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Date: Wed, 11 Feb 2009 14:21:48 -0800
From: Michael McDaniel <couchdb@autosys.us>
To: user@couchdb.apache.org
Subject: Re: Thoughts on document/views...
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On Thu, Feb 12, 2009 at 07:30:17AM +1030, Antony Blakey wrote:
>
> On 12/02/2009, at 7:27 AM, Antony Blakey wrote:
>
>> That guard could be an erlang function, defined in the design doc.
>
> And an obvious extension is to allow view functions to be written in  
> erlang - no JSON conversion, no external.
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

 I am working on an Erlang view server (EVS).  It currently works
 in a rudimentary fashion.  I have been testing from Futon.  I do
 not think it is currently as flexible as the default javascript map
 funs, though do not know all the map fun capabilities available with
 the default. 


 The only CDB changes I have needed are to add the following two
 functions to couch_query_servers.erl  (valid as of CDB v740870)
 before their respective existing funs.  

 The start_doc_map/2 fun is different only in that it provides EVS
 Pid rather than using get_os_process(Lang).  Because the EVS is
 started as a daemon and not like main.js is started, 
 get_os_process(Lang) does not work as written. 


 The map_docs/2 fun is different in that no JSON <-> Erlang term()
 conversion is done (not needed when talking Erlang <-> Erlang).


start_doc_map(<<"erlang">>, Functions) ->
  Pid = erlang:whereis(erlview) ,   % need to parameterize erlview
                                    % Pid is fed to map_docs
    lists:foreach(fun(FunctionSource) ->
                      true = couch_os_process:prompt(Pid, 
                                                    [<<"add_fun">>, 
                                                     FunctionSource])
                  end, Functions),
    {ok, {<<"erlang">>, Pid}}
;


% Pid comes from start_doc_map/2 (see call in couch_view_updater)
map_docs({<<"erlang">>, Pid}, Docs) ->
    Results = lists:map( fun( Doc ) ->
                           couch_os_process:prompt(Pid, [<<"map_doc">>, Doc])
                         end,
                         Docs) ,
    {ok, Results}
;


NOTE that the additional start_doc_map/2 fun can be eliminated if you
want to change get_os_process/1 to ...

get_os_process(Lang) ->
   case Lang 
     of <<"erlang">> -> erlang:whereis(erlview) ;
     _               -> gen_server:call(couch_query_servers,
                                       {get_proc, Lang})
   end
.


 Also note that, obviously, the EVS name 'erlview' is hard-coded and
 should be a local.ini parameter.


 Above is a first-cut; possibly a message passing protocol to the
 EVS would be faster though that would require more CDB hacking.


 I'll post some code when it does a bit more than gurgle bubbles.


 Please, if there is some activity to change CDB internals to 
 simplify a native Erlang view server or create a more direct
 interface, let me know so I don't go too far down this road.

 No sense replicating effort (only databases!).

~Michael


>
> Antony Blakey
> -------------
> CTO, Linkuistics Pty Ltd
> Ph: 0438 840 787
>
> One should respect public opinion insofar as is necessary to avoid  
> starvation and keep out of prison, but anything that goes beyond this is 
> voluntary submission to an unnecessary tyranny.
>   -- Bertrand Russell
>
>

-- 
Michael McDaniel
Portland, Oregon, USA
http://trip.autosys.us
http://autosys.us