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The Apache Cassandra Project develops a highly scalable second-generation distributed database, bringing together Dynamo's fully distributed design and Bigtable's ColumnFamily-based data model.

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package org.apache.cassandra.service.paxos;
/*
 * 
 * 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.
 * 
 */


import java.net.InetAddress;
import java.nio.ByteBuffer;
import java.util.Map;
import java.util.concurrent.ConcurrentHashMap;

import com.google.common.base.Predicate;
import com.google.common.collect.Iterables;
import org.apache.cassandra.db.ConsistencyLevel;
import org.apache.cassandra.db.DecoratedKey;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

import org.apache.cassandra.config.CFMetaData;
import org.apache.cassandra.net.MessageIn;

public class PrepareCallback extends AbstractPaxosCallback
{
    private static final Logger logger = LoggerFactory.getLogger(PrepareCallback.class);

    public boolean promised = true;
    public Commit mostRecentCommit;
    public Commit mostRecentInProgressCommit;
    public Commit mostRecentInProgressCommitWithUpdate;

    private final Map commitsByReplica = new ConcurrentHashMap();

    public PrepareCallback(DecoratedKey key, CFMetaData metadata, int targets, ConsistencyLevel consistency)
    {
        super(targets, consistency);
        // need to inject the right key in the empty commit so comparing with empty commits in the reply works as expected
        mostRecentCommit = Commit.emptyCommit(key, metadata);
        mostRecentInProgressCommit = Commit.emptyCommit(key, metadata);
        mostRecentInProgressCommitWithUpdate = Commit.emptyCommit(key, metadata);
    }

    public synchronized void response(MessageIn message)
    {
        PrepareResponse response = message.payload;
        logger.debug("Prepare response {} from {}", response, message.from);

        // In case of clock skew, another node could be proposing with ballot that are quite a bit
        // older than our own. In that case, we record the more recent commit we've received to make
        // sure we re-prepare on an older ballot.
        if (response.inProgressCommit.isAfter(mostRecentInProgressCommit))
            mostRecentInProgressCommit = response.inProgressCommit;

        if (!response.promised)
        {
            promised = false;
            while (latch.getCount() > 0)
                latch.countDown();
            return;
        }

        commitsByReplica.put(message.from, response.mostRecentCommit);
        if (response.mostRecentCommit.isAfter(mostRecentCommit))
            mostRecentCommit = response.mostRecentCommit;

        // If some response has an update, then we should replay the update with the highest ballot. So find
        // the the highest commit that actually have an update
        if (response.inProgressCommit.isAfter(mostRecentInProgressCommitWithUpdate) && !response.inProgressCommit.update.isEmpty())
            mostRecentInProgressCommitWithUpdate = response.inProgressCommit;

        latch.countDown();
    }

    public Iterable replicasMissingMostRecentCommit()
    {
        return Iterables.filter(commitsByReplica.keySet(), new Predicate()
        {
            public boolean apply(InetAddress inetAddress)
            {
                return (!commitsByReplica.get(inetAddress).ballot.equals(mostRecentCommit.ballot));
            }
        });
    }
}