Mapreduce for dummies

Question

Ok, I am attempting to learn Hadoop and mapreduce. I really want to start with mapreduce and what I find are many, many simplified examples of mappers and reducers, etc. However, I seen to be missing something.

While an example showing how many occurrences of a word are in a document is simple to understand it does not really help me solve any "real world" problems. Does anybody know of a good tutorial of implementing mapreduce in a psuedo-realistic situation. Say, for instance, I want to use hadoop and mapreduce on top of a data store similar to Adventureworks. Now I want to get orders for a given product in the month of may. How would that look from a hadoop/mapreduce perspective? (I realize this may not be the type of problem mapreduce is intended to solve but, it just came to mind quickly.)

Any direction would help.

Answer 1

The book Hadoop: The Definitive Guide is a good place to start. The introductory chapters should be really useful to you to figure out where MapReduce is useful and when you should use it. The more advanced chapters have plenty of more realistic examples than word count.

If you want to dive deeper, you may want to check out Data-Intensive Text Processing with MapReduce. This definitely has plenty of "real-world" use cases, but it doesn't sound like you are interested in doing text processing.

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For your particular example, the main things to realize are:

• The map phase is mostly for parsing, transforming data, and filtering out data. Think record-by-record, shared-nothing approach to record processing. In word count, this is parsing the line and splitting out the words.
• The reduce phase is all about aggregation: counting, averaging, min/max, etc. In word count, this is counting up the instances of the word.

So, if you would want all the records for a given product in the month of May, you could use a map-only job to filter through all the data and only keep the records you want. However, you really should read about what Hadoop is useful for. The question that would fit Hadoop better would be: give me a count of how many times every item was purchased in every month (to build a matrix, perhaps). Very rarely are you looking for specific records like you suggest.

If you are looking for a more real-time access platform, you should check out HBase once you are done learning about Hadoop.

Answer 2

Hadoop can be used for a wide variety of problems. Check this blog entry from atbrox. Also, there is a lot of information on the internet about Hadoop and MapReduce and it's easy to get lost. So, here is the consolidated list of resources on Hadoop.

BTW, Hadoop - The Definitive Guide 3rd edition is due in May. Looks like it also covers MRv2 (NextGen MapReduce) and also includes more case studies. The 2nd edition is worth as mentioned by orangeoctopus.