private[spark] case class PutResult(size: Long, data: Either[Iterator[_], ByteBuffer])

/**
 * Abstract class to store blocks
 */
private[spark]
abstract class BlockStore(val blockManager: BlockManager) extends Logging {
  def putBytes(blockId: String, bytes: ByteBuffer, level: StorageLevel)

  /**
   * Put in a block and, possibly, also return its content as either bytes or another Iterator.
   * This is used to efficiently write the values to multiple locations (e.g. for replication).
   *
   * @return a PutResult that contains the size of the data, as well as the values put if
   *         returnValues is true (if not, the result's data field can be null)
   */
  def putValues(blockId: String, values: ArrayBuffer[Any], level: StorageLevel, 
    returnValues: Boolean) : PutResult

  /**
   * Return the size of a block in bytes.
   */
  def getSize(blockId: String): Long

  def getBytes(blockId: String): Option[ByteBuffer]

  def getValues(blockId: String): Option[Iterator[Any]]

  /**
   * Remove a block, if it exists.
   * @param blockId the block to remove.
   * @return True if the block was found and removed, False otherwise.
   */
  def remove(blockId: String): Boolean

  def contains(blockId: String): Boolean

  def clear() { }
}

/**
 * Stores BlockManager blocks on disk.
 */
private class DiskStore(blockManager: BlockManager, rootDirs: String)
  extends BlockStore(blockManager) with Logging {

  override def putBytes(blockId: String, _bytes: ByteBuffer, level: StorageLevel) {
    // So that we do not modify the input offsets !
    // duplicate does not copy buffer, so inexpensive
    val bytes = _bytes.duplicate()
    val file = createFile(blockId)
    val channel = new RandomAccessFile(file, "rw").getChannel()
    while (bytes.remaining > 0) {
      channel.write(bytes)
    }
    channel.close()
  }

  override def putValues(
      blockId: String,
      values: ArrayBuffer[Any],
      level: StorageLevel,
      returnValues: Boolean)
    : PutResult = {
    val file = createFile(blockId)
    val fileOut = blockManager.wrapForCompression(blockId,
      new FastBufferedOutputStream(new FileOutputStream(file)))
    val objOut = blockManager.defaultSerializer.newInstance().serializeStream(fileOut)
    objOut.writeAll(values.iterator)
    objOut.close()
    val length = file.length()

    if (returnValues) {
      // Return a byte buffer for the contents of the file
      val buffer = getFileBytes(file)
      PutResult(length, Right(buffer))
    } else {
      PutResult(length, null)
    }
  }

  override def getBytes(blockId: String): Option[ByteBuffer] = {
    val file = getFile(blockId)
    val bytes = getFileBytes(file)
    Some(bytes)
  }

  override def getValues(blockId: String): Option[Iterator[Any]] = {
    getBytes(blockId).map(bytes => blockManager.dataDeserialize(blockId, bytes))
  }

}

/**
 * Stores blocks in memory, either as ArrayBuffers of deserialized Java objects or as
 * serialized ByteBuffers.
 */
private class MemoryStore(blockManager: BlockManager, maxMemory: Long)
  extends BlockStore(blockManager) {
  // 使用Entry来表示block内容
  case class Entry(value: Any, size: Long, deserialized: Boolean, var dropPending: Boolean = false) 

  private val entries = new LinkedHashMap[String, Entry](32, 0.75f, true) // 使用LinkedHashMap来表示整个MemoryStore
  private var currentMemory = 0L
  // Object used to ensure that only one thread is putting blocks and if necessary, dropping
  // blocks from the memory store.
  private val putLock = new Object() // HashMap不是线程安全的, 需要锁同步

  override def putBytes(blockId: String, _bytes: ByteBuffer, level: StorageLevel) {
    // Work on a duplicate - since the original input might be used elsewhere.
    val bytes = _bytes.duplicate()
    bytes.rewind()  // 对于NIO的ByteBuffer, 使用前最好rewind
    if (level.deserialized) { // 如果storage level需要非序列化的
      val values = blockManager.dataDeserialize(blockId, bytes) // 需要先反序列化
      val elements = new ArrayBuffer[Any]
      elements ++= values
      val sizeEstimate = SizeEstimator.estimate(elements.asInstanceOf[AnyRef])
      tryToPut(blockId, elements, sizeEstimate, true)
    } else {
      tryToPut(blockId, bytes, bytes.limit, false)
    }
  }

  // putValues的返回值取决于storage level, 如果是deserialized, 返回iterator, 否则ByteBuffer
  override def putValues(
      blockId: String,
      values: ArrayBuffer[Any],
      level: StorageLevel,
      returnValues: Boolean)
    : PutResult = {
    if (level.deserialized) {
      val sizeEstimate = SizeEstimator.estimate(values.asInstanceOf[AnyRef])
      tryToPut(blockId, values, sizeEstimate, true)
      PutResult(sizeEstimate, Left(values.iterator))
    } else {
      val bytes = blockManager.dataSerialize(blockId, values.iterator)
      tryToPut(blockId, bytes, bytes.limit, false)
      PutResult(bytes.limit(), Right(bytes.duplicate()))
    }
  }

  override def getBytes(blockId: String): Option[ByteBuffer] = {
    val entry = entries.synchronized {
      entries.get(blockId)
    }
    if (entry == null) {
      None
    } else if (entry.deserialized) {
      Some(blockManager.dataSerialize(blockId, entry.value.asInstanceOf[ArrayBuffer[Any]].iterator))
    } else {
      Some(entry.value.asInstanceOf[ByteBuffer].duplicate())   // Doesn't actually copy the data
    }
  }

  override def getValues(blockId: String): Option[Iterator[Any]] = {
    val entry = entries.synchronized {
      entries.get(blockId)
    }
    if (entry == null) {
      None
    } else if (entry.deserialized) {
      Some(entry.value.asInstanceOf[ArrayBuffer[Any]].iterator)
    } else {
      val buffer = entry.value.asInstanceOf[ByteBuffer].duplicate() // Doesn't actually copy data
      Some(blockManager.dataDeserialize(blockId, buffer))
    }
  }

  /**
   * Try to put in a set of values, if we can free up enough space. The value should either be
   * an ArrayBuffer if deserialized is true or a ByteBuffer otherwise. Its (possibly estimated)
   * size must also be passed by the caller.
   *
   * Locks on the object putLock to ensure that all the put requests and its associated block
   * dropping is done by only on thread at a time. Otherwise while one thread is dropping
   * blocks to free memory for one block, another thread may use up the freed space for
   * another block.
   */
  private def tryToPut(blockId: String, value: Any, size: Long, deserialized: Boolean): Boolean = {
    // TODO: Its possible to optimize the locking by locking entries only when selecting blocks
    // to be dropped. Once the to-be-dropped blocks have been selected, and lock on entries has been
    // released, it must be ensured that those to-be-dropped blocks are not double counted for
    // freeing up more space for another block that needs to be put. Only then the actually dropping
    // of blocks (and writing to disk if necessary) can proceed in parallel.
    putLock.synchronized {
      if (ensureFreeSpace(blockId, size)) { // 如果可用分配足够的memory
        val entry = new Entry(value, size, deserialized)
        entries.synchronized { entries.put(blockId, entry) }
        currentMemory += size
        true
      } else { // 如果memory无法放下这个block, 那么只有从memory删除, 如果可以用disk, 那么在dropFromMemory中会put到disk中 
        // Tell the block manager that we couldn't put it in memory so that it can drop it to
        // disk if the block allows disk storage.
        val data = if (deserialized) {
          Left(value.asInstanceOf[ArrayBuffer[Any]])
        } else {
          Right(value.asInstanceOf[ByteBuffer].duplicate())
        }
        blockManager.dropFromMemory(blockId, data)
        false
      }
    }
  }

  /**
   * Tries to free up a given amount of space to store a particular block, but can fail and return
   * false if either the block is bigger than our memory or it would require replacing another
   * block from the same RDD (which leads to a wasteful cyclic replacement pattern for RDDs that
   * don't fit into memory that we want to avoid).
   *
   * Assumes that a lock is held by the caller to ensure only one thread is dropping blocks.
   * Otherwise, the freed space may fill up before the caller puts in their new value.
   */
  private def ensureFreeSpace(blockIdToAdd: String, space: Long): Boolean = {
    if (space > maxMemory) {
      logInfo("Will not store " + blockIdToAdd + " as it is larger than our memory limit")
      return false
    }

    if (maxMemory - currentMemory < space) {
      val rddToAdd = getRddId(blockIdToAdd)
      val selectedBlocks = new ArrayBuffer[String]()
      var selectedMemory = 0L

      // This is synchronized to ensure that the set of entries is not changed
      // (because of getValue or getBytes) while traversing the iterator, as that
      // can lead to exceptions.
      entries.synchronized {
        val iterator = entries.entrySet().iterator()  // 会依次删除现有的block, 直到可以放下新的block
        while (maxMemory - (currentMemory - selectedMemory) < space && iterator.hasNext) {
          val pair = iterator.next()
          val blockId = pair.getKey
          if (rddToAdd != null && rddToAdd == getRddId(blockId)) {
            logInfo("Will not store " + blockIdToAdd + " as it would require dropping another " +
              "block from the same RDD")
            return false
          }
          selectedBlocks += blockId
          selectedMemory += pair.getValue.size
        }
      }

      if (maxMemory - (currentMemory - selectedMemory) >= space) {
        logInfo(selectedBlocks.size + " blocks selected for dropping")
        for (blockId <- selectedBlocks) {  // 删除selectedBlocks, 释放空间
          val entry = entries.synchronized { entries.get(blockId) }
          // This should never be null as only one thread should be dropping
          // blocks and removing entries. However the check is still here for
          // future safety.
          if (entry != null) {
            val data = if (entry.deserialized) {
              Left(entry.value.asInstanceOf[ArrayBuffer[Any]])
            } else {
              Right(entry.value.asInstanceOf[ByteBuffer].duplicate())
            }
            blockManager.dropFromMemory(blockId, data)
          }
        }
        return true
      } else {
        return false
      }
    }
    return true
  }

本文章摘自博客园，原文发布日期：2014-01-09