001    /**
002     * Licensed to the Apache Software Foundation (ASF) under one
003     * or more contributor license agreements.  See the NOTICE file
004     * distributed with this work for additional information
005     * regarding copyright ownership.  The ASF licenses this file
006     * to you under the Apache License, Version 2.0 (the
007     * "License"); you may not use this file except in compliance
008     * with the License.  You may obtain a copy of the License at
009     *
010     *     http://www.apache.org/licenses/LICENSE-2.0
011     *
012     * Unless required by applicable law or agreed to in writing, software
013     * distributed under the License is distributed on an "AS IS" BASIS,
014     * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
015     * See the License for the specific language governing permissions and
016     * limitations under the License.
017     */
018    
019    package org.apache.hadoop.mapred.join;
020    
021    import java.io.IOException;
022    import java.util.ArrayList;
023    import java.util.PriorityQueue;
024    
025    import org.apache.hadoop.classification.InterfaceAudience;
026    import org.apache.hadoop.classification.InterfaceStability;
027    import org.apache.hadoop.io.Writable;
028    import org.apache.hadoop.io.WritableComparable;
029    import org.apache.hadoop.io.WritableComparator;
030    import org.apache.hadoop.mapred.JobConf;
031    
032    /**
033     * Prefer the "rightmost" data source for this key.
034     * For example, <tt>override(S1,S2,S3)</tt> will prefer values
035     * from S3 over S2, and values from S2 over S1 for all keys
036     * emitted from all sources.
037     */
038    @InterfaceAudience.Public
039    @InterfaceStability.Stable
040    public class OverrideRecordReader<K extends WritableComparable,
041                                      V extends Writable>
042        extends MultiFilterRecordReader<K,V> {
043    
044      OverrideRecordReader(int id, JobConf conf, int capacity,
045          Class<? extends WritableComparator> cmpcl) throws IOException {
046        super(id, conf, capacity, cmpcl);
047      }
048    
049      /**
050       * Emit the value with the highest position in the tuple.
051       */
052      @SuppressWarnings("unchecked") // No static typeinfo on Tuples
053      protected V emit(TupleWritable dst) {
054        return (V) dst.iterator().next();
055      }
056    
057      /**
058       * Instead of filling the JoinCollector with iterators from all
059       * data sources, fill only the rightmost for this key.
060       * This not only saves space by discarding the other sources, but
061       * it also emits the number of key-value pairs in the preferred
062       * RecordReader instead of repeating that stream n times, where
063       * n is the cardinality of the cross product of the discarded
064       * streams for the given key.
065       */
066      protected void fillJoinCollector(K iterkey) throws IOException {
067        final PriorityQueue<ComposableRecordReader<K,?>> q = getRecordReaderQueue();
068        if (!q.isEmpty()) {
069          int highpos = -1;
070          ArrayList<ComposableRecordReader<K,?>> list =
071            new ArrayList<ComposableRecordReader<K,?>>(kids.length);
072          q.peek().key(iterkey);
073          final WritableComparator cmp = getComparator();
074          while (0 == cmp.compare(q.peek().key(), iterkey)) {
075            ComposableRecordReader<K,?> t = q.poll();
076            if (-1 == highpos || list.get(highpos).id() < t.id()) {
077              highpos = list.size();
078            }
079            list.add(t);
080            if (q.isEmpty())
081              break;
082          }
083          ComposableRecordReader<K,?> t = list.remove(highpos);
084          t.accept(jc, iterkey);
085          for (ComposableRecordReader<K,?> rr : list) {
086            rr.skip(iterkey);
087          }
088          list.add(t);
089          for (ComposableRecordReader<K,?> rr : list) {
090            if (rr.hasNext()) {
091              q.add(rr);
092            }
093          }
094        }
095      }
096    
097    }