Hadoop：二级排序不起作用

Question

我在 Hadoop 1.2.1 中实现了一个算法，其中 reducer 代码依赖于二次排序。但是，当我运行算法时，一个 reducer 接收排序的元组，但另一个没有。我花了很多时间试图找出原因，但没有任何成功。

有谁知道可能是什么问题？我认为它与二级排序代码有关。

下面是实现二次排序的代码：

复合键

    public class CompositeKey implements WritableComparable<CompositeKey>{
        public String key;
        public Integer position;
        @Override
        public void readFields(DataInput arg0) throws IOException {
            key = WritableUtils.readString(arg0);
            position = arg0.readInt();
        }
        @Override
        public void write(DataOutput arg0) throws IOException {
            WritableUtils.writeString(arg0, key);
            arg0.writeLong(position);
        }
        @Override
        public int compareTo(CompositeKey o) {
            int result = key.compareTo(o.key);
            if(0 == result) {
                result = position.compareTo(o.position);
            }
            return result;
        }
    }

KeyComparator

    public class CompositeKeyComparator extends WritableComparator {
         protected CompositeKeyComparator() {
                super(CompositeKey.class, true);
            }   
            @SuppressWarnings("rawtypes")
            @Override
            public int compare(WritableComparable w1, WritableComparable w2) {
                CompositeKey k1 = (CompositeKey)w1;
                CompositeKey k2 = (CompositeKey)w2;

                int result = k1.key.compareTo(k2.key);
                if(0 == result) {
                    result = -1* k1.position.compareTo(k2.position);
                }
                return result;
            }

    }

分组比较器

    public class NaturalKeyGroupingComparator extends WritableComparator {
        protected NaturalKeyGroupingComparator() {
            super(CompositeKey.class, true);
        }   
        @SuppressWarnings("rawtypes")
        @Override
        public int compare(WritableComparable w1, WritableComparable w2) {
            CompositeKey k1 = (CompositeKey)w1;
            CompositeKey k2 = (CompositeKey)w2;

            return k1.key.compareTo(k2.key);
        }

    }

分区器

    public class NaturalKeyPartitioner extends Partitioner<CompositeKey, ReduceValue> {
        @Override
        public int getPartition(CompositeKey key, ReduceValue val, int numPartitions) {
            int hash = key.key.hashCode();
            int partition = hash & Integer.MAX_VALUE % numPartitions;
            return partition;
        }

工作配置

    //secondary sort
    job.setPartitionerClass(NaturalKeyPartitioner.class);
    job.setGroupingComparatorClass(NaturalKeyGroupingComparator.class);
    job.setSortComparatorClass(CompositeKeyComparator.class);

如果我在伪分布式环境或集群上都执行此操作，我注意到一个 reducer 得到了排序的元组，而另一个没有。例如这里是两个reducer接收到的元组的摘录（第一列是主要的ket，第二列是次要的）：

    First reducer:
    a1 0 
    a1 1 
    a1 11 
    a1 16 
    a1 27 
    a1 28 
    a1 34 
    a1 35 
    a1 37 
    a1 38 
    a1 43 
    a1 44 
    a1 46 
    a1 48 
    a1 50 
    a1 54 
    a1 55 
    a1 56 
    a1 57 
    a1 60 
    a1 61 
    a1 63 
    a1 64 
    a1 66 
    a1 69 
    a1 70 
    a1 72 
    a1 75 
    a1 76 
    a1 78 
    a1 79 
    a1 80 
    a1 84 
    a1 85 
    a1 86 
    a1 87 
    a1 88 
    a1 91 
    a1 92 
    a1 97 
    a1 102   
    a1 106    
    a1 108  
    a1 109 
    a1 110 
    a1 111 
    a1 116     
    a1 118  
    a1 119 
    a1 120  

    Second reducer:
    a2 87 
    a2 115
    a2 65 
    a2 90 
    a2 68 
    a2 119    
    a2 91 
    a2 0 
    a2 70 
    a2 3 
    a2 8 
    a2 9 
    a2 10 
    a2 71 
    a2 110   
    a2 16 
    a2 17 
    a2 20 
    a2 21 
    a2 23 
    a2 26 
    a2 72 
    a2 27 
    a2 94 
    a2 29 
    a2 30 
    a2 31 
    a2 75 
    a2 95 
    a2 36 
    a2 76 
    a2 117  
    a2 39 
    a2 40 
    a2 41 
    a2 42 
    a2 97 
    a2 79 
    a2 44 
    a2 45 
    a2 98 
    a2 46 
    a2 80 
    a2 49 
    a2 82 
    a2 50 
    a2 83 
    a2 100 
    a2 84 
    a2 112     
    a2 57 
    a2 59 
    a2 113      
    a2 60 
    a2 114       
    a2 61 

Answer 1

我认为这是因为在 CompositeKey 的序列化/反序列化逻辑中，您将位置写为 long 但将其读取为整数。这会弄乱比较逻辑，因为您没有测试您在上下文中写入的完全相同的内容。