MapReduce | 易学教程

How do i set an Object as the Value for Map output in Hadoop MapReduce?

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问题 In the Hadoop MapReduce, for the intermediate Output (generated by the map()), i want the Value for the Intermediate output to be the following object. MyObject{ date:Date balance:Double } How would i do this. Should i create my own Writable Class? I am a newbie to MapReduce. Thanks. 回答1: You can write your custom type which you can emit as the mapper value. But whatever you want to emit as value, must implement the Writable Interface. You can do something like this : public class MyObj

MRUnit with Avro NullPointerException in Serialization

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问题 I'm trying to test a Hadoop .mapreduce Avro job using MRUnit. I am receiving a NullPointerException as seen below. I've attached a portion of the pom and source code. Any assistance would be appreciated. Thanks The error I'm getting is : java.lang.NullPointerException at org.apache.hadoop.mrunit.internal.io.Serialization.copy(Serialization.java:73) at org.apache.hadoop.mrunit.internal.io.Serialization.copy(Serialization.java:91) at org.apache.hadoop.mrunit.internal.io.Serialization

Tips to improve MapReduce Job performance in Hadoop

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问题 I have 100 mapper and 1 reducer running in a job. How to improve the job performance? As per my understanding: Use of combiner can improve the performance to great extent. But what else we need to configure to improve the jobs performance? 回答1: With the limited data in this question ( Input file size, HDFS block size, Average map processing time, Number of Mapper slots & Reduce slots in cluster etc.), we can't suggest tips. But there are some general guidelines to improve the performance. If

Oozie > Java action > why property oozie.launcher.mapred.child.java.opts does not work

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问题 I am working on Oozie with a Java action. The Java action should use Java option -Xmx15g. Accordingly I set the property oozie.mapreduce.map.memory.mb to 25600 (25G) in case some extra memory is needed. After this simple setting, I ran the Oozie job, then there was of course OutofMemory (heap out of space) error during Java runtime. So I set oozie.launcher.mapred.child.java.opts as -Xmx15g accordingly in the property node of the Java action based on the link: http://downright-amazed.blogspot

Oozie > Java action > why property oozie.launcher.mapred.child.java.opts does not work

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Too many open files in EMR

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问题 I am getting the following excpetion in my reducers: EMFILE: Too many open files at org.apache.hadoop.io.nativeio.NativeIO.open(Native Method) at org.apache.hadoop.io.SecureIOUtils.createForWrite(SecureIOUtils.java:161) at org.apache.hadoop.mapred.TaskLog.writeToIndexFile(TaskLog.java:296) at org.apache.hadoop.mapred.TaskLog.syncLogs(TaskLog.java:369) at org.apache.hadoop.mapred.Child$4.run(Child.java:257) at java.security.AccessController.doPrivileged(Native Method) at javax.security.auth

Hadoop: java.lang.IncompatibleClassChangeError: Found interface org.apache.hadoop.mapreduce.JobContext, but class was expected

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问题 My MapReduce jobs runs ok when assembled in Eclipse with all possible Hadoop and Hive jars included in Eclipse project as dependencies. (These are the jars that come with single node, local Hadoop installation). Yet when trying to run the same program assembled using Maven project (see below) I get: Exception in thread "main" java.lang.IncompatibleClassChangeError: Found interface org.apache.hadoop.mapreduce.JobContext, but class was expected This exception happens when program is assembled

Type mismatch in key from map: expected org.apache.hadoop.io.Text, recieved org.apache.hadoop.io.LongWritable

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问题 I am trying to run a map/reducer in java. Below are my files WordCount.java package counter; public class WordCount extends Configured implements Tool { public int run(String[] arg0) throws Exception { Configuration conf = new Configuration(); Job job = new Job(conf, "wordcount"); job.setOutputKeyClass(Text.class); job.setOutputValueClass(IntWritable.class); job.setMapperClass(WordCountMapper.class); job.setReducerClass(WordCountReducer.class); job.setInputFormatClass(TextInputFormat.class);

How to get Filename/File Contents as key/value input for MAP when running a Hadoop MapReduce Job?

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问题 I am creating a program to analyze PDF, DOC and DOCX files. These files are stored in HDFS. When I start my MapReduce job, I want the map function to have the Filename as key and the Binary Contents as value. I then want to create a stream reader which I can pass to the PDF parser library. How can I achieve that the key/value pair for the Map Phase is filename/filecontents? I am using Hadoop 0.20.2 This is older code that starts a job: public static void main(String[] args) throws Exception {

MapReduce优化参数

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资源相关参数 /*在MapReduce 应用程序中配置就可以生效*/ (1) mapreduce.map.memory.mb: 一个 Map Task 可使用的内存上限（单位 :MB ），默认为 1024 。如果 Map Task 实际使用的资源量超过该值，则会被强制杀死。 (2) mapreduce.reduce.memory.mb: 一个 Reduce Task 可使用的资源上限（单位 :MB ），默认为 1024 。如果 Reduce Task 实际使用的资源量超过该值，则会被强制杀死。 (3) mapreduce.map.cpu.vcores : 每个 Maptask 可用的最多 cpu core 数目 , 默认值 : 1 (4) mapreduce.reduce.cpu.vcores : 每个 Reducetask 可用最多 cpu core 数目默认值 : 1 (5) mapreduce.map.java.opts: Map Task 的 JVM 参数，你可以在此配置默认的 java heap size 等参数, 例如：“-Xmx1024m -verbose:gc -Xloggc:/tmp/@taskid@.gc” （ @taskid@ 会被 Hadoop 框架自动换为相应的 taskid ） , 默认值 : “” (6) mapreduce.reduce.java