[GitHub] [druid] FrankChen021 commented on a change in pull request #11257: Reduce method invocation of reservoir sampling

[GitBox <gi...@apache.org>]

FrankChen021 commented on a change in pull request #11257:
URL: https://github.com/apache/druid/pull/11257#discussion_r649194445



##########
File path: server/src/main/java/org/apache/druid/server/coordinator/ReservoirSegmentSampler.java
##########
@@ -31,6 +32,42 @@
 
   private static final EmittingLogger log = new EmittingLogger(ReservoirSegmentSampler.class);
 
+  static List<BalancerSegmentHolder> getRandomBalancerSegmentHolders(
+      final List<ServerHolder> serverHolders,
+      Set<String> broadcastDatasources,
+      int k
+  )
+  {
+    List<BalancerSegmentHolder> holders = new ArrayList<>(k);
+    int numSoFar = 0;
+
+    for (ServerHolder server : serverHolders) {
+      if (!server.getServer().getType().isSegmentReplicationTarget()) {
+        // if the server only handles broadcast segments (which don't need to be rebalanced), we have nothing to do
+        continue;
+      }
+
+      for (DataSegment segment : server.getServer().iterateAllSegments()) {
+        if (broadcastDatasources.contains(segment.getDataSource())) {
+          // we don't need to rebalance segments that were assigned via broadcast rules
+          continue;
+        }
+
+        if (numSoFar < k) {
+          holders.add(new BalancerSegmentHolder(server.getServer(), segment));
+          numSoFar++;
+          continue;
+        }
+        int randNum = ThreadLocalRandom.current().nextInt(numSoFar + 1);
+        if (randNum < k) {
+          holders.set(randNum, new BalancerSegmentHolder(server.getServer(), segment));
+        }

Review comment:
       Random picking here is a standard implementation of a reservoir sampling algorithm. It makes all segments have the same probability to be selected. The original code also uses this trick, so no need to worry :-)




-- 
This is an automated message from the Apache Git Service.
To respond to the message, please log on to GitHub and use the
URL above to go to the specific comment.

For queries about this service, please contact Infrastructure at:
users@infra.apache.org



---------------------------------------------------------------------
To unsubscribe, e-mail: commits-unsubscribe@druid.apache.org
For additional commands, e-mail: commits-help@druid.apache.org