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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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/*
 * 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.service;

import java.nio.ByteBuffer;
import java.util.concurrent.TimeUnit;

import org.apache.cassandra.db.*;
import org.apache.cassandra.db.partitions.PartitionIterator;
import org.apache.cassandra.db.partitions.UnfilteredPartitionIterators;
import org.apache.cassandra.net.MessageIn;

public class DigestResolver extends ResponseResolver
{
    private volatile ReadResponse dataResponse;

    public DigestResolver(Keyspace keyspace, ReadCommand command, ConsistencyLevel consistency, int maxResponseCount)
    {
        super(keyspace, command, consistency, maxResponseCount);
    }

    @Override
    public void preprocess(MessageIn message)
    {
        super.preprocess(message);
        if (dataResponse == null && !message.payload.isDigestResponse())
            dataResponse = message.payload;
    }

    /**
     * Special case of resolve() so that CL.ONE reads never throw DigestMismatchException in the foreground
     */
    public PartitionIterator getData()
    {
        assert isDataPresent();
        return UnfilteredPartitionIterators.filter(dataResponse.makeIterator(command), command.nowInSec());
    }

    /*
     * This method handles two different scenarios:
     *
     * a) we're handling the initial read of data from the closest replica + digests
     *    from the rest. In this case we check the digests against each other,
     *    throw an exception if there is a mismatch, otherwise return the data row.
     *
     * b) we're checking additional digests that arrived after the minimum to handle
     *    the requested ConsistencyLevel, i.e. asynchronous read repair check
     */
    public PartitionIterator resolve() throws DigestMismatchException
    {
        int responseCount = responses.size();
        
        assert responseCount > 0 : "Attempted response match comparison while no responses have been received.";
        
        if (responseCount == 1)
            return getData();

        if (logger.isTraceEnabled())
            logger.trace("resolving {} responses", responseCount);

        compareResponses();

        return UnfilteredPartitionIterators.filter(dataResponse.makeIterator(command), command.nowInSec());
    }

    public void compareResponses() throws DigestMismatchException
    {
        long start = System.nanoTime();

        // validate digests against each other; throw immediately on mismatch.
        ByteBuffer digest = null;
        for (MessageIn message : responses)
        {
            ReadResponse response = message.payload;

            ByteBuffer newDigest = response.digest(command);
            if (digest == null)
                digest = newDigest;
            else if (!digest.equals(newDigest))
                // rely on the fact that only single partition queries use digests
                throw new DigestMismatchException(((SinglePartitionReadCommand)command).partitionKey(), digest, newDigest);
        }

        if (logger.isTraceEnabled())
            logger.trace("resolve: {} ms.", TimeUnit.NANOSECONDS.toMillis(System.nanoTime() - start));
    }

    public boolean isDataPresent()
    {
        return dataResponse != null;
    }
}