[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

 [HBase]Region location_第1张图片

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.");
        }
      }
    }

 

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