rreddy-22 commented on code in PR #13443:
URL: https://github.com/apache/kafka/pull/13443#discussion_r1177156668


##########
group-coordinator/src/main/java/org/apache/kafka/coordinator/group/assignor/RangeAssignor.java:
##########
@@ -0,0 +1,261 @@
+/*
+ * 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.apache.kafka.common.Uuid;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+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.Set;
+
+import static java.lang.Math.min;
+
+/**
+ * The Server Side Sticky Range Assignor inherits properties of both the range 
assignor and the sticky assignor.
+ * Properties are as follows:
+ * <ol>
+ * <li> Each member must get at least one partition for 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) </li>
+ * 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> The topics involved must have the same number of partitions.
+ * </ul>
+ * <li> Members should retain as much of their previous assignment as possible 
to reduce the number of partition movements during reassignment. (Sticky) </li>
+ * </ol>
+ */
+public class RangeAssignor implements PartitionAssignor {
+
+    private static final Logger log = 
LoggerFactory.getLogger(RangeAssignor.class);
+
+    public static final String RANGE_ASSIGNOR_NAME = "range";
+
+    @Override
+    public String name() {
+        return RANGE_ASSIGNOR_NAME;
+    }
+
+    static class RemainingAssignmentsForMember<String, Integer> {
+        private final String memberId;
+        private final Integer remaining;

Review Comment:
   done



##########
group-coordinator/src/main/java/org/apache/kafka/coordinator/group/assignor/RangeAssignor.java:
##########
@@ -0,0 +1,261 @@
+/*
+ * 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.apache.kafka.common.Uuid;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+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.Set;
+
+import static java.lang.Math.min;
+
+/**
+ * The Server Side Sticky Range Assignor inherits properties of both the range 
assignor and the sticky assignor.
+ * Properties are as follows:
+ * <ol>
+ * <li> Each member must get at least one partition for 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) </li>
+ * 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> The topics involved must have the same number of partitions.
+ * </ul>
+ * <li> Members should retain as much of their previous assignment as possible 
to reduce the number of partition movements during reassignment. (Sticky) </li>
+ * </ol>
+ */
+public class RangeAssignor implements PartitionAssignor {
+
+    private static final Logger log = 
LoggerFactory.getLogger(RangeAssignor.class);
+
+    public static final String RANGE_ASSIGNOR_NAME = "range";
+
+    @Override
+    public String name() {
+        return RANGE_ASSIGNOR_NAME;
+    }
+
+    static class RemainingAssignmentsForMember<String, Integer> {
+        private final String memberId;
+        private final Integer remaining;
+
+        public RemainingAssignmentsForMember(String memberId, Integer 
remaining) {
+            this.memberId = memberId;
+            this.remaining = remaining;
+        }
+
+        public String memberId() {
+            return memberId;
+        }
+
+        public Integer remaining() {
+            return remaining;
+        }
+
+    }
+
+    private Map<Uuid, List<String>> membersPerTopic(final AssignmentSpec 
assignmentSpec) {
+        Map<Uuid, List<String>> membersPerTopic = new HashMap<>();
+        Map<String, AssignmentMemberSpec> membersData = 
assignmentSpec.members();
+
+        membersData.forEach((memberId, memberMetadata) -> {
+            Collection<Uuid> topics = memberMetadata.subscribedTopicIds();
+            for (Uuid topicId: topics) {
+                // Only topics that are present in both the subscribed topics 
list and the topic metadata should be considered for assignment.
+                if (assignmentSpec.topics().containsKey(topicId)) {
+                    membersPerTopic
+                        .computeIfAbsent(topicId, k -> new ArrayList<>())
+                        .add(memberId);
+                } else {
+                    log.warn(memberId + " subscribed to topic " + topicId + " 
which doesn't exist in the topic metadata");
+                }
+            }
+        });
+
+        return membersPerTopic;
+    }
+
+    private Map<Uuid, List<Integer>> getUnassignedPartitionsPerTopic(final 
AssignmentSpec assignmentSpec, Map<Uuid, Set<Integer>> 
assignedStickyPartitionsPerTopic) {
+        Map<Uuid, List<Integer>> unassignedPartitionsPerTopic = new 
HashMap<>();
+        Map<Uuid, AssignmentTopicMetadata> topicsMetadata = 
assignmentSpec.topics();
+
+        topicsMetadata.forEach((topicId, assignmentTopicMetadata) -> {
+            ArrayList<Integer> unassignedPartitionsForTopic = new 
ArrayList<>();
+            int numPartitions = assignmentTopicMetadata.numPartitions();
+            // List of unassigned partitions per topic contains the partitions 
in ascending order.
+            Set<Integer> assignedStickyPartitionsForTopic = 
assignedStickyPartitionsPerTopic.getOrDefault(topicId, new HashSet<>());
+            for (int i = 0; i < numPartitions; i++) {
+                if (!assignedStickyPartitionsForTopic.contains(i)) {
+                    unassignedPartitionsForTopic.add(i);
+                }
+            }
+            unassignedPartitionsPerTopic.put(topicId, 
unassignedPartitionsForTopic);
+        });
+
+        return unassignedPartitionsPerTopic;
+    }
+
+/**
+ * <p> The algorithm includes the following steps:
+ * <ol>
+ * <li> Generate a map of <code>membersPerTopic</code> using the given member 
subscriptions.</li>
+ * <li> Generate a list of members (<code>potentiallyUnfilledMembers</code>) 
that have not met the minimum required quota for assignment AND
+ * get a list (<code>assignedStickyPartitionsPerTopic</code>) of partitions 
that we want to retain in the new assignment.</li>
+ * <li> Add members from the <code>potentiallyUnfilledMembers</code> list to 
the <code>unfilledMembers</code> list if they haven't met the total required 
quota i.e. minimum number of partitions per member + 1 (if member is designated 
to receive one of the excess partitions). </li>
+ * <li> Generate a list of unassigned partitions by calculating the difference 
between total partitions and already assigned (sticky) partitions. </li>
+ * <li> Iterate through the unfilled members and assign partitions from the 
unassigned partitions. </li>
+ * </ol>
+ * </p>
+ */
+    @Override
+    public GroupAssignment assign(final AssignmentSpec assignmentSpec) throws 
PartitionAssignorException {
+        Map<String, Map<Uuid, Set<Integer>>> membersWithNewAssignmentPerTopic 
= new HashMap<>();
+        // Step 1
+        Map<Uuid, List<String>> membersPerTopic = 
membersPerTopic(assignmentSpec);
+        // Step 2
+        Map<Uuid, List<RemainingAssignmentsForMember<String, Integer>>> 
unfilledMembersPerTopic = new HashMap<>();
+        Map<Uuid, Set<Integer>> assignedStickyPartitionsPerTopic = new 
HashMap<>();
+
+        membersPerTopic.forEach((topicId, membersForTopic) -> {
+            // For each topic we have a temporary list of members stored in 
potentiallyUnfilledMembers.
+            // The list is populated with members that satisfy one of the two 
conditions:
+            // 1) Members that already have the minimum required number of 
partitions i.e. total partitions divided by total subscribed members
+            //    BUT they could be assigned an extra partition later on. 
Extra partitions exist if total partitions % number of members is greater than 
0.
+            //    In this case we add the member to the unfilled members map 
iff an extra partition needs to be assigned to it.
+            // 2) Members that don't have the minimum required partitions, so 
irrespective of whether they get an extra partition or not they get added to 
the unfilled map later.
+            List<RemainingAssignmentsForMember<String, Integer>> 
potentiallyUnfilledMembers = new ArrayList<>();
+
+            AssignmentTopicMetadata topicData = 
assignmentSpec.topics().get(topicId);
+            int numPartitionsForTopic = topicData.numPartitions();
+
+            // Idle members case : When the number of members subscribed to a 
topic is greater than the total number of partitions,
+            // all members get assigned via the "extra partitions" logic since 
minRequiredQuota = 0.
+            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();
+
+            for (String memberId: membersForTopic) {
+                Set<Integer> assignedPartitionsForTopic = 
assignmentSpec.members().get(memberId)
+                    .assignedPartitions().getOrDefault(topicId, 
Collections.emptySet());
+
+                int currentAssignmentSize = assignedPartitionsForTopic.size();
+                List<Integer> currentAssignmentListForTopic = new 
ArrayList<>(assignedPartitionsForTopic);
+
+                // If there were previously assigned partitions present, we 
want to retain them.
+                if (currentAssignmentSize > 0) {
+                    // We need to retain:
+                    // 1) currentSize number of partitions (when currentSize < 
required) OR
+                    // 2) required number of partitions (when currentSize > 
required).
+                    int retainedPartitionsCount = min(currentAssignmentSize, 
minRequiredQuota);
+                    // The partitions need to be in ascending order since we 
want the same partition numbers from each topic
+                    // to go to the same member to facilitate joins in case of 
co-partitioned topics.
+                    Collections.sort(currentAssignmentListForTopic);
+                    for (int i = 0; i < retainedPartitionsCount; i++) {
+                        assignedStickyPartitionsPerTopic
+                            .computeIfAbsent(topicId, k -> new HashSet<>())
+                            .add(currentAssignmentListForTopic.get(i));
+                        membersWithNewAssignmentPerTopic
+                            .computeIfAbsent(memberId, k -> new HashMap<>())
+                            .computeIfAbsent(topicId, k -> new HashSet<>())
+                            .add(currentAssignmentListForTopic.get(i));
+                    }
+                }
+
+                // Number of partitions left to reach the minRequiredQuota.
+                int remaining = minRequiredQuota - currentAssignmentSize;
+
+                // 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.
+                if (remaining < 0 && numMembersWithExtraPartition > 0) {
+                    // In order to remain as sticky as possible, since the 
order of members can be different, we want the members that already had extra
+                    // partitions to retain them if it's still required, 
instead of assigning the extras to the first few members directly.
+                    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.
+                    assignedStickyPartitionsPerTopic
+                        .computeIfAbsent(topicId, k -> new HashSet<>())
+                        
.add(currentAssignmentListForTopic.get(minRequiredQuota));
+                    membersWithNewAssignmentPerTopic
+                        .computeIfAbsent(memberId, k -> new HashMap<>())
+                        .computeIfAbsent(topicId, k -> new HashSet<>())
+                        
.add(currentAssignmentListForTopic.get(minRequiredQuota));
+                } else {
+                    // 2) If remaining = 0: it has min req partitions but 
there is scope for getting an extra partition later on, so it is a potential 
unfilled member.

Review Comment:
   I think they're the same thing tbh :(
   



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