Hadoop Streaming 实战： aggregate

1. aggregate概述
aggregate是Hadoop提供的一个软件包，其用来做一些通用的计算和聚合。
Generally speaking, in order to implement an application using Map/Reduce model, the developer needs to implement Map and Reduce functions (and possibly Combine function). However, for a lot of applications related to counting and statistics computing, these functions have very similarcharacteristics. This provides a package implementing those patterns. In particular, the package provides a generic mapper class,a reducer class and a combiner class, and a set of built-in value aggregators.It also provides a generic utility class, ValueAggregatorJob, that offers a static function that creates map/reduce jobs。
在Streaming中通常使用Aggregate包作为reducer来做聚合统计。

2. aggregate class summary

DoubleValueSum	This class implements a value aggregator that sums up a sequence of double values.
LongValueMax	This class implements a value aggregator that maintain the maximum of a sequence of long values.
LongValueMin	This class implements a value aggregator that maintain the minimum of a sequence of long values.
LongValueSum	This class implements a value aggregator that sums up a sequence of long values.
StringValueMax	This class implements a value aggregator that maintain the biggest of a sequence of strings.
StringValueMin	This class implements a value aggregator that maintain the smallest of a sequence of strings.
UniqValueCount	This class implements a value aggregator that dedupes a sequence of objects.
UserDefinedValueAggregatorDescriptor	This class implements a wrapper for a user defined value aggregator descriptor.
ValueAggregatorBaseDescriptor	This class implements the common functionalities of the subclasses of ValueAggregatorDescriptor class.
ValueAggregatorCombiner<K1 extends WritableComparable,V1 extends Writable>	This class implements the generic combiner of Aggregate.
ValueAggregatorJob	This is the main class for creating a map/reduce job using Aggregate framework.
ValueAggregatorJobBase<K1 extends WritableComparable,V1 extends Writable>	This abstract class implements some common functionalities of the the generic mapper, reducer and combiner classes of Aggregate.
ValueAggregatorMapper<K1 extends WritableComparable,V1 extends Writable>	This class implements the generic mapper of Aggregate.
ValueAggregatorReducer<K1 extends WritableComparable,V1 extends Writable>	This class implements the generic reducer of Aggregate.
ValueHistogram	This class implements a value aggregator that computes the histogram of a sequence of strings

3. streaming中使用aggregate
在mapper任务的输出中添加控制，如下：
function：key\tvalue
eg：
LongValueSum：key\tvalue
此外，置-reducer = aggregate。此时，Reducer使用aggregate中对应的function类对相同key的value进行操作，例如，设置function为LongValueSum则将对每个键值对应的value求和。

4. 实例1（value求和）
测试文件test.txt

a       15      1
a       17      1
a       18      1
a       19      1
a       19      1
a       19      1
a       19      1
b       20      1
c       15      1
c       15      1
d       16      1
a       16      1

mapper程序：

#include <iostream>
#include <string>

using namespace std;

int main(int argc, char** argv)
{
        string a,b,c;
        while(cin >> a >> b >> c)
        {
                cout  << "LongValueSum:"<< a << "\t" << b  <<  endl;
        }
        return 0;
}

运行：
$hadoop streaming -input /app/test.txt -output /app/test -mapper ./mapper -reducer aggregate -file mapper  -jobconf mapred.reduce.tasks=1 -jobconf mapre.job.name="test"
输出：
a       142
b       20
c       30
d       16

5. 实例2（强大ValueHistogram）
ValueHistogram是aggregate package中最强大的类，基于每个键，对其value做以下统计
1）唯一值个数
2）最小值个数
3）中位置个数
4）最大值个数
5）平均值个数
6）标准方差
上述例子基础上修改mapper.cpp为：

#include <iostream>
#include <string>

using namespace std;

int main(int argc, char** argv)
{
        string a,b,c;
        while(cin >> a >> b >> c)
        {
                cout  << "ValueHistogram:"<< a << "\t" << b  <<  endl;
        }
        return 0;
}

运行命令同上
运行结果：
a       5       1       1       4       1.6     1.2
b       1       1       1       1       1.0     0.0
c       1       2       2       2       2.0     0.0
d       1       1       1       1       1.0     0.0

参考：
http://hadoop.apache.org/common/docs/r0.15.2/api/index.html?org/apache/hadoop/mapred/lib/aggregate/package-summary.html
book：Hadoop实战