On Tue, Sep 23, 2008 at 5:40 PM, Sandy <[EMAIL PROTECTED]> wrote: > > I just wrote a combiner class to try and speed things up. However, now I > want to do something like the following: > ==map phase== > input: key = LongWritable value = Text, > output: key = Text, value = Longwritable > > ==combiner== > input: key = Text, value = iterator<LongWritable> > output: key = Text, value = Text
The input and output types for the combiner *must* be the same. The combiner may be applied 0, 1, or many times between the map and the reduce. So, combiners must be: * not depend on being run exactly once * not have side effects InputFormat -> Map -> Combiner* -> Reduce -> OutputFormat Since the Combiner may run more than once, it can't do type transformations. -- Owen
