Hi Daniel, the values for a single key will be passed to reduce() in a non-predictable order. Actually, when running the same job on the same data again, the order is most likely different every time.
If you want the values to be in a sorted way, you need to apply a 'secondary sort'. The basic idea is to attach your values to the key, and then benefit from the sorting Hadoop does on the key. However, you need to write some code to make that happen. Josh wrote a nice series of articles on it, and you will find more if you google for "secondary sort". http://www.cloudera.com/blog/2011/03/simple-moving-average-secondary-sort-and-mapreduce-part-1/ http://www.cloudera.com/blog/2011/03/simple-moving-average-secondary-sort-and-mapreduce-part-2/ http://www.cloudera.com/blog/2011/04/simple-moving-average-secondary-sort-and-mapreduce-part-3/ Kai Am 20.09.2011 um 07:43 schrieb Daniel Yehdego: > > Good evening, > I have a certain value output from a mapper and I want to concatenate the > string outputs using a Reducer (one reducer).But the order of the > concatenated string values is not in order. How can I use a reducer that > receives a value from a mapper output and concatenate the strings in order. > waiting your response and thanks in advance. > > Regards, > > Daniel T. Yehdego > Computational Science Program > University of Texas at El Paso, UTEP > [email protected] -- Kai Voigt [email protected]
