如何使用Python为Hadoop编写一个简单的MapReduce程序

       MichaelG.Noll在他的Blog中提到如何在Hadoop中用Python编写MapReduce程序，韩国的gogamza在其Bolg中也提到如何用C编写MapReduce程序（我稍微修改了一下原程序,源码android 源码密钥因为他的Map对单词切分使用tab键）。我合并他们两人的文章，也让国内的Hadoop用户能够使用别的语言来编写MapReduce程序。　　首先您得配好您的Hadoop集群，这方面的介绍网上比较多，这儿给个链接（Hadoop学习笔记二安装部署）。HadoopStreaming帮助我们用非Java的编程语言使用MapReduce，Streaming用STDIN(标准输入)和STDOUT(标准输出)来和我们编写的Map和Reduce进行数据的交换数据。任何能够使用STDIN和STDOUT都可以用来编写MapReduce程序，比如我们用Python的sys.stdin和sys.stdout，或者是C中的stdin和stdout。　　我们还是使用Hadoop的例子WordCount来做示范如何编写MapReduce，在WordCount的例子中我们要解决计算在一批文档中每一个单词的出现频率。首先我们在Map程序中会接受到这批文档每一行的数据，然后我们编写的Map程序把这一行按空格切开成一个数组。并对这个数组遍历按"1"用标准的输出输出来，代表这个单词出现了一次。在Reduce中我们来统计单词的出现频率。　　　　PythonCode　　Map:mapper.py　　#!/usr/bin/envpythonimportsys#mapswordstotheircountsword2count={ }#inputcomesfromSTDIN(standardinput)forlineinsys.stdin:#removeleadingandtrailingwhitespaceline=line.strip()#splitthelineintowordswhileremovinganyemptystringswords=filter(lambdaword:word,line.split())#increasecountersforwordinwords:#writetheresultstoSTDOUT(standardoutput);#whatweoutputherewillbetheinputforthe#Reducestep,i.e.theinputforreducer.py##tab-delimited;thetrivialwordcountis1print'%s\t%s'%(word,1)　　Reduce:reducer.py　　#!/usr/bin/envpythonfromoperatorimportitemgetterimportsys#mapswordstotheircountsword2count={ }#inputcomesfromSTDINforlineinsys.stdin:#removeleadingandtrailingwhitespaceline=line.strip()#parsetheinputwegotfrommapper.pyword,count=line.split()#convertcount(currentlyastring)tointtry:count=int(count)word2count[word]=word2count.get(word,0)+countexceptValueError:#countwasnotanumber,sosilently#ignore/discardthislinepass#sortthewordslexigraphically;##thisstepisNOTrequired,wejustdoitsothatour#finaloutputwilllookmoreliketheofficialHadoop#wordcountexamplessorted_word2count=sorted(word2count.items(),key=itemgetter(0))#writetheresultstoSTDOUT(standardoutput)forword,countinsorted_word2count:print'%s\t%s'%(word,count)　　CCode　　Map:Mapper.c　　#include#include#include#include#defineBUF_SIZE#defineDELIM"\n"intmain(intargc,char*argv[]){ charbuffer[BUF_SIZE];while(fgets(buffer,BUF_SIZE-1,stdin)){ intlen=strlen(buffer);if(buffer[len-1]=='\n')buffer[len-1]=0;char*querys=index(buffer,'');char*query=NULL;if(querys==NULL)continue;querys+=1;/*nottoinclude'\t'*/query=strtok(buffer,"");while(query){ printf("%s\t1\n",query);query=strtok(NULL,"");}}return0;}h>h>h>h>　　Reduce:Reducer.c　　#include#include#include#include#defineBUFFER_SIZE#defineDELIM"\t"intmain(intargc,char*argv[]){ charstrLastKey[BUFFER_SIZE];charstrLine[BUFFER_SIZE];intcount=0;*strLastKey='\0';*strLine='\0';while(fgets(strLine,BUFFER_SIZE-1,stdin)){ char*strCurrKey=NULL;char*strCurrNum=NULL;strCurrKey=strtok(strLine,DELIM);strCurrNum=strtok(NULL,DELIM);/*necessarytocheckerrorbut.*/if(strLastKey[0]=='\0'){ strcpy(strLastKey,strCurrKey);}if(strcmp(strCurrKey,strLastKey)){ printf("%s\t%d\n",strLastKey,count);count=atoi(strCurrNum);}else{ count+=atoi(strCurrNum);}strcpy(strLastKey,strCurrKey);}printf("%s\t%d\n",strLastKey,count);/*flushthecount*/return0;}h>h>h>h>　　首先我们调试一下源码：　　chmod+xmapper.pychmod+xreducer.pyecho"foofooquuxlabsfoobarquux"|./mapper.py|./reducer.pybar1foo3labs1quux2g++Mapper.c-oMapperg++Reducer.c-oReducerchmod+xMapperchmod+xReducerecho"foofooquuxlabsfoobarquux"|./Mapper|./Reducerbar1foo2labs1quux1foo1quux1　　你可能看到C的输出和Python的不一样,因为Python是把他放在词典里了.我们在Hadoop时,会对这进行排序,然后相同的单词会连续在标准输出中输出.　　在Hadoop中运行程序　　首先我们要下载我们的测试文档wget页面中摘下的用php编写的MapReduce程序,供php程序员参考：Map:mapper.php　　#!/usr/bin/php$word2count=array();//inputcomesfromSTDIN(standardinput)while(($line=fgets(STDIN))!==false){ //removeleadingandtrailingwhitespaceandlowercase$line=strtolower(trim($line));//splitthelineintowordswhileremovinganyemptystring$words=preg_split('/\W/',$line,0,PREG_SPLIT_NO_EMPTY);//increasecountersforeach($wordsas$word){ $word2count[$word]+=1;}}//writetheresultstoSTDOUT(standardoutput)//whatweoutputherewillbetheinputforthe//Reducestep,i.e.theinputforreducer.pyforeach($word2countas$word=>$count){ //tab-delimitedecho$word,chr(9),$count,PHP_EOL;}?>　　Reduce:mapper.php　　#!/usr/bin/php$word2count=array();//inputcomesfromSTDINwhile(($line=fgets(STDIN))!==false){ //removeleadingandtrailingwhitespace$line=trim($line);//parsetheinputwegotfrommapper.phplist($word,$count)=explode(chr(9),$line);//convertcount(currentlyastring)toint$count=intval($count);//sumcountsif($count>0)$word2count[$word]+=$count;}//sortthewordslexigraphically////thissetisNOTrequired,wejustdoitsothatour//finaloutputwilllookmoreliketheofficialHadoop//wordcountexamplesksort($word2count);//writetheresultstoSTDOUT(standardoutput)foreach($word2countas$word=>$count){ echo$word,chr(9),$count,PHP_EOL;}?>　　作者：马士华发表于：--