Hadoop - 复合键 [英] Hadoop - composite key

问题描述

假设我有一个以制表符分隔的文件，其中包含格式如下的用户活动数据:

Suppose I have a tab delimited file containing user activity data formatted like this:

timestamp  user_id  page_id  action_id

我想编写一个 hadoop 作业来计算每个页面上的用户操作，因此输出文件应如下所示:

I want to write a hadoop job to count user actions on each page, so the output file should look like this:

user_id  page_id  number_of_actions

我需要像复合键这样的东西 - 它会包含 user_id 和 page_id.有没有什么通用的方法可以用 hadoop 做到这一点?我找不到任何有用的东西.到目前为止，我在映射器中发出这样的键:

I need something like composite key here - it would contain user_id and page_id. Is there any generic way to do this with hadoop? I couldn't find anything helpful. So far I'm emitting key like this in mapper:

context.write(new Text(user_id + "	" + page_id), one);

它有效，但我觉得这不是最好的解决方案.

It works, but I feel that it's not the best solution.

推荐答案

只需编写您自己的 Writable.在您的示例中，解决方案可能如下所示:

Just compose your own Writable. In your example a solution could look like this:

public class UserPageWritable implements WritableComparable<UserPageWritable> {

  private String userId;
  private String pageId;

  @Override
  public void readFields(DataInput in) throws IOException {
    userId = in.readUTF();
    pageId = in.readUTF();
  }

  @Override
  public void write(DataOutput out) throws IOException {
    out.writeUTF(userId);
    out.writeUTF(pageId);
  }

  @Override
  public int compareTo(UserPageWritable o) {
    return ComparisonChain.start().compare(userId, o.userId)
        .compare(pageId, o.pageId).result();
  }

}

虽然我认为您的 ID 可能是 long，但这里您有 String 版本.基本上只是对 Writable 接口的正常序列化，注意它需要默认构造函数，所以你应该总是提供一个.

Although I think your IDs could be a long, here you have the String version. Basically just the normal serialization over the Writable interface, note that it needs the default constructor so you should always provide one.

compareTo 逻辑清楚地告诉了如何对数据集进行排序，并告诉化简器哪些元素是相等的，以便可以将它们分组.

The compareTo logic tells obviously how to sort the dataset and also tells the reducer what elements are equal so they can be grouped.

ComparisionChain 是 Guava 的一个很好的工具.

ComparisionChain is a nice util of Guava.

不要忘记覆盖equals和hashcode！partitioner会根据key的hashcode来确定reducer.

Don't forget to override equals and hashcode! The partitioner will determine the reducer by the hashcode of the key.

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