Unexpected issue w/ m/r Riak.reduceSlice

Jonathan Langevin jlangevin at loomlearning.com
Tue Nov 29 17:21:44 EST 2011


When attempting to run m/r queries that execute Riak.reduceSlice to create
paginated result sets, I've found an unexpected result.

For instance, if I call Riak.reduceSlice with start = 80, end = 85, which
you would expect to return 5 results (knowing that you have a total of 115
objects stored in Riak), you might instead get 16 objects returned back.
This is due to the logic of Riak.reduceSlice, which just returns the
results-thus-far when it finds that the end value (85) is greater than the
current length of value (16 objects).

I suppose ultimately this is due to how Riak sends data in chunks between
phases (at least, that's my understanding)? So it happens to send a small
chunk of 16 results from the map phase to the reduce phase, and reduce
returns back the current "value" chunk since 16 results is less than 85
results...


How can I plan for this, so that I can ensure I'm getting the expected #
results back, and in fair context to the "page" of results that I should be
viewing?
And just a note, while I tested with & without sorted results, the final
code would be with sorted results that are then paginated.


Example data payload:
{
   "inputs":"demo_3_Course",
   "query":[
      {
         "map":{
            "keep":false,
            "language":"javascript",
            "arg":null,
            "source":"function(value){return [value.key];}"
         }
      },
      {
         "reduce":{
            "keep":true,
            "language":"javascript",
            "arg":[
               80,
               85
            ],
            "name":"Riak.reduceSlice"
         }
      }
   ]
}
*

 <http://www.loomlearning.com/>
 Jonathan Langevin
Manager, Information Technology
Loom Inc.
Wilmington, NC: (910) 241-0433 - jlangevin at loomlearning.com -
www.loomlearning.com - Skype: intel352
*
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