I hope I'm asking this in the right way. I'm learning my way around Elastic MapReduce and I've seen numerous references to the "Aggregate" reducer that can be used with "Streaming" job flows.
In Amazon's "Introduction to Amazon Elastic MapReduce" PDF it states "Amazon Elastic MapReduce has a default reducer called aggregrate"
What I would like to know is: are there other default reducers availiable?
I understand that I can write my own reducer, but I don't want to end up writing something that already exists and "reinvent the wheel" because I'm sure my wheel won't be as good as the original.
I'm in a similar situation. I infer from Google results etc that the answer right now is "No, there are no other default reducers in Hadoop", which kind of sucks, because it would be obviously useful to have default reducers like, say, "average" or "median" so you don't have to write your own.
http://hadoop.apache.org/common/docs/current/api/org/apache/hadoop/mapred/lib/aggregate/package-summary.html shows a number of useful aggregator uses but I cannot find documentation for how to access other functionality than the very basic key/value sum described in the documentation and in Erik Forsberg's answer. Perhaps this functionality is only exposed in the Java API, which I don't want to use.
Incidentally, I'm afraid Erik Forsberg's answer is not a good answer to this particular question. Another question for which it could be a useful answer can be constructed, but it is not what the OP is asking.