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Apache Jena Fuseki server Kafka connector

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/*
 * 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.common.replica;

import org.apache.kafka.common.TopicPartition;

import java.util.Optional;
import java.util.Set;
import java.util.stream.Collectors;

/**
 * Returns a replica whose rack id is equal to the rack id specified in the client request metadata. If no such replica
 * is found, returns the leader.
 */
public class RackAwareReplicaSelector implements ReplicaSelector {

    @Override
    public Optional select(TopicPartition topicPartition,
                                        ClientMetadata clientMetadata,
                                        PartitionView partitionView) {
        if (clientMetadata.rackId() != null && !clientMetadata.rackId().isEmpty()) {
            Set sameRackReplicas = partitionView.replicas().stream()
                    .filter(replicaInfo -> clientMetadata.rackId().equals(replicaInfo.endpoint().rack()))
                    .collect(Collectors.toSet());
            if (sameRackReplicas.isEmpty()) {
                return Optional.of(partitionView.leader());
            } else {
                if (sameRackReplicas.contains(partitionView.leader())) {
                    // Use the leader if it's in this rack
                    return Optional.of(partitionView.leader());
                } else {
                    // Otherwise, get the most caught-up replica
                    return sameRackReplicas.stream().max(ReplicaView.comparator());
                }
            }
        } else {
            return Optional.of(partitionView.leader());
        }
    }
}