Github user rxin commented on a diff in the pull request:
https://github.com/apache/spark/pull/13152#discussion_r76170909
--- Diff:
core/src/main/scala/org/apache/spark/storage/BlockReplicationPolicy.scala ---
@@ -0,0 +1,112 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements. See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You under the Apache License, Version 2.0
+ * (the "License"); you may not use this file except in compliance with
+ * the License. You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.apache.spark.storage
+
+import scala.collection.mutable
+import scala.util.Random
+
+import org.apache.spark.annotation.DeveloperApi
+import org.apache.spark.internal.Logging
+
+/**
+ * ::DeveloperApi::
+ * BlockReplicationPrioritization provides logic for prioritizing a
sequence of peers for
+ * replicating blocks. BlockManager will replicate to each peer returned
in order until the
+ * desired replication order is reached. If a replication fails,
prioritize() will be called
+ * again to get a fresh prioritization.
+ */
+@DeveloperApi
+trait BlockReplicationPolicy {
+
+ /**
+ * Method to prioritize a bunch of candidate peers of a block
+ *
+ * @param blockManagerId Id of the current BlockManager for self
identification
+ * @param peers A list of peers of a BlockManager
+ * @param peersReplicatedTo Set of peers already replicated to
+ * @param blockId BlockId of the block being replicated. This can be
used as a source of
+ * randomness if needed.
+ * @param numPeersToReplicateTo Number of peers we need to replicate to
+ * @return A prioritized list of peers. Lower the index of a peer,
higher its priority.
+ * This returns a list of size at most `numPeersToReplicateTo`.
+ */
+ def prioritize(
+ blockManagerId: BlockManagerId,
+ peers: Seq[BlockManagerId],
+ peersReplicatedTo: mutable.HashSet[BlockManagerId],
+ blockId: BlockId,
+ numPeersToReplicateTo: Int): List[BlockManagerId]
+}
+
+@DeveloperApi
+class RandomBlockReplicationPolicy
+ extends BlockReplicationPolicy
+ with Logging {
+
+ /**
+ * Method to prioritize a bunch of candidate peers of a block. This is a
basic implementation,
+ * that just makes sure we put blocks on different hosts, if possible
+ *
+ * @param blockManagerId Id of the current BlockManager for self
identification
+ * @param peers A list of peers of a BlockManager
+ * @param peersReplicatedTo Set of peers already replicated to
+ * @param blockId BlockId of the block being replicated. This can be
used as a source of
+ * randomness if needed.
+ * @return A prioritized list of peers. Lower the index of a peer,
higher its priority
+ */
+ override def prioritize(
+ blockManagerId: BlockManagerId,
+ peers: Seq[BlockManagerId],
+ peersReplicatedTo: mutable.HashSet[BlockManagerId],
+ blockId: BlockId,
+ numReplicas: Int): List[BlockManagerId] = {
+ val random = new Random(blockId.hashCode)
+ logDebug(s"Input peers : ${peers.mkString(", ")}")
+ val prioritizedPeers = if (peers.size > numReplicas) {
+ getSampleIds(peers.size, numReplicas, random).map(peers(_))
+ } else {
+ if (peers.size < numReplicas) {
+ logWarning(s"Expecting ${numReplicas} replicas with only
${peers.size} peer/s.")
+ }
+ random.shuffle(peers).toList
+ }
+ logDebug(s"Prioritized peers : ${prioritizedPeers.mkString(", ")}")
+ prioritizedPeers
+ }
+
+ /**
+ * Uses sampling algorithm by Robert Floyd. Finds a random sample in
O(n) while
+ * minimizing space usage
+ * [[http://math.stackexchange.com/questions/178690/
+ *
whats-the-proof-of-correctness-for-robert-floyds-algorithm-for-selecting-a-sin]]
+ *
+ * @param n total number of indices
+ * @param m number of samples needed
+ * @param r random number generator
+ * @return list of m random unique indices
+ */
+ private def getSampleIds(n: Int, m: Int, r: Random): List[Int] = {
+ val indices = (n - m + 1 to n).foldLeft(Set.empty[Int]) {case (s, i) =>
--- End diff --
this is a bit difficult to read (actually very rarely do foldLeft make code
easy to read in my experience).
Perhaps just write it in a more imperative way?
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