How to call mapreduce from Riak client?

Miroslav Urbanek miroslav.urbanek at
Tue Feb 26 16:39:11 EST 2013

Dear Riak users,

I have an Erlang script for counting the number of objects in a Riak bucket:

Inputs = <<"buckets">>,
Phases = [{reduce,{modfun,riak_kv_mapreduce,reduce_count_inputs},none,true}],
{ok, Client} = riak:client_connect(Host)
Client:mapred(Inputs, Phases)

After the update to 1.3, this no longer works. In the mailing list
archive I found an alternative method:

riak_kv_mrc_pipe:mapred(Inputs, Phases)

However, I'm getting the following error:

escript: exception exit: {noproc,{gen_server,call,
  in function  gen_server:call/3 (gen_server.erl, line 188)
  in call from riak_kv_mrc_pipe:mapred_stream_sink/3
(src/riak_kv_mrc_pipe.erl, line 266)
  in call from riak_kv_mrc_pipe:mapred/3 (src/riak_kv_mrc_pipe.erl, line 219)

What does it mean? What's the optimal way to count number of keys in a
bucket in Riak 1.3 anyway?

Here's the complete script:

#!/usr/bin/env escript

main(_) ->

    Host = 'riak at domain',
    Inputs = <<"bucket">>,
    Phases = [{reduce,{modfun,riak_kv_mapreduce,reduce_count_inputs},none,true}],

    ok = application:start(inets),
    {ok, _} = net_kernel:start([count]),
    true = auth:set_cookie('riak'),
    true = net_kernel:connect_node(Host),

    case riak_kv_mrc_pipe:mapred(Inputs, Phases) of
        {ok, Result} -> io:format("OK: ~p~n", Result);
        {error, Reason} -> io:format("Error: ~p~n", Reason)


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