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   * Licensed to the Apache Software Foundation (ASF) under one
   * or more contributor license agreements.  See the NOTICE file
   * distributed with this work for additional information
   * regarding copyright ownership.  The ASF licenses this file
   * to you under the Apache License, Version 2.0 (the
   * "License"); you may not use this file except in compliance
   * with the License.  You may obtain a copy of the License at
   *
   *   http://www.apache.org/licenses/LICENSE-2.0
   *
   * Unless required by applicable law or agreed to in writing,
   * software distributed under the License is distributed on an
   * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
   * KIND, either express or implied.  See the License for the
   * specific language governing permissions and limitations
   * under the License.
   */
  package org.apache.cassandra.hints;
  
  import java.util.UUID;
  
  import org.slf4j.Logger;
  import org.slf4j.LoggerFactory;
  
  import org.apache.cassandra.db.partitions.PartitionUpdate;
  import org.apache.cassandra.exceptions.RetryOnDifferentSystemException;
  import org.apache.cassandra.locator.InetAddressAndPort;
  import org.apache.cassandra.metrics.HintsServiceMetrics;
  import org.apache.cassandra.net.IVerbHandler;
  import org.apache.cassandra.net.Message;
  import org.apache.cassandra.net.MessagingService;
  import org.apache.cassandra.serializers.MarshalException;
  import org.apache.cassandra.service.StorageProxy;
  import org.apache.cassandra.service.StorageService;
  import org.apache.cassandra.tcm.ClusterMetadata;
  import org.apache.cassandra.tcm.membership.NodeId;
  
  import static org.apache.cassandra.exceptions.RequestFailureReason.RETRY_ON_DIFFERENT_TRANSACTION_SYSTEM;
  
  /**
   * Verb handler used both for hint dispatch and streaming.
   *
   * With the non-sstable format, we cannot just stream hint sstables on node decommission. So sometimes, at decommission
   * time, we might have to stream hints to a non-owning host (say, if the owning host B is down during decommission of host A).
   * In that case the handler just stores the received hint in its local hint store.
   */
  public final class HintVerbHandler implements IVerbHandler<HintMessage>
  {
      public static final HintVerbHandler instance = new HintVerbHandler();
  
      private static final Logger logger = LoggerFactory.getLogger(HintVerbHandler.class);
  
      public void doVerb(Message<HintMessage> message)
      {
          UUID hostId = message.payload.hostId;
          Hint hint = message.payload.hint;
          InetAddressAndPort address = StorageService.instance.getEndpointForHostId(hostId);
  
          // If we see an unknown table id, it means the table, or one of the tables in the mutation, had been dropped.
          // In that case there is nothing we can really do, or should do, other than log it go on.
          // This will *not* happen due to a not-yet-seen table, because we don't transfer hints unless there
          // is schema agreement between the sender and the receiver.
          if (hint == null)
          {
              if (logger.isTraceEnabled())
                  logger.trace("Failed to decode and apply a hint for {}: {} - table with id {} is unknown",
                               address,
                               hostId,
                               message.payload.unknownTableID);
              respond(message);
              return;
          }
  
          // We must perform validation before applying the hint, and there is no other place to do it other than here.
          try
          {
              hint.mutation.getPartitionUpdates().forEach(PartitionUpdate::validate);
          }
          catch (MarshalException e)
          {
              logger.warn("Failed to validate a hint for {}: {} - skipped", address, hostId);
              respond(message);
              return;
          }
  
          ClusterMetadata metadata = ClusterMetadata.current();
          NodeId localId = metadata.myNodeId();
          if (!hostId.equals(localId.toUUID()) && !hostId.equals(metadata.directory.hostId(localId)))
          {
              // the hint may have been written prior to upgrading, in which case it would be addressing the old
              // host id for its target node. If the id in the hint matches neither the pre-upgrade host id nor the
              // post-upgrade node id for this peer, the node is not the final destination of the hint (must have gotten
              // it from a decommissioning node), so just store it locally, to be delivered later.
              HintsService.instance.write(hostId, hint);
              respond(message);
          }
          else if (!StorageProxy.instance.appliesLocally(hint.mutation))
          {
              // the topology has changed, and we are no longer a replica of the mutation - since we don't know which node(s)
              // it has been handed over to, re-address the hint to all replicas; see CASSANDRA-5902.
              HintsService.instance.writeForAllReplicas(hint);
              respond(message);
          }
          else
          {
              try
              {
                  // the common path - the node is both the destination and a valid replica for the hint.
                  hint.applyFuture().addCallback(
                  o -> {
                      HintsServiceMetrics.hintsApplySucceeded.mark();
                      respond(message);
                  },
                  e -> {
                      HintsServiceMetrics.hintsApplyFailed.mark();
                      logger.debug("Failed to apply hint", e);
                  });
              }
              catch (RetryOnDifferentSystemException e)
              {
                  MessagingService.instance().respondWithFailure(RETRY_ON_DIFFERENT_TRANSACTION_SYSTEM, message);
              }
          }
      }
  
      private static void respond(Message<HintMessage> respondTo)
      {
          MessagingService.instance().send(respondTo.emptyResponse(), respondTo.from());
      }
  }
  