rreddy-22 commented on code in PR #13443:
URL: https://github.com/apache/kafka/pull/13443#discussion_r1152313744


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group-coordinator/src/main/java/org/apache/kafka/coordinator/group/assignor/ServerSideStickyRangeAssignor.java:
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@@ -0,0 +1,268 @@
+/*
+ * 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.
+ */
+
+/**
+ * <p>The Server Side Sticky Range Assignor inherits properties of both the 
range assignor and the sticky assignor.
+ * Properties are :-
+ * <ul>
+ * <li> 1) Each consumer must get at least one partition per topic that it is 
subscribed to whenever the number of consumers is
+ *    less than or equal to the number of partitions for that topic. (Range) 
</li>
+ * <li> 2) Partitions should be assigned to consumers in a way that 
facilitates join operations where required. (Range) </li>
+ *    This can only be done if the topics are co-partitioned in the first place
+ *    Co-partitioned:-
+ *    Two streams are co-partitioned if the following conditions are met:-
+ * ->The keys must have the same schemas
+ * ->The topics involved must have the same number of partitions
+ * <li> 3) Consumers should retain as much as their previous assignment as 
possible. (Sticky) </li>
+ * </ul>
+ * </p>
+ *
+ * <p>The algorithm works mainly in 5 steps described below
+ * <ul>
+ * <li> 1) Get a map of the consumersPerTopic created using the member 
subscriptions.</li>
+ * <li> 2) Get a list of consumers (potentiallyUnfilled) that have not met the 
minimum required quota for assignment AND
+ * get a list of sticky partitions that we want to retain in the new 
assignment.</li>
+ * <li> 3) Add consumers from potentiallyUnfilled to Unfilled if they haven't 
met the total required quota = minQuota + (if necessary) extraPartition </li>
+ * <li> 4) Get a list of available partitions by calculating the difference 
between total partitions and assigned sticky partitions </li>
+ * <li> 5) Iterate through unfilled consumers and assign partitions from 
available partitions </li>
+ * </ul>
+ * </p>
+ *
+ */
+package org.apache.kafka.coordinator.group.assignor;
+
+
+import org.apache.kafka.common.Uuid;
+
+import java.util.List;
+import java.util.Map;
+import java.util.Set;
+import java.util.ArrayList;
+import java.util.HashSet;
+import java.util.HashMap;
+import java.util.Collections;
+
+import static java.lang.Math.min;
+
+public class ServerSideStickyRangeAssignor implements PartitionAssignor {
+
+    public static final String RANGE_ASSIGNOR_NAME = "range-sticky";
+
+    @Override
+    public String name() {
+        return RANGE_ASSIGNOR_NAME;
+    }
+
+    protected static <K, V> void putList(Map<K, List<V>> map, K key, V value) {
+        List<V> list = map.computeIfAbsent(key, k -> new ArrayList<>());
+        list.add(value);
+    }
+
+    protected static <K, V> void putSet(Map<K, Set<V>> map, K key, V value) {
+        Set<V> set = map.computeIfAbsent(key, k -> new HashSet<>());
+        set.add(value);
+    }
+
+    static class Pair<T, U> {
+        private final T first;
+        private final U second;
+
+        public Pair(T first, U second) {
+            this.first = first;
+            this.second = second;
+        }
+
+        public T getFirst() {
+            return first;
+        }
+
+        public U getSecond() {
+            return second;
+        }
+
+        @Override
+        public String toString() {
+            return "(" + first + ", " + second + ")";
+        }
+    }
+
+    // Returns a map of the list of consumers per Topic (keyed by topicId)
+    private Map<Uuid, List<String>> consumersPerTopic(AssignmentSpec 
assignmentSpec) {
+        Map<Uuid, List<String>> mapTopicsToConsumers = new HashMap<>();
+        Map<String, AssignmentMemberSpec> membersData = assignmentSpec.members;
+
+        for (Map.Entry<String, AssignmentMemberSpec> memberEntry : 
membersData.entrySet()) {
+            String memberId = memberEntry.getKey();
+            AssignmentMemberSpec memberMetadata = memberEntry.getValue();
+            List<Uuid> topics = new 
ArrayList<>(memberMetadata.subscribedTopics);
+            for (Uuid topicId: topics) {
+                putList(mapTopicsToConsumers, topicId, memberId);
+            }
+        }
+        return mapTopicsToConsumers;
+    }
+
+    private Map<Uuid, List<Integer>> 
getAvailablePartitionsPerTopic(AssignmentSpec assignmentSpec, Map<Uuid, 
Set<Integer>> assignedStickyPartitionsPerTopic) {
+        Map<Uuid, List<Integer>> availablePartitionsPerTopic = new HashMap<>();
+        Map<Uuid, AssignmentTopicMetadata> topicsMetadata = 
assignmentSpec.topics;
+        // Iterate through the topics map provided in assignmentSpec
+        for (Map.Entry<Uuid, AssignmentTopicMetadata> topicMetadataEntry : 
topicsMetadata.entrySet()) {
+            Uuid topicId = topicMetadataEntry.getKey();
+            availablePartitionsPerTopic.put(topicId, new ArrayList<>());
+            int numPartitions = topicsMetadata.get(topicId).numPartitions;
+            // since the loop iterates from 0 to n, the partitions will be in 
ascending order within the list of available partitions per topic
+            Set<Integer> assignedStickyPartitionsForTopic = 
assignedStickyPartitionsPerTopic.get(topicId);
+            for (int i = 0; i < numPartitions; i++) {
+                if (assignedStickyPartitionsForTopic == null || 
!assignedStickyPartitionsForTopic.contains(i)) {
+                    availablePartitionsPerTopic.get(topicId).add(i);
+                }
+            }
+        }
+        return availablePartitionsPerTopic;
+    }
+
+    @Override
+    public GroupAssignment assign(AssignmentSpec assignmentSpec) throws 
PartitionAssignorException {
+        Map<String, Map<Uuid, Set<Integer>>> membersWithNewAssignmentPerTopic 
= new HashMap<>();
+        // Step 1
+        Map<Uuid, List<String>> consumersPerTopic = 
consumersPerTopic(assignmentSpec);
+        // Step 2
+        Map<Uuid, List<Pair<String, Integer>>> unfilledConsumersPerTopic = new 
HashMap<>();
+        Map<Uuid, Set<Integer>> assignedStickyPartitionsPerTopic = new 
HashMap<>();
+
+        for (Map.Entry<Uuid, List<String>> topicEntry : 
consumersPerTopic.entrySet()) {
+            Uuid topicId = topicEntry.getKey();
+            // For each topic we have a temporary list of consumers stored in 
potentiallyUnfilledConsumers.
+            // The list is populated with consumers that satisfy one of the 
two conditions :-
+            // 1) Consumers that have the minimum required number of 
partitions .i.e numPartitionsPerConsumer BUT they could be assigned an extra 
partition later on.
+            //    In this case we add the consumer to the unfilled consumers 
map iff an extra partition needs to be assigned to it.
+            // 2) Consumers 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<Pair<String, Integer>> potentiallyUnfilledConsumers = new 
ArrayList<>();
+            List<String> consumersForTopic = topicEntry.getValue();
+
+            AssignmentTopicMetadata topicData = 
assignmentSpec.topics.get(topicId);
+            int numPartitionsForTopic = topicData.numPartitions;
+            int numPartitionsPerConsumer = numPartitionsForTopic / 
consumersForTopic.size();
+            // Each consumer can get only one extra partition per topic after 
receiving the minimum quota = numPartitionsPerConsumer
+            int numConsumersWithExtraPartition = numPartitionsForTopic % 
consumersForTopic.size();
+
+            for (String memberId: consumersForTopic) {
+                // Size of the older assignment, this will be 0 when assign is 
called for the first time.
+                // The older assignment is required when a reassignment occurs 
to ensure stickiness.
+                int currentAssignmentSize = 0;
+                List<Integer> currentAssignmentListForTopic = new 
ArrayList<>();
+                // Convert the set to a list first and sort the partitions in 
numeric order since we want the same partition numbers from each topic
+                // to go to the same consumer in case of co-partitioned topics.
+                Set<Integer> currentAssignmentSetForTopic =  
assignmentSpec.members.get(memberId).currentAssignmentPerTopic.get(topicId);
+                // We want to make sure that the currentAssignment is not null 
to avoid a null pointer exception
+                if (currentAssignmentSetForTopic != null) {
+                    currentAssignmentListForTopic = new 
ArrayList<>(currentAssignmentSetForTopic);

Review Comment:
   We need the order of the partitions to be guaranteed and sorted when we're 
retaining the prev assignment, that's why we had to convert it to a list first.



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