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
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@@ -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 :-)
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