MapReduce | 易学教程

How to tell MapReduce how many mappers to use?

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问题 I am trying to speed optimize MapReduce job. Is there any way I can tell hadoop to use a particular number of mapper/reducer processes? Or, at least, minimal number of mapper processes? In the documentation, it is specified, that you can do that with the method public void setNumMapTasks(int n) of the JobConf class. That way is not obsolete, so I am starting the Job with Job class. What is the right way of doing this? 回答1: The number of map tasks is determined by the number of blocks in the

custom inputformat for reading json in hadoop

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问题 i am a beginner of hadoop,i have been told to create a custom inputformat class to read json data,i have googled up and learnt how to create a custom inputformat class to read data from the file.but i am stuck on parsing the json data. my json data looks like this [ { "_count": 30, "_start": 0, "_total": 180, "values": [ { "attachment": { "contentDomain": "techcarnival2013.eventbrite.com", "contentUrl": "http://techcarnival2013.eventbrite.com/", "imageUrl": "http://ebmedia.eventbrite.com/s3

Replace multiple joins in SQL with CouchDB views

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问题 I am implementing a filter feature for my application and having trouble with writing the view on CouchDB. While in SQL, this would be a statement with multiple join. How to replace multiple join in CouchDB. This article covers single join: http://www.cmlenz.net/archives/2007/10/couchdb-joins. However, it's not obvious to me how to extend this approach to multiple join. Imagine my object has a dozen properties and each property can have its individual filter. For simplicity, let's assume it

Mapreduce error: Failed to setup local dir

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问题 I'm running mapreduce wordcount example on hadoop installed on windows 8. I got the error as below. It sounds like a security permission issue. But I'm not very sure. I added a property to yarn-site.xml file as <property> <name>yarn.nodemanager.local-dirs</name> <value>c:\hadoop\tmp-nm</value> </property> Any idea would be very helpful! 15/07/15 11:01:54 INFO client.RMProxy: Connecting to ResourceManager at /0.0.0.0:8032 15/07/15 11:01:55 WARN mapreduce.JobResourceUploader: Hadoop command

Run a MapReduce job via rest api

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问题 I use hadoop2.7.1's rest apis to run a mapreduce job outside the cluster. This example "http://hadoop-forum.org/forum/general-hadoop-discussion/miscellaneous/2136-how-can-i-run-mapreduce-job-by-rest-api" really helped me. But when I submit a post response, some strange things happen: I look at "http://master:8088/cluster/apps" and a post response produce two jobs as following picture: strange things: a response produces two jobs After wait a long time, the job which I defined in the http

Mapfile as a input to a MapReduce job

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问题 I recently started to use Hadoop and I have a problem while using a Mapfile as a input to a MapReduce job. The following working code, writes a simple MapFile called "TestMap" in hdfs where there are three keys of type Text and three values of type BytesWritable. Here the contents of TestMap: $ hadoop fs -text /user/hadoop/TestMap/data 11/01/20 11:17:58 INFO util.NativeCodeLoader: Loaded the native-hadoop library 11/01/20 11:17:58 INFO zlib.ZlibFactory: Successfully loaded & initialized

谷歌三遍论文读后感

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谷歌的三篇论文分别介绍了Google-Bigtable, Google-MapReduce和Google-File-System三个谷歌的重要工具。三个工具都有一个共同的特征——分布式系统，所谓的分布式就是将一个业务分拆多个子业务，分别部署在不同的服务器上。分布式文件系统的设计是基于客户机/服务器模式。一个典型的网络可能包括多个供多用户访问的服务器。其中首先讲了GFS是一个面向大规模数据密集型应用的、可伸缩的分布式文件系统。GFS 虽然运行在廉价的普遍硬件设备上，但是它依然了提供灾难冗余的能力，为大量客户机提供了高性能的服务。GFS基于普通的分布式文件系统，逐步发展延伸，基本上完全满足了用户对存储的需求。然后讲的是Google Bigtable，它是一个分布式的结构化数据存储系统，被用来处理海量数据。例如Google里面的web索引、Google earth、Google finance等等项目都在使用Bigtable来存储数据，论文描述了Bigtable提供的简单的数据模型，利用模型用户可以动态的控制数据的分布和格式。Bigtable已在60个Google的产品和项目上的到了应用。最后一篇讲是Google MapReduce。MapReduce是一个编程模型，也是一个处理和生成超大数据集的算法模型的相关实现。MapReduce在 Google 内部也已经成功应用于多个领域

Hadoop: Reducer writing Mapper output into Output File

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问题 I met a very very strange problem. The reducers do work but if I check the output files, I only found the output from the mappers. When I was trying to debug, I found the same problem with the word count sample after I changed the mappers' output value type from Longwritable to Text package org.myorg; import java.io.IOException; import java.util.*; import org.apache.hadoop.fs.Path; import org.apache.hadoop.conf.*; import org.apache.hadoop.io.*; import org.apache.hadoop.mapreduce.*; import org

Hadoop: Reducer writing Mapper output into Output File

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Parsing PDF files in Hadoop Map Reduce

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问题 I have to parse PDF files , that are in HDFS in a Map Reduce Program in Hadoop. So i get the PDF file from HDFS as Input splits and it has to be parsed and sent to the Mapper Class. For implementing this InputFormat I had gone through this link . How can the these input splits be parsed and converted into text format ? 回答1: Processing PDF files in Hadoop can be done by extending FileInputFormat Class. Let the class extending it be WholeFileInputFormat. In the WholeFileInputFormat class you