大数据：Spark Storage（一） 集群下的区块管理

Storage模块

在Spark中提及最多的是RDD，而RDD所交互的数据是通过Storage来实现和管理

Storage模块整体架构

1. 存储层

在Spark里，单节点的Storage的管理是通过block来管理的，每个Block的存储可以在内存里或者在磁盘中，在BlockManager里既可以管理内存的存储，同时也管理硬盘的存储，存储的标识是通过块的ID来区分的。

2. 集群下的架构

2.1 架构

在集群下Spark的Block的管理架构使用Master-Slave模式

• Master : 拥有所有block的具体信息（本地和Slave节点）
• Slave ： 通过master获取block的信息，并且汇报自己的信息

这里的Master并不是Spark集群中分配任务的Master，而是 提交task的客户端Driver，这里并没有主备设计，因为Driver client是单点的，通常Driver client crash了，计算也没有结果了，在Storage 的集群管理中Master是由driver承担。

在Executor在运行task的时候，通过blockManager获取本地的block块，如果本地找不到，尝试通过master去获取远端的块

 for (pid <- Random.shuffle(Seq.range(0, numBlocks))) {
      val pieceId = BroadcastBlockId(id, "piece" + pid)
      logDebug(s"Reading piece $pieceId of $broadcastId")
      // First try getLocalBytes because there is a chance that previous attempts to fetch the
      // broadcast blocks have already fetched some of the blocks. In that case, some blocks
      // would be available locally (on this executor).
      bm.getLocalBytes(pieceId) match {
        case Some(block) =>
          blocks(pid) = block
          releaseLock(pieceId)
        case None =>
          bm.getRemoteBytes(pieceId) match {
            case Some(b) =>
              if (checksumEnabled) {
                val sum = calcChecksum(b.chunks(0))
                if (sum != checksums(pid)) {
                  throw new SparkException(s"corrupt remote block $pieceId of $broadcastId:" +
                    s" $sum != ${checksums(pid)}")
                }
              }
              // We found the block from remote executors/driver's BlockManager, so put the block
              // in this executor's BlockManager.
              if (!bm.putBytes(pieceId, b, StorageLevel.MEMORY_AND_DISK_SER, tellMaster = true)) {
                throw new SparkException(
                  s"Failed to store $pieceId of $broadcastId in local BlockManager")
              }
              blocks(pid) = b
            case None =>
              throw new SparkException(s"Failed to get $pieceId of $broadcastId")
          }
      }
    }

2.2 Executor获取块内容的位置

唯一的 blockID: 

broadcast_0_piece0
请求Master获取该BlockID所在的 Location，也就是BlockManagerId的集合

/** Get locations of the blockId from the driver */
  def getLocations(blockId: BlockId): Seq[BlockManagerId] = {
    driverEndpoint.askWithRetry[Seq[BlockManagerId]](GetLocations(blockId))
  }

唯一的 BlockManagerId

BlockManagerId(driver, 192.168.121.101, 55153, None)

Executor ID, executor ID, 对driver来说就是driver

Host： executor/driver IP

Port:    executor/driver Port

每一个executor, 和driver 都生成唯一的BlockManagerId

2.3 Executor获取块的内容

def getRemoteBytes(blockId: BlockId): Option[ChunkedByteBuffer] = {
    logDebug(s"Getting remote block $blockId")
    require(blockId != null, "BlockId is null")
    var runningFailureCount = 0
    var totalFailureCount = 0
    val locations = getLocations(blockId)
    val maxFetchFailures = locations.size
    var locationIterator = locations.iterator
    while (locationIterator.hasNext) {
      val loc = locationIterator.next()
      logDebug(s"Getting remote block $blockId from $loc")
      val data = try {
        blockTransferService.fetchBlockSync(
          loc.host, loc.port, loc.executorId, blockId.toString).nioByteBuffer()
      } catch {
        case NonFatal(e) =>
          runningFailureCount += 1
          totalFailureCount += 1

          if (totalFailureCount >= maxFetchFailures) {
            // Give up trying anymore locations. Either we've tried all of the original locations,
            // or we've refreshed the list of locations from the master, and have still
            // hit failures after trying locations from the refreshed list.
            logWarning(s"Failed to fetch block after $totalFailureCount fetch failures. " +
              s"Most recent failure cause:", e)
            return None
          }

          logWarning(s"Failed to fetch remote block $blockId " +
            s"from $loc (failed attempt $runningFailureCount)", e)

          // If there is a large number of executors then locations list can contain a
          // large number of stale entries causing a large number of retries that may
          // take a significant amount of time. To get rid of these stale entries
          // we refresh the block locations after a certain number of fetch failures
          if (runningFailureCount >= maxFailuresBeforeLocationRefresh) {
            locationIterator = getLocations(blockId).iterator
            logDebug(s"Refreshed locations from the driver " +
              s"after ${runningFailureCount} fetch failures.")
            runningFailureCount = 0
          }

          // This location failed, so we retry fetch from a different one by returning null here
          null
      }

      if (data != null) {
        return Some(new ChunkedByteBuffer(data))
      }
      logDebug(s"The value of block $blockId is null")
    }
    logDebug(s"Block $blockId not found")
    None
  }

通过获取的BlockManagerId的集合列表，顺序的从列表中取出一个拥有该Block的服务器，通过

blockTransferService.fetchBlockSync(
          loc.host, loc.port, loc.executorId, blockId.toString).nioByteBuffer()

同步的获取块的内容，如果该块不存在，则换下一个拥有该Block的服务器

2.4 BlockManager注册

Driver 初始化SparkContext.init 的时候，会初始化BlockManager.initialize

val idFromMaster = master.registerBlockManager(
      id,
      maxMemory,
      slaveEndpoint)

会通过master 注册BlockManager

  def registerBlockManager(
      blockManagerId: BlockManagerId,
      maxMemSize: Long,
      slaveEndpoint: RpcEndpointRef): BlockManagerId = {
    logInfo(s"Registering BlockManager $blockManagerId")
    val updatedId = driverEndpoint.askWithRetry[BlockManagerId](
      RegisterBlockManager(blockManagerId, maxMemSize, slaveEndpoint))
    logInfo(s"Registered BlockManager $updatedId")
    updatedId
  }

在BlockManagerMaster里，我们看到了endpoint是强制的driver，也就是默认是driver 是master

无论driver,还是executor都是初始化后BlockManager，发消息给driver master进行注册，唯一不同的是driver标识自己的ID是driver，而executor是按照executor id来标识自己的

2.5 Driver Master的endpoint

前面一节已经介绍过无论driver还是executor 都会发送消息到Driver的Master，在Driver 和Executor里SparkEnv.create的时候会初始化BlockManagerMaster

val blockManagerMaster = new BlockManagerMaster(registerOrLookupEndpoint(
      BlockManagerMaster.DRIVER_ENDPOINT_NAME,
      new BlockManagerMasterEndpoint(rpcEnv, isLocal, conf, listenerBus)),
      conf, isDriver)

注册一个lookup的endpoint

def registerOrLookupEndpoint(
        name: String, endpointCreator: => RpcEndpoint):
      RpcEndpointRef = {
      if (isDriver) {
        logInfo("Registering " + name)
        rpcEnv.setupEndpoint(name, endpointCreator)
      } else {
        RpcUtils.makeDriverRef(name, conf, rpcEnv)
      }
    }

代码中可以看到只有isDriver的时候才会setup一个rpc的endpoint，默认是netty的rpc环境，命名为：BlockManagerMaster

spark://[email protected]:40978

所有的driver, executor都会向master 40978发消息

2.6 Master和Executor消息格式

下面的代码每个case都是master和executor的消息格式

override def receiveAndReply(context: RpcCallContext): PartialFunction[Any, Unit] = {
    case RegisterBlockManager(blockManagerId, maxMemSize, slaveEndpoint) =>
      context.reply(register(blockManagerId, maxMemSize, slaveEndpoint))

    case _updateBlockInfo @
        UpdateBlockInfo(blockManagerId, blockId, storageLevel, deserializedSize, size) =>
      context.reply(updateBlockInfo(blockManagerId, blockId, storageLevel, deserializedSize, size))
      listenerBus.post(SparkListenerBlockUpdated(BlockUpdatedInfo(_updateBlockInfo)))

    case GetLocations(blockId) =>
      context.reply(getLocations(blockId))

    case GetLocationsMultipleBlockIds(blockIds) =>
      context.reply(getLocationsMultipleBlockIds(blockIds))

    case GetPeers(blockManagerId) =>
      context.reply(getPeers(blockManagerId))

    case GetExecutorEndpointRef(executorId) =>
      context.reply(getExecutorEndpointRef(executorId))

    case GetMemoryStatus =>
      context.reply(memoryStatus)

    case GetStorageStatus =>
      context.reply(storageStatus)

    case GetBlockStatus(blockId, askSlaves) =>
      context.reply(blockStatus(blockId, askSlaves))

    case GetMatchingBlockIds(filter, askSlaves) =>
      context.reply(getMatchingBlockIds(filter, askSlaves))

    case RemoveRdd(rddId) =>
      context.reply(removeRdd(rddId))

    case RemoveShuffle(shuffleId) =>
      context.reply(removeShuffle(shuffleId))

    case RemoveBroadcast(broadcastId, removeFromDriver) =>
      context.reply(removeBroadcast(broadcastId, removeFromDriver))

    case RemoveBlock(blockId) =>
      removeBlockFromWorkers(blockId)
      context.reply(true)

    case RemoveExecutor(execId) =>
      removeExecutor(execId)
      context.reply(true)

    case StopBlockManagerMaster =>
      context.reply(true)
      stop()

    case BlockManagerHeartbeat(blockManagerId) =>
      context.reply(heartbeatReceived(blockManagerId))

    case HasCachedBlocks(executorId) =>
      blockManagerIdByExecutor.get(executorId) match {
        case Some(bm) =>
          if (blockManagerInfo.contains(bm)) {
            val bmInfo = blockManagerInfo(bm)
            context.reply(bmInfo.cachedBlocks.nonEmpty)
          } else {
            context.reply(false)
          }
        case None => context.reply(false)
      }
  }

2.7 Master结构关系

在Master上会保存每一个executor所对应的BlockManagerID和BlockManagerInfo，而在BlockManagerInfo中保存了每个block的状态

Executor通过心跳主动汇报自己的状态，Master更新EndPoint中Executor的状态

Executor 中的block的状态更新也会汇报给Master，只是跟新Master状态，但不会通知其他的Executor

在Executor和Master交互中是Executor主动推和获取数据的，Master只是管理executor的状态，以及Block的所在的Driver、Executor的位置及其状态，负载较小，Master没有考虑可用性，通常Master节点就是提交任务的Driver的节点。