1、Spark的数据存储
Spark计算速度远胜于Hadoop的原因之一就在于中间结果是缓存在内存而不是直接写入到disk，本文尝试分析Spark中存储子系统的构成，并以数据写入和数据读取为例，讲述清楚存储子系统中各部件的交互关系。
1.1存储子系统概览
Storage模块主要分为两层：
1)通信层：storage模块采用的是master-slave结构来实现通信层，master和slave之间传输控制信息、状态信息，这些都是通过通信层来实现的。
2)存储层：storage模块需要把数据存储到disk或是memory上面，有可能还需replicate到远端，这都是由存储层来实现和提供相应接口。
而其他模块若要和storage模块进行交互，storage模块提供了统一的操作类BlockManager，外部类与storage模块打交道都需要通过调用BlockManager相应接口来实现。
上图是Spark存储子系统中几个主要模块的关系示意图，现简要说明如下
1)CacheManager RDD在进行计算的时候，通过CacheManager来获取数据，并通过CacheManager来存储计算结果
2)BlockManager CacheManager在进行数据读取和存取的时候主要是依赖BlockManager接口来操作，BlockManager决定数据是从内存(MemoryStore)还是从磁盘(DiskStore)中获取
3)MemoryStore 负责将数据保存在内存或从内存读取
4)DiskStore 负责将数据写入磁盘或从磁盘读入
5)BlockManagerWorker 数据写入本地的MemoryStore或DiskStore是一个同步操作，为了容错还需要将数据复制到别的计算结点，以防止数据丢失的时候还能够恢复，数据复制的操作是异步完成，由BlockManagerWorker来处理这一部分事情
6)ConnectionManager 负责与其它计算结点建立连接，并负责数据的发送和接收
7)BlockManagerMaster 注意该模块只运行在Driver Application所在的Executor，功能是负责记录下所有BlockIds存储在哪个SlaveWorker上，比如RDD Task运行在机器A，所需要的BlockId为3，但在机器A上没有BlockId为3的数值，这个时候Slave worker需要通过BlockManager向BlockManagerMaster询问数据存储的位置，然后再通过ConnectionManager去获取。
1.2启动过程分析
上述的各个模块由SparkEnv来创建，创建过程在SparkEnv.create中完成
val blockManagerMaster = new BlockManagerMaster(registerOrLookup(
"BlockManagerMaster",
new BlockManagerMasterActor(isLocal, conf)), conf)
val blockManager = new BlockManager(executorId, actorSystem, blockManagerMaster, serializer, conf)
val connectionManager = blockManager.connectionManager
val broadcastManager = new BroadcastManager(isDriver, conf)
val cacheManager = new CacheManager(blockManager)
这段代码容易让人疑惑，看起来像是在所有的cluster node上都创建了BlockManagerMasterActor，其实不然，仔细看registerOrLookup函数的实现。如果当前节点是driver则创建这个actor，否则建立到driver的连接。
def registerOrLookup(name: String, newActor: => Actor): ActorRef = {
if (isDriver) {
logInfo("Registering " + name)
actorSystem.actorOf(Props(newActor), name = name)
} else {
val driverHost: String = conf.get("spark.driver.host", "localhost")
val driverPort: Int = conf.getInt("spark.driver.port", 7077)
Utils.checkHost(driverHost, "Expected hostname")
val url = s"akka.tcp://spark@$driverHost:$driverPort/user/$name"
val timeout = AkkaUtils.lookupTimeout(conf)
logInfo(s"Connecting to $name: $url")
Await.result(actorSystem.actorSelection(url).resolveOne(timeout), timeout)
}
}
初始化过程中一个主要的动作就是BlockManager需要向BlockManagerMaster发起注册
1.3通信层
BlockManager包装了BlockManagerMaster，发送信息包装成BlockManagerInfo。Spark在Driver和Worker端都创建各自的BlockManager，并通过BlockManagerMaster进行通信，通过BlockManager对Storage模块进行操作。
BlockManager对象在SparkEnv.create函数中进行创建：
def registerOrLookupEndpoint(
name: String, endpointCreator: => RpcEndpoint):
RpcEndpointRef = {
if (isDriver) {
logInfo("Registering " + name)
rpcEnv.setupEndpoint(name, endpointCreator)
} else {
RpcUtils.makeDriverRef(name, conf, rpcEnv)
}
}
…………
val blockManagerMaster = new BlockManagerMaster(registerOrLookupEndpoint(
BlockManagerMaster.DRIVER_ENDPOINT_NAME,
new BlockManagerMasterEndpoint(rpcEnv, isLocal, conf, listenerBus)),
conf, isDriver)
// NB: blockManager is not valid until initialize() is called later.
val blockManager = new BlockManager(executorId, rpcEnv, blockManagerMaster,
serializer, conf, mapOutputTracker, shuffleManager, blockTransferService, securityManager,numUsableCores)
并且在创建之前对当前节点是否是Driver进行了判断。如果是，则创建这个Endpoint；否则，创建Driver的连接。
在创建BlockManager之后，BlockManager会调用initialize方法初始化自己。并且初始化的时候，会调用BlockManagerMaster向Driver注册自己，同时，在注册时也启动了Slave Endpoint。另外，向本地shuffle服务器注册Executor配置，如果存在的话。
def initialize(appId: String): Unit = {
…………
master.registerBlockManager(blockManagerId, maxMemory, slaveEndpoint)
// Register Executors' configuration with the local shuffle service, if one should exist.
if (externalShuffleServiceEnabled && !blockManagerId.isDriver) {
registerWithExternalShuffleServer()
}
}
而BlockManagerMaster将注册请求包装成RegisterBlockManager注册到Driver。Driver的BlockManagerMasterEndpoint会调用register方法，通过对消息BlockManagerInfo检查，向Driver注册。
private def register(id: BlockManagerId, maxMemSize: Long, slaveEndpoint: RpcEndpointRef) {
val time = System.currentTimeMillis()
if (!blockManagerInfo.contains(id)) {
blockManagerIdByExecutor.get(id.executorId) match {
case Some(oldId) =>
// A block manager of the same executor already exists, so remove it (assumed dead)
logError("Got two different block manager registrations on same executor - "
+ s" will replace old one $oldId with new one $id")
removeExecutor(id.executorId)
case None =>
}
logInfo("Registering block manager %s with %s RAM, %s".format(
id.hostPort, Utils.bytesToString(maxMemSize), id))
blockManagerIdByExecutor(id.executorId) = id
blockManagerInfo(id) = new BlockManagerInfo(
id, System.currentTimeMillis(), maxMemSize, slaveEndpoint)
}
listenerBus.post(SparkListenerBlockManagerAdded(time, id, maxMemSize))
}
不难发现BlockManagerInfo对象被保存到Map映射中。
在通信层中BlockManagerMaster控制着消息的流向，这里采用了模式匹配，所有的消息模式都在BlockManagerMessage中。
1.4存储层
Spark Storage的最小存储单位是block，所有的操作都是以block为单位进行的。
在BlockManager被创建的时候MemoryStore和DiskStore对象就被创建出来了
val diskBlockManager = new DiskBlockManager(this, conf)
private[spark] val memoryStore = new MemoryStore(this, maxMemory)
private[spark] val diskStore = new DiskStore(this, diskBlockManager)
1.4.1Disk Store
由于当前的Spark版本对Disk Store进行了更细粒度的分工，把对文件的操作提取出来放到了DiskBlockManager中，DiskStore仅仅负责数据的存储和读取。
Disk Store会配置多个文件目录，Spark会在不同的文件目录下创建文件夹，其中文件夹的命名方式是：spark-UUID（随机UUID码）。Disk Store在存储的时候创建文件夹。并且根据“高内聚，低耦合”原则，这种服务型的工具代码就放到了Utils中（调用路径：DiskStore.putBytes—>DiskBlockManager.createLocalDirs—>Utils.createDirectory）：
def createDirectory(root: String, namePrefix: String = "spark"): File = {
var attempts = 0
val maxAttempts = MAX_DIR_CREATION_ATTEMPTS
var dir: File = null
while (dir == null) {
attempts += 1
if (attempts > maxAttempts) {
throw new IOException("Failed to create a temp directory (under " + root + ") after " +
maxAttempts + " attempts!")
}
try {
dir = new File(root, namePrefix + "-" + UUID.randomUUID.toString)
if (dir.exists() || !dir.mkdirs()) {
dir = null
}
} catch {
case e: SecurityException => dir = null; }
}
dir.getCanonicalFile
}
在DiskBlockManager里，每个block都被存储为一个file，通过计算blockId的hash值，将block映射到文件中。
def getFile(filename: String): File = {
// Figure out which local directory it hashes to, and which subdirectory in that
val hash = Utils.nonNegativeHash(filename)
val dirId = hash % localDirs.length
val subDirId = (hash / localDirs.length) % subDirsPerLocalDir
// Create the subdirectory if it doesn't already exist
val subDir = subDirs(dirId).synchronized {
val old = subDirs(dirId)(subDirId)
if (old != null) {
old
} else {
val newDir = new File(localDirs(dirId), "%02x".format(subDirId))
if (!newDir.exists() && !newDir.mkdir()) {
throw new IOException(s"Failed to create local dir in $newDir.")
}
subDirs(dirId)(subDirId) = newDir
newDir
}
}
new File(subDir, filename)
}
def getFile(blockId: BlockId): File = getFile(blockId.name)
通过hash值的取模运算，求出dirId和subDirId。然后，在从subDirs中找到subDir，如果subDir不存在，则创建一个新subDir。最后，以subDir为路径，blockId的name属性为文件名，新建该文件。
文件创建完之后，那么Spark就会在DiskStore中向文件写与之映射的block：
override def putBytes(blockId: BlockId, _bytes: ByteBuffer, level: StorageLevel): PutResult = {
val bytes = _bytes.duplicate()
logDebug(s"Attempting to put block $blockId")
val startTime = System.currentTimeMillis
val file = diskManager.getFile(blockId)
val channel = new FileOutputStream(file).getChannel
Utils.tryWithSafeFinally {
while (bytes.remaining > 0) {
channel.write(bytes)
}
} {
channel.close()
}
val finishTime = System.currentTimeMillis
logDebug("Block %s stored as %s file on disk in %d ms".format(
file.getName, Utils.bytesToString(bytes.limit), finishTime - startTime))
PutResult(bytes.limit(), Right(bytes.duplicate()))
}
读取过程就简单了，DiskStore根据blockId读取与之映射的file内容，当然，这中间需要从DiskBlockManager中得到文件信息。
private def getBytes(file: File, offset: Long, length: Long): Option[ByteBuffer] = {
val channel = new RandomAccessFile(file, "r").getChannel
Utils.tryWithSafeFinally {
// For small files, directly read rather than memory map
if (length < minMemoryMapBytes) {
val buf = ByteBuffer.allocate(length.toInt)
channel.position(offset)
while (buf.remaining() != 0) {
if (channel.read(buf) == -1) {
throw new IOException("Reached EOF before filling buffer\n" +
s"offset=$offset\nfile=${file.getAbsolutePath}\nbuf.remaining=${buf.remaining}")
}
}
buf.flip()
Some(buf)
} else {
Some(channel.map(MapMode.READ_ONLY, offset, length))
}
} {
channel.close()
}
}
override def getBytes(blockId: BlockId): Option[ByteBuffer] = {
val file = diskManager.getFile(blockId.name)
getBytes(file, 0, file.length)
}
1.4.2Memory Store
相对Disk Store，Memory Store就显得容易很多。Memory Store用一个LinkedHashMap来管理，其中Key是blockId，Value是MemoryEntry样例类，MemoryEntry存储着数据信息。
private case class MemoryEntry(value: Any, size: Long, deserialized: Boolean)
private val entries = new LinkedHashMap[BlockId, MemoryEntry](32, 0.75f, true)
在MemoryStore中存储block的前提是当前内存有足够的空间存放。通过对tryToPut函数的调用对内存空间进行判断。
def putBytes(blockId: BlockId, size: Long, _bytes: () => ByteBuffer): PutResult = {
// Work on a duplicate - since the original input might be used elsewhere.
lazy val bytes = _bytes().duplicate().rewind().asInstanceOf[ByteBuffer]
val putAttempt = tryToPut(blockId, () => bytes, size, deserialized = false)
val data =
if (putAttempt.success) {
assert(bytes.limit == size)
Right(bytes.duplicate())
} else {
null
}
PutResult(size, data, putAttempt.droppedBlocks)
}
在tryToPut函数中，通过调用enoughFreeSpace函数判断内存空间。如果内存空间足够，那么就把block放到LinkedHashMap中；如果内存不足，那么就告诉BlockManager内存不足，如果允许Disk Store，那么就把该block放到disk上。
private def tryToPut(blockId: BlockId, value: () => Any, size: Long, deserialized: Boolean): ResultWithDroppedBlocks = {
var putSuccess = false
val droppedBlocks = new ArrayBuffer[(BlockId, BlockStatus)]
accountingLock.synchronized {
val freeSpaceResult = ensureFreeSpace(blockId, size)
val enoughFreeSpace = freeSpaceResult.success
droppedBlocks ++= freeSpaceResult.droppedBlocks
if (enoughFreeSpace) {
val entry = new MemoryEntry(value(), size, deserialized)
entries.synchronized {
entries.put(blockId, entry)
currentMemory += size
}
val valuesOrBytes = if (deserialized) "values" else "bytes"
logInfo("Block %s stored as %s in memory (estimated size %s, free %s)".format(
blockId, valuesOrBytes, Utils.bytesToString(size), Utils.bytesToString(freeMemory)))
putSuccess = true
} else {
lazy val data = if (deserialized) {
Left(value().asInstanceOf[Array[Any]])
} else {
Right(value().asInstanceOf[ByteBuffer].duplicate())
}
val droppedBlockStatus = blockManager.dropFromMemory(blockId, () => data)
droppedBlockStatus.foreach {
status => droppedBlocks += ((blockId, status)) }
}
releasePendingUnrollMemoryForThisTask()
}
ResultWithDroppedBlocks(putSuccess, droppedBlocks)
}
Memory Store读取block也很简单，只需要从LinkedHashMap中取出blockId的Value即可。
override def getValues(blockId: BlockId): Option[Iterator[Any]] = {
val entry = entries.synchronized {
entries.get(blockId)
}
if (entry == null) {
None
} else if (entry.deserialized) {
Some(entry.value.asInstanceOf[Array[Any]].iterator)
} else {
val buffer = entry.value.asInstanceOf[ByteBuffer].duplicate() // Doesn't actually copy data
Some(blockManager.dataDeserialize(blockId, buffer))
}
}
1.5数据写入过程分析
数据写入的简要流程
1)RDD.iterator是与storage子系统交互的入口
2)CacheManager.getOrCompute调用BlockManager的put接口来写入数据
3)数据优先写入到MemoryStore即内存，如果MemoryStore中的数据已满则将最近使用次数不频繁的数据写入到磁盘
4)通知BlockManagerMaster有新的数据写入，在BlockManagerMaster中保存元数据
5)将写入的数据与其它slave worker进行同步，一般来说在本机写入的数据，都会另先一台机器来进行数据的备份，即replicanumber=1
其实，我们在put和get block的时候并没有那么复杂，前面的细节BlockManager都包装好了，我们只需要调用BlockManager中的put和get函数即可。
def putBytes(
blockId: BlockId,
bytes: ByteBuffer,
level: StorageLevel,
tellMaster: Boolean = true,
effectiveStorageLevel: Option[StorageLevel] = None): Seq[(BlockId, BlockStatus)] = {
require(bytes != null, "Bytes is null")
doPut(blockId, ByteBufferValues(bytes), level, tellMaster, effectiveStorageLevel)
}
private def doPut(
blockId: BlockId,
data: BlockValues,
level: StorageLevel,
tellMaster: Boolean = true,
effectiveStorageLevel: Option[StorageLevel] = None)
: Seq[(BlockId, BlockStatus)] = {
require(blockId != null, "BlockId is null")
require(level != null && level.isValid, "StorageLevel is null or invalid")
effectiveStorageLevel.foreach {
level =>
require(level != null && level.isValid, "Effective StorageLevel is null or invalid")
}
val updatedBlocks = new ArrayBuffer[(BlockId, BlockStatus)]
val putBlockInfo = {
val tinfo = new BlockInfo(level, tellMaster)
val oldBlockOpt = blockInfo.putIfAbsent(blockId, tinfo)
if (oldBlockOpt.isDefined) {
if (oldBlockOpt.get.waitForReady()) {
logWarning(s"Block $blockId already exists on this machine; not re-adding it")
return updatedBlocks
}
oldBlockOpt.get
} else {
tinfo
}
}
val startTimeMs = System.currentTimeMillis
var valuesAfterPut: Iterator[Any] = null
var bytesAfterPut: ByteBuffer = null
var size = 0L
val putLevel = effectiveStorageLevel.getOrElse(level)
val replicationFuture = data match {
case b: ByteBufferValues if putLevel.replication > 1 =>
// Duplicate doesn't copy the bytes, but just creates a wrapper
val bufferView = b.buffer.duplicate()
Future {
replicate(blockId, bufferView, putLevel)
}(futureExecutionContext)
case _ => null
}
putBlockInfo.synchronized {
logTrace("Put for block %s took %s to get into synchronized block"
.format(blockId, Utils.getUsedTimeMs(startTimeMs)))
var marked = false
try {
val (returnValues, blockStore: BlockStore) = {
if (putLevel.useMemory) {
(true, memoryStore)
} else if (putLevel.useOffHeap) {
(false, externalBlockStore)
} else if (putLevel.useDisk) {
(putLevel.replication > 1, diskStore)
} else {
assert(putLevel == StorageLevel.NONE)
throw new BlockException(
blockId, s"Attempted to put block $blockId without specifying storage level!")
}
}
val result = data match {
case IteratorValues(iterator) =>
blockStore.putIterator(blockId, iterator, putLevel, returnValues)
case ArrayValues(array) =>
blockStore.putArray(blockId, array, putLevel, returnValues)
case ByteBufferValues(bytes) =>
bytes.rewind()
blockStore.putBytes(blockId, bytes, putLevel)
}
size = result.size
result.data match {
case Left (newIterator) if putLevel.useMemory => valuesAfterPut = newIterator
case Right (newBytes) => bytesAfterPut = newBytes
case _ =>
}
if (putLevel.useMemory) {
result.droppedBlocks.foreach { updatedBlocks += _ }
}
val putBlockStatus = getCurrentBlockStatus(blockId, putBlockInfo)
if (putBlockStatus.storageLevel != StorageLevel.NONE) {
marked = true
putBlockInfo.markReady(size)
if (tellMaster) {
reportBlockStatus(blockId, putBlockInfo, putBlockStatus)
}
updatedBlocks += ((blockId, putBlockStatus))
}
} finally {
if (!marked) {
blockInfo.remove(blockId)
putBlockInfo.markFailure()
logWarning(s"Putting block $blockId failed")
}
}
}
logDebug("Put block %s locally took %s".format(blockId, Utils.getUsedTimeMs(startTimeMs)))
if (putLevel.replication > 1) {
data match {
case ByteBufferValues(bytes) =>
if (replicationFuture != null) {
Await.ready(replicationFuture, Duration.Inf)
}
case _ =>
val remoteStartTime = System.currentTimeMillis
if (bytesAfterPut == null) {
if (valuesAfterPut == null) {
throw new SparkException(
"Underlying put returned neither an Iterator nor bytes! This shouldn't happen.")
}
bytesAfterPut = dataSerialize(blockId, valuesAfterPut)
}
replicate(blockId, bytesAfterPut, putLevel)
logDebug("Put block %s remotely took %s"
.format(blockId, Utils.getUsedTimeMs(remoteStartTime)))
}
}
BlockManager.dispose(bytesAfterPut)
if (putLevel.replication > 1) {
logDebug("Putting block %s with replication took %s"
.format(blockId, Utils.getUsedTimeMs(startTimeMs)))
} else {
logDebug("Putting block %s without replication took %s"
.format(blockId, Utils.getUsedTimeMs(startTimeMs)))
}
updatedBlocks
}
对于doPut函数，主要做了以下几个操作
创建BlockInfo对象存储block信息；
将BlockInfo加锁，然后根据Storage Level判断存储到Memory还是Disk。同时，对于已经准备好读的BlockInfo要进行解锁。
根据block的副本数量决定是否向远程发送副本。
1.5.1序列化与否
写入的具体内容可以是序列化之后的bytes也可以是没有序列化的value. 此处有一个对scala的语法中Either, Left, Right关键字的理解。
1.6数据读取过程分析
def get(blockId: BlockId): Option[Iterator[Any]] = {
val local = getLocal(blockId)
if (local.isDefined) {
logInfo("Found block %s locally".format(blockId))
return local
}
val remote = getRemote(blockId)
if (remote.isDefined) {
logInfo("Found block %s remotely".format(blockId))
return remote
}
None
}
1.6.1本地读取
首先在查询本机的MemoryStore和DiskStore中是否有所需要的block数据存在，如果没有则发起远程数据获取。
1.6.2远程读取
远程获取调用路径， getRemote->doGetRemote, 在doGetRemote中最主要的就是调用BlockManagerWorker.syncGetBlock来从远程获得数据
def syncGetBlock(msg: GetBlock, toConnManagerId: ConnectionManagerId): ByteBuffer = {
val blockManager = blockManagerWorker.blockManager
val connectionManager = blockManager.connectionManager
val blockMessage = BlockMessage.fromGetBlock(msg)
val blockMessageArray = new BlockMessageArray(blockMessage)
val responseMessage = connectionManager.sendMessageReliablySync(
toConnManagerId, blockMessageArray.toBufferMessage)
responseMessage match {
case Some(message) => {
val bufferMessage = message.asInstanceOf[BufferMessage]
logDebug("Response message received " + bufferMessage)
BlockMessageArray.fromBufferMessage(bufferMessage).foreach(blockMessage => {
logDebug("Found " + blockMessage)
return blockMessage.getData
})
}
case None => logDebug("No response message received")
}
null
}
上述这段代码中最有意思的莫过于sendMessageReliablySync，远程数据读取毫无疑问是一个异步i/o操作，这里的代码怎么写起来就像是在进行同步的操作一样呢。也就是说如何知道对方发送回来响应的呢？
别急，继续去看看sendMessageReliablySync的定义
def sendMessageReliably(connectionManagerId: ConnectionManagerId, message: Message)
: Future[Option[Message]] = {
val promise = Promise[Option[Message]]
val status = new MessageStatus(
message, connectionManagerId, s => promise.success(s.ackMessage))
messageStatuses.synchronized {
messageStatuses += ((message.id, status))
}
sendMessage(connectionManagerId, message)
promise.future
}
要是我说秘密在这里，你肯定会说我在扯淡，但确实在此处。注意到关键字Promise和Future没。
如果这个future执行完毕，返回s.ackMessage。我们再看看这个ackMessage是在什么地方被写入的呢。看一看ConnectionManager.handleMessage中的代码片段
case bufferMessage: BufferMessage =>
{
if (authEnabled) {
val res = handleAuthentication(connection, bufferMessage)
if (res == true) {
// message was security negotiation so skip the rest
logDebug("After handleAuth result was true, returning")
return
}
}
if (bufferMessage.hasAckId) {
val sentMessageStatus = messageStatuses. synchronized {
messageStatuses.get(bufferMessage.ackId) match {
case Some(status) =>{
messageStatuses -= bufferMessage.ackId
status
}
case None =>{
throw new Exception("Could not find reference for received ack message " +
message.id)
null
}
}
}
sentMessageStatus. synchronized {
sentMessageStatus.ackMessage = Some(message)
sentMessageStatus.attempted = true
sentMessageStatus.acked = true
sentMessageStaus.markDone()
}
}
}
注意，此处的所调用的sentMessageStatus.markDone就会调用在sendMessageReliablySync中定义的promise.Success. 不妨看看MessageStatus的定义。
class MessageStatus(
val message: Message,
val connectionManagerId: ConnectionManagerId,
completionHandler: MessageStatus => Unit) {
var ackMessage: Option[Message] = None
var attempted = false
var acked = false
def markDone() {
completionHandler(this) }
}
1.7Partition如何转化为Block
在storage模块里面所有的操作都是和block相关的，但是在RDD里面所有的运算都是基于partition的，那么partition是如何与block对应上的呢？
RDD计算的核心函数是iterator()函数：
final def iterator(split: Partition, context: TaskContext): Iterator[T] = {
if (storageLevel != StorageLevel.NONE) {
SparkEnv.get.cacheManager.getOrCompute(this, split, context, storageLevel)
} else {
computeOrReadCheckpoint(split, context)
}
}
如果当前RDD的storage level不是NONE的话，表示该RDD在BlockManager中有存储，那么调用CacheManager中的getOrCompute()函数计算RDD，在这个函数中partition和block发生了关系：
首先根据RDD id和partition index构造出block id (rdd_xx_xx)，接着从BlockManager中取出相应的block。
如果该block存在，表示此RDD在之前已经被计算过和存储在BlockManager中，因此取出即可，无需再重新计算。
如果该block不存在则需要调用RDD的computeOrReadCheckpoint()函数计算出新的block，并将其存储到BlockManager中。
需要注意的是block的计算和存储是阻塞的，若另一线程也需要用到此block则需等到该线程block的loading结束。
def getOrCompute[T](rdd:RDD[T],split:Partition,context:TaskContext,storageLevel:StorageLevel):Iterator[T]=
{
val key = "rdd_%d_%d".format(rdd.id, split.index)
logDebug("Looking for partition " + key)
blockManager.get(key) match {
case Some(values) =>
// Partition is already materialized, so just return its values
return values.asInstanceOf[Iterator[T]]
case None =>
// Mark the split as loading (unless someone else marks it first)
loading. synchronized {
if (loading.contains(key)) {
logInfo("Another thread is loading %s, waiting for it to finish...".format(key))
while (loading.contains(key)) {
try {
loading.wait()
} catch {
case _:
Throwable =>}
}
logInfo("Finished waiting for %s".format(key))
// See whether someone else has successfully loaded it. The main way this would fail
// is for the RDD-level cache eviction policy if someone else has loaded the same RDD
// partition but we didn't want to make space for it. However, that case is unlikely
// because it's unlikely that two threads would work on the same RDD partition. One
// downside of the current code is that threads wait serially if this does happen.
blockManager.get(key) match {
case Some(values) =>
return values.asInstanceOf[Iterator[T]]
case None =>
logInfo("Whoever was loading %s failed; we'll try it ourselves".format(key))
loading.add(key)
}
} else {
loading.add(key)
}
}
try {
// If we got here, we have to load the split
logInfo("Partition %s not found, computing it".format(key))
val computedValues = rdd.computeOrReadCheckpoint(split, context)
// Persist the result, so long as the task is not running locally
if (context.runningLocally) {
return computedValues
}
val elements = new ArrayBuffer[Any]
elements++ = computedValues
blockManager.put(key, elements, storageLevel, true)
return elements.iterator.asInstanceOf[Iterator[T]]
} finally {
loading. synchronized {
loading.remove(key)
loading.notifyAll()
}
}
}
这样RDD的transformation、action就和block数据建立了联系，虽然抽象上我们的操作是在partition层面上进行的，但是partition最终还是被映射成为block，因此实际上我们的所有操作都是对block的处理和存取。
1.8partition和block的对应关系
在RDD中，核心的函数是iterator：
final def iterator(split: Partition, context: TaskContext): Iterator[T] = {
if (storageLevel != StorageLevel.NONE) {
SparkEnv.get.cacheManager.getOrCompute(this, split, context, storageLevel)
} else {
computeOrReadCheckpoint(split, context)
}
}
如果当前RDD的storage level不是NONE的话，表示该RDD在BlockManager中有存储，那么调用CacheManager中的getOrCompute函数计算RDD，在这个函数中partition和block就对应起来了：
getOrCompute函数会先构造RDDBlockId，其中RDDBlockId就把block和partition联系起来了，RDDBlockId产生的name就是BlockId的name属性，形式是：rdd_rdd.id_partition.index。
def getOrCompute[T](
rdd: RDD[T],
partition: Partition,
context: TaskContext,
storageLevel: StorageLevel): Iterator[T] = {
val key = RDDBlockId(rdd.id, partition.index)
logDebug(s"Looking for partition $key")
blockManager.get(key) match {
case Some(blockResult) =>
val existingMetrics = context.taskMetrics
.getInputMetricsForReadMethod(blockResult.readMethod)
existingMetrics.incBytesRead(blockResult.bytes)
val iter = blockResult.data.asInstanceOf[Iterator[T]]
new InterruptibleIterator[T](context, iter) {
override def next(): T = {
existingMetrics.incRecordsRead(1)
delegate.next()
}
}
case None =>
val storedValues = acquireLockForPartition[T](key)
if (storedValues.isDefined) {
return new InterruptibleIterator[T](context, storedValues.get)
}
try {
logInfo(s"Partition $key not found, computing it")
val computedValues = rdd.computeOrReadCheckpoint(partition, context)
if (context.isRunningLocally) {
return computedValues
}
val updatedBlocks = new ArrayBuffer[(BlockId, BlockStatus)]
val cachedValues = putInBlockManager(key, computedValues, storageLevel, updatedBlocks)
val metrics = context.taskMetrics
val lastUpdatedBlocks = metrics.updatedBlocks.getOrElse(Seq[(BlockId, BlockStatus)]())
metrics.updatedBlocks = Some(lastUpdatedBlocks ++ updatedBlocks.toSeq)
new InterruptibleIterator(context, cachedValues)
} finally {
loading.synchronized {
loading.remove(key)
loading.notifyAll()
}
}
}
}
同时getOrCompute函数会对block进行判断：
如果该block存在，表示此RDD在之前已经被计算过和存储在BlockManager中，因此取出即可，无需再重新计算。
如果该block不存在则需要调用RDD的computeOrReadCheckpoint()函数计算出新的block，并将其存储到BlockManager中。
需要注意的是block的计算和存储是阻塞的，若另一线程也需要用到此block则需等到该线程block的loading结束。
