转自：使用Python实现Hadoop MapReduce程序
英文原文：Writing an Hadoop MapReduce Program in Python
根据上面两篇文章，下面是我在自己的ubuntu上的运行过程。文字基本采用博文使用Python实现Hadoop MapReduce程序， 打字很浪费时间滴。
在这个实例中，我将会向大家介绍如何使用Python 为 Hadoop编写一个简单的MapReduce程序。
我们想要做什么？
我们将编写一个简单的 MapReduce 程序，使用的是C-Python，而不是Jython编写后打包成jar包的程序。
我们的这个例子将模仿 WordCount 并使用Python来实现，例子通过读取文本文件来统计出单词的出现次数。结果也以文本形式输出，每一行包含一个单词和单词出现的次数，两者中间使用制表符来想间隔。
先决条件
编写这个程序之前，你学要架设好Hadoop 集群，这样才能不会在后期工作抓瞎。如果你没有架设好，那么在后面有个简明教程来教你在Ubuntu Linux 上搭建（同样适用于其他发行版linux、unix）
如何在Ubuntu Linux 上搭建hadoop的单节点模式和伪分布模式，请参阅博文 Ubuntu上搭建Hadoop环境（单机模式+伪分布模式）
Python的MapReduce代码
使用Python编写MapReduce代码的技巧就在于我们使用了 HadoopStreaming 来帮助我们在Map 和 Reduce间传递数据通过STDIN (标准输入)和STDOUT (标准输出).我们仅仅使用Python的sys.stdin来输入数据，使用sys.stdout输出数据，这样做是因为HadoopStreaming会帮我们办好其他事。这是真的，别不相信！
Map: mapper.py
将下列的代码保存在/usr/local/hadoop/mapper.py中，他将从STDIN读取数据并将单词成行分隔开，生成一个列表映射单词与发生次数的关系：
注意：要确保这个脚本有足够权限（chmod +x mapper.py）。
- #!/usr/bin/env python
- import sys
- # input comes from STDIN (standard input)
- for line in sys.stdin:
- # remove leading and trailing whitespace
- line = line.strip()
- # split the line into words
- words = line.split()
- # increase counters
- for word in words:
- # write the results to STDOUT (standard output);
- # what we output here will be the input for the
- # Reduce step, i.e. the input for reducer.py
- #
- # tab-delimited; the trivial word count is 1
- print '%s\t%s' % (word, 1)
在这个脚本中，并不计算出单词出现的总数，它将输出 "<word> 1" 迅速地，尽管<word>可能会在输入中出现多次，计算是留给后来的Reduce步骤（或叫做程序）来实现。当然你可以改变下编码风格，完全尊重你的习惯。Reduce: reducer.py
将代码存储在/usr/local/hadoop/reducer.py 中，这个脚本的作用是从mapper.py 的STDIN中读取结果，然后计算每个单词出现次数的总和，并输出结果到STDOUT。
同样，要注意脚本权限：chmod +x reducer.py
- #!/usr/bin/env python
- from operator import itemgetter
- import sys
- current_word = None
- current_count = 0
- word = None
- # input comes from STDIN
- for line in sys.stdin:
- # remove leading and trailing whitespace
- line = line.strip()
- # parse the input we got from mapper.py
- word, count = line.split('\t', 1)
- # convert count (currently a string) to int
- try:
- count = int(count)
- except ValueError:
- # count was not a number, so silently
- # ignore/discard this line
- continue
- # this IF-switch only works because Hadoop sorts map output
- # by key (here: word) before it is passed to the reducer
- if current_word == word:
- current_count += count
- else:
- if current_word:
- # write result to STDOUT
- print '%s\t%s' % (current_word, current_count)
- current_count = count
- current_word = word
- # do not forget to output the last word if needed!
- if current_word == word:
- print '%s\t%s' % (current_word, current_count)
测试你的代码（cat data | map | sort | reduce）
我建议你在运行MapReduce job测试前尝试手工测试你的mapper.py 和 reducer.py脚本，以免得不到任何返回结果
这里有一些建议，关于如何测试你的Map和Reduce的功能：
- hadoop@derekUbun:/usr/local/hadoop$ echo "foo foo quux labs foo bar quux" | ./mapper.py
- foo 1
- foo 1
- quux 1
- labs 1
- foo 1
- bar 1
- quux 1
- hadoop@derekUbun:/usr/local/hadoop$ echo "foo foo quux labs foo bar quux" |./mapper.py | sort |./reducer.py
- bar 1
- foo 3
- labs 1
- quux 2
# using one of the ebooks as example input
# (see below on where to get the ebooks)
- hadoop@derekUbun:/usr/local/hadoop$ cat book/book.txt |./mapper.pysubscribe 1
- to 1
- our 1
- email 1
- newsletter 1
- to 1
- hear 1
- about 1
- new 1
- eBooks. 1
在Hadoop平台上运行Python脚本
为了这个例子，我们将需要一本电子书，把它放在/usr/local/hadpoop/book/book.txt之下
- hadoop@derekUbun:/usr/local/hadoop$ ls -l book
- 总用量 636
- -rw-rw-r-- 1 derek derek 649669 3月 12 12:22 book.txt
复制本地数据到HDFS
在我们运行MapReduce job 前，我们需要将本地的文件复制到HDFS中：
- hadoop@derekUbun:/usr/local/hadoop$ hadoop dfs -copyFromLocal /usr/local/hadoop/book book
- hadoop@derekUbun:/usr/local/hadoop$ hadoop dfs -ls
- Found 3 items
- drwxr-xr-x - hadoop supergroup 0 2013-03-12 15:56 /user/hadoop/book
执行 MapReduce job现在，一切准备就绪，我们将在运行Python MapReduce job 在Hadoop集群上。像我上面所说的，我们使用的是HadoopStreaming 帮助我们传递数据在Map和Reduce间并通过STDIN和STDOUT，进行标准化输入输出。
- hadoop@derekUbun:/usr/local/hadoop$hadoop jar contrib/streaming/hadoop-streaming-1.1.2.jar
- -mapper /usr/local/hadoop/mapper.py
- -reducer /usr/local/hadoop/reducer.py
- -input book/*
- -output book-output
在运行中，如果你想更改Hadoop的一些设置，如增加Reduce任务的数量，你可以使用“-jobconf”选项：
- hadoop@derekUbun:/usr/local/hadoop$hadoop jar contrib/streaming/hadoop-streaming-1.1.2.jar
- -jobconf mapred.reduce.tasks=4
- -mapper /usr/local/hadoop/mapper.py
- -reducer /usr/local/hadoop/reducer.py
- -input book/*
- -output book-output
如果上面两个运行出错，请参考下面一段代码。注意，重新运行，需要删除dfs中的output文件
- bin/hadoop jar contrib/streaming/hadoop-streaming-1.1.2.jar
- -mapper task1/mapper.py
- -file task1/mapper.py
- -reducer task1/reducer.py
- -file task1/reducer.py
- -input url
- -output url-output
- -jobconf mapred.reduce.tasks=3
一个重要的备忘是关于Hadoop does not honor mapred.map.tasks 这个任务将会读取HDFS目录下的book并处理他们，将结果存储在独立的结果文件中，并存储在HDFS目录下的book-output目录。之前执行的结果如下：
- hadoop@derekUbun:/usr/local/hadoop$ hadoop jar contrib/streaming/hadoop-streaming-1.1.2.jar -jobconf mapred.reduce.tasks=4 -mapper /usr/local/hadoop/mapper.py -reducer /usr/local/hadoop/reducer.py -input book/* -output book-output
- 13/03/12 16:01:05 WARN streaming.StreamJob: -jobconf option is deprecated, please use -D instead.
- packageJobJar: [/usr/local/hadoop/tmp/hadoop-unjar4835873410426602498/] [] /tmp/streamjob5047485520312501206.jar tmpDir=null
- 13/03/12 16:01:06 INFO util.NativeCodeLoader: Loaded the native-hadoop library
- 13/03/12 16:01:06 WARN snappy.LoadSnappy: Snappy native library not loaded
- 13/03/12 16:01:06 INFO mapred.FileInputFormat: Total input paths to process : 1
- 13/03/12 16:01:06 INFO streaming.StreamJob: getLocalDirs(): [/usr/local/hadoop/tmp/mapred/local]
- 13/03/12 16:01:06 INFO streaming.StreamJob: Running job: job_201303121448_0010
- 13/03/12 16:01:06 INFO streaming.StreamJob: To kill this job, run:
- 13/03/12 16:01:06 INFO streaming.StreamJob: /usr/local/hadoop/libexec/../bin/hadoop job -Dmapred.job.tracker=localhost:9001 -kill job_201303121448_0010
- 13/03/12 16:01:06 INFO streaming.StreamJob: Tracking URL: http://localhost:50030/jobdetails.jsp?jobid=job_201303121448_0010
- 13/03/12 16:01:07 INFO streaming.StreamJob: map 0% reduce 0%
- 13/03/12 16:01:10 INFO streaming.StreamJob: map 100% reduce 0%
- 13/03/12 16:01:17 INFO streaming.StreamJob: map 100% reduce 8%
- 13/03/12 16:01:18 INFO streaming.StreamJob: map 100% reduce 33%
- 13/03/12 16:01:19 INFO streaming.StreamJob: map 100% reduce 50%
- 13/03/12 16:01:26 INFO streaming.StreamJob: map 100% reduce 67%
- 13/03/12 16:01:27 INFO streaming.StreamJob: map 100% reduce 83%
- 13/03/12 16:01:28 INFO streaming.StreamJob: map 100% reduce 100%
- 13/03/12 16:01:29 INFO streaming.StreamJob: Job complete: job_201303121448_0010
- 13/03/12 16:01:29 INFO streaming.StreamJob: Output: book-output
- hadoop@derekUbun:/usr/local/hadoop$
如你所见到的上面的输出结果，Hadoop 同时还提供了一个基本的WEB接口显示统计结果和信息。
当Hadoop集群在执行时，你可以使用浏览器访问 http://localhost:50030/ ：
检查结果是否输出并存储在HDFS目录下的book-output中：
- hadoop@derekUbun:/usr/local/hadoop$ hadoop dfs -ls book-output
- Found 6 items
- -rw-r--r-- 2 hadoop supergroup 0 2013-03-12 16:01 /user/hadoop/book-output/_SUCCESS
- drwxr-xr-x - hadoop supergroup 0 2013-03-12 16:01 /user/hadoop/book-output/_logs
- -rw-r--r-- 2 hadoop supergroup 33 2013-03-12 16:01 /user/hadoop/book-output/part-00000
- -rw-r--r-- 2 hadoop supergroup 60 2013-03-12 16:01 /user/hadoop/book-output/part-00001
- -rw-r--r-- 2 hadoop supergroup 54 2013-03-12 16:01 /user/hadoop/book-output/part-00002
- -rw-r--r-- 2 hadoop supergroup 47 2013-03-12 16:01 /user/hadoop/book-output/part-00003
- hadoop@derekUbun:/usr/local/hadoop$
可以使用dfs -cat 命令检查文件目录
- hadoop@derekUbun:/usr/local/hadoop$ hadoop dfs -cat book-output/part-00000
- about 1
- eBooks. 1
- the 1
- to 2
- hadoop@derekUbun:/usr/local/hadoop$
下面是原英文作者mapper.py和reducer.py的两个修改版本:
mapper.py
- #!/usr/bin/env python
- """A more advanced Mapper, using Python iterators and generators."""
- import sys
- def read_input(file):
- for line in file:
- # split the line into words
- yield line.split()
- def main(separator='\t'):
- # input comes from STDIN (standard input)
- data = read_input(sys.stdin)
- for words in data:
- # write the results to STDOUT (standard output);
- # what we output here will be the input for the
- # Reduce step, i.e. the input for reducer.py
- #
- # tab-delimited; the trivial word count is 1
- for word in words:
- print '%s%s%d' % (word, separator, 1)
- if __name__ == "__main__":
- main()
reducer.py
- #!/usr/bin/env python
- """A more advanced Reducer, using Python iterators and generators."""
- from itertools import groupby
- from operator import itemgetter
- import sys
- def read_mapper_output(file, separator='\t'):
- for line in file:
- yield line.rstrip().split(separator, 1)
- def main(separator='\t'):
- # input comes from STDIN (standard input)
- data = read_mapper_output(sys.stdin, separator=separator)
- # groupby groups multiple word-count pairs by word,
- # and creates an iterator that returns consecutive keys and their group:
- # current_word - string containing a word (the key)
- # group - iterator yielding all ["<current_word>", "<count>"] items
- for current_word, group in groupby(data, itemgetter(0)):
- try:
- total_count = sum(int(count) for current_word, count in group)
- print "%s%s%d" % (current_word, separator, total_count)
- except ValueError:
- # count was not a number, so silently discard this item
- pass
- if __name__ == "__main__":
- main()
