Getting a value: get vs map

Jeremiah Peschka jeremiah.peschka at gmail.com
Fri Jul 29 10:27:34 EDT 2011


I would have suspected that an MR job where you supply a Bucket, Key pair would be just as fast as a Get request. Shows what I know.
---
Jeremiah Peschka
Founder, Brent Ozar PLF, LLC

On Jul 29, 2011, at 1:37 AM, Antonio Rohman Fernandez wrote:

> MapReduce ( or a simply Map ) gets really slow when database has a significant amount of data ( or distributed over several servers ). Get instead is always faster as Riak doesn't have to search for the key ( you tell Riak exactly where to GET the data in your url )
> 
> Rohman
> 
> On Thu, 28 Jul 2011 23:43:06 +0400, mss at mawhrin.net wrote:
> 
>> Hi,
>> 
>> (I looked at various places for the information, however I could not
>> find anything that would answer the question.  It's not completely ruled
>> out that not all places were checked though :))
>> 
>> I use PB erlang interface to access the database.  Given a bucket name
>> and a key, the value can easily be extracted using:
>> 
>>     {ok, Object} = riakc_pb_socket:get(Conn, Bucket, Key),
>>     Value = riakc_obj:get_value(Object)
>> 
>> Alternatively, a mapred (actually, just map) request could be issued:
>> 
>>     {ok, [{_, Value}]} = riakc_pb_socket:mapred(Conn, [
>>         {Bucket, Key}
>>     ], [
>>         {map, {modfun, riak_kv, map_object_value}, none, true}
>>     ])
>> 
>> I would expect that the result is the same while in the second case, the
>> amount of data transferred to the client is smaller (which might be good
>> for certain situations).
>> 
>> So the [open] question is: are there any reasons for using the first
>> approach over the second?
>> 
>> --
>> Misha
>> 
> -- 
> 
> 	 	Antonio Rohman Fernandez
> CEO, Founder & Lead Engineer
> rohman at mahalostudio.com	 	Projects
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