[GitHub] egor-ryashin commented on a change in pull request #6829: Don't force rebalance of the whole cluster all the time

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egor-ryashin commented on a change in pull request #6829: Don't force rebalance of the whole cluster all the time
URL: https://github.com/apache/incubator-druid/pull/6829#discussion_r246907556
 
 

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 File path: server/src/main/java/org/apache/druid/server/coordinator/helper/DruidCoordinatorBalancer.java
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 @@ -106,8 +108,18 @@ private void balanceTier(
       return;
     }
 
-    final List<ServerHolder> toMoveFrom = Lists.newArrayList(servers);
-    final List<ServerHolder> toMoveTo = Lists.newArrayList(servers);
+    // Sort order is ascending based on available size. We reverse the "from" to shed the largest nodes first
+    final List<ServerHolder> toMoveFrom;
+    final List<ServerHolder> toMoveTo;
+
+    final int nodeLimit = params.getCoordinatorDynamicConfig().getBalancerNodeLimit();
+    if (nodeLimit > 0) {
+      toMoveFrom = Lists.reverse(randomSortedSubList(servers, nodeLimit));
+      toMoveTo = randomSortedSubList(servers, nodeLimit);
 
 Review comment:
   I think it doesn't matter if `toMoveTo` is in the sorted order or not, it seems to me the balancing strategy checks all servers (in the collection) anyway.

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