[GitHub] [helix] MarkGaox commented on a diff in pull request #2595: Implement the on-demand rebalance service

["MarkGaox (via GitHub)" <gi...@apache.org>]

MarkGaox commented on code in PR #2595:
URL: https://github.com/apache/helix/pull/2595#discussion_r1294938531


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helix-core/src/main/java/org/apache/helix/model/ClusterConfig.java:
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@@ -151,7 +151,10 @@ public enum ClusterConfigProperty {
     HELIX_ENABLED_DISABLE_TIMESTAMP,
     HELIX_DISABLED_REASON,
     // disabled type should be a enum of org.apache.helix.constants.InstanceConstants.InstanceDisabledType
-    HELIX_DISABLED_TYPE
+    HELIX_DISABLED_TYPE,
+
+    // The last time when the on-demand rebalance is triggered.
+    LAST_ON_DEMAND_REBALANCE_TIMESTAMP

Review Comment:
   When there are two calls C1 and C2 and C1 < C2, it's true that C1 will be overwritten by C2. It's fine for the rebalance triggered by C1 to be based on C2 timestamp because it won't make any difference. All offline/disabled nodes are still going to be rebalanced as long as T1 < C2 && C2 < T3. As for the rebalance call of C2, when it takes place the cluster is already rebalanced, therefore making no changes. Generally, we don't expect users to make concurrent cluster management API calls (but I understand it's not a good idea to make such assumption hahaha....). Hope this explains your concern. 
   
   For now, we only trigger the cluster level rebalance. We won't implement the fine-grained control of selecting which set of servers to be rebalanced unless the user explicitly requests that.



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