lianetm commented on code in PR #20055:
URL: https://github.com/apache/kafka/pull/20055#discussion_r2186107711


##########
group-coordinator/src/main/java/org/apache/kafka/coordinator/group/modern/consumer/CurrentAssignmentBuilder.java:
##########
@@ -317,4 +451,29 @@ private ConsumerGroupMember computeNextAssignment(
                 .build();
         }
     }
+
+    /**
+     * Gets the set of topic IDs that the member is subscribed to.
+     *
+     * @return The set of topic IDs that the member is subscribed to.
+     */
+    private Set<Uuid> subscribedTopicIds() {
+        Set<String> subscriptions = member.subscribedTopicNames();
+        String subscribedTopicRegex = member.subscribedTopicRegex();
+        if (subscribedTopicRegex != null && !subscribedTopicRegex.isEmpty()) {
+            ResolvedRegularExpression resolvedRegularExpression = 
resolvedRegularExpressions.get(subscribedTopicRegex);
+            if (resolvedRegularExpression != null) {
+                if (subscriptions.isEmpty()) {
+                    subscriptions = resolvedRegularExpression.topics;
+                } else if (!resolvedRegularExpression.topics.isEmpty()) {
+                    subscriptions = new UnionSet<>(subscriptions, 
resolvedRegularExpression.topics);
+                }
+            } else {
+                // Treat an unresolved regex as matching no topics, to be 
conservative.
+            }
+        }
+
+        TopicIds subscribedTopicIds = new TopicIds(subscriptions, 
metadataImage.topics());
+        return subscribedTopicIds;

Review Comment:
   ```suggestion
           return new TopicIds(subscriptions, metadataImage.topics());
   ```



##########
group-coordinator/src/main/java/org/apache/kafka/coordinator/group/GroupMetadataManager.java:
##########
@@ -2243,21 +2243,30 @@ private 
CoordinatorResult<ConsumerGroupHeartbeatResponseData, CoordinatorRecord>
         // epoch 0 and that it is fully initialized.
         boolean bumpGroupEpoch = group.groupEpoch() == 0;
 
-        bumpGroupEpoch |= hasMemberSubscriptionChanged(
+        boolean subscribedTopicNamesChanged = hasMemberSubscriptionChanged(
             groupId,
             member,
             updatedMember,
             records
         );
-
-        bumpGroupEpoch |= maybeUpdateRegularExpressions(
+        UpdateRegularExpressionsResult updateRegularExpressionsResult = 
maybeUpdateRegularExpressions(
             context,
             group,
             member,
             updatedMember,
             records
         );
 
+        // The subscription has changed when either the subscribed topic names 
or subscribed topic
+        // regex has changed.
+        boolean hasSubscriptionChanged = subscribedTopicNamesChanged || 
updateRegularExpressionsResult.subscribedTopicRegexChanged;

Review Comment:
   nit: this seems only needed further down on `maybeReconcile`  , should we 
move it closer?



##########
group-coordinator/src/main/java/org/apache/kafka/coordinator/group/modern/consumer/CurrentAssignmentBuilder.java:
##########
@@ -215,6 +284,64 @@ private boolean ownsRevokedPartitions(
         return false;
     }
 
+    /**
+     * Updates the current assignment, removing any partitions that are not 
part of the subscribed topics.
+     * This method is a lot faster than running the full reconciliation logic 
in computeNextAssignment.
+     *
+     * @param memberAssignedPartitions  The assigned partitions of the member 
to use.
+     * @return A new ConsumerGroupMember.
+     */
+    private ConsumerGroupMember updateCurrentAssignment(
+        Map<Uuid, Set<Integer>> memberAssignedPartitions
+    ) {
+        Set<Uuid> subscribedTopicIds = subscribedTopicIds();
+
+        // Reuse the original map if no topics need to be removed.
+        Map<Uuid, Set<Integer>> newAssignedPartitions = 
memberAssignedPartitions;
+        Map<Uuid, Set<Integer>> newPartitionsPendingRevocation = new 
HashMap<>(member.partitionsPendingRevocation());
+        for (Map.Entry<Uuid, Set<Integer>> entry : 
memberAssignedPartitions.entrySet()) {

Review Comment:
   I guess it would be common for regex case to end up with empty 
`subscribedTopicIds` here (regex not resolved), but still we would iterate the 
whole set of assigned partitions. 
   
   Would it make sense to treat that case separately to avoid iterating to 
remove 1 by 1 if subscribedTopicIds.isEmpty?



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