[GitHub] [kafka] cadonna commented on a change in pull request #8696: KAFKA-6145: KIP-441: Enforce Standby Task Stickiness

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

cadonna commented on a change in pull request #8696:
URL: https://github.com/apache/kafka/pull/8696#discussion_r429196241



##########
File path: streams/src/main/java/org/apache/kafka/streams/processor/internals/assignment/TaskMovement.java
##########
@@ -132,22 +132,97 @@ static boolean assignTaskMovements(final Map<TaskId, SortedSet<UUID>> tasksToCau
         return movementsNeeded;
     }
 
+    static int assignStandbyTaskMovements(final Map<TaskId, SortedSet<UUID>> tasksToCaughtUpClients,
+                                          final Map<UUID, ClientState> clientStates,
+                                          final AtomicInteger remainingWarmupReplicas,
+                                          final Map<UUID, Set<TaskId>> warmups) {
+        final BiFunction<UUID, TaskId, Boolean> caughtUpPredicate =
+            (client, task) -> taskIsCaughtUpOnClientOrNoCaughtUpClientsExist(task, client, tasksToCaughtUpClients);
+
+        final ConstrainedPrioritySet caughtUpClientsByTaskLoad = new ConstrainedPrioritySet(
+            caughtUpPredicate,
+            client -> clientStates.get(client).assignedTaskLoad()
+        );
+
+        final Queue<TaskMovement> taskMovements = new PriorityQueue<>(
+            Comparator.comparing(TaskMovement::numCaughtUpClients).thenComparing(TaskMovement::task)
+        );
+
+        for (final Map.Entry<UUID, ClientState> clientStateEntry : clientStates.entrySet()) {
+            final UUID destination = clientStateEntry.getKey();
+            final ClientState state = clientStateEntry.getValue();
+            for (final TaskId task : state.standbyTasks()) {
+                if (warmups.getOrDefault(destination, Collections.emptySet()).contains(task)) {
+                    // this is a warmup, so we won't move it.
+                } else if (!taskIsCaughtUpOnClientOrNoCaughtUpClientsExist(task, destination, tasksToCaughtUpClients)) {

Review comment:
       prop: Could you add a method `taskIsNotCaughtUpOnClientAndCaughtUpClientsExist()`? Applying De Morgan's law every time I read this code gives me headache.

##########
File path: streams/src/main/java/org/apache/kafka/streams/processor/internals/assignment/HighAvailabilityTaskAssignor.java
##########
@@ -57,14 +59,42 @@ public boolean assign(final Map<UUID, ClientState> clients,
             configs.numStandbyReplicas
         );
 
-        final boolean probingRebalanceNeeded = assignTaskMovements(
-            tasksToCaughtUpClients(statefulTasks, clientStates, configs.acceptableRecoveryLag),
+        final AtomicInteger remainingWarmupReplicas = new AtomicInteger(configs.maxWarmupReplicas);
+
+        final Map<TaskId, SortedSet<UUID>> tasksToCaughtUpClients = tasksToCaughtUpClients(
+            statefulTasks,
+            clientStates,
+            configs.acceptableRecoveryLag
+        );
+
+        // We temporarily need to know which standby tasks were intended as warmups
+        // for active tasks, so that we don't move them (again) when we plan standby
+        // task movements. We can then immediately treat warmups exactly the same as
+        // hot-standby replicas, so we just track it right here as metadata, rather
+        // than add "warmup" assignments to ClientState, for example.
+        final Map<UUID, Set<TaskId>> warmups = new TreeMap<>();
+
+        final int neededActiveTaskMovements = assignActiveTaskMovements(
+            tasksToCaughtUpClients,
             clientStates,
-            configs.maxWarmupReplicas
+            warmups,
+            remainingWarmupReplicas
+        );
+
+        final int neededStandbyTaskMovements = assignStandbyTaskMovements(
+            tasksToCaughtUpClients,
+            clientStates,
+            remainingWarmupReplicas,
+            warmups
         );
 
         assignStatelessActiveTasks(clientStates, diff(TreeSet::new, allTaskIds, statefulTasks));
 
+        // We shouldn't plan a probing rebalance if we _needed_ task movements, but couldn't do any
+        // due to being configured for no warmups.

Review comment:
       To be clear, according to `StreamsConfig`, we do NOT allow `max.warmup.replicas = 0`. It must at least be 1.  Or was your statement hypothetical, that it would be OK to allow it? Anyway, I am in favour of keeping the `> 0` check here. 
   
   




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