3.3. Erlang

Note

The Erlang query server is disabled by default. Read configuration guide about reasons why and how to enable it.

Emit(Id, Value)

Emits key-value pairs to view indexer process.

fun({Doc}) ->
    <<K,_/binary>> = proplists:get_value(<<"_rev">>, Doc, null),
    V = proplists:get_value(<<"_id">>, Doc, null),
    Emit(<<K>>, V)
end.

FoldRows(Fun, Acc)

Helper to iterate over all rows in a list function.

• Arguments

  • Fun – Function object.

  • Acc – The value previously returned by Fun.

fun(Head, {Req}) ->
    Fun = fun({Row}, Acc) ->
        Id = couch_util:get_value(<<"id">>, Row),
        Send(list_to_binary(io_lib:format("Previous doc id: ~p~n", [Acc]))),
        Send(list_to_binary(io_lib:format("Current  doc id: ~p~n", [Id]))),
        {ok, Id}
    end,
    FoldRows(Fun, nil),
    ""
end.

GetRow()

Retrieves the next row from a related view result.

%% FoldRows background implementation.
%% https://git-wip-us.apache.org/repos/asf?p=couchdb.git;a=blob;f=src/couchdb/couch_native_process.erl;hb=HEAD#l368
%%
foldrows(GetRow, ProcRow, Acc) ->
    case GetRow() of
        nil ->
            {ok, Acc};
        Row ->
            case (catch ProcRow(Row, Acc)) of
                {ok, Acc2} ->
                    foldrows(GetRow, ProcRow, Acc2);
                {stop, Acc2} ->
                    {ok, Acc2}
            end
end.

Log(Msg)

• Arguments

  • Msg – Log a message at the INFO level.

fun({Doc}) ->
    <<K,_/binary>> = proplists:get_value(<<"_rev">>, Doc, null),
    V = proplists:get_value(<<"_id">>, Doc, null),
    Log(lists:flatten(io_lib:format("Hello from ~s doc!", [V]))),
    Emit(<<K>>, V)
end.

After the map function has run, the following line can be found in CouchDB logs (e.g. at /var/log/couchdb/couch.log):

[Sun, 04 Nov 2012 11:33:58 GMT] [info] [<0.9144.2>] Hello from 8d300b86622d67953d102165dbe99467 doc!

Send(Chunk)

Sends a single string Chunk in response.

fun(Head, {Req}) ->
    Send("Hello,"),
    Send(" "),
    Send("Couch"),
    "!"
end.

The function above produces the following response:

Hello, Couch!

Start(Headers)

• Arguments

  • Headers – Proplist of response object.

Initialize List Functions response. At this point, response code and headers may be defined. For example, this function redirects to the CouchDB web site:

fun(Head, {Req}) ->
    Start({[{<<"code">>, 302},
            {<<"headers">>, {[
                {<<"Location">>, <<"http://couchdb.apache.org">>}]
            }}
        ]}),
    "Relax!"
end.