在Python中使用Hadoop Streaming中的文件 [英] Using files in Hadoop Streaming with Python

问题描述

我对Hadoop和MapReduce完全陌生，正试图通过它来工作。
我想在Python中开发一个mapreduce应用程序，其中使用了2个.CSV文件中的数据。我只是读取mapper中的两个文件，然后将键值对从文件打印到sys.stdout中。

当我在单个程序中使用它时，程序运行正常机器，但与Hadoop流，我得到一个错误。我认为我在读取Hadoop上的映射器中的文件时犯了一些错误。请帮助我解决代码问题，并告诉我如何在Hadoop Streaming中使用文件处理。 mapper.py代码如下。 （您可以从注释中了解代码）：

 ＃！/ usr / bin / env python 
 import sys 
 from numpy import genfromtxt 
 
 def read_input（inVal）：
 inVal中的行：
＃将行分割成单词
 yield line.strip（ ）
 
 def main（separator ='\t'）：
＃输入来自STDIN（标准输入）
 labels = [] 
 data = [] 
 incoming = read_input（sys.stdin）
传入vals：
＃将结果写入STDOUT（标准输出）; 
＃这里我们输出的是
＃Reduce步骤的输入，即reducer.py的输入
＃
＃tab-delimited; 
 if len（vals）> 10：
 data.append（vals）
 else：
 labels.append（vals）
 
在范围内（0，len（标签））：
打印％s％s％s\\\
％（labels [i]，separator，data [i]）
 
 
 if __name__ ==__main__：
 main（）
  

从两个.csv文件输入到此映射器的记录有60000条如下所示（在单机上，不是hadoop集群）：

  cat mnist_train_labels.csv mnist_train_data.csv | ./mapper.py 
  

解决方案

我能解决在搜索解决方案后3天后发布。

问题出在Hadoop的新版本（在我的情况下是2.2.0）。当从文件读取值时，映射器代码在某个时间点给出非零的退出代码（可能是因为它一次读取了大量值（784））。 Hadoop 2.2.0中有一个设置，它告诉Hadoop系统给出一个通用错误（子进程失败，代码为1）。该设置默认设置为True。我只需将此属性的值设置为False，并使代码运行时没有任何错误。

设置为： stream.non.zero。 exit.is.failure 即可。流式传输时将其设置为false。因此，流式命令会有点像：

  ** hadoop jar ... -D stream.non.zero.exit。 is.failure = false ... ** 
  

希望它可以帮助某人，并节省3天.. 。;）

I am completely new to Hadoop and MapReduce and am trying to work my way through it. I am trying to develop a mapreduce application in python, in which I use data from 2 .CSV files. I am just reading the two files in mapper and then printing the key value pair from the files to the sys.stdout

The program runs fine when I use it on a single machine, but with the Hadoop Streaming, I get an error. I think I am making some mistake in reading files in the mapper on Hadoop. Please help me out with the code, and tell me how to use file-handling in Hadoop Streaming. The mapper.py code is as below. (You can understand the code from the comments):

#!/usr/bin/env python
import sys
from numpy import genfromtxt

def read_input(inVal):
    for line in inVal:
        # split the line into words
        yield line.strip()

def main(separator='\t'):
    # input comes from STDIN (standard input)
    labels=[]
    data=[]    
    incoming = read_input(sys.stdin)
    for vals in incoming:
        # 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;
        if len(vals) > 10:
            data.append(vals)
        else:
            labels.append(vals)

    for i in range(0,len(labels)):
        print "%s%s%s\n" % (labels[i], separator, data[i])


if __name__ == "__main__":
    main()

There are 60000 records which are entered to this mapper from two .csv files as follows (on single machine, not hadoop cluster):

cat mnist_train_labels.csv mnist_train_data.csv | ./mapper.py

解决方案

I was able to resolve the issue after searching a solution for like 3 days.

The problem is with the newer version of Hadoop (2.2.0 in my case). The mapper code, when reading values from files was giving an exit code of non-zero at some point (maybe because it was reading a huge list of values(784) at a time). There is a setting in the Hadoop 2.2.0, which tells the Hadoop System to give a general error (subprocess failed with code 1). This setting is set to True by default. I just had to set the value of this property to False, and it made my code run without any errors.

Setting is: stream.non.zero.exit.is.failure. Just set it to false when streaming. So the streaming command would be somewhat like:

**hadoop jar ... -D stream.non.zero.exit.is.failure=false ...**

Hope it helps someone, and saves 3 days... ;)

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