spork-master/src/org/apache/pig/OrderedLoadFunc.java
/*
 * 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.pig;

import java.io.IOException;

import org.apache.hadoop.io.WritableComparable;
import org.apache.hadoop.mapreduce.InputSplit;

import org.apache.pig.classification.InterfaceAudience;
import org.apache.pig.classification.InterfaceStability;

/**
 * Implementing this interface indicates to Pig that a given loader
 * can be used for MergeJoin. It does not mean the data itself is ordered,
 * but rather that the splits returned by the underlying InputFormat
 * can be ordered to match the order of the data they are loading.  For 
 * example, files splits have a natural order (that of the file they are
 * from) while splits of RDBMS does not (since tables have no inherent order).
 * The position as represented by the
 * WritableComparable object is stored in the index created by
 * a MergeJoin sampling MapReduce job to get an ordered sequence of splits.
 * It is necessary to read splits in order during a merge join to assure 
 * data is being read according to the sort order.
 * @since Pig 0.7
 */
@InterfaceAudience.Public
@InterfaceStability.Evolving // Since we haven't done outer join for merge join yet
public interface OrderedLoadFunc {

    /**
     * The WritableComparable object returned will be used to compare
     * the position of different splits in an ordered stream
     * @param split An InputSplit from the InputFormat underlying this loader.
     * @return WritableComparable representing the position of the split in input
     * @throws IOException
     */
    public WritableComparable<?> getSplitComparable(InputSplit split) 
    throws IOException;

}
