KAFKA-16944; Rewrite Range Assignor (#16504)

The server side range assignor was made to be sticky i.e. partitions from the existing assignment are retained as much as possible. During a rebalance, the expected behavior is to achieve co-partitioning for members that are subscribed to the same set of topics with equal number of partitions.

However, there are cases where this cannot be achieved efficiently with the current algorithm. There is no easy way to implement stickiness and co-partitioning and hence we have resorted to recomputing the target assignment every time.

In case of static membership, instanceIds are leveraged to ensure some form of stickiness.

```
Benchmark                                       (assignmentType)  (assignorType)  (isRackAware)  (memberCount)  (partitionsToMemberRatio)  (subscriptionType)  (topicCount)  Mode  Cnt    Score    Error  Units
ServerSideAssignorBenchmark.doAssignment             INCREMENTAL           RANGE          false            100                         10         HOMOGENEOUS           100  avgt    5    0.052 ±  0.001  ms/op
ServerSideAssignorBenchmark.doAssignment             INCREMENTAL           RANGE          false            100                         10         HOMOGENEOUS          1000  avgt    5    0.454 ±  0.003  ms/op
ServerSideAssignorBenchmark.doAssignment             INCREMENTAL           RANGE          false           1000                         10         HOMOGENEOUS           100  avgt    5    0.476 ±  0.046  ms/op
ServerSideAssignorBenchmark.doAssignment             INCREMENTAL           RANGE          false           1000                         10         HOMOGENEOUS          1000  avgt    5    3.102 ±  0.055  ms/op
ServerSideAssignorBenchmark.doAssignment             INCREMENTAL           RANGE          false          10000                         10         HOMOGENEOUS           100  avgt    5    5.640 ±  0.223  ms/op
ServerSideAssignorBenchmark.doAssignment             INCREMENTAL           RANGE          false          10000                         10         HOMOGENEOUS          1000  avgt    5   37.947 ±  1.000  ms/op
ServerSideAssignorBenchmark.doAssignment             INCREMENTAL           RANGE          false            100                         10       HETEROGENEOUS           100  avgt    5    0.172 ±  0.001  ms/op
ServerSideAssignorBenchmark.doAssignment             INCREMENTAL           RANGE          false            100                         10       HETEROGENEOUS          1000  avgt    5    1.882 ±  0.006  ms/op
ServerSideAssignorBenchmark.doAssignment             INCREMENTAL           RANGE          false           1000                         10       HETEROGENEOUS           100  avgt    5    1.730 ±  0.036  ms/op
ServerSideAssignorBenchmark.doAssignment             INCREMENTAL           RANGE          false           1000                         10       HETEROGENEOUS          1000  avgt    5   17.654 ±  1.160  ms/op
ServerSideAssignorBenchmark.doAssignment             INCREMENTAL           RANGE          false          10000                         10       HETEROGENEOUS           100  avgt    5   18.595 ±  0.316  ms/op
ServerSideAssignorBenchmark.doAssignment             INCREMENTAL           RANGE          false          10000                         10       HETEROGENEOUS          1000  avgt    5  172.398 ±  2.251  ms/op
JMH benchmarks done

Benchmark                                     (memberCount)  (partitionsToMemberRatio)  (topicCount)  Mode  Cnt   Score   Error  Units
TargetAssignmentBuilderBenchmark.build                  100                         10           100  avgt    5   0.071 ± 0.004  ms/op
TargetAssignmentBuilderBenchmark.build                  100                         10          1000  avgt    5   0.428 ± 0.026  ms/op
TargetAssignmentBuilderBenchmark.build                 1000                         10           100  avgt    5   0.659 ± 0.028  ms/op
TargetAssignmentBuilderBenchmark.build                 1000                         10          1000  avgt    5   3.346 ± 0.102  ms/op
TargetAssignmentBuilderBenchmark.build                10000                         10           100  avgt    5   8.947 ± 0.386  ms/op
TargetAssignmentBuilderBenchmark.build                10000                         10          1000  avgt    5  40.240 ± 3.113  ms/op
JMH benchmarks done
```

Reviewers: David Jacot <djacot@confluent.io>
diff --git a/group-coordinator/group-coordinator-api/src/main/java/org/apache/kafka/coordinator/group/api/assignor/MemberSubscription.java b/group-coordinator/group-coordinator-api/src/main/java/org/apache/kafka/coordinator/group/api/assignor/MemberSubscription.java
index 4f3cf45..fcf7b84 100644
--- a/group-coordinator/group-coordinator-api/src/main/java/org/apache/kafka/coordinator/group/api/assignor/MemberSubscription.java
+++ b/group-coordinator/group-coordinator-api/src/main/java/org/apache/kafka/coordinator/group/api/assignor/MemberSubscription.java
@@ -35,6 +35,13 @@
     Optional<String> rackId();
 
     /**
+     * Gets the instance Id if present.
+     *
+     * @return An Optional containing the instance Id, or an empty Optional if not present.
+     */
+    Optional<String> instanceId();
+
+    /**
      * Gets the set of subscribed topic Ids.
      *
      * @return The set of subscribed topic Ids.
diff --git a/group-coordinator/src/main/java/org/apache/kafka/coordinator/group/assignor/RangeAssignor.java b/group-coordinator/src/main/java/org/apache/kafka/coordinator/group/assignor/RangeAssignor.java
index b11959d..ee33212 100644
--- a/group-coordinator/src/main/java/org/apache/kafka/coordinator/group/assignor/RangeAssignor.java
+++ b/group-coordinator/src/main/java/org/apache/kafka/coordinator/group/assignor/RangeAssignor.java
@@ -21,40 +21,67 @@
 import org.apache.kafka.coordinator.group.api.assignor.GroupAssignment;
 import org.apache.kafka.coordinator.group.api.assignor.GroupSpec;
 import org.apache.kafka.coordinator.group.api.assignor.MemberAssignment;
+import org.apache.kafka.coordinator.group.api.assignor.MemberSubscription;
 import org.apache.kafka.coordinator.group.api.assignor.PartitionAssignorException;
 import org.apache.kafka.coordinator.group.api.assignor.SubscribedTopicDescriber;
+import org.apache.kafka.coordinator.group.api.assignor.SubscriptionType;
 import org.apache.kafka.coordinator.group.modern.MemberAssignmentImpl;
 
 import java.util.ArrayList;
-import java.util.Collection;
 import java.util.Collections;
 import java.util.HashMap;
-import java.util.HashSet;
 import java.util.List;
 import java.util.Map;
+import java.util.Optional;
 import java.util.Set;
 
-import static java.lang.Math.min;
-import static org.apache.kafka.coordinator.group.api.assignor.SubscriptionType.HOMOGENEOUS;
-
 /**
- * This Range Assignor inherits properties of both the range assignor and the sticky assignor.
- * The properties are as follows:
+ * A range assignor assigns contiguous partition ranges to members of a consumer group such that:
  * <ol>
- *      <li> Each member must get at least one partition from every topic that it is subscribed to.
- *           The only exception is when the number of subscribed members is greater than the
- *           number of partitions for that topic. (Range) </li>
- *      <li> Partitions should be assigned to members in a way that facilitates the join operation when required. (Range)
- *           This can only be done if every member is subscribed to the same topics and the topics are co-partitioned.
- *           Two streams are co-partitioned if the following conditions are met:
- *           <ul>
- *              <li> The keys must have the same schemas. </li>
- *              <li> The topics involved must have the same number of partitions. </li>
- *           </ul>
- *      </li>
- *      <li> Members should retain as much of their previous assignment as possible to reduce the number of partition
- *           movements during reassignment. (Sticky) </li>
+ *   <li>Each subscribed member receives at least one partition from that topic.</li>
+ *   <li>Each member receives the same partition number from every subscribed topic when co-partitioning is possible.</li>
  * </ol>
+ *
+ * Co-partitioning is possible when the below conditions are satisfied:
+ * <ol>
+ *   <li>All the members are subscribed to the same set of topics.</li>
+ *   <li>All the topics have the same number of partitions.</li>
+ * </ol>
+ *
+ * Co-partitioning is useful in performing joins on data streams.
+ *
+ * <p>For example, suppose there are two members M0 and M1, two topics T1 and T2, and each topic has 3 partitions.
+ *
+ * <p>The co-partitioned assignment will be:
+ * <ul>
+ * <li<code>    M0: [T1P0, T1P1, T2P0, T2P1]    </code></li>
+ * <li><code>   M1: [T1P2, T2P2]                </code></li>
+ * </ul>
+ *
+ * Since the introduction of static membership, we could leverage <code>member.instance.id</code> to make the
+ * assignment behavior more sticky.
+ * For the above example, after one rolling bounce, the group coordinator will attempt to assign new member Ids towards
+ * members, for example if <code>M0</code> -&gt; <code>M3</code> <code>M1</code> -&gt; <code>M2</code>.
+ *
+ * <p>The assignment could be completely shuffled to:
+ * <ul>
+ * <li><code>   M3 (was M0): [T1P2, T2P2]               (before it was [T1P0, T1P1, T2P0, T2P1])  </code>
+ * <li><code>   M2 (was M1): [T1P0, T1P1, T2P0, T2P1]   (before it was [T1P2, T2P2])  </code>
+ * </ul>
+ *
+ * The assignment change was caused by the change of <code>member.id</code> relative order, and
+ * can be avoided by setting the instance.id.
+ * Members will have individual instance Ids <code>I0</code>, <code>I1</code>. As long as
+ * 1. Number of members remain the same.
+ * 2. Topic metadata doesn't change.
+ * 3. Subscription pattern doesn't change for any member.
+ *
+ * <p>The assignment will always be:
+ * <ul>
+ * <li><code>   I0: [T1P0, T1P1, T2P0, T2P1]    </code>
+ * <li><code>   I1: [T1P2, T2P2]                </code>
+ * </ul>
+ * <p>
  */
 public class RangeAssignor implements ConsumerGroupPartitionAssignor {
     public static final String RANGE_ASSIGNOR_NAME = "range";
@@ -65,191 +92,225 @@
     }
 
     /**
-     * Pair of memberId and remaining partitions to meet the quota.
+     * Metadata for a topic including partition and subscription details.
      */
-    private static class MemberWithRemainingAssignments {
-        /**
-         * Member Id.
-         */
-        private final String memberId;
+    private static class TopicMetadata {
+        private final Uuid topicId;
+        private final int numPartitions;
+        private int numMembers;
+        private int minQuota = -1;
+        private int extraPartitions = -1;
+        private int nextRange = 0;
 
         /**
-         * Number of partitions required to meet the assignment quota.
+         * Constructs a new TopicMetadata instance.
+         *
+         * @param topicId           The topic Id.
+         * @param numPartitions     The number of partitions.
+         * @param numMembers        The number of subscribed members.
          */
-        private final int remaining;
+        private TopicMetadata(Uuid topicId, int numPartitions, int numMembers) {
+            this.topicId = topicId;
+            this.numPartitions = numPartitions;
+            this.numMembers = numMembers;
+        }
 
-        public MemberWithRemainingAssignments(String memberId, int remaining) {
-            this.memberId = memberId;
-            this.remaining = remaining;
+        /**
+         * Computes the minimum partition quota per member and the extra partitions, if not already computed.
+         */
+        private void maybeComputeQuota() {
+            if (minQuota != -1) return;
+
+            // The minimum number of partitions each member should receive for a balanced assignment.
+            minQuota = numPartitions / numMembers;
+
+            // Extra partitions to be distributed one to each member.
+            extraPartitions = numPartitions % numMembers;
+        }
+
+        @Override
+        public String toString() {
+            return "TopicMetadata(topicId=" + topicId +
+                ", numPartitions=" + numPartitions +
+                ", numMembers=" + numMembers +
+                ", minQuota=" + minQuota +
+                ", extraPartitions=" + extraPartitions +
+                ", nextRange=" + nextRange +
+                ')';
         }
     }
 
     /**
-     * Returns a map of topic Ids to a list of members subscribed to them,
-     * based on the given assignment specification and metadata.
-     *
-     * @param groupSpec                     The specification required for group assignments.
-     * @param subscribedTopicDescriber      The metadata describer for subscribed topics and clusters.
-     * @return A map of topic Ids to a list of member Ids subscribed to them.
-     *
-     * @throws PartitionAssignorException If a member is subscribed to a non-existent topic.
+     * Assigns partitions to members of a homogeneous group. All members are subscribed to the same set of topics.
+     * Assignment will be co-partitioned when all the topics have an equal number of partitions.
      */
-    private Map<Uuid, Collection<String>> membersPerTopic(
-        final GroupSpec groupSpec,
-        final SubscribedTopicDescriber subscribedTopicDescriber
-    ) {
-        Map<Uuid, Collection<String>> membersPerTopic = new HashMap<>();
+    private GroupAssignment assignHomogeneousGroup(
+        GroupSpec groupSpec,
+        SubscribedTopicDescriber subscribedTopicDescriber
+    ) throws PartitionAssignorException {
+        List<String> memberIds = sortMemberIds(groupSpec);
+        int numMembers = groupSpec.memberIds().size();
 
-        if (groupSpec.subscriptionType().equals(HOMOGENEOUS)) {
-            Collection<String> allMembers = groupSpec.memberIds();
-            Collection<Uuid> topics = groupSpec.memberSubscription(groupSpec.memberIds().iterator().next())
-                .subscribedTopicIds();
+        MemberSubscription subs = groupSpec.memberSubscription(memberIds.get(0));
+        List<TopicMetadata> topics = new ArrayList<>(subs.subscribedTopicIds().size());
 
-            for (Uuid topicId : topics) {
-                if (subscribedTopicDescriber.numPartitions(topicId) == -1) {
-                    throw new PartitionAssignorException("Member is subscribed to a non-existent topic");
-                }
-                membersPerTopic.put(topicId, allMembers);
+        for (Uuid topicId : subs.subscribedTopicIds()) {
+            int numPartitions = subscribedTopicDescriber.numPartitions(topicId);
+            if (numPartitions == -1) {
+                throw new PartitionAssignorException("Member is subscribed to a non-existent topic");
             }
-        } else {
-            groupSpec.memberIds().forEach(memberId -> {
-                Collection<Uuid> topics = groupSpec.memberSubscription(memberId).subscribedTopicIds();
-                for (Uuid topicId : topics) {
-                    if (subscribedTopicDescriber.numPartitions(topicId) == -1) {
+            TopicMetadata m = new TopicMetadata(
+                topicId,
+                numPartitions,
+                numMembers
+            );
+            topics.add(m);
+        }
+
+        Map<String, MemberAssignment> assignments = new HashMap<>((int) ((groupSpec.memberIds().size() / 0.75f) + 1));
+        int memberAssignmentInitialCapacity = (int) ((topics.size() / 0.75f) + 1);
+
+        for (String memberId : memberIds) {
+            Map<Uuid, Set<Integer>> assignment = new HashMap<>(memberAssignmentInitialCapacity);
+            for (TopicMetadata topicMetadata : topics) {
+                topicMetadata.maybeComputeQuota();
+                addPartitionsToAssignment(topicMetadata, assignment);
+            }
+            assignments.put(memberId, new MemberAssignmentImpl(assignment));
+        }
+
+        return new GroupAssignment(assignments);
+    }
+
+    /**
+     * Assigns partitions to members of a heterogeneous group. Not all members are subscribed to the same topics.
+     */
+    private GroupAssignment assignHeterogeneousGroup(
+        GroupSpec groupSpec,
+        SubscribedTopicDescriber subscribedTopicDescriber
+    ) throws PartitionAssignorException {
+        List<String> memberIds = sortMemberIds(groupSpec);
+
+        Map<Uuid, TopicMetadata> topics = new HashMap<>();
+
+        for (String memberId : memberIds) {
+            MemberSubscription subs = groupSpec.memberSubscription(memberId);
+            for (Uuid topicId : subs.subscribedTopicIds()) {
+                TopicMetadata topicMetadata = topics.computeIfAbsent(topicId, __ -> {
+                    int numPartitions = subscribedTopicDescriber.numPartitions(topicId);
+                    if (numPartitions == -1) {
                         throw new PartitionAssignorException("Member is subscribed to a non-existent topic");
                     }
-                    membersPerTopic
-                        .computeIfAbsent(topicId, k -> new ArrayList<>())
-                        .add(memberId);
-                }
-            });
+
+                    return new TopicMetadata(
+                        topicId,
+                        numPartitions,
+                        0
+                    );
+                });
+                topicMetadata.numMembers++;
+            }
         }
 
-        return membersPerTopic;
+        Map<String, MemberAssignment> assignments = new HashMap<>((int) ((groupSpec.memberIds().size() / 0.75f) + 1));
+
+        for (String memberId : memberIds) {
+            MemberSubscription subs = groupSpec.memberSubscription(memberId);
+            Map<Uuid, Set<Integer>> assignment = new HashMap<>((int) ((subs.subscribedTopicIds().size() / 0.75f) + 1));
+            for (Uuid topicId : subs.subscribedTopicIds()) {
+                TopicMetadata metadata = topics.get(topicId);
+                metadata.maybeComputeQuota();
+                addPartitionsToAssignment(metadata, assignment);
+            }
+            assignments.put(memberId, new MemberAssignmentImpl(assignment));
+        }
+
+        return new GroupAssignment(assignments);
     }
 
     /**
-     * The algorithm includes the following steps:
-     * <ol>
-     *      <li> Generate a map of members per topic using the given member subscriptions. </li>
-     *      <li> Generate a list of members called potentially unfilled members, which consists of members that have not
-     *           met the minimum required quota of partitions for the assignment AND get a list called assigned sticky
-     *           partitions for topic, which has the partitions that will be retained in the new assignment. </li>
-     *      <li> Generate a list of unassigned partitions by calculating the difference between the total partitions
-     *           for the topic and the assigned (sticky) partitions. </li>
-     *      <li> Find members from the potentially unfilled members list that haven't met the total required quota
-     *           i.e. minRequiredQuota + 1, if the member is designated to receive one of the excess partitions OR
-     *           minRequiredQuota otherwise. </li>
-     *      <li> Assign partitions to them in ranges from the unassigned partitions per topic
-     *           based on the remaining partitions value. </li>
-     * </ol>
+     * Sorts members based on their instance Ids if available or by member Ids if not.
+     *
+     * Static members are placed first and non-static members follow.
+     *
+     * Prioritizing static members helps them retain their partitions, enhancing stickiness
+     * and stability. Non-static members, which do not have guaranteed rejoining Ids, are placed
+     * later, allowing for more dynamic and flexible partition assignments.
+     *
+     * @param groupSpec     The group specification containing the member information.
+     * @return A sorted list of member Ids.
+     */
+    private List<String> sortMemberIds(
+        GroupSpec groupSpec
+    ) {
+        List<String> sortedMemberIds = new ArrayList<>(groupSpec.memberIds());
+
+        sortedMemberIds.sort((memberId1, memberId2) -> {
+            Optional<String> instanceId1 = groupSpec.memberSubscription(memberId1).instanceId();
+            Optional<String> instanceId2 = groupSpec.memberSubscription(memberId2).instanceId();
+
+            if (instanceId1.isPresent() && instanceId2.isPresent()) {
+                return instanceId1.get().compareTo(instanceId2.get());
+            } else if (instanceId1.isPresent()) {
+                return -1;
+            } else if (instanceId2.isPresent()) {
+                return 1;
+            } else {
+                return memberId1.compareTo(memberId2);
+            }
+        });
+        return sortedMemberIds;
+    }
+
+    /**
+     * Assigns a range of partitions to the specified topic based on the provided metadata.
+     *
+     * @param topicMetadata         Metadata containing the topic details, including the number of partitions,
+     *                              the next range to assign, minQuota, and extra partitions.
+     * @param memberAssignment      Map from topic Id to the set of assigned partition Ids.
+     */
+    private void addPartitionsToAssignment(
+        TopicMetadata topicMetadata,
+        Map<Uuid, Set<Integer>> memberAssignment
+    ) {
+        int start = topicMetadata.nextRange;
+        int quota = topicMetadata.minQuota;
+
+        // Adjust quota to account for extra partitions if available.
+        if (topicMetadata.extraPartitions > 0) {
+            quota++;
+            topicMetadata.extraPartitions--;
+        }
+
+        // Calculate the end using the quota.
+        int end = Math.min(start + quota, topicMetadata.numPartitions);
+
+        topicMetadata.nextRange = end;
+
+        if (start < end) {
+            memberAssignment.put(topicMetadata.topicId, new RangeSet(start, end));
+        }
+    }
+
+    /**
+     * Assigns partitions to members based on their topic subscriptions and the properties of a range assignor:
+     *
+     * @param groupSpec                     The group specification containing the member information.
+     * @param subscribedTopicDescriber      The describer for subscribed topics to get the number of partitions.
+     * @return The group's assignment with the partition assignments for each member.
+     * @throws PartitionAssignorException if any member is subscribed to a non-existent topic.
      */
     @Override
     public GroupAssignment assign(
-        final GroupSpec groupSpec,
-        final SubscribedTopicDescriber subscribedTopicDescriber
+        GroupSpec groupSpec,
+        SubscribedTopicDescriber subscribedTopicDescriber
     ) throws PartitionAssignorException {
-
-        Map<String, MemberAssignment> newTargetAssignment = new HashMap<>();
-
-        // Step 1
-        Map<Uuid, Collection<String>> membersPerTopic = membersPerTopic(
-            groupSpec,
-            subscribedTopicDescriber
-        );
-
-        membersPerTopic.forEach((topicId, membersForTopic) -> {
-            int numPartitionsForTopic = subscribedTopicDescriber.numPartitions(topicId);
-            int minRequiredQuota = numPartitionsForTopic / membersForTopic.size();
-            // Each member can get only ONE extra partition per topic after receiving the minimum quota.
-            int numMembersWithExtraPartition = numPartitionsForTopic % membersForTopic.size();
-
-            // Step 2
-            Set<Integer> assignedStickyPartitionsForTopic = new HashSet<>();
-            List<MemberWithRemainingAssignments> potentiallyUnfilledMembers = new ArrayList<>();
-
-            for (String memberId : membersForTopic) {
-                Set<Integer> assignedPartitionsForTopic = groupSpec
-                    .memberAssignment(memberId)
-                    .partitions()
-                    .getOrDefault(topicId, Collections.emptySet());
-
-                int currentAssignmentSize = assignedPartitionsForTopic.size();
-                List<Integer> currentAssignmentListForTopic = new ArrayList<>(assignedPartitionsForTopic);
-
-                // If there were partitions from this topic that were previously assigned to this member, retain as many as possible.
-                // Sort the current assignment in ascending order since we want the same partition numbers from each topic
-                // to go to the same member, in order to facilitate joins in case of co-partitioned topics.
-                if (currentAssignmentSize > 0) {
-                    int retainedPartitionsCount = min(currentAssignmentSize, minRequiredQuota);
-                    Collections.sort(currentAssignmentListForTopic);
-                    for (int i = 0; i < retainedPartitionsCount; i++) {
-                        assignedStickyPartitionsForTopic
-                            .add(currentAssignmentListForTopic.get(i));
-                        newTargetAssignment.computeIfAbsent(memberId, k -> new MemberAssignmentImpl(new HashMap<>()))
-                            .partitions()
-                            .computeIfAbsent(topicId, k -> new HashSet<>())
-                            .add(currentAssignmentListForTopic.get(i));
-                    }
-                }
-
-                // Number of partitions required to meet the minRequiredQuota.
-                // There are 3 cases w.r.t the value of remaining:
-                // 1) remaining < 0: this means that the member has more than the min required amount.
-                // 2) If remaining = 0: member has the minimum required partitions, but it may get an extra partition, so it is a potentially unfilled member.
-                // 3) If remaining > 0: member doesn't have the minimum required partitions, so it should be added to potentiallyUnfilledMembers.
-                int remaining = minRequiredQuota - currentAssignmentSize;
-
-                // Retain extra partitions as well when applicable.
-                if (remaining < 0 && numMembersWithExtraPartition > 0) {
-                    numMembersWithExtraPartition--;
-                    // Since we already added the minimumRequiredQuota of partitions in the previous step (until minReq - 1), we just need to
-                    // add the extra partition that will be present at the index right after min quota was satisfied.
-                    assignedStickyPartitionsForTopic
-                        .add(currentAssignmentListForTopic.get(minRequiredQuota));
-                    newTargetAssignment.computeIfAbsent(memberId, k -> new MemberAssignmentImpl(new HashMap<>()))
-                        .partitions()
-                        .computeIfAbsent(topicId, k -> new HashSet<>())
-                        .add(currentAssignmentListForTopic.get(minRequiredQuota));
-                } else {
-                    MemberWithRemainingAssignments newPair = new MemberWithRemainingAssignments(memberId, remaining);
-                    potentiallyUnfilledMembers.add(newPair);
-                }
-            }
-
-            // Step 3
-            // Find the difference between the total partitions per topic and the already assigned sticky partitions for the topic to get the unassigned partitions.
-            // List of unassigned partitions for topic contains the partitions in ascending order.
-            List<Integer> unassignedPartitionsForTopic = new ArrayList<>();
-            for (int i = 0; i < numPartitionsForTopic; i++) {
-                if (!assignedStickyPartitionsForTopic.contains(i)) {
-                    unassignedPartitionsForTopic.add(i);
-                }
-            }
-
-            // Step 4 and Step 5
-            // Account for the extra partitions if necessary and increase the required quota by 1.
-            // If remaining > 0 after increasing the required quota, assign the remaining number of partitions from the unassigned partitions list.
-            int unassignedPartitionsListStartPointer = 0;
-            for (MemberWithRemainingAssignments pair : potentiallyUnfilledMembers) {
-                String memberId = pair.memberId;
-                int remaining = pair.remaining;
-                if (numMembersWithExtraPartition > 0) {
-                    remaining++;
-                    numMembersWithExtraPartition--;
-                }
-                if (remaining > 0) {
-                    List<Integer> partitionsToAssign = unassignedPartitionsForTopic
-                        .subList(unassignedPartitionsListStartPointer, unassignedPartitionsListStartPointer + remaining);
-                    unassignedPartitionsListStartPointer += remaining;
-                    newTargetAssignment.computeIfAbsent(memberId, k -> new MemberAssignmentImpl(new HashMap<>()))
-                        .partitions()
-                        .computeIfAbsent(topicId, k -> new HashSet<>())
-                        .addAll(partitionsToAssign);
-                }
-            }
-        });
-
-        return new GroupAssignment(newTargetAssignment);
+        if (groupSpec.memberIds().isEmpty()) {
+            return new GroupAssignment(Collections.emptyMap());
+        } else if (groupSpec.subscriptionType() == SubscriptionType.HOMOGENEOUS) {
+            return assignHomogeneousGroup(groupSpec, subscribedTopicDescriber);
+        } else {
+            return assignHeterogeneousGroup(groupSpec, subscribedTopicDescriber);
+        }
     }
 }
diff --git a/group-coordinator/src/main/java/org/apache/kafka/coordinator/group/assignor/RangeSet.java b/group-coordinator/src/main/java/org/apache/kafka/coordinator/group/assignor/RangeSet.java
new file mode 100644
index 0000000..867cdd9
--- /dev/null
+++ b/group-coordinator/src/main/java/org/apache/kafka/coordinator/group/assignor/RangeSet.java
@@ -0,0 +1,185 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements. See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You under the Apache License, Version 2.0
+ * (the "License"); you may not use this file except in compliance with
+ * the License. You may obtain a copy of the License at
+ *
+ *    http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.apache.kafka.coordinator.group.assignor;
+
+import java.lang.reflect.Array;
+import java.util.Collection;
+import java.util.Iterator;
+import java.util.NoSuchElementException;
+import java.util.Set;
+
+/**
+ * A {@code RangeSet} represents a range of integers from {@code from} (inclusive)
+ * to {@code to} (exclusive).
+ * This implementation provides a view over a continuous range of integers without actually storing them.
+ */
+class RangeSet implements Set<Integer> {
+    private final int from;
+    private final int to;
+
+    /**
+     * Constructs a {@code RangeSet} with the specified range.
+     *
+     * @param from      The starting value (inclusive) of the range.
+     * @param to        The ending value (exclusive) of the range.
+     */
+    public RangeSet(int from, int to) {
+        this.from = from;
+        this.to = to;
+    }
+
+    @Override
+    public int size() {
+        return to - from;
+    }
+
+    @Override
+    public boolean isEmpty() {
+        return size() == 0;
+    }
+
+    @Override
+    public boolean contains(Object o) {
+        if (o instanceof Integer) {
+            int value = (Integer) o;
+            return value >= from && value < to;
+        }
+        return false;
+    }
+
+    @Override
+    public Iterator<Integer> iterator() {
+        return new Iterator<Integer>() {
+            private int current = from;
+
+            @Override
+            public boolean hasNext() {
+                return current < to;
+            }
+
+            @Override
+            public Integer next() {
+                if (!hasNext()) throw new NoSuchElementException();
+                return current++;
+            }
+        };
+    }
+
+    @Override
+    public Object[] toArray() {
+        Object[] array = new Object[size()];
+        for (int i = 0; i < size(); i++) {
+            array[i] = from + i;
+        }
+        return array;
+    }
+
+    @Override
+    @SuppressWarnings("unchecked")
+    public <T> T[] toArray(T[] a) {
+        int size = size();
+        if (a.length < size) {
+            // Create a new array of the same type as a with the correct size
+            a = (T[]) Array.newInstance(a.getClass().getComponentType(), size);
+        }
+        for (int i = 0; i < size; i++) {
+            a[i] = (T) Integer.valueOf(from + i);
+        }
+        if (a.length > size) {
+            a[size] = null;
+        }
+        return a;
+    }
+
+    @Override
+    public boolean add(Integer integer) {
+        throw new UnsupportedOperationException();
+    }
+
+    @Override
+    public boolean remove(Object o) {
+        throw new UnsupportedOperationException();
+    }
+
+    @Override
+    public boolean containsAll(Collection<?> c) {
+        for (Object o : c) {
+            if (!contains(o)) return false;
+        }
+        return true;
+    }
+
+    @Override
+    public boolean addAll(Collection<? extends Integer> c) {
+        throw new UnsupportedOperationException();
+    }
+
+    @Override
+    public boolean retainAll(Collection<?> c) {
+        throw new UnsupportedOperationException();
+    }
+
+    @Override
+    public boolean removeAll(Collection<?> c) {
+        throw new UnsupportedOperationException();
+    }
+
+    @Override
+    public void clear() {
+        throw new UnsupportedOperationException();
+    }
+
+    @Override
+    public String toString() {
+        return "RangeSet(from=" + from + " (inclusive), to=" + to + " (exclusive))";
+    }
+
+    /**
+     * Compares the specified object with this set for equality.
+     * Returns {@code true} if the specified object is also a set,
+     * the two sets have the same size, and every member of the specified
+     * set is contained in this set.
+     *
+     * @param o object to be compared for equality with this set
+     * @return {@code true} if the specified object is equal to this set
+     */
+    @Override
+    public boolean equals(Object o) {
+        if (this == o) return true;
+        if (!(o instanceof Set)) return false;
+
+        if (o instanceof RangeSet) {
+            RangeSet other = (RangeSet) o;
+            return this.from == other.from && this.to == other.to;
+        }
+
+        Set<?> otherSet = (Set<?>) o;
+        if (otherSet.size() != this.size()) return false;
+
+        for (int i = from; i < to; i++) {
+            if (!otherSet.contains(i)) return false;
+        }
+        return true;
+    }
+
+    @Override
+    public int hashCode() {
+        int result = from;
+        result = 31 * result + to;
+        return result;
+    }
+}
diff --git a/group-coordinator/src/main/java/org/apache/kafka/coordinator/group/modern/MemberSubscriptionAndAssignmentImpl.java b/group-coordinator/src/main/java/org/apache/kafka/coordinator/group/modern/MemberSubscriptionAndAssignmentImpl.java
index 77cef2c..c9f3e3d 100644
--- a/group-coordinator/src/main/java/org/apache/kafka/coordinator/group/modern/MemberSubscriptionAndAssignmentImpl.java
+++ b/group-coordinator/src/main/java/org/apache/kafka/coordinator/group/modern/MemberSubscriptionAndAssignmentImpl.java
@@ -30,6 +30,7 @@
  */
 public class MemberSubscriptionAndAssignmentImpl implements MemberSubscription, MemberAssignment {
     private final Optional<String> rackId;
+    private final Optional<String> instanceId;
     private final Set<Uuid> subscribedTopicIds;
     private final Assignment memberAssignment;
 
@@ -42,10 +43,12 @@
      */
     public MemberSubscriptionAndAssignmentImpl(
         Optional<String> rackId,
+        Optional<String> instanceId,
         Set<Uuid> subscribedTopicIds,
         Assignment memberAssignment
     ) {
         this.rackId = Objects.requireNonNull(rackId);
+        this.instanceId = Objects.requireNonNull(instanceId);
         this.subscribedTopicIds = Objects.requireNonNull(subscribedTopicIds);
         this.memberAssignment = Objects.requireNonNull(memberAssignment);
     }
@@ -56,6 +59,11 @@
     }
 
     @Override
+    public Optional<String> instanceId() {
+        return instanceId;
+    }
+
+    @Override
     public Set<Uuid> subscribedTopicIds() {
         return subscribedTopicIds;
     }
@@ -71,6 +79,7 @@
         if (o == null || getClass() != o.getClass()) return false;
         MemberSubscriptionAndAssignmentImpl that = (MemberSubscriptionAndAssignmentImpl) o;
         return rackId.equals(that.rackId) &&
+            instanceId.equals(that.instanceId) &&
             subscribedTopicIds.equals(that.subscribedTopicIds) &&
             memberAssignment.equals(that.memberAssignment);
     }
@@ -78,6 +87,7 @@
     @Override
     public int hashCode() {
         int result = rackId.hashCode();
+        result = 31 * result + instanceId.hashCode();
         result = 31 * result + subscribedTopicIds.hashCode();
         result = 31 * result + memberAssignment.hashCode();
         return result;
@@ -86,6 +96,7 @@
     @Override
     public String toString() {
         return "MemberSubscriptionAndAssignmentImpl(rackId=" + rackId.orElse("N/A") +
+            ", instanceId=" + instanceId +
             ", subscribedTopicIds=" + subscribedTopicIds +
             ", memberAssignment=" + memberAssignment +
             ')';
diff --git a/group-coordinator/src/main/java/org/apache/kafka/coordinator/group/modern/TargetAssignmentBuilder.java b/group-coordinator/src/main/java/org/apache/kafka/coordinator/group/modern/TargetAssignmentBuilder.java
index 4e5d35a..fe0a0e5 100644
--- a/group-coordinator/src/main/java/org/apache/kafka/coordinator/group/modern/TargetAssignmentBuilder.java
+++ b/group-coordinator/src/main/java/org/apache/kafka/coordinator/group/modern/TargetAssignmentBuilder.java
@@ -390,6 +390,7 @@
     ) {
         return new MemberSubscriptionAndAssignmentImpl(
             Optional.ofNullable(member.rackId()),
+            Optional.ofNullable(member.instanceId()),
             new TopicIds(member.subscribedTopicNames(), topicsImage),
             memberAssignment
         );
diff --git a/group-coordinator/src/test/java/org/apache/kafka/coordinator/group/assignor/GroupSpecImplTest.java b/group-coordinator/src/test/java/org/apache/kafka/coordinator/group/assignor/GroupSpecImplTest.java
index 0d7ea3d..379d149 100644
--- a/group-coordinator/src/test/java/org/apache/kafka/coordinator/group/assignor/GroupSpecImplTest.java
+++ b/group-coordinator/src/test/java/org/apache/kafka/coordinator/group/assignor/GroupSpecImplTest.java
@@ -55,6 +55,7 @@
 
         members.put(TEST_MEMBER,  new MemberSubscriptionAndAssignmentImpl(
             Optional.empty(),
+            Optional.empty(),
             mkSet(topicId),
             Assignment.EMPTY
         ));
@@ -102,6 +103,7 @@
         );
         members.put(TEST_MEMBER, new MemberSubscriptionAndAssignmentImpl(
             Optional.empty(),
+            Optional.empty(),
             mkSet(topicId),
             new Assignment(topicPartitions)
         ));
diff --git a/group-coordinator/src/test/java/org/apache/kafka/coordinator/group/assignor/OptimizedUniformAssignmentBuilderTest.java b/group-coordinator/src/test/java/org/apache/kafka/coordinator/group/assignor/OptimizedUniformAssignmentBuilderTest.java
index 65537de..39299a0 100644
--- a/group-coordinator/src/test/java/org/apache/kafka/coordinator/group/assignor/OptimizedUniformAssignmentBuilderTest.java
+++ b/group-coordinator/src/test/java/org/apache/kafka/coordinator/group/assignor/OptimizedUniformAssignmentBuilderTest.java
@@ -80,6 +80,7 @@
             memberA,
             new MemberSubscriptionAndAssignmentImpl(
                 Optional.empty(),
+                Optional.empty(),
                 Collections.emptySet(),
                 Assignment.EMPTY
             )
@@ -117,6 +118,7 @@
             memberA,
             new MemberSubscriptionAndAssignmentImpl(
                 Optional.empty(),
+                Optional.empty(),
                 Collections.singleton(topic2Uuid),
                 Assignment.EMPTY
             )
@@ -152,12 +154,14 @@
 
         members.put(memberA, new MemberSubscriptionAndAssignmentImpl(
             Optional.empty(),
+            Optional.empty(),
             mkSet(topic1Uuid, topic3Uuid),
             Assignment.EMPTY
         ));
 
         members.put(memberB, new MemberSubscriptionAndAssignmentImpl(
             Optional.empty(),
+            Optional.empty(),
             mkSet(topic1Uuid, topic3Uuid),
             Assignment.EMPTY
         ));
@@ -201,18 +205,21 @@
 
         members.put(memberA, new MemberSubscriptionAndAssignmentImpl(
             Optional.empty(),
+            Optional.empty(),
             Collections.singleton(topic3Uuid),
             Assignment.EMPTY
         ));
 
         members.put(memberB, new MemberSubscriptionAndAssignmentImpl(
             Optional.empty(),
+            Optional.empty(),
             Collections.singleton(topic3Uuid),
             Assignment.EMPTY
         ));
 
         members.put(memberC, new MemberSubscriptionAndAssignmentImpl(
             Optional.empty(),
+            Optional.empty(),
             Collections.singleton(topic3Uuid),
             Assignment.EMPTY
         ));
@@ -262,6 +269,7 @@
         for (int i = 1; i < 50; i++) {
             members.put("member" + i, new MemberSubscriptionAndAssignmentImpl(
                 Optional.empty(),
+                Optional.empty(),
                 topicMetadata.keySet(),
                 Assignment.EMPTY
             ));
@@ -302,6 +310,7 @@
 
         members.put(memberA, new MemberSubscriptionAndAssignmentImpl(
             Optional.empty(),
+            Optional.empty(),
             mkSet(topic1Uuid, topic2Uuid),
             new Assignment(mkOrderedAssignment(
                 mkTopicAssignment(topic1Uuid, 0, 1),
@@ -311,6 +320,7 @@
 
         members.put(memberB, new MemberSubscriptionAndAssignmentImpl(
             Optional.empty(),
+            Optional.empty(),
             mkSet(topic1Uuid, topic2Uuid),
             new Assignment(mkOrderedAssignment(
                 mkTopicAssignment(topic1Uuid, 2),
@@ -365,6 +375,7 @@
 
         members.put(memberA, new MemberSubscriptionAndAssignmentImpl(
             Optional.empty(),
+            Optional.empty(),
             mkSet(topic1Uuid, topic2Uuid),
             new Assignment(mkOrderedAssignment(
                 mkTopicAssignment(topic1Uuid, 0, 2),
@@ -374,6 +385,7 @@
 
         members.put(memberB, new MemberSubscriptionAndAssignmentImpl(
             Optional.empty(),
+            Optional.empty(),
             mkSet(topic1Uuid, topic2Uuid),
             new Assignment(mkOrderedAssignment(
                 mkTopicAssignment(topic1Uuid, 1),
@@ -427,6 +439,7 @@
 
         members.put(memberA, new MemberSubscriptionAndAssignmentImpl(
             Optional.empty(),
+            Optional.empty(),
             mkSet(topic1Uuid, topic2Uuid),
             new Assignment(mkOrderedAssignment(
                 mkTopicAssignment(topic1Uuid, 0, 2),
@@ -436,6 +449,7 @@
 
         members.put(memberB, new MemberSubscriptionAndAssignmentImpl(
             Optional.empty(),
+            Optional.empty(),
             mkSet(topic1Uuid, topic2Uuid),
             new Assignment(mkOrderedAssignment(
                 mkTopicAssignment(topic1Uuid, 1),
@@ -446,6 +460,7 @@
         // Add a new member to trigger a re-assignment.
         members.put(memberC, new MemberSubscriptionAndAssignmentImpl(
             Optional.empty(),
+            Optional.empty(),
             mkSet(topic1Uuid, topic2Uuid),
             Assignment.EMPTY
         ));
@@ -498,6 +513,7 @@
 
         members.put(memberA, new MemberSubscriptionAndAssignmentImpl(
             Optional.empty(),
+            Optional.empty(),
             mkSet(topic1Uuid, topic2Uuid),
             new Assignment(mkAssignment(
                 mkTopicAssignment(topic1Uuid, 0),
@@ -507,6 +523,7 @@
 
         members.put(memberB, new MemberSubscriptionAndAssignmentImpl(
             Optional.empty(),
+            Optional.empty(),
             mkSet(topic1Uuid, topic2Uuid),
             new Assignment(mkAssignment(
                 mkTopicAssignment(topic1Uuid, 1),
@@ -563,6 +580,7 @@
 
         members.put(memberA, new MemberSubscriptionAndAssignmentImpl(
             Optional.empty(),
+            Optional.empty(),
             Collections.singleton(topic2Uuid),
             new Assignment(mkAssignment(
                 mkTopicAssignment(topic1Uuid, 0),
@@ -572,6 +590,7 @@
 
         members.put(memberB, new MemberSubscriptionAndAssignmentImpl(
             Optional.empty(),
+            Optional.empty(),
             Collections.singleton(topic2Uuid),
             new Assignment(mkAssignment(
                 mkTopicAssignment(topic1Uuid, 1),
diff --git a/group-coordinator/src/test/java/org/apache/kafka/coordinator/group/assignor/RangeAssignorTest.java b/group-coordinator/src/test/java/org/apache/kafka/coordinator/group/assignor/RangeAssignorTest.java
index a611f33..64b946f 100644
--- a/group-coordinator/src/test/java/org/apache/kafka/coordinator/group/assignor/RangeAssignorTest.java
+++ b/group-coordinator/src/test/java/org/apache/kafka/coordinator/group/assignor/RangeAssignorTest.java
@@ -19,9 +19,13 @@
 import org.apache.kafka.common.Uuid;
 import org.apache.kafka.coordinator.group.api.assignor.GroupAssignment;
 import org.apache.kafka.coordinator.group.api.assignor.GroupSpec;
+import org.apache.kafka.coordinator.group.api.assignor.MemberAssignment;
 import org.apache.kafka.coordinator.group.api.assignor.PartitionAssignorException;
+import org.apache.kafka.coordinator.group.api.assignor.SubscribedTopicDescriber;
+import org.apache.kafka.coordinator.group.api.assignor.SubscriptionType;
 import org.apache.kafka.coordinator.group.modern.Assignment;
 import org.apache.kafka.coordinator.group.modern.GroupSpecImpl;
+import org.apache.kafka.coordinator.group.modern.MemberAssignmentImpl;
 import org.apache.kafka.coordinator.group.modern.MemberSubscriptionAndAssignmentImpl;
 import org.apache.kafka.coordinator.group.modern.SubscribedTopicDescriberImpl;
 import org.apache.kafka.coordinator.group.modern.TopicMetadata;
@@ -42,7 +46,6 @@
 import static org.apache.kafka.coordinator.group.api.assignor.SubscriptionType.HETEROGENEOUS;
 import static org.apache.kafka.coordinator.group.api.assignor.SubscriptionType.HOMOGENEOUS;
 import static org.junit.jupiter.api.Assertions.assertEquals;
-import static org.junit.jupiter.api.Assertions.assertNull;
 import static org.junit.jupiter.api.Assertions.assertThrows;
 
 public class RangeAssignorTest {
@@ -58,7 +61,7 @@
     private final String memberC = "C";
 
     @Test
-    public void testOneConsumerNoTopic() {
+    public void testOneMemberNoTopic() {
         SubscribedTopicDescriberImpl subscribedTopicMetadata = new SubscribedTopicDescriberImpl(
             Collections.singletonMap(
                 topic1Uuid,
@@ -75,6 +78,7 @@
             memberA,
             new MemberSubscriptionAndAssignmentImpl(
                 Optional.empty(),
+                Optional.empty(),
                 Collections.emptySet(),
                 Assignment.EMPTY
             )
@@ -91,11 +95,16 @@
             subscribedTopicMetadata
         );
 
-        assertEquals(Collections.emptyMap(), groupAssignment.members());
+        Map<String, MemberAssignment> expectedAssignment = Collections.singletonMap(
+            memberA,
+            new MemberAssignmentImpl(Collections.emptyMap())
+        );
+
+        assertEquals(expectedAssignment, groupAssignment.members());
     }
 
     @Test
-    public void testOneConsumerSubscribedToNonExistentTopic() {
+    public void testOneMemberSubscribedToNonExistentTopic() {
         SubscribedTopicDescriberImpl subscribedTopicMetadata = new SubscribedTopicDescriberImpl(
             Collections.singletonMap(
                 topic1Uuid,
@@ -112,6 +121,7 @@
             memberA,
             new MemberSubscriptionAndAssignmentImpl(
                 Optional.empty(),
+                Optional.empty(),
                 mkSet(topic2Uuid),
                 Assignment.EMPTY
             )
@@ -128,7 +138,7 @@
     }
 
     @Test
-    public void testFirstAssignmentTwoConsumersTwoTopicsSameSubscriptions() {
+    public void testFirstAssignmentTwoMembersTwoTopicsSameSubscriptions() {
         Map<Uuid, TopicMetadata> topicMetadata = new HashMap<>();
         topicMetadata.put(topic1Uuid, new TopicMetadata(
             topic1Uuid,
@@ -147,12 +157,14 @@
 
         members.put(memberA, new MemberSubscriptionAndAssignmentImpl(
             Optional.empty(),
+            Optional.empty(),
             mkSet(topic1Uuid, topic3Uuid),
             Assignment.EMPTY
         ));
 
         members.put(memberB, new MemberSubscriptionAndAssignmentImpl(
             Optional.empty(),
+            Optional.empty(),
             mkSet(topic1Uuid, topic3Uuid),
             Assignment.EMPTY
         ));
@@ -183,7 +195,7 @@
     }
 
     @Test
-    public void testFirstAssignmentThreeConsumersThreeTopicsDifferentSubscriptions() {
+    public void testFirstAssignmentThreeMembersThreeTopicsDifferentSubscriptions() {
         Map<Uuid, TopicMetadata> topicMetadata = new HashMap<>();
         topicMetadata.put(topic1Uuid, new TopicMetadata(
             topic1Uuid,
@@ -208,18 +220,21 @@
 
         members.put(memberA, new MemberSubscriptionAndAssignmentImpl(
             Optional.empty(),
+            Optional.empty(),
             mkSet(topic1Uuid, topic2Uuid),
             Assignment.EMPTY
         ));
 
         members.put(memberB, new MemberSubscriptionAndAssignmentImpl(
             Optional.empty(),
+            Optional.empty(),
             mkSet(topic3Uuid),
             Assignment.EMPTY
         ));
 
         members.put(memberC, new MemberSubscriptionAndAssignmentImpl(
             Optional.empty(),
+            Optional.empty(),
             mkSet(topic2Uuid, topic3Uuid),
             Assignment.EMPTY
         ));
@@ -253,7 +268,7 @@
     }
 
     @Test
-    public void testFirstAssignmentNumConsumersGreaterThanNumPartitions() {
+    public void testFirstAssignmentNumMembersGreaterThanNumPartitions() {
         Map<Uuid, TopicMetadata> topicMetadata = new HashMap<>();
         topicMetadata.put(topic1Uuid, new TopicMetadata(
             topic1Uuid,
@@ -272,18 +287,21 @@
 
         members.put(memberA, new MemberSubscriptionAndAssignmentImpl(
             Optional.empty(),
+            Optional.empty(),
             mkSet(topic1Uuid, topic3Uuid),
             Assignment.EMPTY
         ));
 
         members.put(memberB, new MemberSubscriptionAndAssignmentImpl(
             Optional.empty(),
+            Optional.empty(),
             mkSet(topic1Uuid, topic3Uuid),
             Assignment.EMPTY
         ));
 
         members.put(memberC, new MemberSubscriptionAndAssignmentImpl(
             Optional.empty(),
+            Optional.empty(),
             mkSet(topic1Uuid, topic3Uuid),
             Assignment.EMPTY
         ));
@@ -301,7 +319,7 @@
         );
 
         Map<String, Map<Uuid, Set<Integer>>> expectedAssignment = new HashMap<>();
-        // Topic 3 has 2 partitions but three consumers subscribed to it - one of them will not get a partition.
+        // Topic 3 has 2 partitions but three Members subscribed to it - one of them will not get a partition.
         expectedAssignment.put(memberA, mkAssignment(
             mkTopicAssignment(topic1Uuid, 0),
             mkTopicAssignment(topic3Uuid, 0)
@@ -318,7 +336,163 @@
     }
 
     @Test
-    public void testReassignmentNumConsumersGreaterThanNumPartitionsWhenOneConsumerAdded() {
+    public void testStaticMembership() throws PartitionAssignorException {
+        SubscribedTopicDescriber subscribedTopicMetadata = new SubscribedTopicDescriberImpl(
+            Collections.singletonMap(
+                topic1Uuid,
+                new TopicMetadata(
+                    topic1Uuid,
+                    topic1Name,
+                    3,
+                    Collections.emptyMap()
+                )
+            )
+        );
+
+        Map<String, MemberSubscriptionAndAssignmentImpl> members = new TreeMap<>();
+        members.put(memberA, new MemberSubscriptionAndAssignmentImpl(
+            Optional.empty(),
+            Optional.of("instanceA"),
+            Collections.singleton(topic1Uuid),
+            Assignment.EMPTY
+        ));
+        members.put(memberB, new MemberSubscriptionAndAssignmentImpl(
+            Optional.empty(),
+            Optional.of("instanceB"),
+            Collections.singleton(topic1Uuid),
+            Assignment.EMPTY
+        ));
+
+        GroupSpec groupSpec = new GroupSpecImpl(
+            members,
+            SubscriptionType.HOMOGENEOUS,
+            invertedTargetAssignment(members)
+        );
+
+        GroupAssignment initialAssignment = assignor.assign(
+            groupSpec,
+            subscribedTopicMetadata
+        );
+
+        // Remove static memberA and add it back with a different member Id but same instance Id.
+        members.remove(memberA);
+        members.put("memberA1", new MemberSubscriptionAndAssignmentImpl(
+            Optional.empty(),
+            Optional.of("instanceA"),
+            Collections.singleton(topic1Uuid),
+            Assignment.EMPTY
+        ));
+
+        groupSpec = new GroupSpecImpl(
+            members,
+            SubscriptionType.HOMOGENEOUS,
+            invertedTargetAssignment(members)
+        );
+
+        GroupAssignment reassignedAssignment = assignor.assign(
+            groupSpec,
+            subscribedTopicMetadata
+        );
+
+        // Assert that the assignment did not change
+        assertEquals(
+            initialAssignment.members().get(memberA).partitions(),
+            reassignedAssignment.members().get("memberA1").partitions()
+        );
+
+        assertEquals(
+            initialAssignment.members().get(memberB).partitions(),
+            reassignedAssignment.members().get(memberB).partitions()
+        );
+    }
+
+    @Test
+    public void testMixedStaticMembership() throws PartitionAssignorException {
+        SubscribedTopicDescriber subscribedTopicMetadata = new SubscribedTopicDescriberImpl(
+            Collections.singletonMap(
+                topic1Uuid,
+                new TopicMetadata(
+                    topic1Uuid,
+                    topic1Name,
+                    5,
+                    Collections.emptyMap()
+                )
+            )
+        );
+
+        // Initialize members with instance Ids.
+        Map<String, MemberSubscriptionAndAssignmentImpl> members = new TreeMap<>();
+        members.put(memberA, new MemberSubscriptionAndAssignmentImpl(
+            Optional.empty(),
+            Optional.of("instanceA"),
+            Collections.singleton(topic1Uuid),
+            Assignment.EMPTY
+        ));
+        members.put(memberC, new MemberSubscriptionAndAssignmentImpl(
+            Optional.empty(),
+            Optional.of("instanceC"),
+            Collections.singleton(topic1Uuid),
+            Assignment.EMPTY
+        ));
+
+        // Initialize member without an instance Id.
+        members.put(memberB, new MemberSubscriptionAndAssignmentImpl(
+            Optional.empty(),
+            Optional.empty(),
+            Collections.singleton(topic1Uuid),
+            Assignment.EMPTY
+        ));
+
+        GroupSpec groupSpec = new GroupSpecImpl(
+            members,
+            SubscriptionType.HOMOGENEOUS,
+            invertedTargetAssignment(members)
+        );
+
+        GroupAssignment initialAssignment = assignor.assign(
+            groupSpec,
+            subscribedTopicMetadata
+        );
+
+        // Remove memberA and add it back with a different member Id but same instance Id.
+        members.remove(memberA);
+        members.put("memberA1", new MemberSubscriptionAndAssignmentImpl(
+            Optional.empty(),
+            Optional.of("instanceA"),
+            Collections.singleton(topic1Uuid),
+            Assignment.EMPTY
+        ));
+
+        groupSpec = new GroupSpecImpl(
+            members,
+            SubscriptionType.HOMOGENEOUS,
+            invertedTargetAssignment(members)
+        );
+
+        GroupAssignment reassignedAssignment = assignor.assign(
+            groupSpec,
+            subscribedTopicMetadata
+        );
+
+        // Assert that the assignments did not change.
+        assertEquals(
+            initialAssignment.members().get(memberA).partitions(),
+            reassignedAssignment.members().get("memberA1").partitions()
+        );
+
+        assertEquals(
+            initialAssignment.members().get(memberB).partitions(),
+            reassignedAssignment.members().get(memberB).partitions()
+        );
+
+        assertEquals(
+            initialAssignment.members().get(memberC).partitions(),
+            reassignedAssignment.members().get(memberC).partitions()
+        );
+    }
+
+    @Test
+    public void testReassignmentNumMembersGreaterThanNumPartitionsWhenOneMemberAdded() {
         Map<Uuid, TopicMetadata> topicMetadata = new HashMap<>();
         topicMetadata.put(topic1Uuid, new TopicMetadata(
             topic1Uuid,
@@ -337,6 +511,7 @@
 
         members.put(memberA, new MemberSubscriptionAndAssignmentImpl(
             Optional.empty(),
+            Optional.empty(),
             mkSet(topic1Uuid, topic2Uuid),
             new Assignment(mkAssignment(
                 mkTopicAssignment(topic1Uuid, 0),
@@ -346,6 +521,7 @@
 
         members.put(memberB, new MemberSubscriptionAndAssignmentImpl(
             Optional.empty(),
+            Optional.empty(),
             mkSet(topic1Uuid, topic2Uuid),
             new Assignment(mkAssignment(
                 mkTopicAssignment(topic1Uuid, 1),
@@ -353,9 +529,10 @@
             ))
         ));
 
-        // Add a new consumer to trigger a re-assignment
+        // Add a new Member to trigger a re-assignment
         members.put(memberC, new MemberSubscriptionAndAssignmentImpl(
             Optional.empty(),
+            Optional.empty(),
             mkSet(topic1Uuid, topic2Uuid),
             Assignment.EMPTY
         ));
@@ -381,14 +558,14 @@
             mkTopicAssignment(topic1Uuid, 1),
             mkTopicAssignment(topic2Uuid, 1)
         ));
+        // Member C shouldn't get any assignment.
+        expectedAssignment.put(memberC, Collections.emptyMap());
 
-        // Consumer C shouldn't get any assignment, due to stickiness A, B retain their assignments
-        assertNull(computedAssignment.members().get(memberC));
         assertAssignment(expectedAssignment, computedAssignment);
     }
 
     @Test
-    public void testReassignmentWhenOnePartitionAddedForTwoConsumersTwoTopics() {
+    public void testReassignmentWhenOnePartitionAddedForTwoMembersTwoTopics() {
         // Simulating adding a partition - originally T1 -> 3 Partitions and T2 -> 3 Partitions
         Map<Uuid, TopicMetadata> topicMetadata = new HashMap<>();
         topicMetadata.put(topic1Uuid, new TopicMetadata(
@@ -408,6 +585,7 @@
 
         members.put(memberA, new MemberSubscriptionAndAssignmentImpl(
             Optional.empty(),
+            Optional.empty(),
             mkSet(topic1Uuid, topic2Uuid),
             new Assignment(mkAssignment(
                 mkTopicAssignment(topic1Uuid, 0, 1),
@@ -417,6 +595,7 @@
 
         members.put(memberB, new MemberSubscriptionAndAssignmentImpl(
             Optional.empty(),
+            Optional.empty(),
             mkSet(topic1Uuid, topic2Uuid),
             new Assignment(mkAssignment(
                 mkTopicAssignment(topic1Uuid, 2),
@@ -450,7 +629,7 @@
     }
 
     @Test
-    public void testReassignmentWhenOneConsumerAddedAfterInitialAssignmentWithTwoConsumersTwoTopics() {
+    public void testReassignmentWhenOneMemberAddedAfterInitialAssignmentWithTwoMembersTwoTopics() {
         Map<Uuid, TopicMetadata> topicMetadata = new HashMap<>();
         topicMetadata.put(topic1Uuid, new TopicMetadata(
             topic1Uuid,
@@ -469,6 +648,7 @@
 
         members.put(memberA, new MemberSubscriptionAndAssignmentImpl(
             Optional.empty(),
+            Optional.empty(),
             mkSet(topic1Uuid, topic2Uuid),
             new Assignment(mkAssignment(
                 mkTopicAssignment(topic1Uuid, 0, 1),
@@ -478,6 +658,7 @@
 
         members.put(memberB, new MemberSubscriptionAndAssignmentImpl(
             Optional.empty(),
+            Optional.empty(),
             mkSet(topic1Uuid, topic2Uuid),
             new Assignment(mkAssignment(
                 mkTopicAssignment(topic1Uuid, 2),
@@ -485,9 +666,10 @@
             ))
         ));
 
-        // Add a new consumer to trigger a re-assignment
+        // Add a new Member to trigger a re-assignment
         members.put(memberC, new MemberSubscriptionAndAssignmentImpl(
             Optional.empty(),
+            Optional.empty(),
             mkSet(topic1Uuid, topic2Uuid),
             Assignment.EMPTY
         ));
@@ -510,19 +692,19 @@
             mkTopicAssignment(topic2Uuid, 0)
         ));
         expectedAssignment.put(memberB, mkAssignment(
-            mkTopicAssignment(topic1Uuid, 2),
-            mkTopicAssignment(topic2Uuid, 2)
-        ));
-        expectedAssignment.put(memberC, mkAssignment(
             mkTopicAssignment(topic1Uuid, 1),
             mkTopicAssignment(topic2Uuid, 1)
         ));
+        expectedAssignment.put(memberC, mkAssignment(
+            mkTopicAssignment(topic1Uuid, 2),
+            mkTopicAssignment(topic2Uuid, 2)
+        ));
 
         assertAssignment(expectedAssignment, computedAssignment);
     }
 
     @Test
-    public void testReassignmentWhenOneConsumerAddedAndOnePartitionAfterInitialAssignmentWithTwoConsumersTwoTopics() {
+    public void testReassignmentWhenOneMemberAddedAndOnePartitionAfterInitialAssignmentWithTwoMembersTwoTopics() {
         // Add a new partition to topic 1, initially T1 -> 3 partitions
         Map<Uuid, TopicMetadata> topicMetadata = new HashMap<>();
         topicMetadata.put(topic1Uuid, new TopicMetadata(
@@ -542,6 +724,7 @@
 
         members.put(memberA, new MemberSubscriptionAndAssignmentImpl(
             Optional.empty(),
+            Optional.empty(),
             mkSet(topic1Uuid, topic2Uuid),
             new Assignment(mkAssignment(
                 mkTopicAssignment(topic1Uuid, 0, 1),
@@ -551,6 +734,7 @@
 
         members.put(memberB, new MemberSubscriptionAndAssignmentImpl(
             Optional.empty(),
+            Optional.empty(),
             mkSet(topic1Uuid, topic2Uuid),
             new Assignment(mkAssignment(
                 mkTopicAssignment(topic1Uuid, 2),
@@ -558,9 +742,10 @@
             ))
         ));
 
-        // Add a new consumer to trigger a re-assignment
+        // Add a new Member to trigger a re-assignment
         members.put(memberC, new MemberSubscriptionAndAssignmentImpl(
             Optional.empty(),
+            Optional.empty(),
             mkSet(topic1Uuid),
             Assignment.EMPTY
         ));
@@ -594,7 +779,7 @@
     }
 
     @Test
-    public void testReassignmentWhenOneConsumerRemovedAfterInitialAssignmentWithTwoConsumersTwoTopics() {
+    public void testReassignmentWhenOneMemberRemovedAfterInitialAssignmentWithTwoMembersTwoTopics() {
         Map<Uuid, TopicMetadata> topicMetadata = new HashMap<>();
         topicMetadata.put(topic1Uuid, new TopicMetadata(
             topic1Uuid,
@@ -611,10 +796,11 @@
 
         Map<String, MemberSubscriptionAndAssignmentImpl> members = new TreeMap<>();
 
-        // Consumer A was removed
+        // Member A was removed
 
         members.put(memberB, new MemberSubscriptionAndAssignmentImpl(
             Optional.empty(),
+            Optional.empty(),
             mkSet(topic1Uuid, topic2Uuid),
             new Assignment(mkAssignment(
                 mkTopicAssignment(topic1Uuid, 2),
@@ -644,7 +830,7 @@
     }
 
     @Test
-    public void testReassignmentWhenMultipleSubscriptionsRemovedAfterInitialAssignmentWithThreeConsumersTwoTopics() {
+    public void testReassignmentWhenMultipleSubscriptionsRemovedAfterInitialAssignmentWithThreeMembersTwoTopics() {
         Map<Uuid, TopicMetadata> topicMetadata = new HashMap<>();
         topicMetadata.put(topic1Uuid, new TopicMetadata(
             topic1Uuid,
@@ -671,6 +857,7 @@
 
         members.put(memberA, new MemberSubscriptionAndAssignmentImpl(
             Optional.empty(),
+            Optional.empty(),
             mkSet(topic1Uuid),
             new Assignment(mkAssignment(
                 mkTopicAssignment(topic1Uuid, 0, 1, 2),
@@ -680,6 +867,7 @@
 
         members.put(memberB, new MemberSubscriptionAndAssignmentImpl(
             Optional.empty(),
+            Optional.empty(),
             mkSet(topic1Uuid, topic2Uuid, topic3Uuid),
             new Assignment(mkAssignment(
                 mkTopicAssignment(topic2Uuid, 1)
@@ -688,6 +876,7 @@
 
         members.put(memberC, new MemberSubscriptionAndAssignmentImpl(
             Optional.empty(),
+            Optional.empty(),
             mkSet(topic2Uuid),
             new Assignment(mkAssignment(
                 mkTopicAssignment(topic2Uuid, 2),
@@ -723,6 +912,15 @@
         assertAssignment(expectedAssignment, computedAssignment);
     }
 
+    /**
+     * Asserts that the computed group assignment matches the expected assignment.
+     *
+     * @param expectedAssignment       A map representing the expected assignment for each member.
+     *                                 The key is the member Id and the value is another map where
+     *                                 the key is the topic Uuid and the value is a set of assigned partition Ids.
+     * @param computedGroupAssignment  The computed group assignment to be checked against the expected assignment.
+     *                                 Contains the actual assignments for each member.
+     */
     private void assertAssignment(
         Map<String, Map<Uuid, Set<Integer>>> expectedAssignment,
         GroupAssignment computedGroupAssignment
diff --git a/group-coordinator/src/test/java/org/apache/kafka/coordinator/group/assignor/RangeSetTest.java b/group-coordinator/src/test/java/org/apache/kafka/coordinator/group/assignor/RangeSetTest.java
new file mode 100644
index 0000000..23dc0c5
--- /dev/null
+++ b/group-coordinator/src/test/java/org/apache/kafka/coordinator/group/assignor/RangeSetTest.java
@@ -0,0 +1,132 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements. See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You under the Apache License, Version 2.0
+ * (the "License"); you may not use this file except in compliance with
+ * the License. You may obtain a copy of the License at
+ *
+ *    http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.apache.kafka.coordinator.group.assignor;
+
+import org.junit.jupiter.api.Test;
+
+import java.util.HashSet;
+import java.util.Iterator;
+import java.util.NoSuchElementException;
+import java.util.Set;
+
+import static org.apache.kafka.common.utils.Utils.mkSet;
+import static org.junit.jupiter.api.Assertions.assertArrayEquals;
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertFalse;
+import static org.junit.jupiter.api.Assertions.assertNotEquals;
+import static org.junit.jupiter.api.Assertions.assertThrows;
+import static org.junit.jupiter.api.Assertions.assertTrue;
+
+
+public class RangeSetTest {
+    @Test
+    void testSize() {
+        RangeSet rangeSet = new RangeSet(5, 10);
+        assertEquals(5, rangeSet.size());
+    }
+
+    @Test
+    void testIsEmpty() {
+        RangeSet rangeSet = new RangeSet(5, 5);
+        assertTrue(rangeSet.isEmpty());
+    }
+
+    @Test
+    void testContains() {
+        RangeSet rangeSet = new RangeSet(5, 10);
+        assertTrue(rangeSet.contains(5));
+        assertTrue(rangeSet.contains(9));
+        assertFalse(rangeSet.contains(10));
+        assertFalse(rangeSet.contains(4));
+    }
+
+    @Test
+    void testIterator() {
+        RangeSet rangeSet = new RangeSet(5, 10);
+        Iterator<Integer> iterator = rangeSet.iterator();
+        for (int i = 5; i < 10; i++) {
+            assertTrue(iterator.hasNext());
+            assertEquals(i, iterator.next());
+        }
+        assertFalse(iterator.hasNext());
+        assertThrows(NoSuchElementException.class, iterator::next);
+    }
+
+    @Test
+    void testUnsupportedOperations() {
+        RangeSet rangeSet = new RangeSet(5, 10);
+        assertThrows(UnsupportedOperationException.class, () -> rangeSet.add(5));
+        assertThrows(UnsupportedOperationException.class, () -> rangeSet.remove(5));
+        assertThrows(UnsupportedOperationException.class, () -> rangeSet.addAll(null));
+        assertThrows(UnsupportedOperationException.class, () -> rangeSet.retainAll(null));
+        assertThrows(UnsupportedOperationException.class, () -> rangeSet.removeAll(null));
+        assertThrows(UnsupportedOperationException.class, rangeSet::clear);
+    }
+
+    @Test
+    void testToArray() {
+        RangeSet rangeSet = new RangeSet(5, 10);
+        Object[] expectedArray = {5, 6, 7, 8, 9};
+        assertArrayEquals(expectedArray, rangeSet.toArray());
+    }
+
+    @Test
+    void testToArrayWithArrayParameter() {
+        RangeSet rangeSet = new RangeSet(5, 10);
+        Integer[] inputArray = new Integer[5];
+        Integer[] expectedArray = {5, 6, 7, 8, 9};
+        assertArrayEquals(expectedArray, rangeSet.toArray(inputArray));
+    }
+
+    @Test
+    void testContainsAll() {
+        RangeSet rangeSet = new RangeSet(5, 10);
+        assertTrue(rangeSet.containsAll(mkSet(5, 6, 7, 8, 9)));
+        assertFalse(rangeSet.containsAll(mkSet(5, 6, 10)));
+    }
+
+    @Test
+    void testToString() {
+        RangeSet rangeSet = new RangeSet(5, 8);
+        assertEquals("RangeSet(from=5 (inclusive), to=8 (exclusive))", rangeSet.toString());
+    }
+
+    @Test
+    void testEquals() {
+        RangeSet rangeSet1 = new RangeSet(5, 10);
+        RangeSet rangeSet2 = new RangeSet(5, 10);
+        RangeSet rangeSet3 = new RangeSet(6, 10);
+        Set<Integer> set = mkSet(5, 6, 7, 8, 9);
+        HashSet<Integer> hashSet = new HashSet<>(mkSet(6, 7, 8, 9));
+
+        assertEquals(rangeSet1, rangeSet2);
+        assertNotEquals(rangeSet1, rangeSet3);
+        assertEquals(rangeSet1, set);
+        assertEquals(rangeSet3, hashSet);
+        assertNotEquals(rangeSet1, new Object());
+    }
+
+    @Test
+    void testHashCode() {
+        RangeSet rangeSet1 = new RangeSet(5, 10);
+        RangeSet rangeSet2 = new RangeSet(5, 10);
+        RangeSet rangeSet3 = new RangeSet(6, 10);
+
+        assertEquals(rangeSet1.hashCode(), rangeSet2.hashCode());
+        assertNotEquals(rangeSet1.hashCode(), rangeSet3.hashCode());
+    }
+}
diff --git a/group-coordinator/src/test/java/org/apache/kafka/coordinator/group/assignor/UniformHeterogeneousAssignmentBuilderTest.java b/group-coordinator/src/test/java/org/apache/kafka/coordinator/group/assignor/UniformHeterogeneousAssignmentBuilderTest.java
index 70f4dec..98d0d20 100644
--- a/group-coordinator/src/test/java/org/apache/kafka/coordinator/group/assignor/UniformHeterogeneousAssignmentBuilderTest.java
+++ b/group-coordinator/src/test/java/org/apache/kafka/coordinator/group/assignor/UniformHeterogeneousAssignmentBuilderTest.java
@@ -76,11 +76,13 @@
         Map<String, MemberSubscriptionAndAssignmentImpl> members = new TreeMap<>();
         members.put(memberA, new MemberSubscriptionAndAssignmentImpl(
             Optional.empty(),
+            Optional.empty(),
             Collections.emptySet(),
             Assignment.EMPTY
         ));
         members.put(memberB, new MemberSubscriptionAndAssignmentImpl(
             Optional.empty(),
+            Optional.empty(),
             Collections.emptySet(),
             Assignment.EMPTY
         ));
@@ -116,11 +118,13 @@
         Map<String, MemberSubscriptionAndAssignmentImpl> members = new TreeMap<>();
         members.put(memberA, new MemberSubscriptionAndAssignmentImpl(
             Optional.empty(),
+            Optional.empty(),
             Collections.singleton(topic3Uuid),
             Assignment.EMPTY
         ));
         members.put(memberB, new MemberSubscriptionAndAssignmentImpl(
             Optional.empty(),
+            Optional.empty(),
             Collections.singleton(topic2Uuid),
             Assignment.EMPTY
         ));
@@ -156,12 +160,14 @@
 
         members.put(memberA, new MemberSubscriptionAndAssignmentImpl(
             Optional.empty(),
+            Optional.empty(),
             mkSet(topic1Uuid, topic3Uuid),
             Assignment.EMPTY
         ));
 
         members.put(memberB, new MemberSubscriptionAndAssignmentImpl(
             Optional.empty(),
+            Optional.empty(),
             Collections.singleton(topic3Uuid),
             Assignment.EMPTY
         ));
@@ -210,18 +216,21 @@
 
         members.put(memberA, new MemberSubscriptionAndAssignmentImpl(
             Optional.empty(),
+            Optional.empty(),
             Collections.singleton(topic3Uuid),
             Assignment.EMPTY
         ));
 
         members.put(memberB, new MemberSubscriptionAndAssignmentImpl(
             Optional.empty(),
+            Optional.empty(),
             Collections.singleton(topic3Uuid),
             Assignment.EMPTY
         ));
 
         members.put(memberC, new MemberSubscriptionAndAssignmentImpl(
             Optional.empty(),
+            Optional.empty(),
             Collections.singleton(topic1Uuid),
             Assignment.EMPTY
         ));
@@ -279,6 +288,7 @@
 
         members.put(memberA, new MemberSubscriptionAndAssignmentImpl(
             Optional.empty(),
+            Optional.empty(),
             Collections.singleton(topic1Uuid),
             new Assignment(mkAssignment(
                 mkTopicAssignment(topic1Uuid, 0, 1, 2)
@@ -287,6 +297,7 @@
 
         members.put(memberB, new MemberSubscriptionAndAssignmentImpl(
             Optional.empty(),
+            Optional.empty(),
             mkSet(topic1Uuid, topic2Uuid),
             new Assignment(mkAssignment(
                 mkTopicAssignment(topic1Uuid, 3),
@@ -296,6 +307,7 @@
 
         members.put(memberC, new MemberSubscriptionAndAssignmentImpl(
             Optional.empty(),
+            Optional.empty(),
             mkSet(topic1Uuid, topic2Uuid, topic3Uuid),
             new Assignment(mkAssignment(
                 mkTopicAssignment(topic1Uuid, 4, 5),
@@ -364,6 +376,7 @@
 
         members.put(memberA, new MemberSubscriptionAndAssignmentImpl(
             Optional.empty(),
+            Optional.empty(),
             mkSet(topic1Uuid, topic3Uuid),
             new Assignment(mkAssignment(
                 mkTopicAssignment(topic1Uuid, 0, 1, 2, 3),
@@ -373,6 +386,7 @@
 
         members.put(memberB, new MemberSubscriptionAndAssignmentImpl(
             Optional.empty(),
+            Optional.empty(),
             mkSet(topic1Uuid, topic2Uuid, topic3Uuid, topic4Uuid),
             new Assignment(mkAssignment(
                 mkTopicAssignment(topic2Uuid, 0, 1, 2),
@@ -426,6 +440,7 @@
 
         members.put(memberA, new MemberSubscriptionAndAssignmentImpl(
             Optional.empty(),
+            Optional.empty(),
             Collections.singleton(topic1Uuid),
             new Assignment(mkAssignment(
                 mkTopicAssignment(topic1Uuid, 0, 2),
@@ -435,6 +450,7 @@
 
         members.put(memberB, new MemberSubscriptionAndAssignmentImpl(
             Optional.empty(),
+            Optional.empty(),
             mkSet(topic1Uuid, topic2Uuid),
             new Assignment(mkAssignment(
                 mkTopicAssignment(topic1Uuid, 1),
@@ -445,6 +461,7 @@
         // Add a new member to trigger a re-assignment.
         members.put(memberC, new MemberSubscriptionAndAssignmentImpl(
             Optional.empty(),
+            Optional.empty(),
             mkSet(topic1Uuid, topic2Uuid),
             Assignment.EMPTY
         ));
@@ -502,6 +519,7 @@
 
         members.put(memberA, new MemberSubscriptionAndAssignmentImpl(
             Optional.empty(),
+            Optional.empty(),
             mkSet(topic1Uuid, topic3Uuid),
             new Assignment(mkAssignment(
                 mkTopicAssignment(topic1Uuid, 0, 1, 2),
@@ -511,6 +529,7 @@
 
         members.put(memberB, new MemberSubscriptionAndAssignmentImpl(
             Optional.empty(),
+            Optional.empty(),
             Collections.singleton(topic2Uuid),
             new Assignment(mkAssignment(
                 mkTopicAssignment(topic2Uuid, 3, 4, 5, 6)
@@ -564,6 +583,7 @@
 
         members.put(memberA, new MemberSubscriptionAndAssignmentImpl(
             Optional.empty(),
+            Optional.empty(),
             Collections.singleton(topic1Uuid),
             new Assignment(mkAssignment(
                 mkTopicAssignment(topic1Uuid, 0, 2),
@@ -573,6 +593,7 @@
 
         members.put(memberB, new MemberSubscriptionAndAssignmentImpl(
             Optional.empty(),
+            Optional.empty(),
             mkSet(topic1Uuid, topic2Uuid),
             new Assignment(mkAssignment(
                 mkTopicAssignment(topic1Uuid, 1),
@@ -617,12 +638,14 @@
 
         members.put(memberA, new MemberSubscriptionAndAssignmentImpl(
             Optional.empty(),
+            Optional.empty(),
             mkSet(topic1Uuid),
             Assignment.EMPTY
         ));
 
         members.put(memberB, new MemberSubscriptionAndAssignmentImpl(
             Optional.empty(),
+            Optional.empty(),
             Collections.emptySet(),
             Assignment.EMPTY
         ));
diff --git a/group-coordinator/src/test/java/org/apache/kafka/coordinator/group/modern/TargetAssignmentBuilderTest.java b/group-coordinator/src/test/java/org/apache/kafka/coordinator/group/modern/TargetAssignmentBuilderTest.java
index 911e2d8..b77936e 100644
--- a/group-coordinator/src/test/java/org/apache/kafka/coordinator/group/modern/TargetAssignmentBuilderTest.java
+++ b/group-coordinator/src/test/java/org/apache/kafka/coordinator/group/modern/TargetAssignmentBuilderTest.java
@@ -286,6 +286,7 @@
 
         assertEquals(new MemberSubscriptionAndAssignmentImpl(
             Optional.of("rackId"),
+            Optional.of("instanceId"),
             new TopicIds(mkSet("bar", "foo", "zar"), topicsImage),
             assignment
         ), subscriptionSpec);
diff --git a/jmh-benchmarks/src/main/java/org/apache/kafka/jmh/assignor/ServerSideAssignorBenchmark.java b/jmh-benchmarks/src/main/java/org/apache/kafka/jmh/assignor/ServerSideAssignorBenchmark.java
index 1ea8d71..cbd074c 100644
--- a/jmh-benchmarks/src/main/java/org/apache/kafka/jmh/assignor/ServerSideAssignorBenchmark.java
+++ b/jmh-benchmarks/src/main/java/org/apache/kafka/jmh/assignor/ServerSideAssignorBenchmark.java
@@ -237,6 +237,7 @@
 
         members.put(memberId, new MemberSubscriptionAndAssignmentImpl(
             rackId,
+            Optional.empty(),
             subscribedTopicIds,
             Assignment.EMPTY
         ));
@@ -270,6 +271,7 @@
 
             updatedMemberSpec.put(memberId, new MemberSubscriptionAndAssignmentImpl(
                 groupSpec.memberSubscription(memberId).rackId(),
+                Optional.empty(),
                 groupSpec.memberSubscription(memberId).subscribedTopicIds(),
                 new Assignment(Collections.unmodifiableMap(memberAssignment.partitions()))
             ));
@@ -285,6 +287,7 @@
         Optional<String> rackId = rackId(memberCount - 1);
         updatedMemberSpec.put("newMember", new MemberSubscriptionAndAssignmentImpl(
             rackId,
+            Optional.empty(),
             subscribedTopicIdsForNewMember,
             Assignment.EMPTY
         ));
diff --git a/jmh-benchmarks/src/main/java/org/apache/kafka/jmh/assignor/TargetAssignmentBuilderBenchmark.java b/jmh-benchmarks/src/main/java/org/apache/kafka/jmh/assignor/TargetAssignmentBuilderBenchmark.java
index c660cbd..4734036 100644
--- a/jmh-benchmarks/src/main/java/org/apache/kafka/jmh/assignor/TargetAssignmentBuilderBenchmark.java
+++ b/jmh-benchmarks/src/main/java/org/apache/kafka/jmh/assignor/TargetAssignmentBuilderBenchmark.java
@@ -197,6 +197,7 @@
 
             members.put(memberId, new MemberSubscriptionAndAssignmentImpl(
                 Optional.empty(),
+                Optional.empty(),
                 new TopicIds(new HashSet<>(allTopicNames), topicsImage),
                 Assignment.EMPTY
             ));