What's the best way to count unique visitors with Hadoop?

Question

hey all, just getting started on hadoop and curious what the best way in mapreduce would be to count unique visitors if your logfiles looked like this...

DAT         


        

Answer 1

You could do it as a 2-stage operation:

First step, emit (username => siteID), and have the reducer just collapse multiple occurrences of siteID using a set - since you'd typically have far less sites than users, this should be fine.

Then in the second step, you can emit (siteID => username) and do a simple count, since the duplicates have been removed.

Answer 2

It is often faster to use HiveQL to sort many simple tasks. Hive will translate your queries into Hadoop MapReduce. In this case you may use

SELECT COUNT(DISTINCT username) FROM logviews

You may find a more advanced example here: http://www.dataminelab.com/blog/calculating-unique-visitors-in-hadoop-and-hive/

Answer 3

My aproach is similar to what tzaman gave with a small twist

1. map output : (username, siteid) => ("")
2. reduce output: (siteid) => (1)
3. map : identity mapper
4. reduce : longsumreducer (i.e. simply summarize)

Note that the first reduce does not need to go over any of the records is gets presented. You can simply examine the key and produce the output.

HTH

Answer 4

Use the secondary sort to sort on user id. That way, you don't need to have anything in memory -- just stream the data through, and increment your distinct counter every time you see the value change for a particular site id.

Here is some documentation.