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From Norman Barker <norman.bar...@gmail.com>
Subject pmap and document update example
Date Mon, 10 Aug 2009 15:01:19 GMT
Hi,

I commented on this in a recent thread, a couple of minutes later I
had a working example (compliments to the modular design of couchdb
and the easy set up with an ini file).

Folloing Joe Armstrong's book on pmap I have

update_db(JsonObject, Db)->
    NewDoc = couch_doc:from_json_obj(JsonObject),
    DocId = couch_util:new_uuid(),
    {ok, _NewRev} = couch_db:update_doc(Db, NewDoc#doc{id=DocId}, []).


pmap(F, L, Db) ->
    S = self(),
    %% make_ref() returns a unique reference
    %% we'll match on this later
    Ref = erlang:make_ref(),
    Pids = map(fun(I) ->
		       spawn(fun() -> do_f(S, Ref, F, I, Db) end)
	       end, L),
    %% gather the results
    gather(Pids, Ref).

do_f(Parent, Ref, F, I, Db) ->
    Parent ! {self(), Ref, (catch F(I, Db))}.

gather([Pid|T], Ref) ->
    receive
	  {Pid, Ref, Ret} -> [Ret|gather(T, Ref)]
    end;

gather([], _) ->
    [].


and then I invoke this with

    {FormResp} = my_update_query_servers:render_update(Lang, UpFun,
nil, nil, Req, Db),
    case proplists:get_value(<<"results">>, FormResp, nil) of
        undefined ->
            throw({error, no_result});
        ResultList ->
            Res = pmap(fun update_db/2, ResultList, Db)
    end,


is there something bad with this? my_update_query_servers returns a
collection of JSON objects and this seems to me to be a parallel
write.  Since I gather all the references I should be able to update
the caller with the list of created documents?

Norman

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