Actually, it would be almost as useful to be able to have a "multi-reduce".
In such a system, you would specify multiple input/map pairs. The reduce
function signature would then be something like:
reduce(WritableComparable key, OutputCollector, Reporter, Iterator ...)
Where the output of each set of maps would be given its own iterator.
I didn't mention this alternative earlier because I figured it would be a
much bigger leap than just ordering the reduce values. It would, however,
be very useful when it comes to co-grouping operations.
On 10/1/07 6:17 PM, "Ted Dunning" <[EMAIL PROTECTED]> wrote:
>
> This is a common requirement.
>
> Left unchanged would be fine but is probably very hard to enforce because of
> the many map tasks and some uncertainty about which maps finished first.
> Similarly useful would be the ability to require a particular sort ordering
> on reduce values.
>
>
> On 10/1/07 6:05 PM, "Chris Dyer" <[EMAIL PROTECTED]> wrote:
>
>> Does anyone know if Hadoop guarantees (can be made to guarantee) that the
>> relative order of keys that are equal will be left unchanged?
>
