Hadoop Using composite-key

Question

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

timestamp  user_id  page_id  action_id

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

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 + "\t" + page_id), one);

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

Answer 1

You can use a Writable, something 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();
  }

}