cmccabe commented on a change in pull request #10070:
URL: https://github.com/apache/kafka/pull/10070#discussion_r573238990



##########
File path: 
metadata/src/main/java/org/apache/kafka/controller/BrokerHeartbeatManager.java
##########
@@ -0,0 +1,491 @@
+/*
+ * 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.common.errors.InvalidReplicationFactorException;
+import org.apache.kafka.common.utils.LogContext;
+import org.apache.kafka.common.utils.Time;
+import org.apache.kafka.metadata.UsableBroker;
+import org.slf4j.Logger;
+
+import java.util.Comparator;
+import java.util.HashMap;
+import java.util.Iterator;
+import java.util.List;
+import java.util.NoSuchElementException;
+import java.util.Optional;
+import java.util.TreeSet;
+import java.util.function.Function;
+
+
+/**
+ * The BrokerHeartbeatManager manages all the soft state associated with 
broker heartbeats.
+ * Soft state is state which does not appear in the metadata log.  This state 
includes
+ * things like the last time each broker sent us a heartbeat, and whether the 
broker is
+ * trying to perform a controlled shutdown.
+ *
+ * Only the active controller has a BrokerHeartbeatManager, since only the 
active
+ * controller handles broker heartbeats.  Standby controllers will create a 
heartbeat
+ * manager as part of the process of activating.  This design minimizes the 
size of the
+ * metadata partition by excluding heartbeats from it.  However, it does mean 
that after
+ * a controller failover, we may take some extra time to fence brokers, since 
the new
+ * active controller does not know when the last heartbeats were received from 
each.
+ */
+public class BrokerHeartbeatManager {
+    static class BrokerHeartbeatState {
+        /**
+         * The broker ID.
+         */
+        private final int id;
+
+        /**
+         * The last time we received a heartbeat from this broker, in 
monotonic nanoseconds.
+         * When this field is updated, we also may have to update the broker's 
position in
+         * the unfenced list.
+         */
+        long lastContactNs;
+
+        /**
+         * The last metadata offset which this broker reported.  When this 
field is updated,
+         * we may also have to update the broker's position in the active set.
+         */
+        long metadataOffset;
+
+        /**
+         * The offset at which the broker should complete its controlled 
shutdown, or -1
+         * if the broker is not performing a controlled shutdown.  When this 
field is
+         * updated, we also have to update the broker's position in the 
shuttingDown set.
+         */
+        private long shutdownOffset;
+
+        /**
+         * The previous entry in the unfenced list, or null if the broker is 
not in that list.
+         */
+        private BrokerHeartbeatState prev;
+
+        /**
+         * The next entry in the unfenced list, or null if the broker is not 
in that list.
+         */
+        private BrokerHeartbeatState next;
+
+        BrokerHeartbeatState(int id) {
+            this.id = id;
+            this.lastContactNs = 0;
+            this.prev = null;
+            this.next = null;
+            this.metadataOffset = -1;
+            this.shutdownOffset = -1;
+        }
+
+        /**
+         * Returns the broker ID.
+         */
+        int id() {
+            return id;
+        }
+
+        /**
+         * Returns true only if the broker is fenced.
+         */
+        boolean fenced() {
+            return prev == null;
+        }
+
+        /**
+         * Returns true only if the broker is in controlled shutdown state.
+         */
+        boolean shuttingDown() {
+            return shutdownOffset >= 0;
+        }
+    }
+
+    static class MetadataOffsetComparator implements 
Comparator<BrokerHeartbeatState> {
+        static final MetadataOffsetComparator INSTANCE = new 
MetadataOffsetComparator();
+
+        @Override
+        public int compare(BrokerHeartbeatState a, BrokerHeartbeatState b) {
+            if (a.metadataOffset < b.metadataOffset) {
+                return -1;
+            } else if (a.metadataOffset > b.metadataOffset) {
+                return 1;
+            } else if (a.id < b.id) {
+                return -1;
+            } else if (a.id > b.id) {
+                return 1;
+            } else {
+                return 0;
+            }
+        }
+    }
+
+    static class BrokerHeartbeatStateList {
+        /**
+         * The head of the list of unfenced brokers.  The list is sorted in 
ascending order
+         * of last contact time.
+         */
+        private final BrokerHeartbeatState head;
+
+        BrokerHeartbeatStateList() {
+            this.head = new BrokerHeartbeatState(-1);
+            head.prev = head;
+            head.next = head;
+        }
+
+        /**
+         * Return the head of the list, or null if the list is empty.
+         */
+        BrokerHeartbeatState first() {
+            BrokerHeartbeatState result = head.next;
+            return result == head ? null : result;
+        }
+
+        /**
+         * Add the broker to the list. We start looking for a place to put it 
at the end
+         * of the list.
+         */
+        void add(BrokerHeartbeatState broker) {
+            BrokerHeartbeatState cur = head.prev;
+            while (true) {
+                if (cur == head || cur.lastContactNs <= broker.lastContactNs) {
+                    broker.next = cur.next;
+                    cur.next.prev = broker;
+                    broker.prev = cur;
+                    cur.next = broker;
+                    break;
+                }
+                cur = cur.prev;
+            }
+        }
+
+        /**
+         * Remove a broker from the list.
+         */
+        void remove(BrokerHeartbeatState broker) {
+            if (broker.next == null) {
+                throw new RuntimeException(broker + " is not in the  list.");
+            }
+            broker.prev.next = broker.next;
+            broker.next.prev = broker.prev;
+            broker.prev = null;
+            broker.next = null;
+        }
+
+        BrokerHeartbeatStateIterator iterator() {
+            return new BrokerHeartbeatStateIterator(head);
+        }
+    }
+
+    static class BrokerHeartbeatStateIterator implements 
Iterator<BrokerHeartbeatState> {
+        private final BrokerHeartbeatState head;
+        private BrokerHeartbeatState cur;
+
+        BrokerHeartbeatStateIterator(BrokerHeartbeatState head) {
+            this.head = head;
+            this.cur = head;
+        }
+
+        @Override
+        public boolean hasNext() {
+            return cur.next != head;
+        }
+
+        @Override
+        public BrokerHeartbeatState next() {
+            if (!hasNext()) {
+                throw new NoSuchElementException();
+            }
+            BrokerHeartbeatState result = cur.next;
+            cur = cur.next;
+            return result;
+        }
+    }
+
+    private final Logger log;
+
+    /**
+     * The Kafka clock object to use.
+     */
+    private final Time time;
+
+    /**
+     * The broker session timeout in nanoseconds.
+     */
+    private final long sessionTimeoutNs;
+
+    /**
+     * Maps broker IDs to heartbeat states.
+     */
+    private final HashMap<Integer, BrokerHeartbeatState> brokers;
+
+    /**
+     * The list of unfenced brokers, sorted by last contact time.
+     */
+    private final BrokerHeartbeatStateList unfenced;
+
+    /**
+     * The set of active brokers.  A broker is active if it is unfenced, and 
not shutting
+     * down.
+     */
+    private final TreeSet<BrokerHeartbeatState> active;
+
+    BrokerHeartbeatManager(LogContext logContext,
+                           Time time,
+                           long sessionTimeoutNs) {
+        this.log = logContext.logger(ClusterControlManager.class);
+        this.time = time;
+        this.sessionTimeoutNs = sessionTimeoutNs;
+        this.brokers = new HashMap<>();
+        this.unfenced = new BrokerHeartbeatStateList();
+        this.active = new TreeSet<>(MetadataOffsetComparator.INSTANCE);
+    }
+
+    // VisibleForTesting
+    Time time() {
+        return time;
+    }
+
+    // VisibleForTesting
+    BrokerHeartbeatStateList unfenced() {
+        return unfenced;
+    }
+
+    /**
+     * Remove broker state.
+     *
+     * @param brokerId      The broker ID to remove.
+     */
+    void remove(int brokerId) {
+        BrokerHeartbeatState broker = brokers.remove(brokerId);
+        if (broker != null) {
+            if (!broker.fenced()) {
+                unfenced.remove(broker);
+            }
+            if (!broker.shuttingDown()) {
+                active.remove(broker);
+            }
+        }
+    }
+
+    /**
+     * Check if the given broker has a valid session.
+     *
+     * @param brokerId      The broker ID to check.
+     *
+     * @return              True if the given broker has a valid session.
+     */
+    boolean hasValidSession(int brokerId) {
+        BrokerHeartbeatState broker = brokers.get(brokerId);
+        if (broker == null) return false;
+        return hasValidSession(broker);
+    }
+
+    /**
+     * Check if the given broker has a valid session.
+     *
+     * @param broker        The broker to check.
+     *
+     * @return              True if the given broker has a valid session.
+     */
+    private boolean hasValidSession(BrokerHeartbeatState broker) {
+        if (broker.fenced()) {
+            return false;
+        } else {
+            return broker.lastContactNs + sessionTimeoutNs >= 
time.nanoseconds();
+        }
+    }
+
+    /**
+     * Update broker state, including lastContactNs.
+     *
+     * @param brokerId          The broker ID.
+     * @param fenced            True only if the broker is currently fenced.
+     * @param metadataOffset    The latest metadata offset of the broker.
+     */
+    void touch(int brokerId, boolean fenced, long metadataOffset) {
+        BrokerHeartbeatState broker = brokers.get(brokerId);
+        if (broker == null) {
+            broker = new BrokerHeartbeatState(brokerId);
+            brokers.put(brokerId, broker);
+        } else {
+            if (!broker.fenced()) {
+                unfenced.remove(broker);
+                if (!broker.shuttingDown()) {
+                    active.remove(broker);
+                }
+            }
+        }
+        broker.lastContactNs = time.nanoseconds();
+        broker.metadataOffset = metadataOffset;
+        if (fenced) {
+            broker.shutdownOffset = -1;

Review comment:
       Good question.  A fenced broker will not have leaders, so there should 
be no leaders to move away.  More specifically, if any fenced broker tries to 
enter controlled shutdown, it will be shut down immediately.  I'll add a 
comment.




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