为什么减速器在我的情况下不能正常工作？

Question

为什么 "set" 只有一个元素，而例如前 5 行具有相同 URL 和四个不同 IP 的输入应该有 4 个元素。我也使用 "for-each" 而不是 "iterator" 但不起作用。有人可以帮助我吗？

映射器

public class WordCount {

    public static class TokenizerMapper extends Mapper<Object, Text, Text, Text> {

        private Text IP = new Text();
        private Text word = new Text();

        public void map(Object key, Text value, Context context) throws IOException, InterruptedException {
            String line = value.toString();
            String[] tokens = line.split(",");
            word.set(tokens[2]);
            IP.set(tokens[0]);
            context.write(word, IP);
        }
    }

减速机

    public static class IntSumReducer extends Reducer<Text, Text, Text, Text> {

        public void reduce(Text key, Iterable<Text> values, Context context) throws IOException, InterruptedException {
            Set<String> set = new HashSet<String>();
            Iterator<Text> iterator = values.iterator();
            while (iterator.hasNext()) {
                set.add(iterator.next().toString());
            }
            int a = set.size();
            String str = String.format("%d", a);
            context.write(key, new Text(str));
        }
    }

工作

    public static void main(String[] args) throws Exception {
        Job job = new Job();
        job.setJarByClass(WordCount.class);
        job.setMapperClass(TokenizerMapper.class);
        job.setCombinerClass(IntSumReducer.class);
        job.setReducerClass(IntSumReducer.class);
        job.setOutputKeyClass(Text.class);
        job.setOutputValueClass(Text.class);
        FileInputFormat.addInputPath(job, new Path(args[0]));
        FileOutputFormat.setOutputPath(job, new Path(args[1]));
        System.exit(job.waitForCompletion(true) ? 0 : 1);       
    }
}

输入

"10.131.0.1","[29/Nov/2017:14:31:33","GET / HTTP/1.1","200"
"10.131.0.2","[29/Nov/2017:14:31:38","GET / HTTP/1.1","200"
"10.131.0.3","[29/Nov/2017:14:31:56","GET / HTTP/1.1","200"
"10.131.0.4","[29/Nov/2017:14:32:02","GET / HTTP/1.1","404"
"10.131.0.5","[29/Nov/2017:16:31:39","GET / HTTP/1.1","200"
"10.131.0.1","[29/Nov/2017:14:05:35","GET /contest.php HTTP/1.1","200"
"10.131.0.2","[29/Nov/2017:14:05:38","GET /contest.php HTTP/1.1","200"
"10.131.0.3","[29/Nov/2017:14:05:50","GET /contest.php HTTP/1.1","404"
"10.131.0.1","[29/Nov/2017:13:51:41","GET /login.php HTTP/1.1","200"
"10.131.0.2","[29/Nov/2017:13:51:49","GET /login.php HTTP/1.1","200"
"10.131.0.1","[29/Nov/2017:13:51:46","GET /contestproblem.php?name=RUET%20OJ%20Server%20Testing%20Contest HTTP/1.1","200"
"10.131.0.8","[29/Nov/2017:13:51:46","GET /contestproblen.php?name=RUET%20OJ%20Server%20Testing%20Contest HTTP/1.1","200"

我的结果是

"GET / HTTP/1.1"    1
"GET /contest.php HTTP/1.1" 1
"GET /contestproblem.php?name=RUET%20OJ%20Server%20Testing%20Contest HTTP/1.1"  1
"GET /contestproblen.php?name=RUET%20OJ%20Server%20Testing%20Contest HTTP/1.1"  1
"GET /login.php HTTP/1.1"   1

Answer 1

Reducer 工作正常，但 Combiner 没有按照您的想法进行。 Combiner 打开时发生的情况是：

MAPPER 输出：

("GET / HTTP/1.1", "10.31.0.1")
("GET / HTTP/1.1", "10.31.0.2")

合成器输入：

("GET / HTTP/1.1", {"10.31.0.1", "10.31.0.2"})

组合器输出：

("GET / HTTP/1.1", "2") //You have the right answer here...

减速器输入：

("GET / HTTP/1.1", {"2"}) //...but then it gets passed into the Reducer again

减速器输出：

("GET / HTTP/1.1", "1")

只有一个元素进入Reducer，所以它减少到“1”。

移除 Combiner（删除 job.setCombinerClass(IntSumReducer.class); 即可。

其他建议更改：

让 Reducer 输出 IntWritable 而不是将数字转换为 Text。
将 Set 设为 Set<Text> 而不是 Set<String>，以节省昂贵的 Text -> String 转换。

为什么减速器在我的情况下不能正常工作？

Why reducer does not work correctly in my case?

java

hadoop

mapreduce