package sarama import ( "container/heap" "math" "sort" "strings" ) const ( // RangeBalanceStrategyName identifies strategies that use the range partition assignment strategy RangeBalanceStrategyName = "range" // RoundRobinBalanceStrategyName identifies strategies that use the round-robin partition assignment strategy RoundRobinBalanceStrategyName = "roundrobin" // StickyBalanceStrategyName identifies strategies that use the sticky-partition assignment strategy StickyBalanceStrategyName = "sticky" defaultGeneration = -1 ) // BalanceStrategyPlan is the results of any BalanceStrategy.Plan attempt. // It contains an allocation of topic/partitions by memberID in the form of // a `memberID -> topic -> partitions` map. type BalanceStrategyPlan map[string]map[string][]int32 // Add assigns a topic with a number partitions to a member. func (p BalanceStrategyPlan) Add(memberID, topic string, partitions ...int32) { if len(partitions) == 0 { return } if _, ok := p[memberID]; !ok { p[memberID] = make(map[string][]int32, 1) } p[memberID][topic] = append(p[memberID][topic], partitions...) } // -------------------------------------------------------------------- // BalanceStrategy is used to balance topics and partitions // across members of a consumer group type BalanceStrategy interface { // Name uniquely identifies the strategy. Name() string // Plan accepts a map of `memberID -> metadata` and a map of `topic -> partitions` // and returns a distribution plan. Plan(members map[string]ConsumerGroupMemberMetadata, topics map[string][]int32) (BalanceStrategyPlan, error) // AssignmentData returns the serialized assignment data for the specified // memberID AssignmentData(memberID string, topics map[string][]int32, generationID int32) ([]byte, error) } // -------------------------------------------------------------------- // BalanceStrategyRange is the default and assigns partitions as ranges to consumer group members. // Example with one topic T with six partitions (0..5) and two members (M1, M2): // M1: {T: [0, 1, 2]} // M2: {T: [3, 4, 5]} var BalanceStrategyRange = &balanceStrategy{ name: RangeBalanceStrategyName, coreFn: func(plan BalanceStrategyPlan, memberIDs []string, topic string, partitions []int32) { step := float64(len(partitions)) / float64(len(memberIDs)) for i, memberID := range memberIDs { pos := float64(i) min := int(math.Floor(pos*step + 0.5)) max := int(math.Floor((pos+1)*step + 0.5)) plan.Add(memberID, topic, partitions[min:max]...) } }, } // BalanceStrategyRoundRobin assigns partitions to members in alternating order. // Example with topic T with six partitions (0..5) and two members (M1, M2): // M1: {T: [0, 2, 4]} // M2: {T: [1, 3, 5]} var BalanceStrategyRoundRobin = &balanceStrategy{ name: RoundRobinBalanceStrategyName, coreFn: func(plan BalanceStrategyPlan, memberIDs []string, topic string, partitions []int32) { for i, part := range partitions { memberID := memberIDs[i%len(memberIDs)] plan.Add(memberID, topic, part) } }, } // BalanceStrategySticky assigns partitions to members with an attempt to preserve earlier assignments // while maintain a balanced partition distribution. // Example with topic T with six partitions (0..5) and two members (M1, M2): // M1: {T: [0, 2, 4]} // M2: {T: [1, 3, 5]} // // On reassignment with an additional consumer, you might get an assignment plan like: // M1: {T: [0, 2]} // M2: {T: [1, 3]} // M3: {T: [4, 5]} // var BalanceStrategySticky = &stickyBalanceStrategy{} // -------------------------------------------------------------------- type balanceStrategy struct { name string coreFn func(plan BalanceStrategyPlan, memberIDs []string, topic string, partitions []int32) } // Name implements BalanceStrategy. func (s *balanceStrategy) Name() string { return s.name } // Plan implements BalanceStrategy. func (s *balanceStrategy) Plan(members map[string]ConsumerGroupMemberMetadata, topics map[string][]int32) (BalanceStrategyPlan, error) { // Build members by topic map mbt := make(map[string][]string) for memberID, meta := range members { for _, topic := range meta.Topics { mbt[topic] = append(mbt[topic], memberID) } } // Sort members for each topic for topic, memberIDs := range mbt { sort.Sort(&balanceStrategySortable{ topic: topic, memberIDs: memberIDs, }) } // Assemble plan plan := make(BalanceStrategyPlan, len(members)) for topic, memberIDs := range mbt { s.coreFn(plan, memberIDs, topic, topics[topic]) } return plan, nil } // AssignmentData simple strategies do not require any shared assignment data func (s *balanceStrategy) AssignmentData(memberID string, topics map[string][]int32, generationID int32) ([]byte, error) { return nil, nil } type balanceStrategySortable struct { topic string memberIDs []string } func (p balanceStrategySortable) Len() int { return len(p.memberIDs) } func (p balanceStrategySortable) Swap(i, j int) { p.memberIDs[i], p.memberIDs[j] = p.memberIDs[j], p.memberIDs[i] } func (p balanceStrategySortable) Less(i, j int) bool { return balanceStrategyHashValue(p.topic, p.memberIDs[i]) < balanceStrategyHashValue(p.topic, p.memberIDs[j]) } func balanceStrategyHashValue(vv ...string) uint32 { h := uint32(2166136261) for _, s := range vv { for _, c := range s { h ^= uint32(c) h *= 16777619 } } return h } type stickyBalanceStrategy struct { movements partitionMovements } // Name implements BalanceStrategy. func (s *stickyBalanceStrategy) Name() string { return StickyBalanceStrategyName } // Plan implements BalanceStrategy. func (s *stickyBalanceStrategy) Plan(members map[string]ConsumerGroupMemberMetadata, topics map[string][]int32) (BalanceStrategyPlan, error) { // track partition movements during generation of the partition assignment plan s.movements = partitionMovements{ Movements: make(map[topicPartitionAssignment]consumerPair), PartitionMovementsByTopic: make(map[string]map[consumerPair]map[topicPartitionAssignment]bool), } // prepopulate the current assignment state from userdata on the consumer group members currentAssignment, prevAssignment, err := prepopulateCurrentAssignments(members) if err != nil { return nil, err } // determine if we're dealing with a completely fresh assignment, or if there's existing assignment state isFreshAssignment := false if len(currentAssignment) == 0 { isFreshAssignment = true } // create a mapping of all current topic partitions and the consumers that can be assigned to them partition2AllPotentialConsumers := make(map[topicPartitionAssignment][]string) for topic, partitions := range topics { for _, partition := range partitions { partition2AllPotentialConsumers[topicPartitionAssignment{Topic: topic, Partition: partition}] = []string{} } } // create a mapping of all consumers to all potential topic partitions that can be assigned to them // also, populate the mapping of partitions to potential consumers consumer2AllPotentialPartitions := make(map[string][]topicPartitionAssignment, len(members)) for memberID, meta := range members { consumer2AllPotentialPartitions[memberID] = make([]topicPartitionAssignment, 0) for _, topicSubscription := range meta.Topics { // only evaluate topic subscriptions that are present in the supplied topics map if _, found := topics[topicSubscription]; found { for _, partition := range topics[topicSubscription] { topicPartition := topicPartitionAssignment{Topic: topicSubscription, Partition: partition} consumer2AllPotentialPartitions[memberID] = append(consumer2AllPotentialPartitions[memberID], topicPartition) partition2AllPotentialConsumers[topicPartition] = append(partition2AllPotentialConsumers[topicPartition], memberID) } } } // add this consumer to currentAssignment (with an empty topic partition assignment) if it does not already exist if _, exists := currentAssignment[memberID]; !exists { currentAssignment[memberID] = make([]topicPartitionAssignment, 0) } } // create a mapping of each partition to its current consumer, where possible currentPartitionConsumers := make(map[topicPartitionAssignment]string, len(currentAssignment)) unvisitedPartitions := make(map[topicPartitionAssignment]bool, len(partition2AllPotentialConsumers)) for partition := range partition2AllPotentialConsumers { unvisitedPartitions[partition] = true } var unassignedPartitions []topicPartitionAssignment for memberID, partitions := range currentAssignment { var keepPartitions []topicPartitionAssignment for _, partition := range partitions { // If this partition no longer exists at all, likely due to the // topic being deleted, we remove the partition from the member. if _, exists := partition2AllPotentialConsumers[partition]; !exists { continue } delete(unvisitedPartitions, partition) currentPartitionConsumers[partition] = memberID if !strsContains(members[memberID].Topics, partition.Topic) { unassignedPartitions = append(unassignedPartitions, partition) continue } keepPartitions = append(keepPartitions, partition) } currentAssignment[memberID] = keepPartitions } for unvisited := range unvisitedPartitions { unassignedPartitions = append(unassignedPartitions, unvisited) } // sort the topic partitions in order of priority for reassignment sortedPartitions := sortPartitions(currentAssignment, prevAssignment, isFreshAssignment, partition2AllPotentialConsumers, consumer2AllPotentialPartitions) // at this point we have preserved all valid topic partition to consumer assignments and removed // all invalid topic partitions and invalid consumers. Now we need to assign unassignedPartitions // to consumers so that the topic partition assignments are as balanced as possible. // an ascending sorted set of consumers based on how many topic partitions are already assigned to them sortedCurrentSubscriptions := sortMemberIDsByPartitionAssignments(currentAssignment) s.balance(currentAssignment, prevAssignment, sortedPartitions, unassignedPartitions, sortedCurrentSubscriptions, consumer2AllPotentialPartitions, partition2AllPotentialConsumers, currentPartitionConsumers) // Assemble plan plan := make(BalanceStrategyPlan, len(currentAssignment)) for memberID, assignments := range currentAssignment { if len(assignments) == 0 { plan[memberID] = make(map[string][]int32) } else { for _, assignment := range assignments { plan.Add(memberID, assignment.Topic, assignment.Partition) } } } return plan, nil } // AssignmentData serializes the set of topics currently assigned to the // specified member as part of the supplied balance plan func (s *stickyBalanceStrategy) AssignmentData(memberID string, topics map[string][]int32, generationID int32) ([]byte, error) { return encode(&StickyAssignorUserDataV1{ Topics: topics, Generation: generationID, }, nil) } func strsContains(s []string, value string) bool { for _, entry := range s { if entry == value { return true } } return false } // Balance assignments across consumers for maximum fairness and stickiness. func (s *stickyBalanceStrategy) balance(currentAssignment map[string][]topicPartitionAssignment, prevAssignment map[topicPartitionAssignment]consumerGenerationPair, sortedPartitions []topicPartitionAssignment, unassignedPartitions []topicPartitionAssignment, sortedCurrentSubscriptions []string, consumer2AllPotentialPartitions map[string][]topicPartitionAssignment, partition2AllPotentialConsumers map[topicPartitionAssignment][]string, currentPartitionConsumer map[topicPartitionAssignment]string) { initializing := false if len(sortedCurrentSubscriptions) == 0 || len(currentAssignment[sortedCurrentSubscriptions[0]]) == 0 { initializing = true } // assign all unassigned partitions for _, partition := range unassignedPartitions { // skip if there is no potential consumer for the partition if len(partition2AllPotentialConsumers[partition]) == 0 { continue } sortedCurrentSubscriptions = assignPartition(partition, sortedCurrentSubscriptions, currentAssignment, consumer2AllPotentialPartitions, currentPartitionConsumer) } // narrow down the reassignment scope to only those partitions that can actually be reassigned for partition := range partition2AllPotentialConsumers { if !canTopicPartitionParticipateInReassignment(partition, partition2AllPotentialConsumers) { sortedPartitions = removeTopicPartitionFromMemberAssignments(sortedPartitions, partition) } } // narrow down the reassignment scope to only those consumers that are subject to reassignment fixedAssignments := make(map[string][]topicPartitionAssignment) for memberID := range consumer2AllPotentialPartitions { if !canConsumerParticipateInReassignment(memberID, currentAssignment, consumer2AllPotentialPartitions, partition2AllPotentialConsumers) { fixedAssignments[memberID] = currentAssignment[memberID] delete(currentAssignment, memberID) sortedCurrentSubscriptions = sortMemberIDsByPartitionAssignments(currentAssignment) } } // create a deep copy of the current assignment so we can revert to it if we do not get a more balanced assignment later preBalanceAssignment := deepCopyAssignment(currentAssignment) preBalancePartitionConsumers := make(map[topicPartitionAssignment]string, len(currentPartitionConsumer)) for k, v := range currentPartitionConsumer { preBalancePartitionConsumers[k] = v } reassignmentPerformed := s.performReassignments(sortedPartitions, currentAssignment, prevAssignment, sortedCurrentSubscriptions, consumer2AllPotentialPartitions, partition2AllPotentialConsumers, currentPartitionConsumer) // if we are not preserving existing assignments and we have made changes to the current assignment // make sure we are getting a more balanced assignment; otherwise, revert to previous assignment if !initializing && reassignmentPerformed && getBalanceScore(currentAssignment) >= getBalanceScore(preBalanceAssignment) { currentAssignment = deepCopyAssignment(preBalanceAssignment) currentPartitionConsumer = make(map[topicPartitionAssignment]string, len(preBalancePartitionConsumers)) for k, v := range preBalancePartitionConsumers { currentPartitionConsumer[k] = v } } // add the fixed assignments (those that could not change) back for consumer, assignments := range fixedAssignments { currentAssignment[consumer] = assignments } } // Calculate the balance score of the given assignment, as the sum of assigned partitions size difference of all consumer pairs. // A perfectly balanced assignment (with all consumers getting the same number of partitions) has a balance score of 0. // Lower balance score indicates a more balanced assignment. func getBalanceScore(assignment map[string][]topicPartitionAssignment) int { consumer2AssignmentSize := make(map[string]int, len(assignment)) for memberID, partitions := range assignment { consumer2AssignmentSize[memberID] = len(partitions) } var score float64 for memberID, consumerAssignmentSize := range consumer2AssignmentSize { delete(consumer2AssignmentSize, memberID) for _, otherConsumerAssignmentSize := range consumer2AssignmentSize { score += math.Abs(float64(consumerAssignmentSize - otherConsumerAssignmentSize)) } } return int(score) } // Determine whether the current assignment plan is balanced. func isBalanced(currentAssignment map[string][]topicPartitionAssignment, sortedCurrentSubscriptions []string, allSubscriptions map[string][]topicPartitionAssignment) bool { sortedCurrentSubscriptions = sortMemberIDsByPartitionAssignments(currentAssignment) min := len(currentAssignment[sortedCurrentSubscriptions[0]]) max := len(currentAssignment[sortedCurrentSubscriptions[len(sortedCurrentSubscriptions)-1]]) if min >= max-1 { // if minimum and maximum numbers of partitions assigned to consumers differ by at most one return true return true } // create a mapping from partitions to the consumer assigned to them allPartitions := make(map[topicPartitionAssignment]string) for memberID, partitions := range currentAssignment { for _, partition := range partitions { if _, exists := allPartitions[partition]; exists { Logger.Printf("Topic %s Partition %d is assigned more than one consumer", partition.Topic, partition.Partition) } allPartitions[partition] = memberID } } // for each consumer that does not have all the topic partitions it can get make sure none of the topic partitions it // could but did not get cannot be moved to it (because that would break the balance) for _, memberID := range sortedCurrentSubscriptions { consumerPartitions := currentAssignment[memberID] consumerPartitionCount := len(consumerPartitions) // skip if this consumer already has all the topic partitions it can get if consumerPartitionCount == len(allSubscriptions[memberID]) { continue } // otherwise make sure it cannot get any more potentialTopicPartitions := allSubscriptions[memberID] for _, partition := range potentialTopicPartitions { if !memberAssignmentsIncludeTopicPartition(currentAssignment[memberID], partition) { otherConsumer := allPartitions[partition] otherConsumerPartitionCount := len(currentAssignment[otherConsumer]) if consumerPartitionCount < otherConsumerPartitionCount { return false } } } } return true } // Reassign all topic partitions that need reassignment until balanced. func (s *stickyBalanceStrategy) performReassignments(reassignablePartitions []topicPartitionAssignment, currentAssignment map[string][]topicPartitionAssignment, prevAssignment map[topicPartitionAssignment]consumerGenerationPair, sortedCurrentSubscriptions []string, consumer2AllPotentialPartitions map[string][]topicPartitionAssignment, partition2AllPotentialConsumers map[topicPartitionAssignment][]string, currentPartitionConsumer map[topicPartitionAssignment]string) bool { reassignmentPerformed := false modified := false // repeat reassignment until no partition can be moved to improve the balance for { modified = false // reassign all reassignable partitions (starting from the partition with least potential consumers and if needed) // until the full list is processed or a balance is achieved for _, partition := range reassignablePartitions { if isBalanced(currentAssignment, sortedCurrentSubscriptions, consumer2AllPotentialPartitions) { break } // the partition must have at least two consumers if len(partition2AllPotentialConsumers[partition]) <= 1 { Logger.Printf("Expected more than one potential consumer for partition %s topic %d", partition.Topic, partition.Partition) } // the partition must have a consumer consumer := currentPartitionConsumer[partition] if consumer == "" { Logger.Printf("Expected topic %s partition %d to be assigned to a consumer", partition.Topic, partition.Partition) } if _, exists := prevAssignment[partition]; exists { if len(currentAssignment[consumer]) > (len(currentAssignment[prevAssignment[partition].MemberID]) + 1) { sortedCurrentSubscriptions = s.reassignPartition(partition, currentAssignment, sortedCurrentSubscriptions, currentPartitionConsumer, prevAssignment[partition].MemberID) reassignmentPerformed = true modified = true continue } } // check if a better-suited consumer exists for the partition; if so, reassign it for _, otherConsumer := range partition2AllPotentialConsumers[partition] { if len(currentAssignment[consumer]) > (len(currentAssignment[otherConsumer]) + 1) { sortedCurrentSubscriptions = s.reassignPartitionToNewConsumer(partition, currentAssignment, sortedCurrentSubscriptions, currentPartitionConsumer, consumer2AllPotentialPartitions) reassignmentPerformed = true modified = true break } } } if !modified { return reassignmentPerformed } } } // Identify a new consumer for a topic partition and reassign it. func (s *stickyBalanceStrategy) reassignPartitionToNewConsumer(partition topicPartitionAssignment, currentAssignment map[string][]topicPartitionAssignment, sortedCurrentSubscriptions []string, currentPartitionConsumer map[topicPartitionAssignment]string, consumer2AllPotentialPartitions map[string][]topicPartitionAssignment) []string { for _, anotherConsumer := range sortedCurrentSubscriptions { if memberAssignmentsIncludeTopicPartition(consumer2AllPotentialPartitions[anotherConsumer], partition) { return s.reassignPartition(partition, currentAssignment, sortedCurrentSubscriptions, currentPartitionConsumer, anotherConsumer) } } return sortedCurrentSubscriptions } // Reassign a specific partition to a new consumer func (s *stickyBalanceStrategy) reassignPartition(partition topicPartitionAssignment, currentAssignment map[string][]topicPartitionAssignment, sortedCurrentSubscriptions []string, currentPartitionConsumer map[topicPartitionAssignment]string, newConsumer string) []string { consumer := currentPartitionConsumer[partition] // find the correct partition movement considering the stickiness requirement partitionToBeMoved := s.movements.getTheActualPartitionToBeMoved(partition, consumer, newConsumer) return s.processPartitionMovement(partitionToBeMoved, newConsumer, currentAssignment, sortedCurrentSubscriptions, currentPartitionConsumer) } // Track the movement of a topic partition after assignment func (s *stickyBalanceStrategy) processPartitionMovement(partition topicPartitionAssignment, newConsumer string, currentAssignment map[string][]topicPartitionAssignment, sortedCurrentSubscriptions []string, currentPartitionConsumer map[topicPartitionAssignment]string) []string { oldConsumer := currentPartitionConsumer[partition] s.movements.movePartition(partition, oldConsumer, newConsumer) currentAssignment[oldConsumer] = removeTopicPartitionFromMemberAssignments(currentAssignment[oldConsumer], partition) currentAssignment[newConsumer] = append(currentAssignment[newConsumer], partition) currentPartitionConsumer[partition] = newConsumer return sortMemberIDsByPartitionAssignments(currentAssignment) } // Determine whether a specific consumer should be considered for topic partition assignment. func canConsumerParticipateInReassignment(memberID string, currentAssignment map[string][]topicPartitionAssignment, consumer2AllPotentialPartitions map[string][]topicPartitionAssignment, partition2AllPotentialConsumers map[topicPartitionAssignment][]string) bool { currentPartitions := currentAssignment[memberID] currentAssignmentSize := len(currentPartitions) maxAssignmentSize := len(consumer2AllPotentialPartitions[memberID]) if currentAssignmentSize > maxAssignmentSize { Logger.Printf("The consumer %s is assigned more partitions than the maximum possible", memberID) } if currentAssignmentSize < maxAssignmentSize { // if a consumer is not assigned all its potential partitions it is subject to reassignment return true } for _, partition := range currentPartitions { if canTopicPartitionParticipateInReassignment(partition, partition2AllPotentialConsumers) { return true } } return false } // Only consider reassigning those topic partitions that have two or more potential consumers. func canTopicPartitionParticipateInReassignment(partition topicPartitionAssignment, partition2AllPotentialConsumers map[topicPartitionAssignment][]string) bool { return len(partition2AllPotentialConsumers[partition]) >= 2 } // The assignment should improve the overall balance of the partition assignments to consumers. func assignPartition(partition topicPartitionAssignment, sortedCurrentSubscriptions []string, currentAssignment map[string][]topicPartitionAssignment, consumer2AllPotentialPartitions map[string][]topicPartitionAssignment, currentPartitionConsumer map[topicPartitionAssignment]string) []string { for _, memberID := range sortedCurrentSubscriptions { if memberAssignmentsIncludeTopicPartition(consumer2AllPotentialPartitions[memberID], partition) { currentAssignment[memberID] = append(currentAssignment[memberID], partition) currentPartitionConsumer[partition] = memberID break } } return sortMemberIDsByPartitionAssignments(currentAssignment) } // Deserialize topic partition assignment data to aid with creation of a sticky assignment. func deserializeTopicPartitionAssignment(userDataBytes []byte) (StickyAssignorUserData, error) { userDataV1 := &StickyAssignorUserDataV1{} if err := decode(userDataBytes, userDataV1); err != nil { userDataV0 := &StickyAssignorUserDataV0{} if err := decode(userDataBytes, userDataV0); err != nil { return nil, err } return userDataV0, nil } return userDataV1, nil } // filterAssignedPartitions returns a map of consumer group members to their list of previously-assigned topic partitions, limited // to those topic partitions currently reported by the Kafka cluster. func filterAssignedPartitions(currentAssignment map[string][]topicPartitionAssignment, partition2AllPotentialConsumers map[topicPartitionAssignment][]string) map[string][]topicPartitionAssignment { assignments := deepCopyAssignment(currentAssignment) for memberID, partitions := range assignments { // perform in-place filtering i := 0 for _, partition := range partitions { if _, exists := partition2AllPotentialConsumers[partition]; exists { partitions[i] = partition i++ } } assignments[memberID] = partitions[:i] } return assignments } func removeTopicPartitionFromMemberAssignments(assignments []topicPartitionAssignment, topic topicPartitionAssignment) []topicPartitionAssignment { for i, assignment := range assignments { if assignment == topic { return append(assignments[:i], assignments[i+1:]...) } } return assignments } func memberAssignmentsIncludeTopicPartition(assignments []topicPartitionAssignment, topic topicPartitionAssignment) bool { for _, assignment := range assignments { if assignment == topic { return true } } return false } func sortPartitions(currentAssignment map[string][]topicPartitionAssignment, partitionsWithADifferentPreviousAssignment map[topicPartitionAssignment]consumerGenerationPair, isFreshAssignment bool, partition2AllPotentialConsumers map[topicPartitionAssignment][]string, consumer2AllPotentialPartitions map[string][]topicPartitionAssignment) []topicPartitionAssignment { unassignedPartitions := make(map[topicPartitionAssignment]bool, len(partition2AllPotentialConsumers)) for partition := range partition2AllPotentialConsumers { unassignedPartitions[partition] = true } sortedPartitions := make([]topicPartitionAssignment, 0) if !isFreshAssignment && areSubscriptionsIdentical(partition2AllPotentialConsumers, consumer2AllPotentialPartitions) { // if this is a reassignment and the subscriptions are identical (all consumers can consumer from all topics) // then we just need to simply list partitions in a round robin fashion (from consumers with // most assigned partitions to those with least) assignments := filterAssignedPartitions(currentAssignment, partition2AllPotentialConsumers) // use priority-queue to evaluate consumer group members in descending-order based on // the number of topic partition assignments (i.e. consumers with most assignments first) pq := make(assignmentPriorityQueue, len(assignments)) i := 0 for consumerID, consumerAssignments := range assignments { pq[i] = &consumerGroupMember{ id: consumerID, assignments: consumerAssignments, } i++ } heap.Init(&pq) for { // loop until no consumer-group members remain if pq.Len() == 0 { break } member := pq[0] // partitions that were assigned to a different consumer last time var prevPartitionIndex int for i, partition := range member.assignments { if _, exists := partitionsWithADifferentPreviousAssignment[partition]; exists { prevPartitionIndex = i break } } if len(member.assignments) > 0 { partition := member.assignments[prevPartitionIndex] sortedPartitions = append(sortedPartitions, partition) delete(unassignedPartitions, partition) if prevPartitionIndex == 0 { member.assignments = member.assignments[1:] } else { member.assignments = append(member.assignments[:prevPartitionIndex], member.assignments[prevPartitionIndex+1:]...) } heap.Fix(&pq, 0) } else { heap.Pop(&pq) } } for partition := range unassignedPartitions { sortedPartitions = append(sortedPartitions, partition) } } else { // an ascending sorted set of topic partitions based on how many consumers can potentially use them sortedPartitions = sortPartitionsByPotentialConsumerAssignments(partition2AllPotentialConsumers) } return sortedPartitions } func sortMemberIDsByPartitionAssignments(assignments map[string][]topicPartitionAssignment) []string { // sort the members by the number of partition assignments in ascending order sortedMemberIDs := make([]string, 0, len(assignments)) for memberID := range assignments { sortedMemberIDs = append(sortedMemberIDs, memberID) } sort.SliceStable(sortedMemberIDs, func(i, j int) bool { ret := len(assignments[sortedMemberIDs[i]]) - len(assignments[sortedMemberIDs[j]]) if ret == 0 { return sortedMemberIDs[i] < sortedMemberIDs[j] } return len(assignments[sortedMemberIDs[i]]) < len(assignments[sortedMemberIDs[j]]) }) return sortedMemberIDs } func sortPartitionsByPotentialConsumerAssignments(partition2AllPotentialConsumers map[topicPartitionAssignment][]string) []topicPartitionAssignment { // sort the members by the number of partition assignments in descending order sortedPartionIDs := make([]topicPartitionAssignment, len(partition2AllPotentialConsumers)) i := 0 for partition := range partition2AllPotentialConsumers { sortedPartionIDs[i] = partition i++ } sort.Slice(sortedPartionIDs, func(i, j int) bool { if len(partition2AllPotentialConsumers[sortedPartionIDs[i]]) == len(partition2AllPotentialConsumers[sortedPartionIDs[j]]) { ret := strings.Compare(sortedPartionIDs[i].Topic, sortedPartionIDs[j].Topic) if ret == 0 { return sortedPartionIDs[i].Partition < sortedPartionIDs[j].Partition } return ret < 0 } return len(partition2AllPotentialConsumers[sortedPartionIDs[i]]) < len(partition2AllPotentialConsumers[sortedPartionIDs[j]]) }) return sortedPartionIDs } func deepCopyAssignment(assignment map[string][]topicPartitionAssignment) map[string][]topicPartitionAssignment { copy := make(map[string][]topicPartitionAssignment, len(assignment)) for memberID, subscriptions := range assignment { copy[memberID] = append(subscriptions[:0:0], subscriptions...) } return copy } func areSubscriptionsIdentical(partition2AllPotentialConsumers map[topicPartitionAssignment][]string, consumer2AllPotentialPartitions map[string][]topicPartitionAssignment) bool { curMembers := make(map[string]int) for _, cur := range partition2AllPotentialConsumers { if len(curMembers) == 0 { for _, curMembersElem := range cur { curMembers[curMembersElem]++ } continue } if len(curMembers) != len(cur) { return false } yMap := make(map[string]int) for _, yElem := range cur { yMap[yElem]++ } for curMembersMapKey, curMembersMapVal := range curMembers { if yMap[curMembersMapKey] != curMembersMapVal { return false } } } curPartitions := make(map[topicPartitionAssignment]int) for _, cur := range consumer2AllPotentialPartitions { if len(curPartitions) == 0 { for _, curPartitionElem := range cur { curPartitions[curPartitionElem]++ } continue } if len(curPartitions) != len(cur) { return false } yMap := make(map[topicPartitionAssignment]int) for _, yElem := range cur { yMap[yElem]++ } for curMembersMapKey, curMembersMapVal := range curPartitions { if yMap[curMembersMapKey] != curMembersMapVal { return false } } } return true } // We need to process subscriptions' user data with each consumer's reported generation in mind // higher generations overwrite lower generations in case of a conflict // note that a conflict could exist only if user data is for different generations func prepopulateCurrentAssignments(members map[string]ConsumerGroupMemberMetadata) (map[string][]topicPartitionAssignment, map[topicPartitionAssignment]consumerGenerationPair, error) { currentAssignment := make(map[string][]topicPartitionAssignment) prevAssignment := make(map[topicPartitionAssignment]consumerGenerationPair) // for each partition we create a sorted map of its consumers by generation sortedPartitionConsumersByGeneration := make(map[topicPartitionAssignment]map[int]string) for memberID, meta := range members { consumerUserData, err := deserializeTopicPartitionAssignment(meta.UserData) if err != nil { return nil, nil, err } for _, partition := range consumerUserData.partitions() { if consumers, exists := sortedPartitionConsumersByGeneration[partition]; exists { if consumerUserData.hasGeneration() { if _, generationExists := consumers[consumerUserData.generation()]; generationExists { // same partition is assigned to two consumers during the same rebalance. // log a warning and skip this record Logger.Printf("Topic %s Partition %d is assigned to multiple consumers following sticky assignment generation %d", partition.Topic, partition.Partition, consumerUserData.generation()) continue } else { consumers[consumerUserData.generation()] = memberID } } else { consumers[defaultGeneration] = memberID } } else { generation := defaultGeneration if consumerUserData.hasGeneration() { generation = consumerUserData.generation() } sortedPartitionConsumersByGeneration[partition] = map[int]string{generation: memberID} } } } // prevAssignment holds the prior ConsumerGenerationPair (before current) of each partition // current and previous consumers are the last two consumers of each partition in the above sorted map for partition, consumers := range sortedPartitionConsumersByGeneration { // sort consumers by generation in decreasing order var generations []int for generation := range consumers { generations = append(generations, generation) } sort.Sort(sort.Reverse(sort.IntSlice(generations))) consumer := consumers[generations[0]] if _, exists := currentAssignment[consumer]; !exists { currentAssignment[consumer] = []topicPartitionAssignment{partition} } else { currentAssignment[consumer] = append(currentAssignment[consumer], partition) } // check for previous assignment, if any if len(generations) > 1 { prevAssignment[partition] = consumerGenerationPair{ MemberID: consumers[generations[1]], Generation: generations[1], } } } return currentAssignment, prevAssignment, nil } type consumerGenerationPair struct { MemberID string Generation int } // consumerPair represents a pair of Kafka consumer ids involved in a partition reassignment. type consumerPair struct { SrcMemberID string DstMemberID string } // partitionMovements maintains some data structures to simplify lookup of partition movements among consumers. type partitionMovements struct { PartitionMovementsByTopic map[string]map[consumerPair]map[topicPartitionAssignment]bool Movements map[topicPartitionAssignment]consumerPair } func (p *partitionMovements) removeMovementRecordOfPartition(partition topicPartitionAssignment) consumerPair { pair := p.Movements[partition] delete(p.Movements, partition) partitionMovementsForThisTopic := p.PartitionMovementsByTopic[partition.Topic] delete(partitionMovementsForThisTopic[pair], partition) if len(partitionMovementsForThisTopic[pair]) == 0 { delete(partitionMovementsForThisTopic, pair) } if len(p.PartitionMovementsByTopic[partition.Topic]) == 0 { delete(p.PartitionMovementsByTopic, partition.Topic) } return pair } func (p *partitionMovements) addPartitionMovementRecord(partition topicPartitionAssignment, pair consumerPair) { p.Movements[partition] = pair if _, exists := p.PartitionMovementsByTopic[partition.Topic]; !exists { p.PartitionMovementsByTopic[partition.Topic] = make(map[consumerPair]map[topicPartitionAssignment]bool) } partitionMovementsForThisTopic := p.PartitionMovementsByTopic[partition.Topic] if _, exists := partitionMovementsForThisTopic[pair]; !exists { partitionMovementsForThisTopic[pair] = make(map[topicPartitionAssignment]bool) } partitionMovementsForThisTopic[pair][partition] = true } func (p *partitionMovements) movePartition(partition topicPartitionAssignment, oldConsumer, newConsumer string) { pair := consumerPair{ SrcMemberID: oldConsumer, DstMemberID: newConsumer, } if _, exists := p.Movements[partition]; exists { // this partition has previously moved existingPair := p.removeMovementRecordOfPartition(partition) if existingPair.DstMemberID != oldConsumer { Logger.Printf("Existing pair DstMemberID %s was not equal to the oldConsumer ID %s", existingPair.DstMemberID, oldConsumer) } if existingPair.SrcMemberID != newConsumer { // the partition is not moving back to its previous consumer p.addPartitionMovementRecord(partition, consumerPair{ SrcMemberID: existingPair.SrcMemberID, DstMemberID: newConsumer, }) } } else { p.addPartitionMovementRecord(partition, pair) } } func (p *partitionMovements) getTheActualPartitionToBeMoved(partition topicPartitionAssignment, oldConsumer, newConsumer string) topicPartitionAssignment { if _, exists := p.PartitionMovementsByTopic[partition.Topic]; !exists { return partition } if _, exists := p.Movements[partition]; exists { // this partition has previously moved if oldConsumer != p.Movements[partition].DstMemberID { Logger.Printf("Partition movement DstMemberID %s was not equal to the oldConsumer ID %s", p.Movements[partition].DstMemberID, oldConsumer) } oldConsumer = p.Movements[partition].SrcMemberID } partitionMovementsForThisTopic := p.PartitionMovementsByTopic[partition.Topic] reversePair := consumerPair{ SrcMemberID: newConsumer, DstMemberID: oldConsumer, } if _, exists := partitionMovementsForThisTopic[reversePair]; !exists { return partition } var reversePairPartition topicPartitionAssignment for otherPartition := range partitionMovementsForThisTopic[reversePair] { reversePairPartition = otherPartition } return reversePairPartition } func (p *partitionMovements) isLinked(src, dst string, pairs []consumerPair, currentPath []string) ([]string, bool) { if src == dst { return currentPath, false } if len(pairs) == 0 { return currentPath, false } for _, pair := range pairs { if src == pair.SrcMemberID && dst == pair.DstMemberID { currentPath = append(currentPath, src, dst) return currentPath, true } } for _, pair := range pairs { if pair.SrcMemberID == src { // create a deep copy of the pairs, excluding the current pair reducedSet := make([]consumerPair, len(pairs)-1) i := 0 for _, p := range pairs { if p != pair { reducedSet[i] = pair i++ } } currentPath = append(currentPath, pair.SrcMemberID) return p.isLinked(pair.DstMemberID, dst, reducedSet, currentPath) } } return currentPath, false } func (p *partitionMovements) in(cycle []string, cycles [][]string) bool { superCycle := make([]string, len(cycle)-1) for i := 0; i < len(cycle)-1; i++ { superCycle[i] = cycle[i] } superCycle = append(superCycle, cycle...) for _, foundCycle := range cycles { if len(foundCycle) == len(cycle) && indexOfSubList(superCycle, foundCycle) != -1 { return true } } return false } func (p *partitionMovements) hasCycles(pairs []consumerPair) bool { cycles := make([][]string, 0) for _, pair := range pairs { // create a deep copy of the pairs, excluding the current pair reducedPairs := make([]consumerPair, len(pairs)-1) i := 0 for _, p := range pairs { if p != pair { reducedPairs[i] = pair i++ } } if path, linked := p.isLinked(pair.DstMemberID, pair.SrcMemberID, reducedPairs, []string{pair.SrcMemberID}); linked { if !p.in(path, cycles) { cycles = append(cycles, path) Logger.Printf("A cycle of length %d was found: %v", len(path)-1, path) } } } // for now we want to make sure there is no partition movements of the same topic between a pair of consumers. // the odds of finding a cycle among more than two consumers seem to be very low (according to various randomized // tests with the given sticky algorithm) that it should not worth the added complexity of handling those cases. for _, cycle := range cycles { if len(cycle) == 3 { return true } } return false } func (p *partitionMovements) isSticky() bool { for topic, movements := range p.PartitionMovementsByTopic { movementPairs := make([]consumerPair, len(movements)) i := 0 for pair := range movements { movementPairs[i] = pair i++ } if p.hasCycles(movementPairs) { Logger.Printf("Stickiness is violated for topic %s", topic) Logger.Printf("Partition movements for this topic occurred among the following consumer pairs: %v", movements) return false } } return true } func indexOfSubList(source []string, target []string) int { targetSize := len(target) maxCandidate := len(source) - targetSize nextCand: for candidate := 0; candidate <= maxCandidate; candidate++ { j := candidate for i := 0; i < targetSize; i++ { if target[i] != source[j] { // Element mismatch, try next cand continue nextCand } j++ } // All elements of candidate matched target return candidate } return -1 } type consumerGroupMember struct { id string assignments []topicPartitionAssignment } // assignmentPriorityQueue is a priority-queue of consumer group members that is sorted // in descending order (most assignments to least assignments). type assignmentPriorityQueue []*consumerGroupMember func (pq assignmentPriorityQueue) Len() int { return len(pq) } func (pq assignmentPriorityQueue) Less(i, j int) bool { // order asssignment priority queue in descending order using assignment-count/member-id if len(pq[i].assignments) == len(pq[j].assignments) { return strings.Compare(pq[i].id, pq[j].id) > 0 } return len(pq[i].assignments) > len(pq[j].assignments) } func (pq assignmentPriorityQueue) Swap(i, j int) { pq[i], pq[j] = pq[j], pq[i] } func (pq *assignmentPriorityQueue) Push(x interface{}) { member := x.(*consumerGroupMember) *pq = append(*pq, member) } func (pq *assignmentPriorityQueue) Pop() interface{} { old := *pq n := len(old) member := old[n-1] *pq = old[0 : n-1] return member }