/*
* 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.clients.consumer;
import org.apache.kafka.clients.consumer.internals.AbstractPartitionAssignor;
import org.apache.kafka.common.TopicPartition;
import org.apache.kafka.common.utils.CircularIterator;
import org.apache.kafka.common.utils.Utils;
import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;
import java.util.SortedSet;
import java.util.TreeSet;
/**
* The round robin assignor lays out all the available partitions and all the available consumers. It
* then proceeds to do a round robin assignment from partition to consumer. If the subscriptions of all consumer
* instances are identical, then the partitions will be uniformly distributed. (i.e., the partition ownership counts
* will be within a delta of exactly one across all consumers.)
*
* For example, suppose there are two consumers C0 and C1, two topics t0 and t1, and each topic has 3 partitions,
* resulting in partitions t0p0, t0p1, t0p2, t1p0, t1p1, and t1p2.
*
* The assignment will be:
* C0: [t0p0, t0p2, t1p1]
* C1: [t0p1, t1p0, t1p2]
*
* When subscriptions differ across consumer instances, the assignment process still considers each
* consumer instance in round robin fashion but skips over an instance if it is not subscribed to
* the topic. Unlike the case when subscriptions are identical, this can result in imbalanced
* assignments. For example, we have three consumers C0, C1, C2, and three topics t0, t1, t2,
* with 1, 2, and 3 partitions, respectively. Therefore, the partitions are t0p0, t1p0, t1p1, t2p0,
* t2p1, t2p2. C0 is subscribed to t0; C1 is subscribed to t0, t1; and C2 is subscribed to t0, t1, t2.
*
* Tha assignment will be:
* C0: [t0p0]
* C1: [t1p0]
* C2: [t1p1, t2p0, t2p1, t2p2]
*/
public class RoundRobinAssignor extends AbstractPartitionAssignor {
@Override
public Map<String, List<TopicPartition>> assign(Map<String, Integer> partitionsPerTopic,
Map<String, List<String>> subscriptions) {
Map<String, List<TopicPartition>> assignment = new HashMap<>();
for (String memberId : subscriptions.keySet())
assignment.put(memberId, new ArrayList<TopicPartition>());
CircularIterator<String> assigner = new CircularIterator<>(Utils.sorted(subscriptions.keySet()));
for (TopicPartition partition : allPartitionsSorted(partitionsPerTopic, subscriptions)) {
final String topic = partition.topic();
while (!subscriptions.get(assigner.peek()).contains(topic))
assigner.next();
assignment.get(assigner.next()).add(partition);
}
return assignment;
}
public List<TopicPartition> allPartitionsSorted(Map<String, Integer> partitionsPerTopic,
Map<String, List<String>> subscriptions) {
SortedSet<String> topics = new TreeSet<>();
for (List<String> subscription : subscriptions.values())
topics.addAll(subscription);
List<TopicPartition> allPartitions = new ArrayList<>();
for (String topic : topics) {
Integer numPartitionsForTopic = partitionsPerTopic.get(topic);
if (numPartitionsForTopic != null)
allPartitions.addAll(AbstractPartitionAssignor.partitions(topic, numPartitionsForTopic));
}
return allPartitions;
}
@Override
public String name() {
return "roundrobin";
}
}