MapReduce常见算法

快来打我* 2021-09-29 02:34 260阅读 0赞

MapReduce常见算法

*  单词计数 
 *  数据去重 
 *  排序 
 *  Top K 
 *  选择　　以求最值为例，从100万数据中选出一**行**最小值 
 *  投影　　以求处理手机上网日志为例，从其11个字段选出了五个字段(**列**)来显示我们的手机上网流量 
 *  分组　　相当于**分区**，以求处理手机上网日志为例，喊手机号和非手机号分为两组 
 *  多表连接 
 *  单表关联

使用TopK算法找出文件中的最大数：

1 package suanfa;
     2 
     3 import java.io.IOException;
     4 import java.net.URI;
     5 
     6 import org.apache.hadoop.conf.Configuration;
     7 import org.apache.hadoop.fs.FileSystem;
     8 import org.apache.hadoop.fs.Path;
     9 import org.apache.hadoop.io.LongWritable;
    10 import org.apache.hadoop.io.NullWritable;
    11 import org.apache.hadoop.io.Text;
    12 import org.apache.hadoop.mapreduce.Job;
    13 import org.apache.hadoop.mapreduce.Mapper;
    14 import org.apache.hadoop.mapreduce.Reducer;
    15 import org.apache.hadoop.mapreduce.lib.input.FileInputFormat;
    16 import org.apache.hadoop.mapreduce.lib.output.FileOutputFormat;
    17 
    18 /**
    19  * 找出文件中的最大数
    20  * 
    21  * @author ahu_lichang
    22  * 
    23  */
    24 public class TopKApp {
    25     static final String INPUT_PATH = "hdfs://chaoren:9000/input";
    26     static final String OUT_PATH = "hdfs://chaoren:9000/out";
    27 
    28     public static void main(String[] args) throws Exception {
    29         Configuration conf = new Configuration();
    30         FileSystem fileSystem = FileSystem.get(new URI(INPUT_PATH), conf);
    31         Path outPath = new Path(OUT_PATH);
    32         if (fileSystem.exists(outPath)) {
    33             fileSystem.delete(outPath, true);
    34         }
    35         Job job = new Job(conf, TopKApp.class.getSimpleName());
    36         FileInputFormat.setInputPaths(job, INPUT_PATH);
    37         job.setMapperClass(MyMapper.class);
    38         job.setReducerClass(MyReducer.class);
    39         job.setOutputKeyClass(LongWritable.class);
    40         job.setOutputValueClass(NullWritable.class);
    41         FileOutputFormat.setOutputPath(job, outPath);
    42         job.waitForCompletion(true);
    43     }
    44 
    45     static class MyMapper extends
    46             Mapper<LongWritable, Text, LongWritable, NullWritable> {
    47         long max = Long.MIN_VALUE;
    48 
    49         protected void map(LongWritable k1, Text v1, Context context)
    50                 throws java.io.IOException, InterruptedException {
    51             long temp = Long.parseLong(v1.toString());
    52             if (temp > max) {
    53                 max = temp;
    54             }
    55         }
    56 
    57         @Override
    58         protected void cleanup(
    59                 org.apache.hadoop.mapreduce.Mapper<LongWritable, Text, LongWritable, NullWritable>.Context context)
    60                 throws IOException, InterruptedException {
    61             context.write(new LongWritable(max), NullWritable.get());
    62         }
    63     }
    64 
    65     static class MyReducer extends
    66             Reducer<LongWritable, NullWritable, LongWritable, NullWritable> {
    67         long max = Long.MIN_VALUE;
    68 
    69         protected void reduce(
    70                 LongWritable k2,
    71                 Iterable<NullWritable> v2s,
    72                 org.apache.hadoop.mapreduce.Reducer<LongWritable, NullWritable, LongWritable, NullWritable>.Context context)
    73                 throws IOException, InterruptedException {
    74             long temp = k2.get();
    75             if (temp > max) {
    76                 max = temp;
    77             }
    78         }
    79 
    80         @Override
    81         protected void cleanup(
    82                 org.apache.hadoop.mapreduce.Reducer<LongWritable, NullWritable, LongWritable, NullWritable>.Context context)
    83                 throws IOException, InterruptedException {
    84             context.write(new LongWritable(max), NullWritable.get());
    85         }
    86     }
    87 }

![1110462-20170403115655910-2003317789.png][]

遇见一个问题：在删除HDFS中的文件的时候，说文件时安全模式下，无法删除？

这时候要想删除该文件，必须退出安全模式，Hadoop退出安全模式的命令是：**hadoop dfsadmin -safemode leave**

转载于:https://www.cnblogs.com/ahu-lichang/p/6661596.html

[1110462-20170403115655910-2003317789.png]: https://images2015.cnblogs.com/blog/1110462/201704/1110462-20170403115655910-2003317789.png