Round-robin partition assignments across multiple topics

This is closer to the behavior of Kafka's Java consumer, as described
here:

https://github.com/apache/kafka/blob/15bc405/clients/src/main/java/org/apache/kafka/clients/consumer/RoundRobinAssignor.java#L31-L55

This is valuable in situations where you have highly disparate
partition counts. Given three group members subscribed to many topics,
only a few of which have more than one partition, the first member
will receive far more assignments than the others.

balancer.go

@@ -1,6 +1,7 @@
 package cluster
 
 import (
+	"container/ring"
 	"math"
 	"sort"
 
@@ -70,89 +71,53 @@ func (n *Notification) success(current map[string][]int32) *Notification {
 
 // --------------------------------------------------------------------
 
+type balancer struct {
+	client    sarama.Client
+	memberIDs []string
+	topics    map[string]*topicInfo
+}
+
 type topicInfo struct {
 	Partitions []int32
 	MemberIDs  []string
 }
 
-func (info topicInfo) Perform(s Strategy) map[string][]int32 {
-	if s == StrategyRoundRobin {
-		return info.RoundRobin()
-	}
-	return info.Ranges()
-}
-
-func (info topicInfo) Ranges() map[string][]int32 {
-	sort.Strings(info.MemberIDs)
-
-	mlen := len(info.MemberIDs)
-	plen := len(info.Partitions)
-	res := make(map[string][]int32, mlen)
-
-	for pos, memberID := range info.MemberIDs {
-		n, i := float64(plen)/float64(mlen), float64(pos)
-		min := int(math.Floor(i*n + 0.5))
-		max := int(math.Floor((i+1)*n + 0.5))
-		sub := info.Partitions[min:max]
-		if len(sub) > 0 {
-			res[memberID] = sub
-		}
-	}
-	return res
-}
-
-func (info topicInfo) RoundRobin() map[string][]int32 {
-	sort.Strings(info.MemberIDs)
-
-	mlen := len(info.MemberIDs)
-	res := make(map[string][]int32, mlen)
-	for i, pnum := range info.Partitions {
-		memberID := info.MemberIDs[i%mlen]
-		res[memberID] = append(res[memberID], pnum)
-	}
-	return res
-}
-
-// --------------------------------------------------------------------
-
-type balancer struct {
-	client sarama.Client
-	topics map[string]topicInfo
-}
-
 func newBalancerFromMeta(client sarama.Client, members map[string]sarama.ConsumerGroupMemberMetadata) (*balancer, error) {
-	balancer := newBalancer(client)
+	balancer := &balancer{
+		client:    client,
+		memberIDs: make([]string, 0, len(members)),
+		topics:    make(map[string]*topicInfo),
+	}
 	for memberID, meta := range members {
+		balancer.memberIDs = append(balancer.memberIDs, memberID)
 		for _, topic := range meta.Topics {
-			if err := balancer.Topic(topic, memberID); err != nil {
+			if err := balancer.Topic(memberID, topic); err != nil {
 				return nil, err
 			}
 		}
 	}
-	return balancer, nil
-}
 
-func newBalancer(client sarama.Client) *balancer {
-	return &balancer{
-		client: client,
-		topics: make(map[string]topicInfo),
-	}
+	sort.Strings(balancer.memberIDs)
+	return balancer, nil
 }
 
-func (r *balancer) Topic(name string, memberID string) error {
-	topic, ok := r.topics[name]
+func (r *balancer) Topic(memberID string, name string) error {
+	info, ok := r.topics[name]
 	if !ok {
 		nums, err := r.client.Partitions(name)
 		if err != nil {
 			return err
 		}
-		topic = topicInfo{
+
+		r.topics[name] = &topicInfo{
+			MemberIDs:  []string{memberID},
 			Partitions: nums,
-			MemberIDs:  make([]string, 0, 1),
 		}
+
+		return nil
 	}
-	topic.MemberIDs = append(topic.MemberIDs, memberID)
-	r.topics[name] = topic
+
+	info.MemberIDs = append(info.MemberIDs, memberID)
 	return nil
 }
 
@@ -161,14 +126,87 @@ func (r *balancer) Perform(s Strategy) map[string]map[string][]int32 {
 		return nil
 	}
 
-	res := make(map[string]map[string][]int32, 1)
-	for topic, info := range r.topics {
-		for memberID, partitions := range info.Perform(s) {
-			if _, ok := res[memberID]; !ok {
-				res[memberID] = make(map[string][]int32, 1)
+	switch s {
+	case StrategyRoundRobin:
+		return assignRoundRobin(r.memberIDs, r.topics)
+	default:
+		return assignRange(r.memberIDs, r.topics)
+	}
+}
+
+func assignRange(_ []string, topics map[string]*topicInfo) map[string]map[string][]int32 {
+	tlen := len(topics)
+	res := make(map[string]map[string][]int32)
+
+	for topic, info := range topics {
+		mlen := len(info.MemberIDs)
+		plen := len(info.Partitions)
+
+		sort.Strings(info.MemberIDs)
+		for pos, memberID := range info.MemberIDs {
+			n, i := float64(plen)/float64(mlen), float64(pos)
+			min := int(math.Floor(i*n + 0.5))
+			max := int(math.Floor((i+1)*n + 0.5))
+			sub := info.Partitions[min:max]
+			if len(sub) <= 0 {
+				continue
+			}
+
+			assigned, ok := res[memberID]
+			if !ok {
+				assigned = make(map[string][]int32, tlen)
+				res[memberID] = assigned
 			}
-			res[memberID][topic] = partitions
+			assigned[topic] = sub
 		}
 	}
+
+	return res
+}
+
+func assignRoundRobin(memberIDs []string, topics map[string]*topicInfo) map[string]map[string][]int32 {
+	sort.Strings(memberIDs)
+
+	r := ring.New(len(memberIDs))
+	for i := 0; i < r.Len(); i++ {
+		r.Value = memberIDs[i]
+		r = r.Next()
+	}
+
+	isSubscribed := func(memberID string, topic string) bool {
+		info, ok := topics[topic]
+		if !ok {
+			return false
+		}
+
+		for _, subscriber := range info.MemberIDs {
+			if memberID == subscriber {
+				return true
+			}
+		}
+
+		return false
+	}
+
+	tlen := len(topics)
+	res := make(map[string]map[string][]int32, r.Len())
+
+	for topic, info := range topics {
+		for i := range info.Partitions {
+			for ; !isSubscribed(r.Value.(string), topic); r = r.Next() {
+				continue
+			}
+
+			memberID := r.Value.(string)
+			assigned, ok := res[memberID]
+			if !ok {
+				assigned = make(map[string][]int32, tlen)
+				res[memberID] = assigned
+			}
+			assigned[topic] = append(assigned[topic], info.Partitions[i])
+			r = r.Next()
+		}
+	}
+
 	return res
 }