rreddy-22 commented on code in PR #14182: URL: https://github.com/apache/kafka/pull/14182#discussion_r1308177084
########## group-coordinator/src/main/java/org/apache/kafka/coordinator/group/assignor/OptimizedUniformAssignmentBuilder.java: ########## @@ -0,0 +1,390 @@ +/* + * 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.apache.kafka.coordinator.group.common.TopicIdPartition; +import org.slf4j.Logger; +import org.slf4j.LoggerFactory; + +import java.util.ArrayList; +import java.util.Collections; +import java.util.HashMap; +import java.util.HashSet; +import java.util.Iterator; +import java.util.LinkedList; +import java.util.List; +import java.util.Map; +import java.util.Queue; +import java.util.Set; +import java.util.stream.IntStream; + +import static java.lang.Math.min; + +/** + * Assigns Kafka partitions to members of a consumer group ensuring a balanced distribution with + * considerations for sticky assignments and rack-awareness. + * The order of priority of properties during the assignment will be: balance > rack matching (when applicable) > stickiness. + * + * <p> Here's the step-by-step breakdown of the assignment process: + * + * <ul> + * <li> Compute the quotas of partitions for each member based on the total partitions and member count.</li> + * <li> For existing assignments, retain partitions based on the determined quota and member's rack compatibility. + * <li> If a partition's rack mismatches with its member, track it with its prior owner.</li> + * <li> Identify members that haven't fulfilled their partition quota or are eligible to receive extra partitions.</li> + * <li> Derive the unassigned partitions by taking the difference between total partitions and the sticky assignments.</li> + * <li> Depending on members needing extra partitions, select members from the potentially unfilled list + * and add them to the unfilled list.</li> + * <li> Proceed with a round-robin assignment adhering to rack awareness. + * For each unassigned partition, locate the first compatible member from the unfilled list.</li> + * <li> If no rack-compatible member is found, revert to the tracked current owner. + * If that member can't accommodate the partition due to quota limits, resort to a generic round-robin assignment.</li> + * </ul> + */ +public class OptimizedUniformAssignmentBuilder extends UniformAssignor.AbstractAssignmentBuilder { + private static final Logger log = LoggerFactory.getLogger(OptimizedUniformAssignmentBuilder.class); + private final AssignmentSpec assignmentSpec; + private final SubscribedTopicDescriber subscribedTopicDescriber; + // List of topics subscribed to by all members. + private final List<Uuid> subscriptionList; + private final RackInfo rackInfo; + // Count of members to receive an extra partition beyond the minimum quota, + // to account for the distribution of the remaining partitions. + private int remainingMembersToGetAnExtraPartition; + // Map of members to the remaining number of partitions needed to meet the minimum quota, + // including members eligible for an extra partition. + private final Map<String, Integer> potentiallyUnfilledMembers; + // Members mapped to the remaining number of partitions needed to meet the full quota. + // Full quota = minQuota + one extra partition (if applicable). + private Map<String, Integer> unfilledMembers; + private List<TopicIdPartition> unassignedPartitions; + private final Map<String, MemberAssignment> newAssignment; + // Tracks the current owner of each partition when using rack-aware strategy. + // Current refers to the existing assignment. + private final Map<TopicIdPartition, String> currentPartitionOwners; + // Indicates if a rack aware assignment can be done. + // True if racks are defined for both members and partitions. + boolean useRackAwareStrategy; + + OptimizedUniformAssignmentBuilder(AssignmentSpec assignmentSpec, SubscribedTopicDescriber subscribedTopicDescriber) { + this.assignmentSpec = assignmentSpec; + this.subscribedTopicDescriber = subscribedTopicDescriber; + this.subscriptionList = new ArrayList<>(assignmentSpec.members().values().iterator().next().subscribedTopicIds()); + + this.rackInfo = new RackInfo(assignmentSpec, subscribedTopicDescriber, subscriptionList); + this.potentiallyUnfilledMembers = new HashMap<>(); + this.unfilledMembers = new HashMap<>(); + this.newAssignment = new HashMap<>(); + + // If consumer or member racks are unavailable, rack aware strategy is not used. + this.useRackAwareStrategy = !(rackInfo.memberRacks.isEmpty() || rackInfo.partitionRacks.isEmpty()); + // Without rack-aware strategy, tracking current owners of unassigned partitions is unnecessary + // as all sticky partitions are retained until a member meets its quota. + this. currentPartitionOwners = useRackAwareStrategy ? new HashMap<>() : Collections.emptyMap(); + } + + @Override + protected GroupAssignment buildAssignment() throws PartitionAssignorException{ + int totalPartitionsCount = 0; + // Removes the current topic from subscriptionList if the topic doesn't exist in the topic metadata. + Iterator<Uuid> iterator = subscriptionList.iterator(); + while (iterator.hasNext()) { + Uuid topicId = iterator.next(); + int partitionCount = subscribedTopicDescriber.numPartitions(topicId); + if (partitionCount == -1) { + log.warn("Members are subscribed to topic " + topicId + " which doesn't exist in the topic metadata."); + iterator.remove(); + } else { + totalPartitionsCount += partitionCount; + } + } + + if (subscriptionList.isEmpty()) { + log.info("The subscription list is empty, returning an empty assignment"); + return new GroupAssignment(Collections.emptyMap()); + } + + // The minimum required quota that each member needs to meet for a balanced assignment. + // This is the same for all members. + int numberOfMembers = assignmentSpec.members().size(); + final int minQuota = (int) Math.floor(((double) totalPartitionsCount) / numberOfMembers); + remainingMembersToGetAnExtraPartition = totalPartitionsCount % numberOfMembers; + + assignmentSpec.members().forEach((memberId, assignmentMemberSpec) -> + newAssignment.put(memberId, new MemberAssignment(new HashMap<>())) + ); + + Set<TopicIdPartition> allAssignedStickyPartitions = computeAssignedStickyPartitions(minQuota); + unassignedPartitions = computeUnassignedPartitions(allAssignedStickyPartitions); + unfilledMembers = computeUnfilledMembers(); + + if (!isTotalUnassignedPartitionsEqualsTotalRemainingAssignments()) { + throw new PartitionAssignorException("Number of available partitions is not equal to the total requirement"); + } + + if (useRackAwareStrategy) rackAwareRoundRobinAssignment(); + unassignedPartitionsRoundRobinAssignment(); + + return new GroupAssignment(newAssignment); + } + + /** + * Retrieves a set of partitions that were currently assigned to members and will be retained in the new assignment, + * by ensuring that the partitions are still relevant based on current topic metadata and subscriptions. + * If rack awareness is enabled, it ensures that a partition's rack matches the member's rack. + * + * <p> For each member, it: + * <ul> + * <li> Finds the valid current assignment considering topic subscriptions and metadata.</li> + * <li> If current assignments exist, retains up to the minimum quota of assignments.</li> + * <li> If there are members that should get an extra partition, + * assigns the next partition after the retained ones.</li> + * <li> For members with assignments not exceeding the minimum quota, + * it identifies them as potentially unfilled members and tracks the remaining quota.</li> + * </ul> + * + * @return A set containing all the sticky partitions that have been retained in the new assignment. + */ + private Set<TopicIdPartition> computeAssignedStickyPartitions(Integer minQuota) { + Set<TopicIdPartition> allAssignedStickyPartitions = new HashSet<>(); + + assignmentSpec.members().forEach((memberId, assignmentMemberSpec) -> { + // Remove all the topics that aren't in the subscriptions or the topic metadata anymore. + // If rack awareness is enabled, only add partitions if the consumers rack matches the partitions rack. + List<TopicIdPartition> validCurrentAssignment = validCurrentAssignment( + memberId, + assignmentMemberSpec.assignedPartitions() + ); + + int currentAssignmentSize = validCurrentAssignment.size(); + int remaining = minQuota - currentAssignmentSize; + + if (currentAssignmentSize > 0) { + int retainedPartitionsCount = min(currentAssignmentSize, minQuota); + IntStream.range(0, retainedPartitionsCount).forEach(i -> { + newAssignment.get(memberId) + .targetPartitions() + .computeIfAbsent(validCurrentAssignment.get(i).topicId(), __ -> new HashSet<>()) + .add(validCurrentAssignment.get(i).partition()); + allAssignedStickyPartitions.add(validCurrentAssignment.get(i)); + }); + + // The extra partition is located at the index "retainedPartitionsCount" from the current step. + if (remaining < 0 && remainingMembersToGetAnExtraPartition > 0) { + newAssignment.get(memberId) + .targetPartitions() + .computeIfAbsent(validCurrentAssignment.get(retainedPartitionsCount).topicId(), __ -> new HashSet<>()) + .add(validCurrentAssignment.get(retainedPartitionsCount).partition()); + allAssignedStickyPartitions.add(validCurrentAssignment.get(retainedPartitionsCount)); + remainingMembersToGetAnExtraPartition--; + } + } + + if (remaining >= 0) { + potentiallyUnfilledMembers.put(memberId, remaining); + } + + }); + + return allAssignedStickyPartitions; + } + + /** + * Filters the current assignment of partitions for a given member. + * + * If a partition is assigned to a member not subscribed to its topic or + * if the rack-aware strategy is to be used but there is a mismatch, + * the partition is excluded from the valid assignment and stored for future consideration. + * + * @param memberId The Id of the member whose assignment is being validated. + * @param assignedPartitions The partitions currently assigned to the member. + * + * @return List of valid partitions after applying the filters. + */ + private List<TopicIdPartition> validCurrentAssignment( + String memberId, + Map<Uuid, Set<Integer>> assignedPartitions + ) { + List<TopicIdPartition> validCurrentAssignmentList = new ArrayList<>(); + Set<Uuid> subscriptionSet = new HashSet<>(subscriptionList); + + assignedPartitions.forEach((topicId, currentAssignment) -> { + if (subscriptionSet.contains(topicId)) { + currentAssignment.forEach((partition) -> { + TopicIdPartition topicIdPartition = new TopicIdPartition(topicId, partition); + if (useRackAwareStrategy && rackInfo.racksMismatch(memberId, topicIdPartition)) { + currentPartitionOwners.put(topicIdPartition, memberId); + } else { + validCurrentAssignmentList.add(topicIdPartition); + } + }); + } + }); + + return validCurrentAssignmentList; + } + + /** + * This method iterates over the unassigned partitions and attempts to allocate them + * to members while considering their rack affiliations. + */ + private void rackAwareRoundRobinAssignment() { + Queue<String> roundRobinMembers = new LinkedList<>(unfilledMembers.keySet()); + + // Sorts partitions in ascending order by number of potential consumers with matching racks. + // Partitions with no potential members aren't included in this list. + List<TopicIdPartition> sortedPartitions = rackInfo.sortPartitionsByRackConsumers(unassignedPartitions); + + sortedPartitions.forEach((partition) -> { + boolean assigned = false; + for (int i = 0; i < roundRobinMembers.size() && !assigned; i++) { + String memberId = roundRobinMembers.poll(); + Integer remainingPartitionCount = unfilledMembers.getOrDefault(memberId, 0); + + if (remainingPartitionCount > 0 && !rackInfo.racksMismatch(memberId, partition)) { + assignPartitionToMember(memberId, partition); + assigned = true; + unassignedPartitions.remove(partition); + } + + // Only re-add to the end of the queue if it's still in the unfilledMembers map + if (unfilledMembers.containsKey(memberId)) { + roundRobinMembers.add(memberId); + } + } + }); + } + + /** + * Allocates the unassigned partitions to available members. + * + * If the rack-aware strategy is enabled, partitions are attempted to be assigned back to their current owners first. + * + * If a partition couldn't be assigned to its current owner due to quotas or + * if the rack-aware strategy is not enabled, the partitions are allocated to members in a round-robin fashion.</p> + */ + private void unassignedPartitionsRoundRobinAssignment() { + Queue<String> roundRobinMembers = new LinkedList<>(unfilledMembers.keySet()); + + unassignedPartitions.forEach((partition) -> { + boolean assigned = false; + + if (useRackAwareStrategy && currentPartitionOwners.containsKey(partition)) { + String prevOwner = currentPartitionOwners.get(partition); + if (unfilledMembers.containsKey(prevOwner)) { + assignPartitionToMember(prevOwner, partition); + assigned = true; + if (!unfilledMembers.containsKey(prevOwner)) { + roundRobinMembers.remove(prevOwner); + } + } + } + + // Only re-add the member to the end of the queue if it's still available for assignment. + for (int i = 0; i < unfilledMembers.size() && !assigned; i++) { + String memberId = roundRobinMembers.poll(); + if (unfilledMembers.get(memberId) > 0) { + assignPartitionToMember(memberId, partition); + assigned = true; + } + if (unfilledMembers.containsKey(memberId)) { + roundRobinMembers.add(memberId); + } + } + }); + } + + /** + * Assigns the specified partition to the given member. + * + * <p> + * If the member has met their allocation quota, the member is removed from the + * tracking map of members with their remaining allocations. + * Otherwise, the count of remaining partitions that can be assigned to the member is updated. + * </p> + * + * @param memberId The Id of the member to which the partition will be assigned. + * @param partition The partition to be assigned. + */ + private void assignPartitionToMember(String memberId, TopicIdPartition partition) { + newAssignment.get(memberId) + .targetPartitions() + .computeIfAbsent(partition.topicId(), __ -> new HashSet<>()) + .add(partition.partition()); + + int remainingPartitionCount = unfilledMembers.get(memberId) - 1; + if (remainingPartitionCount == 0) { + unfilledMembers.remove(memberId); + } else { + unfilledMembers.put(memberId, remainingPartitionCount); + } + } + + /** + * Determines which members can still be assigned partitions to meet the full quota. + * + * @return A map of member IDs and their capacity for additional partitions. + */ + private Map<String, Integer> computeUnfilledMembers() { + Map<String, Integer> unfilledMembers = new HashMap<>(); + + potentiallyUnfilledMembers.forEach((memberId, remaining) -> { + if (remainingMembersToGetAnExtraPartition > 0) { + remaining++; + remainingMembersToGetAnExtraPartition--; + } + if (remaining > 0) { + unfilledMembers.put(memberId, remaining); + } + }); + + return unfilledMembers; + } + + /** + * This method compares the full list of partitions against the set of already + * assigned sticky partitions to identify those that still need to be allocated. + * + * @param allAssignedStickyPartitions Set of partitions that have already been assigned. + * @return List of unassigned partitions. + */ + private List<TopicIdPartition> computeUnassignedPartitions(Set<TopicIdPartition> allAssignedStickyPartitions) { + List<TopicIdPartition> unassignedPartitions = new ArrayList<>(); + List<Uuid> sortedAllTopics = new ArrayList<>(subscriptionList); Review Comment: It makes sure that all the unassigned partitions are present topic wise and then when we do a rr distribution while assigning these partitions they are distributed more evenly by topic as well -- This is an automated message from the Apache Git Service. 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