junrao commented on a change in pull request #10070:
URL: https://github.com/apache/kafka/pull/10070#discussion_r577263866



##########
File path: 
metadata/src/main/java/org/apache/kafka/controller/QuorumController.java
##########
@@ -0,0 +1,920 @@
+/*
+ * 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.config.ConfigDef;
+import org.apache.kafka.common.config.ConfigResource;
+import org.apache.kafka.common.errors.ApiException;
+import org.apache.kafka.common.errors.NotControllerException;
+import org.apache.kafka.common.errors.UnknownServerException;
+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.BrokerRegistrationRequestData;
+import org.apache.kafka.common.message.CreateTopicsRequestData;
+import org.apache.kafka.common.message.CreateTopicsResponseData;
+import org.apache.kafka.common.message.ElectLeadersRequestData;
+import org.apache.kafka.common.message.ElectLeadersResponseData;
+import org.apache.kafka.common.metadata.ConfigRecord;
+import org.apache.kafka.common.metadata.FenceBrokerRecord;
+import org.apache.kafka.common.metadata.IsrChangeRecord;
+import org.apache.kafka.common.metadata.MetadataRecordType;
+import org.apache.kafka.common.metadata.PartitionRecord;
+import org.apache.kafka.common.metadata.QuotaRecord;
+import org.apache.kafka.common.metadata.RegisterBrokerRecord;
+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.ApiMessage;
+import org.apache.kafka.common.quota.ClientQuotaAlteration;
+import org.apache.kafka.common.quota.ClientQuotaEntity;
+import org.apache.kafka.common.requests.ApiError;
+import org.apache.kafka.common.utils.LogContext;
+import org.apache.kafka.common.utils.Time;
+import org.apache.kafka.common.utils.Utils;
+import org.apache.kafka.metadata.ApiMessageAndVersion;
+import org.apache.kafka.metadata.BrokerHeartbeatReply;
+import org.apache.kafka.metadata.BrokerRegistrationReply;
+import org.apache.kafka.metadata.FeatureManager;
+import org.apache.kafka.metadata.VersionRange;
+import org.apache.kafka.metalog.MetaLogLeader;
+import org.apache.kafka.metalog.MetaLogListener;
+import org.apache.kafka.metalog.MetaLogManager;
+import org.apache.kafka.queue.EventQueue.EarliestDeadlineFunction;
+import org.apache.kafka.queue.EventQueue;
+import org.apache.kafka.queue.KafkaEventQueue;
+import org.apache.kafka.timeline.SnapshotRegistry;
+import org.slf4j.Logger;
+
+import java.util.ArrayList;
+import java.util.Collection;
+import java.util.Collections;
+import java.util.List;
+import java.util.Map.Entry;
+import java.util.Map;
+import java.util.Optional;
+import java.util.Random;
+import java.util.Set;
+import java.util.concurrent.CompletableFuture;
+import java.util.concurrent.RejectedExecutionException;
+import java.util.concurrent.TimeUnit;
+import java.util.function.Supplier;
+import java.util.stream.Collectors;
+
+
+public final class QuorumController implements Controller {
+    /**
+     * A builder class which creates the QuorumController.
+     */
+    static public class Builder {
+        private final int nodeId;
+        private Time time = Time.SYSTEM;
+        private String threadNamePrefix = null;
+        private LogContext logContext = null;
+        private Map<ConfigResource.Type, ConfigDef> configDefs = 
Collections.emptyMap();
+        private MetaLogManager logManager = null;
+        private Map<String, VersionRange> supportedFeatures = 
Collections.emptyMap();
+        private short defaultReplicationFactor = 3;
+        private int defaultNumPartitions = 1;
+        private ReplicaPlacementPolicy replicaPlacementPolicy =
+            new SimpleReplicaPlacementPolicy(new Random());
+        private long sessionTimeoutNs = TimeUnit.NANOSECONDS.convert(18, 
TimeUnit.SECONDS);
+
+        public Builder(int nodeId) {
+            this.nodeId = nodeId;
+        }
+
+        public Builder setTime(Time time) {
+            this.time = time;
+            return this;
+        }
+
+        public Builder setThreadNamePrefix(String threadNamePrefix) {
+            this.threadNamePrefix = threadNamePrefix;
+            return this;
+        }
+
+        public Builder setLogContext(LogContext logContext) {
+            this.logContext = logContext;
+            return this;
+        }
+
+        public Builder setConfigDefs(Map<ConfigResource.Type, ConfigDef> 
configDefs) {
+            this.configDefs = configDefs;
+            return this;
+        }
+
+        public Builder setLogManager(MetaLogManager logManager) {
+            this.logManager = logManager;
+            return this;
+        }
+
+        public Builder setSupportedFeatures(Map<String, VersionRange> 
supportedFeatures) {
+            this.supportedFeatures = supportedFeatures;
+            return this;
+        }
+
+        public Builder setDefaultReplicationFactor(short 
defaultReplicationFactor) {
+            this.defaultReplicationFactor = defaultReplicationFactor;
+            return this;
+        }
+
+        public Builder setDefaultNumPartitions(int defaultNumPartitions) {
+            this.defaultNumPartitions = defaultNumPartitions;
+            return this;
+        }
+
+        public Builder setReplicaPlacementPolicy(ReplicaPlacementPolicy 
replicaPlacementPolicy) {
+            this.replicaPlacementPolicy = replicaPlacementPolicy;
+            return this;
+        }
+
+        public Builder setSessionTimeoutNs(long sessionTimeoutNs) {
+            this.sessionTimeoutNs = sessionTimeoutNs;
+            return this;
+        }
+
+        public QuorumController build() throws Exception {
+            if (logManager == null) {
+                throw new RuntimeException("You must set a metadata log 
manager.");
+            }
+            if (threadNamePrefix == null) {
+                threadNamePrefix = String.format("Node%d_", nodeId);
+            }
+            if (logContext == null) {
+                logContext = new LogContext(String.format("[Controller %d] ", 
nodeId));
+            }
+            KafkaEventQueue queue = null;
+            try {
+                queue = new KafkaEventQueue(time, logContext, 
threadNamePrefix);
+                return new QuorumController(logContext, nodeId, queue, time, 
configDefs,
+                        logManager, supportedFeatures, 
defaultReplicationFactor,
+                        defaultNumPartitions, replicaPlacementPolicy, 
sessionTimeoutNs);
+            } catch (Exception e) {
+                Utils.closeQuietly(queue, "event queue");
+                throw e;
+            }
+        }
+    }
+
+    private static final String ACTIVE_CONTROLLER_EXCEPTION_TEXT_PREFIX =
+        "The active controller appears to be node ";
+
+    private NotControllerException newNotControllerException() {
+        int latestController = logManager.leader().nodeId();
+        if (latestController < 0) {
+            return new NotControllerException("No controller appears to be 
active.");
+        } else {
+            return new 
NotControllerException(ACTIVE_CONTROLLER_EXCEPTION_TEXT_PREFIX +
+                latestController);
+        }
+    }
+
+    public static int exceptionToApparentController(NotControllerException e) {
+        if 
(e.getMessage().startsWith(ACTIVE_CONTROLLER_EXCEPTION_TEXT_PREFIX)) {
+            return Integer.parseInt(e.getMessage().substring(
+                ACTIVE_CONTROLLER_EXCEPTION_TEXT_PREFIX.length()));
+        } else {
+            return -1;
+        }
+    }
+
+    private void handleEventEnd(String name, long startProcessingTimeNs) {
+        long endProcessingTime = time.nanoseconds();
+        long deltaNs = endProcessingTime - startProcessingTimeNs;
+        log.debug("Processed {} in {} us", name,
+            TimeUnit.MICROSECONDS.convert(deltaNs, TimeUnit.NANOSECONDS));
+    }
+
+    private Throwable handleEventException(String name,
+                                           Optional<Long> 
startProcessingTimeNs,
+                                           Throwable exception) {
+        if (!startProcessingTimeNs.isPresent()) {
+            log.debug("unable to start processing {} because of {}.", name,
+                exception.getClass().getSimpleName());
+            if (exception instanceof ApiException) {
+                return exception;
+            } else {
+                return new UnknownServerException(exception);
+            }
+        }
+        long endProcessingTime = time.nanoseconds();
+        long deltaNs = endProcessingTime - startProcessingTimeNs.get();
+        long deltaUs = TimeUnit.MICROSECONDS.convert(deltaNs, 
TimeUnit.NANOSECONDS);
+        if (exception instanceof ApiException) {
+            log.debug("{}: failed with {} in {} us", name,
+                exception.getClass().getSimpleName(), deltaUs);
+            return exception;
+        }
+        log.warn("{}: failed with unknown server exception {} at epoch {} in 
{} us.  " +
+            "Reverting to last committed offset {}.",
+            this, exception.getClass().getSimpleName(), curClaimEpoch, deltaUs,
+            lastCommittedOffset, exception);
+        renounce();
+        return new UnknownServerException(exception);
+    }
+
+    /**
+     * A controller event for handling internal state changes, such as Raft 
inputs.
+     */
+    class ControlEvent implements EventQueue.Event {
+        private final String name;
+        private final Runnable handler;
+        private Optional<Long> startProcessingTimeNs = Optional.empty();
+
+        ControlEvent(String name, Runnable handler) {
+            this.name = name;
+            this.handler = handler;
+        }
+
+        @Override
+        public void run() throws Exception {
+            startProcessingTimeNs = Optional.of(time.nanoseconds());
+            log.debug("Executing {}.", this);
+            handler.run();
+            handleEventEnd(this.toString(), startProcessingTimeNs.get());
+        }
+
+        @Override
+        public void handleException(Throwable exception) {
+            handleEventException(name, startProcessingTimeNs, exception);
+        }
+
+        @Override
+        public String toString() {
+            return name;
+        }
+    }
+
+    private void appendControlEvent(String name, Runnable handler) {
+        ControlEvent event = new ControlEvent(name, handler);
+        queue.append(event);
+    }
+
+    /**
+     * A controller event that reads the committed internal state in order to 
expose it
+     * to an API.
+     */
+    class ControllerReadEvent<T> implements EventQueue.Event {
+        private final String name;
+        private final CompletableFuture<T> future;
+        private final Supplier<T> handler;
+        private Optional<Long> startProcessingTimeNs = Optional.empty();
+
+        ControllerReadEvent(String name, Supplier<T> handler) {
+            this.name = name;
+            this.future = new CompletableFuture<T>();
+            this.handler = handler;
+        }
+
+        CompletableFuture<T> future() {
+            return future;
+        }
+
+        @Override
+        public void run() throws Exception {
+            startProcessingTimeNs = Optional.of(time.nanoseconds());
+            T value = handler.get();
+            handleEventEnd(this.toString(), startProcessingTimeNs.get());
+            future.complete(value);
+        }
+
+        @Override
+        public void handleException(Throwable exception) {
+            future.completeExceptionally(
+                handleEventException(name, startProcessingTimeNs, exception));
+        }
+
+        @Override
+        public String toString() {
+            return name + "(" + System.identityHashCode(this) + ")";
+        }
+    }
+
+    // VisibleForTesting
+    ReplicationControlManager replicationControl() {
+        return replicationControl;
+    }
+
+    // VisibleForTesting
+    <T> CompletableFuture<T> appendReadEvent(String name, Supplier<T> handler) 
{
+        ControllerReadEvent<T> event = new ControllerReadEvent<T>(name, 
handler);
+        queue.append(event);
+        return event.future();
+    }
+
+    interface ControllerWriteOperation<T> {
+        /**
+         * Generate the metadata records needed to implement this controller 
write
+         * operation.  In general, this operation should not modify the "hard 
state" of
+         * the controller.  That modification will happen later on, when we 
replay the
+         * records generated by this function.
+         *
+         * There are cases where this function modifies the "soft state" of the
+         * controller.  Mainly, this happens when we process cluster 
heartbeats.
+         *
+         * This function also generates an RPC result.  In general, if the RPC 
resulted in
+         * an error, the RPC result will be an error, and the generated record 
list will
+         * be empty.  This would happen if we tried to create a topic with 
incorrect
+         * parameters, for example.  Of course, partial errors are possible 
for batch
+         * operations.
+         *
+         * @return              A tuple containing a list of records, and an 
RPC result.
+         */
+        ControllerResult<T> generateRecordsAndResult() throws Exception;
+
+        /**
+         * Once we've passed the records to the Raft layer, we will invoke 
this function
+         * with the end offset at which those records were placed.  If there 
were no
+         * records to write, we'll just pass the last write offset.
+         */
+        default void processBatchEndOffset(long offset) {}
+    }
+
+    /**
+     * A controller event that modifies the controller state.
+     */
+    class ControllerWriteEvent<T> implements EventQueue.Event, DeferredEvent {
+        private final String name;
+        private final CompletableFuture<T> future;
+        private final ControllerWriteOperation<T> op;
+        private Optional<Long> startProcessingTimeNs = Optional.empty();
+        private ControllerResultAndOffset<T> resultAndOffset;
+
+        ControllerWriteEvent(String name, ControllerWriteOperation<T> op) {
+            this.name = name;
+            this.future = new CompletableFuture<T>();
+            this.op = op;
+            this.resultAndOffset = null;
+        }
+
+        CompletableFuture<T> future() {
+            return future;
+        }
+
+        @Override
+        public void run() throws Exception {
+            long controllerEpoch = curClaimEpoch;
+            if (controllerEpoch == -1) {
+                throw newNotControllerException();
+            }
+            startProcessingTimeNs = Optional.of(time.nanoseconds());
+            ControllerResult<T> result = op.generateRecordsAndResult();
+            if (result.records().isEmpty()) {
+                op.processBatchEndOffset(writeOffset);
+                // If the operation did not return any records, then it was 
actually just
+                //a read after all, and not a read + write.  However, this 
read was done
+                //from the latest in-memory state, which might contain 
uncommitted data.
+                Optional<Long> maybeOffset = purgatory.highestPendingOffset();
+                if (!maybeOffset.isPresent()) {
+                    // If the purgatory is empty, there are no pending 
operations and no
+                    // uncommitted state.  We can return immediately.
+                    this.resultAndOffset = new ControllerResultAndOffset<>(-1,
+                        new ArrayList<>(), result.response());
+                    log.debug("Completing read-only operation {} immediately 
because " +
+                        "the purgatory is empty.", this);
+                    complete(null);
+                    return;
+                }
+                // If there are operations in the purgatory, we want to wait 
for the latest
+                // one to complete before returning our result to the user.
+                this.resultAndOffset = new 
ControllerResultAndOffset<>(maybeOffset.get(),
+                    result.records(), result.response());
+                log.debug("Read-only operation {} will be completed when the 
log " +
+                    "reaches offset {}", this, resultAndOffset.offset());
+            } else {
+                // If the operation returned a batch of records, those records 
need to be
+                // written before we can return our result to the user.  Here, 
we hand off
+                // the batch of records to the metadata log manager.  They 
will be written
+                // out asynchronously.
+                long offset = logManager.scheduleWrite(controllerEpoch, 
result.records());
+                op.processBatchEndOffset(offset);
+                writeOffset = offset;
+                this.resultAndOffset = new ControllerResultAndOffset<>(offset,
+                    result.records(), result.response());
+                for (ApiMessageAndVersion message : result.records()) {
+                    replay(message.message());
+                }
+                snapshotRegistry.createSnapshot(offset);
+                log.debug("Read-write operation {} will be completed when the 
log " +
+                    "reaches offset {}.", this, resultAndOffset.offset());
+            }
+            purgatory.add(resultAndOffset.offset(), this);
+        }
+
+        @Override
+        public void handleException(Throwable exception) {
+            complete(exception);
+        }
+
+        @Override
+        public void complete(Throwable exception) {
+            if (exception == null) {
+                handleEventEnd(this.toString(), startProcessingTimeNs.get());
+                future.complete(resultAndOffset.response());
+            } else {
+                future.completeExceptionally(
+                    handleEventException(name, startProcessingTimeNs, 
exception));
+            }
+        }
+
+        @Override
+        public String toString() {
+            return name + "(" + System.identityHashCode(this) + ")";
+        }
+    }
+
+    private <T> CompletableFuture<T> appendWriteEvent(String name,
+                                                      long timeoutMs,
+                                                      
ControllerWriteOperation<T> op) {
+        ControllerWriteEvent<T> event = new ControllerWriteEvent<>(name, op);
+        queue.appendWithDeadline(time.nanoseconds() +
+            TimeUnit.NANOSECONDS.convert(timeoutMs, TimeUnit.MILLISECONDS), 
event);
+        return event.future();
+    }
+
+    private <T> CompletableFuture<T> appendWriteEvent(String name,
+                                                      
ControllerWriteOperation<T> op) {
+        ControllerWriteEvent<T> event = new ControllerWriteEvent<>(name, op);
+        queue.append(event);
+        return event.future();
+    }
+
+    class QuorumMetaLogListener implements MetaLogListener {
+        @Override
+        public void handleCommits(long offset, List<ApiMessage> messages) {
+            appendControlEvent("handleCommits[" + offset + "]", () -> {
+                if (curClaimEpoch == -1) {
+                    if (log.isDebugEnabled()) {
+                        if (log.isTraceEnabled()) {
+                            log.trace("Replaying commits from the active node 
up to " +
+                                "offset {}: {}.", offset, messages.stream().
+                                map(m -> 
m.toString()).collect(Collectors.joining(", ")));
+                        } else {
+                            log.debug("Replaying commits from the active node 
up to " +
+                                "offset {}.", offset);
+                        }
+                    }
+                    for (ApiMessage message : messages) {
+                        replay(message);
+                    }
+                } else {
+                    log.debug("Completing purgatory items up to offset {}.", 
offset);
+
+                    // Complete any events in the purgatory that were waiting 
for this offset.
+                    purgatory.completeUpTo(offset);
+
+                    // Delete all snapshots older than the offset.
+                    // TODO: add an exception here for when we're writing out 
a log snapshot
+                    snapshotRegistry.deleteSnapshotsUpTo(offset);
+                }
+                lastCommittedOffset = offset;
+            });
+        }
+
+        @Override
+        public void handleNewLeader(MetaLogLeader newLeader) {
+            if (newLeader.nodeId() == nodeId) {
+                final long newEpoch = newLeader.epoch();
+                appendControlEvent("handleClaim[" + newEpoch + "]", () -> {
+                    long curEpoch = curClaimEpoch;
+                    if (curEpoch != -1) {
+                        throw new RuntimeException("Tried to claim controller 
epoch " +
+                            newEpoch + ", but we never renounced controller 
epoch " +
+                            curEpoch);
+                    }
+                    log.info("Becoming active at controller epoch {}.", 
newEpoch);
+                    curClaimEpoch = newEpoch;
+                    writeOffset = lastCommittedOffset;
+                    clusterControl.activate();
+                });
+            }
+        }
+
+        @Override
+        public void handleRenounce(long oldEpoch) {
+            appendControlEvent("handleRenounce[" + oldEpoch + "]", () -> {
+                if (curClaimEpoch == oldEpoch) {
+                    log.info("Renouncing the leadership at oldEpoch {} due to 
a metadata " +
+                            "log event. Reverting to last committed offset 
{}.", curClaimEpoch,
+                        lastCommittedOffset);
+                    renounce();
+                }
+            });
+        }
+
+        @Override
+        public void beginShutdown() {
+            queue.beginShutdown("MetaLogManager.Listener");
+        }
+    }
+
+    private void renounce() {
+        curClaimEpoch = -1;
+        purgatory.failAll(newNotControllerException());
+        snapshotRegistry.revertToSnapshot(lastCommittedOffset);
+        snapshotRegistry.deleteSnapshotsUpTo(lastCommittedOffset);
+        writeOffset = -1;
+        clusterControl.deactivate();
+        cancelMaybeFenceReplicas();
+    }
+
+    private <T> void scheduleDeferredWriteEvent(String name, long deadlineNs,
+                                                ControllerWriteOperation<T> 
op) {
+        ControllerWriteEvent<T> event = new ControllerWriteEvent<>(name, op);
+        queue.scheduleDeferred(name, new EarliestDeadlineFunction(deadlineNs), 
event);
+        event.future.exceptionally(e -> {
+            if (e instanceof UnknownServerException && e.getCause() != null &&
+                    e.getCause() instanceof RejectedExecutionException) {
+                log.error("Cancelling write event {} because the event queue 
is closed.", name);
+                return null;
+            }
+            log.error("Unexpected exception while executing deferred write 
event {}. " +
+                "Rescheduling for a minute from now.", name, e);
+            scheduleDeferredWriteEvent(name,
+                deadlineNs + TimeUnit.NANOSECONDS.convert(1, 
TimeUnit.MINUTES), op);
+            return null;
+        });
+    }
+
+    static final String MAYBE_FENCE_REPLICAS = "maybeFenceReplicas";
+
+    class MaybeFenceReplicas implements ControllerWriteOperation<Void> {
+        @Override
+        public ControllerResult<Void> generateRecordsAndResult() {
+            ControllerResult<Set<Integer>> result =
+                clusterControl.maybeFenceLeastRecentlyContacted();
+            for (int brokerId : result.response()) {
+                replicationControl.removeFromIsr(brokerId, result.records());
+            }
+            rescheduleMaybeFenceReplicas();
+            return new ControllerResult<>(result.records(), null);
+        }
+    }
+
+    private void rescheduleMaybeFenceReplicas() {
+        long nextCheckTimeNs = clusterControl.nextCheckTimeNs();
+        if (nextCheckTimeNs == Long.MAX_VALUE) {
+            cancelMaybeFenceReplicas();
+        } else {
+            scheduleDeferredWriteEvent(MAYBE_FENCE_REPLICAS, nextCheckTimeNs,
+                new MaybeFenceReplicas());
+        }
+    }
+
+    private void cancelMaybeFenceReplicas() {
+        queue.cancelDeferred(MAYBE_FENCE_REPLICAS);
+    }
+
+    @SuppressWarnings("unchecked")
+    private void replay(ApiMessage message) {
+        try {
+            MetadataRecordType type = 
MetadataRecordType.fromId(message.apiKey());
+            switch (type) {
+                case REGISTER_BROKER_RECORD:
+                    clusterControl.replay((RegisterBrokerRecord) message);
+                    break;
+                case UNREGISTER_BROKER_RECORD:
+                    clusterControl.replay((UnregisterBrokerRecord) message);
+                    break;
+                case FENCE_BROKER_RECORD:
+                    clusterControl.replay((FenceBrokerRecord) message);
+                    break;
+                case UNFENCE_BROKER_RECORD:
+                    clusterControl.replay((UnfenceBrokerRecord) message);
+                    break;
+                case TOPIC_RECORD:
+                    replicationControl.replay((TopicRecord) message);
+                    break;
+                case PARTITION_RECORD:
+                    replicationControl.replay((PartitionRecord) message);
+                    break;
+                case CONFIG_RECORD:
+                    configurationControl.replay((ConfigRecord) message);
+                    break;
+                case QUOTA_RECORD:
+                    clientQuotaControlManager.replay((QuotaRecord) message);
+                    break;
+                case ISR_CHANGE_RECORD:
+                    replicationControl.replay((IsrChangeRecord) message);
+                    break;
+                default:
+                    throw new RuntimeException("Unhandled record type " + 
type);
+            }
+        } catch (Exception e) {
+            log.error("Error replaying record {}", message.toString(), e);
+        }
+    }
+
+    private final Logger log;
+
+    /**
+     * The ID of this controller node.
+     */
+    private final int nodeId;
+
+    /**
+     * The single-threaded queue that processes all of our events.
+     * It also processes timeouts.
+     */
+    private final KafkaEventQueue queue;
+
+    /**
+     * The Kafka clock object to use.
+     */
+    private final Time time;
+
+    /**
+     * A registry for snapshot data.  This must be accessed only by the event 
queue thread.
+     */
+    private final SnapshotRegistry snapshotRegistry;
+
+    /**
+     * The purgatory which holds deferred operations which are waiting for the 
metadata
+     * log's high water mark to advance.  This must be accessed only by the 
event queue thread.
+     */
+    private final ControllerPurgatory purgatory;
+
+    /**
+     * An object which stores the controller's dynamic configuration.
+     * This must be accessed only by the event queue thread.
+     */
+    private final ConfigurationControlManager configurationControl;
+
+    /**
+     * An object which stores the controller's dynamic client quotas.
+     * This must be accessed only by the event queue thread.
+     */
+    private final ClientQuotaControlManager clientQuotaControlManager;
+
+    /**
+     * An object which stores the controller's view of the cluster.
+     * This must be accessed only by the event queue thread.
+     */
+    private final ClusterControlManager clusterControl;
+
+    /**
+     * An object which stores the controller's view of the cluster features.
+     * This must be accessed only by the event queue thread.
+     */
+    private final FeatureControlManager featureControl;
+
+    /**
+     * An object which stores the controller's view of topics and partitions.
+     * This must be accessed only by the event queue thread.
+     */
+    private final ReplicationControlManager replicationControl;
+
+    /**
+     * The interface that we use to mutate the Raft log.
+     */
+    private final MetaLogManager logManager;
+
+    /**
+     * The interface that receives callbacks from the Raft log.  These 
callbacks are
+     * invoked from the Raft thread(s), not from the controller thread.
+     */
+    private final QuorumMetaLogListener metaLogListener;
+
+    /**
+     * If this controller is active, this is the non-negative controller epoch.
+     * Otherwise, this is -1.  This variable must be modified only from the 
controller
+     * thread, but it can be read from other threads.
+     */
+    private volatile long curClaimEpoch;

Review comment:
       I thought the issue was that if changing leaderEpoch to Long requires a 
log format change for user data, it has significant performance impact such as 
down conversion.




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