Multiple functions of any phase type are allowed. They're performed on the coordinating node and you can force Riak to wait for all data to arrive before perform a reduce [1].
There's a thread from June 2010 talking about working with multiple Map phases [2] as well as this delightful Basho blog post [3]. [1]: http://wiki.basho.com/MapReduce.html#Configuration-Tuning-for-Reduce-Phases [2]: http://lists.basho.com/pipermail/riak-users_lists.basho.com/2010-June/001418.html [3]: http://basho.com/blog/technical/2010/04/14/practical-map-reduce:-forwarding-and-collecting/ --- Jeremiah Peschka - Founder, Brent Ozar PLF, LLC Microsoft SQL Server MVP On Oct 28, 2011, at 7:05 AM, Dmitry Demeshchuk wrote: > Why do you need such a way? > > If you have some certain list of operations to perform on each object, > you can just combine them inside a body of a single map function. > > If this list is dynamic and may vary depending on a situation – you > can pass the list of functions to execute as the second parameter to > the map function. > > On the contrary, reduce functions cannot be combined like that – you > generally need to gather all the results of a previous reduce to get a > consistent result of the next one. So, multiple reduce functions are > not just allowed, but sometimes even necessary. > > On Fri, Oct 28, 2011 at 5:59 PM, Lyes zaiko <[email protected]> wrote: >> Hi all! >> I want to execute a mapreduce query, in erlang, that contains two map phases >> such that the Map2 function takes the result of the Map1 function as input. >> Is it possible and if, what must be the return value of each map phase >> Thank you! >> _______________________________________________ >> riak-users mailing list >> [email protected] >> http://lists.basho.com/mailman/listinfo/riak-users_lists.basho.com >> >> > > > > -- > Best regards, > Dmitry Demeshchuk > > _______________________________________________ > riak-users mailing list > [email protected] > http://lists.basho.com/mailman/listinfo/riak-users_lists.basho.com _______________________________________________ riak-users mailing list [email protected] http://lists.basho.com/mailman/listinfo/riak-users_lists.basho.com
