/*
 * 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.hadoop.hbase.master;

import static org.apache.hadoop.hbase.util.ConcurrentMapUtils.computeIfAbsent;

import java.io.IOException;
import java.net.InetAddress;
import java.util.ArrayList;
import java.util.Collections;
import java.util.HashSet;
import java.util.List;
import java.util.Map;
import java.util.Map.Entry;
import java.util.Objects;
import java.util.Set;
import java.util.concurrent.ConcurrentNavigableMap;
import java.util.concurrent.ConcurrentSkipListMap;
import java.util.concurrent.CopyOnWriteArrayList;
import java.util.concurrent.atomic.AtomicBoolean;
import java.util.function.Predicate;
import org.apache.hadoop.conf.Configuration;
import org.apache.hadoop.hbase.ClockOutOfSyncException;
import org.apache.hadoop.hbase.HConstants;
import org.apache.hadoop.hbase.NotServingRegionException;
import org.apache.hadoop.hbase.RegionMetrics;
import org.apache.hadoop.hbase.ServerMetrics;
import org.apache.hadoop.hbase.ServerMetricsBuilder;
import org.apache.hadoop.hbase.ServerName;
import org.apache.hadoop.hbase.YouAreDeadException;
import org.apache.hadoop.hbase.client.ClusterConnection;
import org.apache.hadoop.hbase.client.RegionInfo;
import org.apache.hadoop.hbase.conf.ConfigurationObserver;
import org.apache.hadoop.hbase.ipc.DecommissionedHostRejectedException;
import org.apache.hadoop.hbase.ipc.HBaseRpcController;
import org.apache.hadoop.hbase.ipc.RemoteWithExtrasException;
import org.apache.hadoop.hbase.ipc.RpcControllerFactory;
import org.apache.hadoop.hbase.monitoring.MonitoredTask;
import org.apache.hadoop.hbase.procedure2.Procedure;
import org.apache.hadoop.hbase.regionserver.HRegionServer;
import org.apache.hadoop.hbase.util.Bytes;
import org.apache.hadoop.hbase.util.EnvironmentEdgeManager;
import org.apache.hadoop.hbase.zookeeper.ZKUtil;
import org.apache.hadoop.hbase.zookeeper.ZKWatcher;
import org.apache.hadoop.hbase.zookeeper.ZNodePaths;
import org.apache.yetus.audience.InterfaceAudience;
import org.apache.zookeeper.KeeperException;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

import org.apache.hbase.thirdparty.com.google.protobuf.UnsafeByteOperations;

import org.apache.hadoop.hbase.shaded.protobuf.ProtobufUtil;
import org.apache.hadoop.hbase.shaded.protobuf.generated.AdminProtos.AdminService;
import org.apache.hadoop.hbase.shaded.protobuf.generated.ClusterStatusProtos.RegionStoreSequenceIds;
import org.apache.hadoop.hbase.shaded.protobuf.generated.ClusterStatusProtos.StoreSequenceId;
import org.apache.hadoop.hbase.shaded.protobuf.generated.RegionServerStatusProtos.RegionServerStartupRequest;

/**
 * The ServerManager class manages info about region servers.
 * <p>
 * Maintains lists of online and dead servers. Processes the startups, shutdowns, and deaths of
 * region servers.
 * <p>
 * Servers are distinguished in two different ways. A given server has a location, specified by
 * hostname and port, and of which there can only be one online at any given time. A server instance
 * is specified by the location (hostname and port) as well as the startcode (timestamp from when
 * the server was started). This is used to differentiate a restarted instance of a given server
 * from the original instance.
 * <p>
 * If a sever is known not to be running any more, it is called dead. The dead server needs to be
 * handled by a ServerShutdownHandler. If the handler is not enabled yet, the server can't be
 * handled right away so it is queued up. After the handler is enabled, the server will be submitted
 * to a handler to handle. However, the handler may be just partially enabled. If so, the server
 * cannot be fully processed, and be queued up for further processing. A server is fully processed
 * only after the handler is fully enabled and has completed the handling.
 */
@InterfaceAudience.Private
public class ServerManager implements ConfigurationObserver {
  public static final String WAIT_ON_REGIONSERVERS_MAXTOSTART =
    "hbase.master.wait.on.regionservers.maxtostart";

  public static final String WAIT_ON_REGIONSERVERS_MINTOSTART =
    "hbase.master.wait.on.regionservers.mintostart";

  public static final String WAIT_ON_REGIONSERVERS_TIMEOUT =
    "hbase.master.wait.on.regionservers.timeout";

  public static final String WAIT_ON_REGIONSERVERS_INTERVAL =
    "hbase.master.wait.on.regionservers.interval";

  private static final Logger LOG = LoggerFactory.getLogger(ServerManager.class);

  // Set if we are to shutdown the cluster.
  private AtomicBoolean clusterShutdown = new AtomicBoolean(false);

  /**
   * The last flushed sequence id for a region.
   */
  private final ConcurrentNavigableMap<byte[], Long> flushedSequenceIdByRegion =
    new ConcurrentSkipListMap<>(Bytes.BYTES_COMPARATOR);

  /**
   * The last flushed sequence id for a store in a region.
   */
  private final ConcurrentNavigableMap<byte[],
    ConcurrentNavigableMap<byte[], Long>> storeFlushedSequenceIdsByRegion =
      new ConcurrentSkipListMap<>(Bytes.BYTES_COMPARATOR);

  /** Map of registered servers to their current load */
  private final ConcurrentNavigableMap<ServerName, ServerMetrics> onlineServers =
    new ConcurrentSkipListMap<>();

  /** List of region servers that should not get any more new regions. */
  private final ArrayList<ServerName> drainingServers = new ArrayList<>();

  private final MasterServices master;
  private final ClusterConnection connection;
  private final RegionServerList storage;

  private final DeadServer deadservers = new DeadServer();

  private final long maxSkew;
  private final long warningSkew;

  private final RpcControllerFactory rpcControllerFactory;

  /** Listeners that are called on server events. */
  private List<ServerListener> listeners = new CopyOnWriteArrayList<>();

  /** Configured value of HConstants.REJECT_DECOMMISSIONED_HOSTS_KEY */
  private volatile boolean rejectDecommissionedHostsConfig;

  /**
   * Constructor.
   */
  public ServerManager(final MasterServices master, RegionServerList storage) {
    this.master = master;
    this.storage = storage;
    Configuration c = master.getConfiguration();
    maxSkew = c.getLong("hbase.master.maxclockskew", 30000);
    warningSkew = c.getLong("hbase.master.warningclockskew", 10000);
    this.connection = master.getClusterConnection();
    this.rpcControllerFactory =
      this.connection == null ? null : connection.getRpcControllerFactory();
    rejectDecommissionedHostsConfig = getRejectDecommissionedHostsConfig(c);
  }

  /**
   * Implementation of the ConfigurationObserver interface. We are interested in live-loading the
   * configuration value of HConstants.REJECT_DECOMMISSIONED_HOSTS_KEY
   * @param conf Server configuration instance
   */
  @Override
  public void onConfigurationChange(Configuration conf) {
    final boolean newValue = getRejectDecommissionedHostsConfig(conf);
    if (rejectDecommissionedHostsConfig == newValue) {
      // no-op
      return;
    }

    LOG.info("Config Reload for RejectDecommissionedHosts. previous value: {}, new value: {}",
      rejectDecommissionedHostsConfig, newValue);

    rejectDecommissionedHostsConfig = newValue;
  }

  /**
   * Reads the value of HConstants.REJECT_DECOMMISSIONED_HOSTS_KEY from the config and returns it
   * @param conf Configuration instance of the Master
   */
  public boolean getRejectDecommissionedHostsConfig(Configuration conf) {
    return conf.getBoolean(HConstants.REJECT_DECOMMISSIONED_HOSTS_KEY,
      HConstants.REJECT_DECOMMISSIONED_HOSTS_DEFAULT);
  }

  /**
   * Add the listener to the notification list.
   * @param listener The ServerListener to register
   */
  public void registerListener(final ServerListener listener) {
    this.listeners.add(listener);
  }

  /**
   * Remove the listener from the notification list.
   * @param listener The ServerListener to unregister
   */
  public boolean unregisterListener(final ServerListener listener) {
    return this.listeners.remove(listener);
  }

  /**
   * Let the server manager know a new regionserver has come online
   * @param request       the startup request
   * @param versionNumber the version number of the new regionserver
   * @param version       the version of the new regionserver, could contain strings like "SNAPSHOT"
   * @param ia            the InetAddress from which request is received
   * @return The ServerName we know this server as.
   */
  ServerName regionServerStartup(RegionServerStartupRequest request, int versionNumber,
    String version, InetAddress ia) throws IOException {
    // Test for case where we get a region startup message from a regionserver
    // that has been quickly restarted but whose znode expiration handler has
    // not yet run, or from a server whose fail we are currently processing.
    // Test its host+port combo is present in serverAddressToServerInfo. If it
    // is, reject the server and trigger its expiration. The next time it comes
    // in, it should have been removed from serverAddressToServerInfo and queued
    // for processing by ProcessServerShutdown.

    // if use-ip is enabled, we will use ip to expose Master/RS service for client,
    // see HBASE-27304 for details.
    boolean useIp = master.getConfiguration().getBoolean(HConstants.HBASE_SERVER_USEIP_ENABLED_KEY,
      HConstants.HBASE_SERVER_USEIP_ENABLED_DEFAULT);
    String isaHostName = useIp ? ia.getHostAddress() : ia.getHostName();
    final String hostname =
      request.hasUseThisHostnameInstead() ? request.getUseThisHostnameInstead() : isaHostName;
    ServerName sn = ServerName.valueOf(hostname, request.getPort(), request.getServerStartCode());

    // Check if the host should be rejected based on it's decommissioned status
    checkRejectableDecommissionedStatus(sn);

    checkClockSkew(sn, request.getServerCurrentTime());
    checkIsDead(sn, "STARTUP");
    if (!checkAndRecordNewServer(sn, ServerMetricsBuilder.of(sn, versionNumber, version))) {
      LOG.warn("THIS SHOULD NOT HAPPEN, RegionServerStartup could not record the server: {}", sn);
    }
    storage.started(sn);
    return sn;
  }

  /**
   * Updates last flushed sequence Ids for the regions on server sn
   */
  private void updateLastFlushedSequenceIds(ServerName sn, ServerMetrics hsl) {
    for (Entry<byte[], RegionMetrics> entry : hsl.getRegionMetrics().entrySet()) {
      byte[] encodedRegionName = Bytes.toBytes(RegionInfo.encodeRegionName(entry.getKey()));
      Long existingValue = flushedSequenceIdByRegion.get(encodedRegionName);
      long l = entry.getValue().getCompletedSequenceId();
      // Don't let smaller sequence ids override greater sequence ids.
      if (LOG.isTraceEnabled()) {
        LOG.trace(Bytes.toString(encodedRegionName) + ", existingValue=" + existingValue
          + ", completeSequenceId=" + l);
      }
      if (existingValue == null || (l != HConstants.NO_SEQNUM && l > existingValue)) {
        flushedSequenceIdByRegion.put(encodedRegionName, l);
      } else if (l != HConstants.NO_SEQNUM && l < existingValue) {
        LOG.warn("RegionServer " + sn + " indicates a last flushed sequence id (" + l
          + ") that is less than the previous last flushed sequence id (" + existingValue
          + ") for region " + Bytes.toString(entry.getKey()) + " Ignoring.");
      }
      ConcurrentNavigableMap<byte[], Long> storeFlushedSequenceId =
        computeIfAbsent(storeFlushedSequenceIdsByRegion, encodedRegionName,
          () -> new ConcurrentSkipListMap<>(Bytes.BYTES_COMPARATOR));
      for (Entry<byte[], Long> storeSeqId : entry.getValue().getStoreSequenceId().entrySet()) {
        byte[] family = storeSeqId.getKey();
        existingValue = storeFlushedSequenceId.get(family);
        l = storeSeqId.getValue();
        if (LOG.isTraceEnabled()) {
          LOG.trace(Bytes.toString(encodedRegionName) + ", family=" + Bytes.toString(family)
            + ", existingValue=" + existingValue + ", completeSequenceId=" + l);
        }
        // Don't let smaller sequence ids override greater sequence ids.
        if (existingValue == null || (l != HConstants.NO_SEQNUM && l > existingValue.longValue())) {
          storeFlushedSequenceId.put(family, l);
        }
      }
    }
  }

  public void regionServerReport(ServerName sn, ServerMetrics sl) throws YouAreDeadException {
    checkIsDead(sn, "REPORT");
    if (null == this.onlineServers.replace(sn, sl)) {
      // Already have this host+port combo and its just different start code?
      // Just let the server in. Presume master joining a running cluster.
      // recordNewServer is what happens at the end of reportServerStartup.
      // The only thing we are skipping is passing back to the regionserver
      // the ServerName to use. Here we presume a master has already done
      // that so we'll press on with whatever it gave us for ServerName.
      if (!checkAndRecordNewServer(sn, sl)) {
        // Master already registered server with same (host + port) and higher startcode.
        // This can happen if regionserver report comes late from old server
        // (possible race condition), by that time master has already processed SCP for that
        // server and started accepting regionserver report from new server i.e. server with
        // same (host + port) and higher startcode.
        // The exception thrown here is not meant to tell the region server it is dead because if
        // there is a new server on the same host port, the old server should have already been
        // dead in ideal situation.
        // The exception thrown here is to skip the later steps of the whole regionServerReport
        // request processing. Usually, after recording it in ServerManager, we will call the
        // related methods in AssignmentManager to record region states. If the region server
        // is already dead, we should not do these steps anymore, so here we throw an exception
        // to let the upper layer know that they should not continue processing anymore.
        final String errorMsg = "RegionServerReport received from " + sn
          + ", but another server with the same name and higher startcode is already registered,"
          + " ignoring";
        LOG.warn(errorMsg);
        throw new YouAreDeadException(errorMsg);
      }
    }
    updateLastFlushedSequenceIds(sn, sl);
  }

  /**
   * Checks if the Master is configured to reject decommissioned hosts or not. When it's configured
   * to do so, any RegionServer trying to join the cluster will have it's host checked against the
   * list of hosts of currently decommissioned servers and potentially get prevented from reporting
   * for duty; otherwise, we do nothing and we let them pass to the next check. See HBASE-28342 for
   * details.
   * @param sn The ServerName to check for
   * @throws DecommissionedHostRejectedException if the Master is configured to reject
   *                                             decommissioned hosts and this host exists in the
   *                                             list of the decommissioned servers
   */
  private void checkRejectableDecommissionedStatus(ServerName sn)
    throws DecommissionedHostRejectedException {
    LOG.info("Checking decommissioned status of RegionServer {}", sn.getServerName());

    // If the Master is not configured to reject decommissioned hosts, return early.
    if (!rejectDecommissionedHostsConfig) {
      return;
    }

    // Look for a match for the hostname in the list of decommissioned servers
    for (ServerName server : getDrainingServersList()) {
      if (Objects.equals(server.getHostname(), sn.getHostname())) {
        // Found a match and master is configured to reject decommissioned hosts, throw exception!
        LOG.warn(
          "Rejecting RegionServer {} from reporting for duty because Master is configured "
            + "to reject decommissioned hosts and this host was marked as such in the past.",
          sn.getServerName());
        throw new DecommissionedHostRejectedException(String.format(
          "Host %s exists in the list of decommissioned servers and Master is configured to "
            + "reject decommissioned hosts",
          sn.getHostname()));
      }
    }
  }

  /**
   * Check is a server of same host and port already exists, if not, or the existed one got a
   * smaller start code, record it.
   * @param serverName the server to check and record
   * @param sl         the server load on the server
   * @return true if the server is recorded, otherwise, false
   */
  boolean checkAndRecordNewServer(final ServerName serverName, final ServerMetrics sl) {
    ServerName existingServer = null;
    synchronized (this.onlineServers) {
      existingServer = findServerWithSameHostnamePortWithLock(serverName);
      if (existingServer != null && (existingServer.getStartcode() > serverName.getStartcode())) {
        LOG.info("Server serverName=" + serverName + " rejected; we already have "
          + existingServer.toString() + " registered with same hostname and port");
        return false;
      }
      recordNewServerWithLock(serverName, sl);
    }

    // Tell our listeners that a server was added
    if (!this.listeners.isEmpty()) {
      for (ServerListener listener : this.listeners) {
        listener.serverAdded(serverName);
      }
    }

    // Note that we assume that same ts means same server, and don't expire in that case.
    // TODO: ts can theoretically collide due to clock shifts, so this is a bit hacky.
    if (existingServer != null && (existingServer.getStartcode() < serverName.getStartcode())) {
      LOG.info("Triggering server recovery; existingServer " + existingServer
        + " looks stale, new server:" + serverName);
      expireServer(existingServer);
    }
    return true;
  }

  /**
   * Find out the region servers crashed between the crash of the previous master instance and the
   * current master instance and schedule SCP for them.
   * <p/>
   * Since the {@code RegionServerTracker} has already helped us to construct the online servers set
   * by scanning zookeeper, now we can compare the online servers with {@code liveServersFromWALDir}
   * to find out whether there are servers which are already dead.
   * <p/>
   * Must be called inside the initialization method of {@code RegionServerTracker} to avoid
   * concurrency issue.
   * @param deadServersFromPE     the region servers which already have a SCP associated.
   * @param liveServersFromWALDir the live region servers from wal directory.
   */
  void findDeadServersAndProcess(Set<ServerName> deadServersFromPE,
    Set<ServerName> liveServersFromWALDir) {
    deadServersFromPE.forEach(deadservers::putIfAbsent);
    liveServersFromWALDir.stream().filter(sn -> !onlineServers.containsKey(sn))
      .forEach(this::expireServer);
  }

  /**
   * Checks if the clock skew between the server and the master. If the clock skew exceeds the
   * configured max, it will throw an exception; if it exceeds the configured warning threshold, it
   * will log a warning but start normally.
   * @param serverName Incoming servers's name
   * @throws ClockOutOfSyncException if the skew exceeds the configured max value
   */
  private void checkClockSkew(final ServerName serverName, final long serverCurrentTime)
    throws ClockOutOfSyncException {
    long skew = Math.abs(EnvironmentEdgeManager.currentTime() - serverCurrentTime);
    if (skew > maxSkew) {
      String message = "Server " + serverName + " has been "
        + "rejected; Reported time is too far out of sync with master.  " + "Time difference of "
        + skew + "ms > max allowed of " + maxSkew + "ms";
      LOG.warn(message);
      throw new ClockOutOfSyncException(message);
    } else if (skew > warningSkew) {
      String message = "Reported time for server " + serverName + " is out of sync with master "
        + "by " + skew + "ms. (Warning threshold is " + warningSkew + "ms; " + "error threshold is "
        + maxSkew + "ms)";
      LOG.warn(message);
    }
  }

  /**
   * Called when RegionServer first reports in for duty and thereafter each time it heartbeats to
   * make sure it is has not been figured for dead. If this server is on the dead list, reject it
   * with a YouAreDeadException. If it was dead but came back with a new start code, remove the old
   * entry from the dead list.
   * @param what START or REPORT
   */
  private void checkIsDead(final ServerName serverName, final String what)
    throws YouAreDeadException {
    if (this.deadservers.isDeadServer(serverName)) {
      // Exact match: host name, port and start code all match with existing one of the
      // dead servers. So, this server must be dead. Tell it to kill itself.
      String message =
        "Server " + what + " rejected; currently processing " + serverName + " as dead server";
      LOG.debug(message);
      throw new YouAreDeadException(message);
    }
    // Remove dead server with same hostname and port of newly checking in rs after master
    // initialization. See HBASE-5916 for more information.
    if (
      (this.master == null || this.master.isInitialized())
        && this.deadservers.cleanPreviousInstance(serverName)
    ) {
      // This server has now become alive after we marked it as dead.
      // We removed it's previous entry from the dead list to reflect it.
      LOG.debug("{} {} came back up, removed it from the dead servers list", what, serverName);
    }
  }

  /**
   * Assumes onlineServers is locked.
   * @return ServerName with matching hostname and port.
   */
  public ServerName findServerWithSameHostnamePortWithLock(final ServerName serverName) {
    ServerName end =
      ServerName.valueOf(serverName.getHostname(), serverName.getPort(), Long.MAX_VALUE);

    ServerName r = onlineServers.lowerKey(end);
    if (r != null) {
      if (ServerName.isSameAddress(r, serverName)) {
        return r;
      }
    }
    return null;
  }

  /**
   * Adds the onlineServers list. onlineServers should be locked.
   * @param serverName The remote servers name.
   */
  void recordNewServerWithLock(final ServerName serverName, final ServerMetrics sl) {
    LOG.info("Registering regionserver=" + serverName);
    this.onlineServers.put(serverName, sl);
    master.getAssignmentManager().getRegionStates().createServer(serverName);
  }

  public RegionStoreSequenceIds getLastFlushedSequenceId(byte[] encodedRegionName) {
    RegionStoreSequenceIds.Builder builder = RegionStoreSequenceIds.newBuilder();
    Long seqId = flushedSequenceIdByRegion.get(encodedRegionName);
    builder.setLastFlushedSequenceId(seqId != null ? seqId.longValue() : HConstants.NO_SEQNUM);
    Map<byte[], Long> storeFlushedSequenceId =
      storeFlushedSequenceIdsByRegion.get(encodedRegionName);
    if (storeFlushedSequenceId != null) {
      for (Map.Entry<byte[], Long> entry : storeFlushedSequenceId.entrySet()) {
        builder.addStoreSequenceId(StoreSequenceId.newBuilder()
          .setFamilyName(UnsafeByteOperations.unsafeWrap(entry.getKey()))
          .setSequenceId(entry.getValue().longValue()).build());
      }
    }
    return builder.build();
  }

  /** Returns ServerMetrics if serverName is known else null */
  public ServerMetrics getLoad(final ServerName serverName) {
    return this.onlineServers.get(serverName);
  }

  /**
   * Compute the average load across all region servers. Currently, this uses a very naive
   * computation - just uses the number of regions being served, ignoring stats about number of
   * requests.
   * @return the average load
   */
  public double getAverageLoad() {
    int totalLoad = 0;
    int numServers = 0;
    for (ServerMetrics sl : this.onlineServers.values()) {
      numServers++;
      totalLoad += sl.getRegionMetrics().size();
    }
    return numServers == 0 ? 0 : (double) totalLoad / (double) numServers;
  }

  /** Returns the count of active regionservers */
  public int countOfRegionServers() {
    // Presumes onlineServers is a concurrent map
    return this.onlineServers.size();
  }

  /** Returns Read-only map of servers to serverinfo */
  public Map<ServerName, ServerMetrics> getOnlineServers() {
    // Presumption is that iterating the returned Map is OK.
    synchronized (this.onlineServers) {
      return Collections.unmodifiableMap(this.onlineServers);
    }
  }

  public DeadServer getDeadServers() {
    return this.deadservers;
  }

  /**
   * Checks if any dead servers are currently in progress.
   * @return true if any RS are being processed as dead, false if not
   */
  public boolean areDeadServersInProgress() {
    return this.deadservers.areDeadServersInProgress();
  }

  void letRegionServersShutdown() {
    long previousLogTime = 0;
    ServerName sn = master.getServerName();
    ZKWatcher zkw = master.getZooKeeper();
    int onlineServersCt;
    while ((onlineServersCt = onlineServers.size()) > 0) {
      if (EnvironmentEdgeManager.currentTime() > (previousLogTime + 1000)) {
        Set<ServerName> remainingServers = onlineServers.keySet();
        synchronized (onlineServers) {
          if (remainingServers.size() == 1 && remainingServers.contains(sn)) {
            // Master will delete itself later.
            return;
          }
        }
        StringBuilder sb = new StringBuilder();
        // It's ok here to not sync on onlineServers - merely logging
        for (ServerName key : remainingServers) {
          if (sb.length() > 0) {
            sb.append(", ");
          }
          sb.append(key);
        }
        LOG.info("Waiting on regionserver(s) " + sb.toString());
        previousLogTime = EnvironmentEdgeManager.currentTime();
      }

      try {
        List<String> servers = getRegionServersInZK(zkw);
        if (
          servers == null || servers.isEmpty()
            || (servers.size() == 1 && servers.contains(sn.toString()))
        ) {
          LOG.info("ZK shows there is only the master self online, exiting now");
          // Master could have lost some ZK events, no need to wait more.
          break;
        }
      } catch (KeeperException ke) {
        LOG.warn("Failed to list regionservers", ke);
        // ZK is malfunctioning, don't hang here
        break;
      }
      synchronized (onlineServers) {
        try {
          if (onlineServersCt == onlineServers.size()) onlineServers.wait(100);
        } catch (InterruptedException ignored) {
          // continue
        }
      }
    }
  }

  private List<String> getRegionServersInZK(final ZKWatcher zkw) throws KeeperException {
    return ZKUtil.listChildrenNoWatch(zkw, zkw.getZNodePaths().rsZNode);
  }

  /**
   * Expire the passed server. Add it to list of dead servers and queue a shutdown processing.
   * @return pid if we queued a ServerCrashProcedure else {@link Procedure#NO_PROC_ID} if we did not
   *         (could happen for many reasons including the fact that its this server that is going
   *         down or we already have queued an SCP for this server or SCP processing is currently
   *         disabled because we are in startup phase).
   */
  // Redo test so we can make this protected.
  public synchronized long expireServer(final ServerName serverName) {
    return expireServer(serverName, false);

  }

  synchronized long expireServer(final ServerName serverName, boolean force) {
    // THIS server is going down... can't handle our own expiration.
    if (serverName.equals(master.getServerName())) {
      if (!(master.isAborted() || master.isStopped())) {
        master.stop("We lost our znode?");
      }
      return Procedure.NO_PROC_ID;
    }
    if (this.deadservers.isDeadServer(serverName)) {
      LOG.warn("Expiration called on {} but already in DeadServer", serverName);
      return Procedure.NO_PROC_ID;
    }
    moveFromOnlineToDeadServers(serverName);

    // If server is in draining mode, remove corresponding znode
    // In some tests, the mocked HM may not have ZK Instance, hence null check
    if (master.getZooKeeper() != null) {
      String drainingZnode = ZNodePaths
        .joinZNode(master.getZooKeeper().getZNodePaths().drainingZNode, serverName.getServerName());
      try {
        ZKUtil.deleteNodeFailSilent(master.getZooKeeper(), drainingZnode);
      } catch (KeeperException e) {
        LOG.warn(
          "Error deleting the draining znode for stopping server " + serverName.getServerName(), e);
      }
    }

    // If cluster is going down, yes, servers are going to be expiring; don't
    // process as a dead server
    if (isClusterShutdown()) {
      LOG.info("Cluster shutdown set; " + serverName + " expired; onlineServers="
        + this.onlineServers.size());
      if (this.onlineServers.isEmpty()) {
        master.stop("Cluster shutdown set; onlineServer=0");
      }
      return Procedure.NO_PROC_ID;
    }
    LOG.info("Processing expiration of " + serverName + " on " + this.master.getServerName());
    long pid = master.getAssignmentManager().submitServerCrash(serverName, true, force);
    if (pid == Procedure.NO_PROC_ID) {
      // skip later processing as we failed to submit SCP
      return Procedure.NO_PROC_ID;
    }
    storage.expired(serverName);
    // Tell our listeners that a server was removed
    if (!this.listeners.isEmpty()) {
      this.listeners.stream().forEach(l -> l.serverRemoved(serverName));
    }
    return pid;
  }

  /**
   * Called when server has expired.
   */
  // Locking in this class needs cleanup.
  public synchronized void moveFromOnlineToDeadServers(final ServerName sn) {
    synchronized (this.onlineServers) {
      boolean online = this.onlineServers.containsKey(sn);
      if (online) {
        // Remove the server from the known servers lists and update load info BUT
        // add to deadservers first; do this so it'll show in dead servers list if
        // not in online servers list.
        this.deadservers.putIfAbsent(sn);
        this.onlineServers.remove(sn);
        onlineServers.notifyAll();
      } else {
        // If not online, that is odd but may happen if 'Unknown Servers' -- where meta
        // has references to servers not online nor in dead servers list. If
        // 'Unknown Server', don't add to DeadServers else will be there for ever.
        LOG.trace("Expiration of {} but server not online", sn);
      }
    }
  }

  /*
   * Remove the server from the drain list.
   */
  public synchronized boolean removeServerFromDrainList(final ServerName sn) {
    LOG.info("Removing server {} from the draining list.", sn);

    // Remove the server from the draining servers lists.
    return this.drainingServers.remove(sn);
  }

  /**
   * Add the server to the drain list.
   * @return True if the server is added or the server is already on the drain list.
   */
  public synchronized boolean addServerToDrainList(final ServerName sn) {
    // If master is not rejecting decommissioned hosts, warn if the server (sn) is not online.
    // However, we want to add servers even if they're not online if the master is configured
    // to reject decommissioned hosts
    if (!rejectDecommissionedHostsConfig && !this.isServerOnline(sn)) {
      LOG.warn("Server {} is not currently online. Ignoring request to add it to draining list.",
        sn);
      return false;
    }

    // Add the server to the draining servers lists, if it's not already in it.
    if (this.drainingServers.contains(sn)) {
      LOG.warn(
        "Server {} is already in the draining server list. Ignoring request to add it again.", sn);
      return true;
    }

    LOG.info("Server {} added to draining server list.", sn);
    return this.drainingServers.add(sn);
  }

  // RPC methods to region servers

  private HBaseRpcController newRpcController() {
    return rpcControllerFactory == null ? null : rpcControllerFactory.newController();
  }

  /**
   * Sends a WARMUP RPC to the specified server to warmup the specified region.
   * <p>
   * A region server could reject the close request because it either does not have the specified
   * region or the region is being split.
   * @param server server to warmup a region
   * @param region region to warmup
   */
  public void sendRegionWarmup(ServerName server, RegionInfo region) {
    if (server == null) return;
    try {
      AdminService.BlockingInterface admin = getRsAdmin(server);
      HBaseRpcController controller = newRpcController();
      ProtobufUtil.warmupRegion(controller, admin, region);
    } catch (IOException e) {
      LOG.error("Received exception in RPC for warmup server:" + server + "region: " + region
        + "exception: " + e);
    }
  }

  /**
   * Contacts a region server and waits up to timeout ms to close the region. This bypasses the
   * active hmaster. Pass -1 as timeout if you do not want to wait on result.
   */
  public static void closeRegionSilentlyAndWait(ClusterConnection connection, ServerName server,
    RegionInfo region, long timeout) throws IOException, InterruptedException {
    AdminService.BlockingInterface rs = connection.getAdmin(server);
    HBaseRpcController controller = connection.getRpcControllerFactory().newController();
    try {
      ProtobufUtil.closeRegion(controller, rs, server, region.getRegionName());
    } catch (IOException e) {
      LOG.warn("Exception when closing region: " + region.getRegionNameAsString(), e);
    }
    if (timeout < 0) {
      return;
    }
    long expiration = timeout + EnvironmentEdgeManager.currentTime();
    while (EnvironmentEdgeManager.currentTime() < expiration) {
      controller.reset();
      try {
        RegionInfo rsRegion = ProtobufUtil.getRegionInfo(controller, rs, region.getRegionName());
        if (rsRegion == null) return;
      } catch (IOException ioe) {
        if (
          ioe instanceof NotServingRegionException
            || (ioe instanceof RemoteWithExtrasException && ((RemoteWithExtrasException) ioe)
              .unwrapRemoteException() instanceof NotServingRegionException)
        ) {
          // no need to retry again
          return;
        }
        LOG.warn("Exception when retrieving regioninfo from: " + region.getRegionNameAsString(),
          ioe);
      }
      Thread.sleep(1000);
    }
    throw new IOException("Region " + region + " failed to close within" + " timeout " + timeout);
  }

  /**
   * @return Admin interface for the remote regionserver named <code>sn</code>
   * @throws RetriesExhaustedException wrapping a ConnectException if failed
   */
  public AdminService.BlockingInterface getRsAdmin(final ServerName sn) throws IOException {
    LOG.debug("New admin connection to {}", sn);
    if (sn.equals(master.getServerName()) && master instanceof HRegionServer) {
      // A master is also a region server now, see HBASE-10569 for details
      return ((HRegionServer) master).getRSRpcServices();
    } else {
      return this.connection.getAdmin(sn);
    }
  }

  /**
   * Calculate min necessary to start. This is not an absolute. It is just a friction that will
   * cause us hang around a bit longer waiting on RegionServers to check-in.
   */
  private int getMinToStart() {
    if (master.isInMaintenanceMode()) {
      // If in maintenance mode, then master hosting meta will be the only server available
      return 1;
    }

    int minimumRequired = 1;
    if (
      LoadBalancer.isTablesOnMaster(master.getConfiguration())
        && LoadBalancer.isSystemTablesOnlyOnMaster(master.getConfiguration())
    ) {
      // If Master is carrying regions it will show up as a 'server', but is not handling user-
      // space regions, so we need a second server.
      minimumRequired = 2;
    }

    int minToStart = this.master.getConfiguration().getInt(WAIT_ON_REGIONSERVERS_MINTOSTART, -1);
    // Ensure we are never less than minimumRequired else stuff won't work.
    return Math.max(minToStart, minimumRequired);
  }

  /**
   * Wait for the region servers to report in. We will wait until one of this condition is met: -
   * the master is stopped - the 'hbase.master.wait.on.regionservers.maxtostart' number of region
   * servers is reached - the 'hbase.master.wait.on.regionservers.mintostart' is reached AND there
   * have been no new region server in for 'hbase.master.wait.on.regionservers.interval' time AND
   * the 'hbase.master.wait.on.regionservers.timeout' is reached
   */
  public void waitForRegionServers(MonitoredTask status) throws InterruptedException {
    final long interval =
      this.master.getConfiguration().getLong(WAIT_ON_REGIONSERVERS_INTERVAL, 1500);
    final long timeout =
      this.master.getConfiguration().getLong(WAIT_ON_REGIONSERVERS_TIMEOUT, 4500);
    // Min is not an absolute; just a friction making us wait longer on server checkin.
    int minToStart = getMinToStart();
    int maxToStart =
      this.master.getConfiguration().getInt(WAIT_ON_REGIONSERVERS_MAXTOSTART, Integer.MAX_VALUE);
    if (maxToStart < minToStart) {
      LOG.warn(String.format("The value of '%s' (%d) is set less than '%s' (%d), ignoring.",
        WAIT_ON_REGIONSERVERS_MAXTOSTART, maxToStart, WAIT_ON_REGIONSERVERS_MINTOSTART,
        minToStart));
      maxToStart = Integer.MAX_VALUE;
    }

    long now = EnvironmentEdgeManager.currentTime();
    final long startTime = now;
    long slept = 0;
    long lastLogTime = 0;
    long lastCountChange = startTime;
    int count = countOfRegionServers();
    int oldCount = 0;
    // This while test is a little hard to read. We try to comment it in below but in essence:
    // Wait if Master is not stopped and the number of regionservers that have checked-in is
    // less than the maxToStart. Both of these conditions will be true near universally.
    // Next, we will keep cycling if ANY of the following three conditions are true:
    // 1. The time since a regionserver registered is < interval (means servers are actively
    // checking in).
    // 2. We are under the total timeout.
    // 3. The count of servers is < minimum.
    for (ServerListener listener : this.listeners) {
      listener.waiting();
    }
    while (
      !this.master.isStopped() && !isClusterShutdown() && count < maxToStart
        && ((lastCountChange + interval) > now || timeout > slept || count < minToStart)
    ) {
      // Log some info at every interval time or if there is a change
      if (oldCount != count || lastLogTime + interval < now) {
        lastLogTime = now;
        String msg =
          "Waiting on regionserver count=" + count + "; waited=" + slept + "ms, expecting min="
            + minToStart + " server(s), max=" + getStrForMax(maxToStart) + " server(s), "
            + "timeout=" + timeout + "ms, lastChange=" + (now - lastCountChange) + "ms";
        LOG.info(msg);
        status.setStatus(msg);
      }

      // We sleep for some time
      final long sleepTime = 50;
      Thread.sleep(sleepTime);
      now = EnvironmentEdgeManager.currentTime();
      slept = now - startTime;

      oldCount = count;
      count = countOfRegionServers();
      if (count != oldCount) {
        lastCountChange = now;
      }
    }
    // Did we exit the loop because cluster is going down?
    if (isClusterShutdown()) {
      this.master.stop("Cluster shutdown");
    }
    LOG.info("Finished waiting on RegionServer count=" + count + "; waited=" + slept + "ms,"
      + " expected min=" + minToStart + " server(s), max=" + getStrForMax(maxToStart)
      + " server(s)," + " master is " + (this.master.isStopped() ? "stopped." : "running"));
  }

  private String getStrForMax(final int max) {
    return max == Integer.MAX_VALUE ? "NO_LIMIT" : Integer.toString(max);
  }

  /** Returns A copy of the internal list of online servers. */
  public List<ServerName> getOnlineServersList() {
    // TODO: optimize the load balancer call so we don't need to make a new list
    // TODO: FIX. THIS IS POPULAR CALL.
    return new ArrayList<>(this.onlineServers.keySet());
  }

  /**
   * @param keys                 The target server name
   * @param idleServerPredicator Evaluates the server on the given load
   * @return A copy of the internal list of online servers matched by the predicator
   */
  public List<ServerName> getOnlineServersListWithPredicator(List<ServerName> keys,
    Predicate<ServerMetrics> idleServerPredicator) {
    List<ServerName> names = new ArrayList<>();
    if (keys != null && idleServerPredicator != null) {
      keys.forEach(name -> {
        ServerMetrics load = onlineServers.get(name);
        if (load != null) {
          if (idleServerPredicator.test(load)) {
            names.add(name);
          }
        }
      });
    }
    return names;
  }

  /** Returns A copy of the internal list of draining servers. */
  public List<ServerName> getDrainingServersList() {
    return new ArrayList<>(this.drainingServers);
  }

  public boolean isServerOnline(ServerName serverName) {
    return serverName != null && onlineServers.containsKey(serverName);
  }

  public enum ServerLiveState {
    LIVE,
    DEAD,
    UNKNOWN
  }

  /** Returns whether the server is online, dead, or unknown. */
  public synchronized ServerLiveState isServerKnownAndOnline(ServerName serverName) {
    return onlineServers.containsKey(serverName)
      ? ServerLiveState.LIVE
      : (deadservers.isDeadServer(serverName) ? ServerLiveState.DEAD : ServerLiveState.UNKNOWN);
  }

  /**
   * Check if a server is known to be dead. A server can be online, or known to be dead, or unknown
   * to this manager (i.e, not online, not known to be dead either; it is simply not tracked by the
   * master any more, for example, a very old previous instance).
   */
  public synchronized boolean isServerDead(ServerName serverName) {
    return serverName == null || deadservers.isDeadServer(serverName);
  }

  /**
   * Check if a server is unknown. A server can be online, or known to be dead, or unknown to this
   * manager (i.e, not online, not known to be dead either; it is simply not tracked by the master
   * any more, for example, a very old previous instance).
   */
  public boolean isServerUnknown(ServerName serverName) {
    return serverName == null
      || (!onlineServers.containsKey(serverName) && !deadservers.isDeadServer(serverName));
  }

  public void shutdownCluster() {
    String statusStr = "Cluster shutdown requested of master=" + this.master.getServerName();
    LOG.info(statusStr);
    this.clusterShutdown.set(true);
    if (onlineServers.isEmpty()) {
      // we do not synchronize here so this may cause a double stop, but not a big deal
      master.stop("OnlineServer=0 right after cluster shutdown set");
    }
  }

  public boolean isClusterShutdown() {
    return this.clusterShutdown.get();
  }

  /**
   * Stop the ServerManager.
   */
  public void stop() {
    // Nothing to do.
  }

  /**
   * Creates a list of possible destinations for a region. It contains the online servers, but not
   * the draining or dying servers.
   * @param serversToExclude can be null if there is no server to exclude
   */
  public List<ServerName> createDestinationServersList(final List<ServerName> serversToExclude) {
    Set<ServerName> destServers = new HashSet<>();
    onlineServers.forEach((sn, sm) -> {
      if (sm.getLastReportTimestamp() > 0) {
        // This means we have already called regionServerReport at leaset once, then let's include
        // this server for region assignment. This is an optimization to avoid assigning regions to
        // an uninitialized server. See HBASE-25032 for more details.
        destServers.add(sn);
      }
    });

    if (serversToExclude != null) {
      destServers.removeAll(serversToExclude);
    }

    // Loop through the draining server list and remove them from the server list
    final List<ServerName> drainingServersCopy = getDrainingServersList();
    destServers.removeAll(drainingServersCopy);

    return new ArrayList<>(destServers);
  }

  /**
   * Calls {@link #createDestinationServersList} without server to exclude.
   */
  public List<ServerName> createDestinationServersList() {
    return createDestinationServersList(null);
  }

  /**
   * To clear any dead server with same host name and port of any online server
   */
  void clearDeadServersWithSameHostNameAndPortOfOnlineServer() {
    for (ServerName serverName : getOnlineServersList()) {
      deadservers.cleanAllPreviousInstances(serverName);
    }
  }

  /**
   * Called by delete table and similar to notify the ServerManager that a region was removed.
   */
  public void removeRegion(final RegionInfo regionInfo) {
    final byte[] encodedName = regionInfo.getEncodedNameAsBytes();
    storeFlushedSequenceIdsByRegion.remove(encodedName);
    flushedSequenceIdByRegion.remove(encodedName);
  }

  public boolean isRegionInServerManagerStates(final RegionInfo hri) {
    final byte[] encodedName = hri.getEncodedNameAsBytes();
    return (storeFlushedSequenceIdsByRegion.containsKey(encodedName)
      || flushedSequenceIdByRegion.containsKey(encodedName));
  }

  /**
   * Called by delete table and similar to notify the ServerManager that a region was removed.
   */
  public void removeRegions(final List<RegionInfo> regions) {
    for (RegionInfo hri : regions) {
      removeRegion(hri);
    }
  }

  /**
   * May return 0 when server is not online.
   */
  public int getVersionNumber(ServerName serverName) {
    ServerMetrics serverMetrics = onlineServers.get(serverName);
    return serverMetrics != null ? serverMetrics.getVersionNumber() : 0;
  }

  /**
   * May return "0.0.0" when server is not online
   */
  public String getVersion(ServerName serverName) {
    ServerMetrics serverMetrics = onlineServers.get(serverName);
    return serverMetrics != null ? serverMetrics.getVersion() : "0.0.0";
  }

  public int getInfoPort(ServerName serverName) {
    ServerMetrics serverMetrics = onlineServers.get(serverName);
    return serverMetrics != null ? serverMetrics.getInfoServerPort() : 0;
  }
}