cmccabe commented on a change in pull request #10070:
URL: https://github.com/apache/kafka/pull/10070#discussion_r579547731



##########
File path: 
metadata/src/main/java/org/apache/kafka/controller/ReplicationControlManager.java
##########
@@ -0,0 +1,894 @@
+/*
+ * 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.controller;
+
+import org.apache.kafka.clients.admin.AlterConfigOp.OpType;
+import org.apache.kafka.common.ElectionType;
+import org.apache.kafka.common.Uuid;
+import org.apache.kafka.common.config.ConfigResource;
+import org.apache.kafka.common.errors.BrokerIdNotRegisteredException;
+import org.apache.kafka.common.errors.InvalidReplicationFactorException;
+import org.apache.kafka.common.errors.InvalidRequestException;
+import org.apache.kafka.common.errors.InvalidTopicException;
+import org.apache.kafka.common.internals.Topic;
+import org.apache.kafka.common.message.AlterIsrRequestData;
+import org.apache.kafka.common.message.AlterIsrResponseData;
+import org.apache.kafka.common.message.BrokerHeartbeatRequestData;
+import 
org.apache.kafka.common.message.CreateTopicsRequestData.CreatableReplicaAssignment;
+import org.apache.kafka.common.message.CreateTopicsRequestData.CreatableTopic;
+import 
org.apache.kafka.common.message.CreateTopicsRequestData.CreatableTopicCollection;
+import org.apache.kafka.common.message.CreateTopicsRequestData;
+import 
org.apache.kafka.common.message.CreateTopicsResponseData.CreatableTopicResult;
+import org.apache.kafka.common.message.CreateTopicsResponseData;
+import org.apache.kafka.common.message.ElectLeadersRequestData.TopicPartitions;
+import org.apache.kafka.common.message.ElectLeadersRequestData;
+import 
org.apache.kafka.common.message.ElectLeadersResponseData.PartitionResult;
+import 
org.apache.kafka.common.message.ElectLeadersResponseData.ReplicaElectionResult;
+import org.apache.kafka.common.message.ElectLeadersResponseData;
+import org.apache.kafka.common.metadata.FenceBrokerRecord;
+import org.apache.kafka.common.metadata.PartitionChangeRecord;
+import org.apache.kafka.common.metadata.PartitionRecord;
+import org.apache.kafka.common.metadata.TopicRecord;
+import org.apache.kafka.common.metadata.UnfenceBrokerRecord;
+import org.apache.kafka.common.metadata.UnregisterBrokerRecord;
+import org.apache.kafka.common.protocol.Errors;
+import org.apache.kafka.common.requests.ApiError;
+import org.apache.kafka.common.utils.LogContext;
+import org.apache.kafka.controller.BrokersToIsrs.TopicPartition;
+import org.apache.kafka.metadata.ApiMessageAndVersion;
+import org.apache.kafka.metadata.BrokerHeartbeatReply;
+import org.apache.kafka.metadata.BrokerRegistration;
+import org.apache.kafka.timeline.SnapshotRegistry;
+import org.apache.kafka.timeline.TimelineHashMap;
+import org.slf4j.Logger;
+
+import java.util.AbstractMap.SimpleImmutableEntry;
+import java.util.ArrayList;
+import java.util.Arrays;
+import java.util.Collections;
+import java.util.HashMap;
+import java.util.HashSet;
+import java.util.Iterator;
+import java.util.List;
+import java.util.Map;
+import java.util.Map.Entry;
+import java.util.Objects;
+import java.util.Random;
+
+import static org.apache.kafka.clients.admin.AlterConfigOp.OpType.SET;
+import static org.apache.kafka.common.config.ConfigResource.Type.TOPIC;
+
+
+/**
+ * The ReplicationControlManager is the part of the controller which deals 
with topics
+ * and partitions. It is responsible for managing the in-sync replica set and 
leader
+ * of each partition, as well as administrative tasks like creating or 
deleting topics.
+ */
+public class ReplicationControlManager {
+    static class TopicControlInfo {
+        private final Uuid id;
+        private final TimelineHashMap<Integer, PartitionControlInfo> parts;
+
+        TopicControlInfo(SnapshotRegistry snapshotRegistry, Uuid id) {
+            this.id = id;
+            this.parts = new TimelineHashMap<>(snapshotRegistry, 0);
+        }
+    }
+
+    static class PartitionControlInfo {
+        private final int[] replicas;
+        private final int[] isr;
+        private final int[] removingReplicas;
+        private final int[] addingReplicas;
+        private final int leader;
+        private final int leaderEpoch;
+        private final int partitionEpoch;
+
+        PartitionControlInfo(PartitionRecord record) {
+            this(Replicas.toArray(record.replicas()),
+                Replicas.toArray(record.isr()),
+                Replicas.toArray(record.removingReplicas()),
+                Replicas.toArray(record.addingReplicas()),
+                record.leader(),
+                record.leaderEpoch(),
+                record.partitionEpoch());
+        }
+
+        PartitionControlInfo(int[] replicas, int[] isr, int[] removingReplicas,
+                             int[] addingReplicas, int leader, int leaderEpoch,
+                             int partitionEpoch) {
+            this.replicas = replicas;
+            this.isr = isr;
+            this.removingReplicas = removingReplicas;
+            this.addingReplicas = addingReplicas;
+            this.leader = leader;
+            this.leaderEpoch = leaderEpoch;
+            this.partitionEpoch = partitionEpoch;
+        }
+
+        PartitionControlInfo merge(PartitionChangeRecord record) {
+            int[] newIsr = (record.isr() == null) ? isr : 
Replicas.toArray(record.isr());
+            int newLeader;
+            int newLeaderEpoch;
+            if (record.leader() == Integer.MIN_VALUE) {
+                newLeader = leader;
+                newLeaderEpoch = leaderEpoch;
+            } else {
+                newLeader = record.leader();
+                newLeaderEpoch = leaderEpoch + 1;

Review comment:
       merge only bumps the epoch if the leader was set.
   
   ```
           PartitionControlInfo merge(PartitionChangeRecord record) {
               int[] newIsr = (record.isr() == null) ? isr : 
Replicas.toArray(record.isr());
               int newLeader;
               int newLeaderEpoch;
               if (record.leader() == NO_LEADER_CHANGE) {
                   newLeader = leader;
                   newLeaderEpoch = leaderEpoch;
               } else {
                   newLeader = record.leader();
                   newLeaderEpoch = leaderEpoch + 1;
               }
               return new PartitionControlInfo(replicas,
                   newIsr,
                   removingReplicas,
                   addingReplicas,
                   newLeader,
                   newLeaderEpoch,
                   partitionEpoch + 1);
           }
   ```




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