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The MongoDB Connector for Hadoop is a plugin for Hadoop that provides the ability to use MongoDB as an input source and/or an output destination.

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/*
 * Copyright 2010-2013 10gen Inc.
 *
 * Licensed 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 com.mongodb.hadoop.splitter;

import com.mongodb.MongoClientURI;
import com.mongodb.hadoop.input.MongoInputSplit;
import com.mongodb.hadoop.util.MongoConfigUtil;
import org.apache.hadoop.conf.Configuration;
import org.apache.hadoop.mapreduce.InputSplit;

import java.util.ArrayList;
import java.util.List;
import java.util.Map;
import java.util.Map.Entry;


/**
 * This implementation of MongoSplitter can be used for sharded systems. It treats each shard as a single split, by reading all of the data
 * on each shard for that split. This should be used with caution as the input splits could contain duplicate data from ongoing, or aborted
 * migrations.
 */
public class ShardMongoSplitter extends MongoCollectionSplitter {
    public ShardMongoSplitter() {
    }

    public ShardMongoSplitter(final Configuration conf) {
        super(conf);
    }

    // Treat each shard as one split.
    @Override
    public List calculateSplits() throws SplitFailedException {
        final ArrayList returnVal = new ArrayList();

        MongoClientURI inputURI = MongoConfigUtil.getInputURI(getConfiguration());

        Map> shardsMap;
        try {
            shardsMap = getShardsMap();

            for (Entry> entry : shardsMap.entrySet()) {
                List shardHosts = entry.getValue();

                MongoInputSplit chunkSplit = createSplitFromBounds(null, null);
                chunkSplit.setInputURI(rewriteURI(inputURI, shardHosts));
                returnVal.add(chunkSplit);
            }
        } finally {
            // getShardsMap() creates a client to a config server. Close it now.
            MongoConfigUtil.close(getConfigDB().getMongo());
        }
        if (MongoConfigUtil.isFilterEmptySplitsEnabled(getConfiguration())) {
            return filterEmptySplits(returnVal);
        }
        return returnVal;
    }

}