[SPARK-3495] Block replication fails continuously when the replication target node is dead AND [SPARK-3496] Block replication by mistake chooses driver as target

If a block manager (say, A) wants to replicate a block and the node chosen for replication (say, B) is dead, then the attempt to send the block to B fails. However, this continues to fail indefinitely. Even if the driver learns about the demise of the B, A continues to try replicating to B and failing miserably.

The reason behind this bug is that A initially fetches a list of peers from the driver (when B was active), but never updates it after B is dead. This affects Spark Streaming as its receiver uses block replication.

The solution in this patch adds the following.
- Changed BlockManagerMaster to return all the peers of a block manager, rather than the requested number. It also filters out driver BlockManager.
- Refactored BlockManager's replication code to handle peer caching correctly.
    + The peer for replication is randomly selected. This is different from past behavior where for a node A, a node B was deterministically chosen for the lifetime of the application.
    + If replication fails to one node, the peers are refetched.
    + The peer cached has a TTL of 1 second to enable discovery of new peers and using them for replication.
- Refactored use of \<driver\> in BlockManager into a new method `BlockManagerId.isDriver`
- Added replication unit tests (replication was not tested till now, duh!)

This should not make a difference in performance of Spark workloads where replication is not used.

@andrewor14 @JoshRosen

Author: Tathagata Das <[email protected]>

Closes apache#2366 from tdas/replication-fix and squashes the following commits:

9690f57 [Tathagata Das] Moved replication tests to a new BlockManagerReplicationSuite.
0661773 [Tathagata Das] Minor changes based on PR comments.
a55a65c [Tathagata Das] Added a unit test to test replication behavior.
012afa3 [Tathagata Das] Bug fix
89f91a0 [Tathagata Das] Minor change.
68e2c72 [Tathagata Das] Made replication peer selection logic more efficient.
08afaa9 [Tathagata Das] Made peer selection for replication deterministic to block id
3821ab9 [Tathagata Das] Fixes based on PR comments.
08e5646 [Tathagata Das] More minor changes.
d402506 [Tathagata Das] Fixed imports.
4a20531 [Tathagata Das] Filtered driver block manager from peer list, and also consolidated the use of <driver> in BlockManager.
7598f91 [Tathagata Das] Minor changes.
03de02d [Tathagata Das] Change replication logic to correctly refetch peers from master on failure and on new worker addition.
d081bf6 [Tathagata Das] Fixed bug in get peers and unit tests to test get-peers and replication under executor churn.
9f0ac9f [Tathagata Das] Modified replication tests to fail on replication bug.
af0c1da [Tathagata Das] Added replication unit tests to BlockManagerSuite

Author: tdas

core/src/main/scala/org/apache/spark/storage/BlockManager.scala

@@ -22,6 +22,7 @@ import java.nio.{ByteBuffer, MappedByteBuffer}
 
 import scala.concurrent.ExecutionContext.Implicits.global
 
+import scala.collection.mutable
 import scala.collection.mutable.{ArrayBuffer, HashMap}
 import scala.concurrent.{Await, Future}
 import scala.concurrent.duration._
@@ -112,6 +113,11 @@ private[spark] class BlockManager(
   private val broadcastCleaner = new MetadataCleaner(
     MetadataCleanerType.BROADCAST_VARS, this.dropOldBroadcastBlocks, conf)
 
+  // Field related to peer block managers that are necessary for block replication
+  @volatile private var cachedPeers: Seq[BlockManagerId] = _
+  private val peerFetchLock = new Object
+  private var lastPeerFetchTime = 0L
+
   initialize()
 
   /* The compression codec to use. Note that the "lazy" val is necessary because we want to delay
@@ -787,31 +793,111 @@ private[spark] class BlockManager(
   }
 
   /**
-   * Replicate block to another node.
+   * Get peer block managers in the system.
+   */
+  private def getPeers(forceFetch: Boolean): Seq[BlockManagerId] = {
+    peerFetchLock.synchronized {
+      val cachedPeersTtl = conf.getInt("spark.storage.cachedPeersTtl", 60 * 1000) // milliseconds
+      val timeout = System.currentTimeMillis - lastPeerFetchTime > cachedPeersTtl
+      if (cachedPeers == null || forceFetch || timeout) {
+        cachedPeers = master.getPeers(blockManagerId).sortBy(_.hashCode)
+        lastPeerFetchTime = System.currentTimeMillis
+        logDebug("Fetched peers from master: " + cachedPeers.mkString("[", ",", "]"))
+      }
+      cachedPeers
+    }
+  }
+
+  /**
+   * Replicate block to another node. Not that this is a blocking call that returns after
+   * the block has been replicated.
    */
-  @volatile var cachedPeers: Seq[BlockManagerId] = null
   private def replicate(blockId: BlockId, data: ByteBuffer, level: StorageLevel): Unit = {
+    val maxReplicationFailures = conf.getInt("spark.storage.maxReplicationFailures", 1)
+    val numPeersToReplicateTo = level.replication - 1
+    val peersForReplication = new ArrayBuffer[BlockManagerId]
+    val peersReplicatedTo = new ArrayBuffer[BlockManagerId]
+    val peersFailedToReplicateTo = new ArrayBuffer[BlockManagerId]
     val tLevel = StorageLevel(
       level.useDisk, level.useMemory, level.useOffHeap, level.deserialized, 1)
-    if (cachedPeers == null) {
-      cachedPeers = master.getPeers(blockManagerId, level.replication - 1)
+    val startTime = System.currentTimeMillis
+    val random = new Random(blockId.hashCode)
+
+    var replicationFailed = false
+    var failures = 0
+    var done = false
+
+    // Get cached list of peers
+    peersForReplication ++= getPeers(forceFetch = false)
+
+    // Get a random peer. Note that this selection of a peer is deterministic on the block id.
+    // So assuming the list of peers does not change and no replication failures,
+    // if there are multiple attempts in the same node to replicate the same block,
+    // the same set of peers will be selected.
+    def getRandomPeer(): Option[BlockManagerId] = {
+      // If replication had failed, then force update the cached list of peers and remove the peers
+      // that have been already used
+      if (replicationFailed) {
+        peersForReplication.clear()
+        peersForReplication ++= getPeers(forceFetch = true)
+        peersForReplication --= peersReplicatedTo
+        peersForReplication --= peersFailedToReplicateTo
+      }
+      if (!peersForReplication.isEmpty) {
+        Some(peersForReplication(random.nextInt(peersForReplication.size)))
+      } else {
+        None
+      }
     }
-    for (peer: BlockManagerId <- cachedPeers) {
-      val start = System.nanoTime
-      data.rewind()
-      logDebug(s"Try to replicate $blockId once; The size of the data is ${data.limit()} Bytes. " +
-        s"To node: $peer")
 
-      try {
-        blockTransferService.uploadBlockSync(
-          peer.host, peer.port, blockId.toString, new NioByteBufferManagedBuffer(data), tLevel)
-      } catch {
-        case e: Exception =>
-          logError(s"Failed to replicate block to $peer", e)
+    // One by one choose a random peer and try uploading the block to it
+    // If replication fails (e.g., target peer is down), force the list of cached peers
+    // to be re-fetched from driver and then pick another random peer for replication. Also
+    // temporarily black list the peer for which replication failed.
+    //
+    // This selection of a peer and replication is continued in a loop until one of the
+    // following 3 conditions is fulfilled:
+    // (i) specified number of peers have been replicated to
+    // (ii) too many failures in replicating to peers
+    // (iii) no peer left to replicate to
+    //
+    while (!done) {
+      getRandomPeer() match {
+        case Some(peer) =>
+          try {
+            val onePeerStartTime = System.currentTimeMillis
+            data.rewind()
+            logTrace(s"Trying to replicate $blockId of ${data.limit()} bytes to $peer")
+            blockTransferService.uploadBlockSync(
+              peer.host, peer.port, blockId.toString, new NioByteBufferManagedBuffer(data), tLevel)
+            logTrace(s"Replicated $blockId of ${data.limit()} bytes to $peer in %f ms"
+              .format((System.currentTimeMillis - onePeerStartTime)))
+            peersReplicatedTo += peer
+            peersForReplication -= peer
+            replicationFailed = false
+            if (peersReplicatedTo.size == numPeersToReplicateTo) {
+              done = true  // specified number of peers have been replicated to
+            }
+          } catch {
+            case e: Exception =>
+              logWarning(s"Failed to replicate $blockId to $peer, failure #$failures", e)
+              failures += 1
+              replicationFailed = true
+              peersFailedToReplicateTo += peer
+              if (failures > maxReplicationFailures) { // too many failures in replcating to peers
+                done = true
+              }
+          }
+        case None => // no peer left to replicate to
+          done = true
       }
-
-      logDebug("Replicating BlockId %s once used %fs; The size of the data is %d bytes."
-        .format(blockId, (System.nanoTime - start) / 1e6, data.limit()))
+    }
+    val timeTakeMs = (System.currentTimeMillis - startTime)
+    logDebug(s"Replicating $blockId of ${data.limit()} bytes to " +
+      s"${peersReplicatedTo.size} peer(s) took $timeTakeMs ms")
+    if (peersReplicatedTo.size < numPeersToReplicateTo) {
+      logWarning(s"Block $blockId replicated to only " +
+        s"${peersReplicatedTo.size} peer(s) instead of $numPeersToReplicateTo peers")
     }
   }
 

core/src/main/scala/org/apache/spark/storage/BlockManagerId.scala

@@ -59,6 +59,8 @@ class BlockManagerId private (
 
   def port: Int = port_
 
+  def isDriver: Boolean = (executorId == "<driver>")
+
   override def writeExternal(out: ObjectOutput) {
     out.writeUTF(executorId_)
     out.writeUTF(host_)

core/src/main/scala/org/apache/spark/storage/BlockManagerMaster.scala

@@ -84,13 +84,8 @@ class BlockManagerMaster(
   }
 
   /** Get ids of other nodes in the cluster from the driver */
-  def getPeers(blockManagerId: BlockManagerId, numPeers: Int): Seq[BlockManagerId] = {
-    val result = askDriverWithReply[Seq[BlockManagerId]](GetPeers(blockManagerId, numPeers))
-    if (result.length != numPeers) {
-      throw new SparkException(
-        "Error getting peers, only got " + result.size + " instead of " + numPeers)
-    }
-    result
+  def getPeers(blockManagerId: BlockManagerId): Seq[BlockManagerId] = {
+    askDriverWithReply[Seq[BlockManagerId]](GetPeers(blockManagerId))
   }
 
   /**

core/src/main/scala/org/apache/spark/storage/BlockManagerMasterActor.scala

@@ -83,8 +83,8 @@ class BlockManagerMasterActor(val isLocal: Boolean, conf: SparkConf, listenerBus
     case GetLocationsMultipleBlockIds(blockIds) =>
       sender ! getLocationsMultipleBlockIds(blockIds)
 
-    case GetPeers(blockManagerId, size) =>
-      sender ! getPeers(blockManagerId, size)
+    case GetPeers(blockManagerId) =>
+      sender ! getPeers(blockManagerId)
 
     case GetMemoryStatus =>
       sender ! memoryStatus
@@ -173,11 +173,10 @@ class BlockManagerMasterActor(val isLocal: Boolean, conf: SparkConf, listenerBus
    * from the executors, but not from the driver.
    */
   private def removeBroadcast(broadcastId: Long, removeFromDriver: Boolean): Future[Seq[Int]] = {
-    // TODO: Consolidate usages of <driver>
     import context.dispatcher
     val removeMsg = RemoveBroadcast(broadcastId, removeFromDriver)
     val requiredBlockManagers = blockManagerInfo.values.filter { info =>
-      removeFromDriver || info.blockManagerId.executorId != "<driver>"
+      removeFromDriver || !info.blockManagerId.isDriver
     }
     Future.sequence(
       requiredBlockManagers.map { bm =>
@@ -212,7 +211,7 @@ class BlockManagerMasterActor(val isLocal: Boolean, conf: SparkConf, listenerBus
     val minSeenTime = now - slaveTimeout
     val toRemove = new mutable.HashSet[BlockManagerId]
     for (info <- blockManagerInfo.values) {
-      if (info.lastSeenMs < minSeenTime && info.blockManagerId.executorId != "<driver>") {
+      if (info.lastSeenMs < minSeenTime && !info.blockManagerId.isDriver) {
         logWarning("Removing BlockManager " + info.blockManagerId + " with no recent heart beats: "
           + (now - info.lastSeenMs) + "ms exceeds " + slaveTimeout + "ms")
         toRemove += info.blockManagerId
@@ -232,7 +231,7 @@ class BlockManagerMasterActor(val isLocal: Boolean, conf: SparkConf, listenerBus
    */
   private def heartbeatReceived(blockManagerId: BlockManagerId): Boolean = {
     if (!blockManagerInfo.contains(blockManagerId)) {
-      blockManagerId.executorId == "<driver>" && !isLocal
+      blockManagerId.isDriver && !isLocal
     } else {
       blockManagerInfo(blockManagerId).updateLastSeenMs()
       true
@@ -355,7 +354,7 @@ class BlockManagerMasterActor(val isLocal: Boolean, conf: SparkConf, listenerBus
       tachyonSize: Long) {
 
     if (!blockManagerInfo.contains(blockManagerId)) {
-      if (blockManagerId.executorId == "<driver>" && !isLocal) {
+      if (blockManagerId.isDriver && !isLocal) {
         // We intentionally do not register the master (except in local mode),
         // so we should not indicate failure.
         sender ! true
@@ -403,16 +402,14 @@ class BlockManagerMasterActor(val isLocal: Boolean, conf: SparkConf, listenerBus
     blockIds.map(blockId => getLocations(blockId))
   }
 
-  private def getPeers(blockManagerId: BlockManagerId, size: Int): Seq[BlockManagerId] = {
-    val peers: Array[BlockManagerId] = blockManagerInfo.keySet.toArray
-
-    val selfIndex = peers.indexOf(blockManagerId)
-    if (selfIndex == -1) {
-      throw new SparkException("Self index for " + blockManagerId + " not found")
+  /** Get the list of the peers of the given block manager */
+  private def getPeers(blockManagerId: BlockManagerId): Seq[BlockManagerId] = {
+    val blockManagerIds = blockManagerInfo.keySet
+    if (blockManagerIds.contains(blockManagerId)) {
+      blockManagerIds.filterNot { _.isDriver }.filterNot { _ == blockManagerId }.toSeq
+    } else {
+      Seq.empty
     }
-
-    // Note that this logic will select the same node multiple times if there aren't enough peers
-    Array.tabulate[BlockManagerId](size) { i => peers((selfIndex + i + 1) % peers.length) }.toSeq
   }
 }
 

core/src/main/scala/org/apache/spark/storage/BlockManagerMessages.scala

@@ -88,7 +88,7 @@ private[spark] object BlockManagerMessages {
 
   case class GetLocationsMultipleBlockIds(blockIds: Array[BlockId]) extends ToBlockManagerMaster
 
-  case class GetPeers(blockManagerId: BlockManagerId, size: Int) extends ToBlockManagerMaster
+  case class GetPeers(blockManagerId: BlockManagerId) extends ToBlockManagerMaster
 
   case class RemoveExecutor(execId: String) extends ToBlockManagerMaster
 

core/src/test/scala/org/apache/spark/broadcast/BroadcastSuite.scala

@@ -132,7 +132,7 @@ class BroadcastSuite extends FunSuite with LocalSparkContext {
       val statuses = bmm.getBlockStatus(blockId, askSlaves = true)
       assert(statuses.size === 1)
       statuses.head match { case (bm, status) =>
-        assert(bm.executorId === "<driver>", "Block should only be on the driver")
+        assert(bm.isDriver, "Block should only be on the driver")
         assert(status.storageLevel === StorageLevel.MEMORY_AND_DISK)
         assert(status.memSize > 0, "Block should be in memory store on the driver")
         assert(status.diskSize === 0, "Block should not be in disk store on the driver")