AndrewJSchofield commented on code in PR #18864:
URL: https://github.com/apache/kafka/pull/18864#discussion_r1958257137


##########
group-coordinator/src/main/java/org/apache/kafka/coordinator/group/assignor/SimpleAssignor.java:
##########
@@ -72,36 +77,178 @@ private GroupAssignment assignHomogenous(
         if (subscribeTopicIds.isEmpty())
             return new GroupAssignment(Collections.emptyMap());
 
-        Map<Uuid, Set<Integer>> targetPartitions = computeTargetPartitions(
+        // Subscribed topic partitions for the share group.
+        List<TargetPartition> targetPartitions = computeTargetPartitions(
             subscribeTopicIds, subscribedTopicDescriber);
 
-        return new 
GroupAssignment(groupSpec.memberIds().stream().collect(Collectors.toMap(
-            Function.identity(), memberId -> new 
MemberAssignmentImpl(targetPartitions))));
+        // The current assignment from topic partition to members.
+        Map<TargetPartition, List<String>> currentAssignment = 
currentAssignment(groupSpec);
+        return newAssignmentHomogeneous(groupSpec, subscribeTopicIds, 
targetPartitions, currentAssignment);
     }
 
     private GroupAssignment assignHeterogeneous(
         GroupSpec groupSpec,
         SubscribedTopicDescriber subscribedTopicDescriber
     ) {
-        Map<String, MemberAssignment> members = new HashMap<>();
+        Map<String, List<TargetPartition>> memberToPartitionsSubscription = 
new HashMap<>();
         for (String memberId : groupSpec.memberIds()) {
             MemberSubscription spec = groupSpec.memberSubscription(memberId);
             if (spec.subscribedTopicIds().isEmpty())
                 continue;
 
-            Map<Uuid, Set<Integer>> targetPartitions = computeTargetPartitions(
+            // Subscribed topic partitions for the share group member.
+            List<TargetPartition> targetPartitions = computeTargetPartitions(
                 spec.subscribedTopicIds(), subscribedTopicDescriber);
+            memberToPartitionsSubscription.put(memberId, targetPartitions);
+        }
+
+        // The current assignment from topic partition to members.
+        Map<TargetPartition, List<String>> currentAssignment = 
currentAssignment(groupSpec);
+        return newAssignmentHeterogeneous(groupSpec, 
memberToPartitionsSubscription, currentAssignment);
+    }
 
-            members.put(memberId, new MemberAssignmentImpl(targetPartitions));
+    // Get the current assignment for subscribed topic partitions to share 
group members.
+    private Map<TargetPartition, List<String>> currentAssignment(GroupSpec 
groupSpec) {
+        Map<TargetPartition, List<String>> assignment = new HashMap<>();
+
+        for (String member : groupSpec.memberIds()) {
+            Map<Uuid, Set<Integer>> assignedTopicPartitions = 
groupSpec.memberAssignment(member).partitions();
+            assignedTopicPartitions.forEach((topicId, partitions) -> 
partitions.forEach(
+                partition -> assignment.computeIfAbsent(new 
TargetPartition(topicId, partition), k -> new ArrayList<>()).add(member)));
         }
+        return assignment;
+    }
+
+    private GroupAssignment newAssignmentHomogeneous(
+        GroupSpec groupSpec,
+        Set<Uuid> subscribeTopicIds,
+        List<TargetPartition> targetPartitions,
+        Map<TargetPartition, List<String>> currentAssignment) {
+
+        Map<TargetPartition, List<String>> newAssignment = new HashMap<>();
+        // Step 1: Hash member IDs to partitions.

Review Comment:
   Blank line before this comment please.



##########
group-coordinator/src/main/java/org/apache/kafka/coordinator/group/assignor/SimpleAssignor.java:
##########
@@ -72,36 +77,178 @@ private GroupAssignment assignHomogenous(
         if (subscribeTopicIds.isEmpty())
             return new GroupAssignment(Collections.emptyMap());
 
-        Map<Uuid, Set<Integer>> targetPartitions = computeTargetPartitions(
+        // Subscribed topic partitions for the share group.
+        List<TargetPartition> targetPartitions = computeTargetPartitions(
             subscribeTopicIds, subscribedTopicDescriber);
 
-        return new 
GroupAssignment(groupSpec.memberIds().stream().collect(Collectors.toMap(
-            Function.identity(), memberId -> new 
MemberAssignmentImpl(targetPartitions))));
+        // The current assignment from topic partition to members.
+        Map<TargetPartition, List<String>> currentAssignment = 
currentAssignment(groupSpec);
+        return newAssignmentHomogeneous(groupSpec, subscribeTopicIds, 
targetPartitions, currentAssignment);
     }
 
     private GroupAssignment assignHeterogeneous(
         GroupSpec groupSpec,
         SubscribedTopicDescriber subscribedTopicDescriber
     ) {
-        Map<String, MemberAssignment> members = new HashMap<>();
+        Map<String, List<TargetPartition>> memberToPartitionsSubscription = 
new HashMap<>();
         for (String memberId : groupSpec.memberIds()) {
             MemberSubscription spec = groupSpec.memberSubscription(memberId);
             if (spec.subscribedTopicIds().isEmpty())
                 continue;
 
-            Map<Uuid, Set<Integer>> targetPartitions = computeTargetPartitions(
+            // Subscribed topic partitions for the share group member.
+            List<TargetPartition> targetPartitions = computeTargetPartitions(
                 spec.subscribedTopicIds(), subscribedTopicDescriber);
+            memberToPartitionsSubscription.put(memberId, targetPartitions);
+        }
+
+        // The current assignment from topic partition to members.
+        Map<TargetPartition, List<String>> currentAssignment = 
currentAssignment(groupSpec);
+        return newAssignmentHeterogeneous(groupSpec, 
memberToPartitionsSubscription, currentAssignment);
+    }
 
-            members.put(memberId, new MemberAssignmentImpl(targetPartitions));
+    // Get the current assignment for subscribed topic partitions to share 
group members.
+    private Map<TargetPartition, List<String>> currentAssignment(GroupSpec 
groupSpec) {
+        Map<TargetPartition, List<String>> assignment = new HashMap<>();
+
+        for (String member : groupSpec.memberIds()) {
+            Map<Uuid, Set<Integer>> assignedTopicPartitions = 
groupSpec.memberAssignment(member).partitions();
+            assignedTopicPartitions.forEach((topicId, partitions) -> 
partitions.forEach(
+                partition -> assignment.computeIfAbsent(new 
TargetPartition(topicId, partition), k -> new ArrayList<>()).add(member)));
         }
+        return assignment;
+    }
+
+    private GroupAssignment newAssignmentHomogeneous(
+        GroupSpec groupSpec,
+        Set<Uuid> subscribeTopicIds,
+        List<TargetPartition> targetPartitions,
+        Map<TargetPartition, List<String>> currentAssignment) {
+
+        Map<TargetPartition, List<String>> newAssignment = new HashMap<>();
+        // Step 1: Hash member IDs to partitions.
+        memberHashAssignment(targetPartitions, groupSpec.memberIds(), 
newAssignment);
+
+        // Step 2: Round-robin assignment for unassigned partitions which do 
not have members already assigned in the current assignment.
+        Set<TargetPartition> assignedPartitions = new 
HashSet<>(newAssignment.keySet());
+        List<TargetPartition> unassignedPartitions = targetPartitions.stream()
+            .filter(targetPartition -> 
!assignedPartitions.contains(targetPartition))
+            .filter(targetPartition -> 
!currentAssignment.containsKey(targetPartition))
+            .collect(Collectors.toList());

Review Comment:
   The `.collect(Collectors.toList())` can be replaced with `.toList()`.



##########
group-coordinator/src/main/java/org/apache/kafka/coordinator/group/assignor/SimpleAssignor.java:
##########
@@ -72,36 +77,178 @@ private GroupAssignment assignHomogenous(
         if (subscribeTopicIds.isEmpty())
             return new GroupAssignment(Collections.emptyMap());
 
-        Map<Uuid, Set<Integer>> targetPartitions = computeTargetPartitions(
+        // Subscribed topic partitions for the share group.
+        List<TargetPartition> targetPartitions = computeTargetPartitions(
             subscribeTopicIds, subscribedTopicDescriber);
 
-        return new 
GroupAssignment(groupSpec.memberIds().stream().collect(Collectors.toMap(
-            Function.identity(), memberId -> new 
MemberAssignmentImpl(targetPartitions))));
+        // The current assignment from topic partition to members.
+        Map<TargetPartition, List<String>> currentAssignment = 
currentAssignment(groupSpec);
+        return newAssignmentHomogeneous(groupSpec, subscribeTopicIds, 
targetPartitions, currentAssignment);
     }
 
     private GroupAssignment assignHeterogeneous(
         GroupSpec groupSpec,
         SubscribedTopicDescriber subscribedTopicDescriber
     ) {
-        Map<String, MemberAssignment> members = new HashMap<>();
+        Map<String, List<TargetPartition>> memberToPartitionsSubscription = 
new HashMap<>();
         for (String memberId : groupSpec.memberIds()) {
             MemberSubscription spec = groupSpec.memberSubscription(memberId);
             if (spec.subscribedTopicIds().isEmpty())
                 continue;
 
-            Map<Uuid, Set<Integer>> targetPartitions = computeTargetPartitions(
+            // Subscribed topic partitions for the share group member.
+            List<TargetPartition> targetPartitions = computeTargetPartitions(
                 spec.subscribedTopicIds(), subscribedTopicDescriber);
+            memberToPartitionsSubscription.put(memberId, targetPartitions);
+        }
+
+        // The current assignment from topic partition to members.
+        Map<TargetPartition, List<String>> currentAssignment = 
currentAssignment(groupSpec);
+        return newAssignmentHeterogeneous(groupSpec, 
memberToPartitionsSubscription, currentAssignment);
+    }
 
-            members.put(memberId, new MemberAssignmentImpl(targetPartitions));
+    // Get the current assignment for subscribed topic partitions to share 
group members.
+    private Map<TargetPartition, List<String>> currentAssignment(GroupSpec 
groupSpec) {
+        Map<TargetPartition, List<String>> assignment = new HashMap<>();
+
+        for (String member : groupSpec.memberIds()) {
+            Map<Uuid, Set<Integer>> assignedTopicPartitions = 
groupSpec.memberAssignment(member).partitions();
+            assignedTopicPartitions.forEach((topicId, partitions) -> 
partitions.forEach(
+                partition -> assignment.computeIfAbsent(new 
TargetPartition(topicId, partition), k -> new ArrayList<>()).add(member)));
         }
+        return assignment;
+    }
+
+    private GroupAssignment newAssignmentHomogeneous(
+        GroupSpec groupSpec,
+        Set<Uuid> subscribeTopicIds,
+        List<TargetPartition> targetPartitions,
+        Map<TargetPartition, List<String>> currentAssignment) {
+
+        Map<TargetPartition, List<String>> newAssignment = new HashMap<>();
+        // Step 1: Hash member IDs to partitions.
+        memberHashAssignment(targetPartitions, groupSpec.memberIds(), 
newAssignment);
+
+        // Step 2: Round-robin assignment for unassigned partitions which do 
not have members already assigned in the current assignment.
+        Set<TargetPartition> assignedPartitions = new 
HashSet<>(newAssignment.keySet());
+        List<TargetPartition> unassignedPartitions = targetPartitions.stream()
+            .filter(targetPartition -> 
!assignedPartitions.contains(targetPartition))
+            .filter(targetPartition -> 
!currentAssignment.containsKey(targetPartition))
+            .collect(Collectors.toList());
+
+        roundRobinAssignment(groupSpec.memberIds(), unassignedPartitions, 
newAssignment);
+
+        Map<String, Set<TargetPartition>> finalAssignment = new HashMap<>();
+
+        // When combining current assignment, we need to only consider the 
topics in current assignment that are also being
+        // subscribed in the new assignment as well.
+        currentAssignment.forEach((targetPartition, members) -> {
+            if (subscribeTopicIds.contains(targetPartition.topicId))
+                members.forEach(member -> {
+                    if (groupSpec.memberIds().contains(member))
+                        finalAssignment.computeIfAbsent(member, k -> new 
HashSet<>()).add(targetPartition);
+                });
+        });
+        newAssignment.forEach((targetPartition, members) -> 
members.forEach(member ->
+            finalAssignment.computeIfAbsent(member, k -> new 
HashSet<>()).add(targetPartition)));
+
+        return groupAssignment(finalAssignment, groupSpec.memberIds());
+    }
+
+    private GroupAssignment newAssignmentHeterogeneous(
+        GroupSpec groupSpec,
+        Map<String, List<TargetPartition>> memberToPartitionsSubscription,
+        Map<TargetPartition, List<String>> currentAssignment) {
+
+        // Exhaustive set of all subscribed topic partitions.
+        Set<TargetPartition> targetPartitions = new LinkedHashSet<>();
+        
memberToPartitionsSubscription.values().forEach(targetPartitions::addAll);
+
+        // Create a map for topic to members subscription.
+        Map<Uuid, Set<String>> topicToMemberSubscription = new HashMap<>();
+        memberToPartitionsSubscription.forEach((member, partitions) ->
+            partitions.forEach(partition -> 
topicToMemberSubscription.computeIfAbsent(partition.topicId(), k -> new 
LinkedHashSet<>()).add(member)));
+
+        Map<TargetPartition, List<String>> newAssignment = new HashMap<>();
+        // Step 1: Hash member IDs to partitions.

Review Comment:
   Blank line before this comment please.



##########
group-coordinator/src/main/java/org/apache/kafka/coordinator/group/assignor/SimpleAssignor.java:
##########
@@ -72,36 +77,178 @@ private GroupAssignment assignHomogenous(
         if (subscribeTopicIds.isEmpty())

Review Comment:
   This should be better named `subscribedTopicIds`.



##########
group-coordinator/src/main/java/org/apache/kafka/coordinator/group/assignor/SimpleAssignor.java:
##########
@@ -72,36 +77,178 @@ private GroupAssignment assignHomogenous(
         if (subscribeTopicIds.isEmpty())
             return new GroupAssignment(Collections.emptyMap());
 
-        Map<Uuid, Set<Integer>> targetPartitions = computeTargetPartitions(
+        // Subscribed topic partitions for the share group.
+        List<TargetPartition> targetPartitions = computeTargetPartitions(
             subscribeTopicIds, subscribedTopicDescriber);
 
-        return new 
GroupAssignment(groupSpec.memberIds().stream().collect(Collectors.toMap(
-            Function.identity(), memberId -> new 
MemberAssignmentImpl(targetPartitions))));
+        // The current assignment from topic partition to members.
+        Map<TargetPartition, List<String>> currentAssignment = 
currentAssignment(groupSpec);
+        return newAssignmentHomogeneous(groupSpec, subscribeTopicIds, 
targetPartitions, currentAssignment);
     }
 
     private GroupAssignment assignHeterogeneous(
         GroupSpec groupSpec,
         SubscribedTopicDescriber subscribedTopicDescriber
     ) {
-        Map<String, MemberAssignment> members = new HashMap<>();
+        Map<String, List<TargetPartition>> memberToPartitionsSubscription = 
new HashMap<>();
         for (String memberId : groupSpec.memberIds()) {
             MemberSubscription spec = groupSpec.memberSubscription(memberId);
             if (spec.subscribedTopicIds().isEmpty())
                 continue;
 
-            Map<Uuid, Set<Integer>> targetPartitions = computeTargetPartitions(
+            // Subscribed topic partitions for the share group member.
+            List<TargetPartition> targetPartitions = computeTargetPartitions(
                 spec.subscribedTopicIds(), subscribedTopicDescriber);
+            memberToPartitionsSubscription.put(memberId, targetPartitions);
+        }
+
+        // The current assignment from topic partition to members.
+        Map<TargetPartition, List<String>> currentAssignment = 
currentAssignment(groupSpec);
+        return newAssignmentHeterogeneous(groupSpec, 
memberToPartitionsSubscription, currentAssignment);
+    }
 
-            members.put(memberId, new MemberAssignmentImpl(targetPartitions));
+    // Get the current assignment for subscribed topic partitions to share 
group members.
+    private Map<TargetPartition, List<String>> currentAssignment(GroupSpec 
groupSpec) {
+        Map<TargetPartition, List<String>> assignment = new HashMap<>();
+
+        for (String member : groupSpec.memberIds()) {
+            Map<Uuid, Set<Integer>> assignedTopicPartitions = 
groupSpec.memberAssignment(member).partitions();
+            assignedTopicPartitions.forEach((topicId, partitions) -> 
partitions.forEach(
+                partition -> assignment.computeIfAbsent(new 
TargetPartition(topicId, partition), k -> new ArrayList<>()).add(member)));
         }
+        return assignment;
+    }
+
+    private GroupAssignment newAssignmentHomogeneous(
+        GroupSpec groupSpec,
+        Set<Uuid> subscribeTopicIds,
+        List<TargetPartition> targetPartitions,
+        Map<TargetPartition, List<String>> currentAssignment) {
+
+        Map<TargetPartition, List<String>> newAssignment = new HashMap<>();
+        // Step 1: Hash member IDs to partitions.
+        memberHashAssignment(targetPartitions, groupSpec.memberIds(), 
newAssignment);
+
+        // Step 2: Round-robin assignment for unassigned partitions which do 
not have members already assigned in the current assignment.
+        Set<TargetPartition> assignedPartitions = new 
HashSet<>(newAssignment.keySet());
+        List<TargetPartition> unassignedPartitions = targetPartitions.stream()
+            .filter(targetPartition -> 
!assignedPartitions.contains(targetPartition))
+            .filter(targetPartition -> 
!currentAssignment.containsKey(targetPartition))
+            .collect(Collectors.toList());
+
+        roundRobinAssignment(groupSpec.memberIds(), unassignedPartitions, 
newAssignment);
+
+        Map<String, Set<TargetPartition>> finalAssignment = new HashMap<>();
+
+        // When combining current assignment, we need to only consider the 
topics in current assignment that are also being
+        // subscribed in the new assignment as well.
+        currentAssignment.forEach((targetPartition, members) -> {
+            if (subscribeTopicIds.contains(targetPartition.topicId))
+                members.forEach(member -> {
+                    if (groupSpec.memberIds().contains(member))
+                        finalAssignment.computeIfAbsent(member, k -> new 
HashSet<>()).add(targetPartition);
+                });
+        });
+        newAssignment.forEach((targetPartition, members) -> 
members.forEach(member ->
+            finalAssignment.computeIfAbsent(member, k -> new 
HashSet<>()).add(targetPartition)));
+
+        return groupAssignment(finalAssignment, groupSpec.memberIds());
+    }
+
+    private GroupAssignment newAssignmentHeterogeneous(
+        GroupSpec groupSpec,
+        Map<String, List<TargetPartition>> memberToPartitionsSubscription,
+        Map<TargetPartition, List<String>> currentAssignment) {
+
+        // Exhaustive set of all subscribed topic partitions.
+        Set<TargetPartition> targetPartitions = new LinkedHashSet<>();
+        
memberToPartitionsSubscription.values().forEach(targetPartitions::addAll);
+
+        // Create a map for topic to members subscription.
+        Map<Uuid, Set<String>> topicToMemberSubscription = new HashMap<>();
+        memberToPartitionsSubscription.forEach((member, partitions) ->
+            partitions.forEach(partition -> 
topicToMemberSubscription.computeIfAbsent(partition.topicId(), k -> new 
LinkedHashSet<>()).add(member)));
+
+        Map<TargetPartition, List<String>> newAssignment = new HashMap<>();
+        // Step 1: Hash member IDs to partitions.
+        memberToPartitionsSubscription.forEach((member, partitions) ->
+            memberHashAssignment(partitions, 
Collections.singletonList(member), newAssignment));

Review Comment:
   `List.of`



##########
group-coordinator/src/main/java/org/apache/kafka/coordinator/group/assignor/SimpleAssignor.java:
##########
@@ -72,36 +77,178 @@ private GroupAssignment assignHomogenous(
         if (subscribeTopicIds.isEmpty())
             return new GroupAssignment(Collections.emptyMap());
 
-        Map<Uuid, Set<Integer>> targetPartitions = computeTargetPartitions(
+        // Subscribed topic partitions for the share group.
+        List<TargetPartition> targetPartitions = computeTargetPartitions(
             subscribeTopicIds, subscribedTopicDescriber);
 
-        return new 
GroupAssignment(groupSpec.memberIds().stream().collect(Collectors.toMap(
-            Function.identity(), memberId -> new 
MemberAssignmentImpl(targetPartitions))));
+        // The current assignment from topic partition to members.
+        Map<TargetPartition, List<String>> currentAssignment = 
currentAssignment(groupSpec);
+        return newAssignmentHomogeneous(groupSpec, subscribeTopicIds, 
targetPartitions, currentAssignment);
     }
 
     private GroupAssignment assignHeterogeneous(
         GroupSpec groupSpec,
         SubscribedTopicDescriber subscribedTopicDescriber
     ) {
-        Map<String, MemberAssignment> members = new HashMap<>();
+        Map<String, List<TargetPartition>> memberToPartitionsSubscription = 
new HashMap<>();
         for (String memberId : groupSpec.memberIds()) {
             MemberSubscription spec = groupSpec.memberSubscription(memberId);
             if (spec.subscribedTopicIds().isEmpty())
                 continue;
 
-            Map<Uuid, Set<Integer>> targetPartitions = computeTargetPartitions(
+            // Subscribed topic partitions for the share group member.
+            List<TargetPartition> targetPartitions = computeTargetPartitions(
                 spec.subscribedTopicIds(), subscribedTopicDescriber);
+            memberToPartitionsSubscription.put(memberId, targetPartitions);
+        }
+
+        // The current assignment from topic partition to members.
+        Map<TargetPartition, List<String>> currentAssignment = 
currentAssignment(groupSpec);
+        return newAssignmentHeterogeneous(groupSpec, 
memberToPartitionsSubscription, currentAssignment);
+    }
 
-            members.put(memberId, new MemberAssignmentImpl(targetPartitions));
+    // Get the current assignment for subscribed topic partitions to share 
group members.
+    private Map<TargetPartition, List<String>> currentAssignment(GroupSpec 
groupSpec) {
+        Map<TargetPartition, List<String>> assignment = new HashMap<>();
+
+        for (String member : groupSpec.memberIds()) {
+            Map<Uuid, Set<Integer>> assignedTopicPartitions = 
groupSpec.memberAssignment(member).partitions();
+            assignedTopicPartitions.forEach((topicId, partitions) -> 
partitions.forEach(
+                partition -> assignment.computeIfAbsent(new 
TargetPartition(topicId, partition), k -> new ArrayList<>()).add(member)));
         }
+        return assignment;
+    }
+
+    private GroupAssignment newAssignmentHomogeneous(
+        GroupSpec groupSpec,
+        Set<Uuid> subscribeTopicIds,
+        List<TargetPartition> targetPartitions,
+        Map<TargetPartition, List<String>> currentAssignment) {
+
+        Map<TargetPartition, List<String>> newAssignment = new HashMap<>();
+        // Step 1: Hash member IDs to partitions.
+        memberHashAssignment(targetPartitions, groupSpec.memberIds(), 
newAssignment);
+
+        // Step 2: Round-robin assignment for unassigned partitions which do 
not have members already assigned in the current assignment.
+        Set<TargetPartition> assignedPartitions = new 
HashSet<>(newAssignment.keySet());
+        List<TargetPartition> unassignedPartitions = targetPartitions.stream()
+            .filter(targetPartition -> 
!assignedPartitions.contains(targetPartition))
+            .filter(targetPartition -> 
!currentAssignment.containsKey(targetPartition))
+            .collect(Collectors.toList());
+
+        roundRobinAssignment(groupSpec.memberIds(), unassignedPartitions, 
newAssignment);
+
+        Map<String, Set<TargetPartition>> finalAssignment = new HashMap<>();
+
+        // When combining current assignment, we need to only consider the 
topics in current assignment that are also being

Review Comment:
   This code to combine the current assignment with the new assignment to 
create the final assignment can be in common I think.



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