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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 org.slf4j.Logger;
import org.slf4j.LoggerFactory;

import org.apache.cassandra.db.*;
import org.apache.cassandra.db.partitions.*;
import org.apache.cassandra.net.MessageIn;
import org.apache.cassandra.utils.concurrent.Accumulator;

public abstract class ResponseResolver
{
    protected static final Logger logger = LoggerFactory.getLogger(ResponseResolver.class);

    protected final Keyspace keyspace;
    protected final ReadCommand command;
    protected final ConsistencyLevel consistency;

    // Accumulator gives us non-blocking thread-safety with optimal algorithmic constraints
    protected final Accumulator> responses;

    public ResponseResolver(Keyspace keyspace, ReadCommand command, ConsistencyLevel consistency, int maxResponseCount)
    {
        this.keyspace = keyspace;
        this.command = command;
        this.consistency = consistency;
        this.responses = new Accumulator<>(maxResponseCount);
    }

    public abstract PartitionIterator getData();
    public abstract PartitionIterator resolve() throws DigestMismatchException;

    /**
     * Compares received responses, potentially triggering a digest mismatch (for a digest resolver) and read-repairs
     * (for a data resolver).
     * 
     * This is functionally equivalent to calling {@link #resolve()} and consuming the result, but can be slightly more
     * efficient in some case due to the fact that we don't care about the result itself. This is used when doing
     * asynchronous read-repairs.
     *
     * @throws DigestMismatchException if it's a digest resolver and the responses don't match.
     */
    public abstract void compareResponses() throws DigestMismatchException;

    public abstract boolean isDataPresent();

    public void preprocess(MessageIn message)
    {
        responses.add(message);
    }

    public Iterable> getMessages()
    {
        return responses;
    }
}