Re: [PR] KAFKA-21001: Keep warm-up tasks sticky in the streams sticky assignor [kafka]

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

Copilot commented on code in PR #23314:
URL: https://github.com/apache/kafka/pull/23314#discussion_r3917938262


##########
group-coordinator/src/main/java/org/apache/kafka/coordinator/group/streams/assignor/StickyTaskAssignor.java:
##########
@@ -163,14 +163,28 @@ private static void collectStandbyCandidates(final Map<TaskId, ArrayList<Standby
             }
         }
 
+        // prev warm-up tasks: a warm-up task is a member restoring the state of a task whose active task is being
+        // migrated to it, so for stickiness it counts as a prev standby -- the member that has already restored the
+        // state gets the task, instead of the restore work being thrown away. Its reported offset sum ranks it among
+        // the standbys, most caught-up first.
+        for (final Map.Entry<String, Set<Integer>> entry : memberAssignmentState.warmupTasks().entrySet()) {
+            final String subtopologyId = entry.getKey();
+            final Set<Integer> partitionNoSet = entry.getValue();
+            for (final int partitionNo : partitionNoSet) {
+                standbyCandidates
+                    .computeIfAbsent(new TaskId(subtopologyId, partitionNo), task -> new ArrayList<>())
+                    .add(new StandbyCandidate(member, true, reportedOffsetSum(memberAssignmentState, subtopologyId, partitionNo)));

Review Comment:
   This does not preserve the normal in-progress migration: `assignActive` first reassigns a task to its current active owner (lines 280–292) and only consults these standby candidates for tasks left over (lines 294–308). During warm-up, the source intentionally remains active while the destination holds the warm-up, so this candidate is never considered and a recomputation cancels the migration. The new test avoids that state by giving task 8 no active owner. Warm-up ownership needs to take precedence over current active ownership, with a source-active/destination-warm-up test.



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