,99999999999999
      byte [] metaKey = HRegionInfo.createRegionName(tableName, row,
        HConstants.NINES, false);
	//默认重试10次
      for (int tries = 0; true; tries++) {
	//找不到
        if (tries >= numRetries) {
          throw new NoServerForRegionException("Unable to find region for "
            + Bytes.toStringBinary(row) + " after " + numRetries + " tries.");
        }
	
        HRegionLocation metaLocation = null;
        try {
          // locate the root or meta region
		//递归查找parentTable
          metaLocation = locateRegion(parentTable, metaKey);
          // If null still, go around again.
          if (metaLocation == null) continue;
		//找到对应Region server地址之后，可以发起RPC请求了。
		//这里先生成一个RPC Proxy对象，具体RPC分析见后文
          HRegionInterface server =
            getHRegionConnection(metaLocation.getHostname(), metaLocation.getPort());

          Result regionInfoRow = null;
          // This block guards against two threads trying to load the meta
          // region at the same time. The first will load the meta region and
          // the second will use the value that the first one found.
          synchronized (regionLockObject) {
            // If the parent table is META, we may want to pre-fetch some
            // region info into the global region cache for this table.
		//如果parentTable是.META.表，则预先获取.META.的一些数据，默认10条
            if (Bytes.equals(parentTable, HConstants.META_TABLE_NAME) &&
                (getRegionCachePrefetch(tableName)) )  {
              prefetchRegionCache(tableName, row);
            }

            // Check the cache again for a hit in case some other thread made the
            // same query while we were waiting on the lock. If not supposed to
            // be using the cache, delete any existing cached location so it won't
            // interfere.
            if (useCache) {
              location = getCachedLocation(tableName, row);
              if (location != null) {
                return location;
              }
            } 
		//如果不使用cache，则清除之，比如row对应的region发生了分裂，用老的location启动rpc时会抛异常，此时通过useCache=fasle重新
		//寻址，并把老的cache删掉
	    else {
              deleteCachedLocation(tableName, row);
            }

            // Query the root or meta region for the location of the meta region
		//发起RPC请求，获取<=该key的行
            regionInfoRow = server.getClosestRowBefore(
            metaLocation.getRegionInfo().getRegionName(), metaKey,
            HConstants.CATALOG_FAMILY);
          }
          if (regionInfoRow == null) {
            throw new TableNotFoundException(Bytes.toString(tableName));
          }
		//region信息，做校验，region会处于不稳定状态
          byte [] value = regionInfoRow.getValue(HConstants.CATALOG_FAMILY,
              HConstants.REGIONINFO_QUALIFIER);
          if (value == null || value.length == 0) {
            throw new IOException("HRegionInfo was null or empty in " +
              Bytes.toString(parentTable) + ", row=" + regionInfoRow);
          }
          // convert the row result into the HRegionLocation we need!
		//反序列化
          HRegionInfo regionInfo = (HRegionInfo) Writables.getWritable(
              value, new HRegionInfo());
          // possible we got a region of a different table...
		//一些校验
          if (!Bytes.equals(regionInfo.getTableName(), tableName)) {
            throw new TableNotFoundException(
                  "Table '" + Bytes.toString(tableName) + "' was not found, got: " +
                  Bytes.toString(regionInfo.getTableName()) + ".");
          }
          if (regionInfo.isSplit()) {
            throw new RegionOfflineException("the only available region for" +
              " the required row is a split parent," +
              " the daughters should be online soon: " +
              regionInfo.getRegionNameAsString());
          }
          if (regionInfo.isOffline()) {
            throw new RegionOfflineException("the region is offline, could" +
              " be caused by a disable table call: " +
              regionInfo.getRegionNameAsString());
          }
		//该region的server location
          value = regionInfoRow.getValue(HConstants.CATALOG_FAMILY,
              HConstants.SERVER_QUALIFIER);
          String hostAndPort = "";
          if (value != null) {
            hostAndPort = Bytes.toString(value);
          }
          ......

          // Instantiate the location
          String hostname = Addressing.parseHostname(hostAndPort);
          int port = Addressing.parsePort(hostAndPort);
          location = new HRegionLocation(regionInfo, hostname, port);
		//cache之
          cacheLocation(tableName, location);
          return location;
        } 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 (IOException e) {
          if (e instanceof RemoteException) {
            e = RemoteExceptionHandler.decodeRemoteException((RemoteException) e);
          }
          if (tries < numRetries - 1) {
            .......
          } else {
            throw e;
          }
          // Only relocate the parent region if necessary
		//网络有问题，则重新找
          if(!(e instanceof RegionOfflineException ||
              e instanceof NoServerForRegionException)) {
            relocateRegion(parentTable, metaKey);
          }
        }
		//重试次数越多，sleep越长，interrupt则退出重试
        try{
          Thread.sleep(ConnectionUtils.getPauseTime(this.pause, tries));
        } catch (InterruptedException e) {
          Thread.currentThread().interrupt();
          throw new IOException("Giving up trying to location region in " +
            "meta: thread is interrupted.");
        }
      }
    }