My mochiweb webservice...

Now that mochiweb is popular, and that I use it for some of my work, I can publish here some webservice thingy that I use.

I wanted to have this simple thing: modulename,funname, and parameters inside the URL:

http://www.server.tld/modulename/fun/arg1/arg2 
wmodulename:Fun(arg1, arg2)

(Note the 'w' prefix, I explain it later)

Here's the code:

-module(webservice).
-export([start/0, start/2, loop/2, stop/0, test/0]).
-export([dolog/2]).
-define(PORT, 8080).

start() ->
 start("/home/rolphin/Devel/Web", ?PORT).

start(Wwwroot, Port) ->
 Loop = fun (Req) ->
  ?MODULE:loop(Req, Wwwroot)
 end,
 mochiweb_http:start([{loop, Loop}, {name, ?MODULE}, {port, Port}]).

stop() ->
    mochiweb_http:stop(?MODULE).

loop(Req, DocRoot) ->
 log(Req),
 case string:tokens(Req:get(path), "/") of
  [ "dump" ] ->
   Req:ok({"text/plain",
    io_lib:format("~p~n", [Req:dump()])});

  [ "favicon.ico" ] ->
   Req:respond({404, [], ""});

  [ "codepath" ] ->
   Req:ok({"text/plain",
    io_lib:format("codepath: ~p~n", [code:get_path()])});

  [ "codepath", "json" ] ->
   Req:ok({"text/plain",
    mochijson:encode({array, code:get_path()})});

  [ Path, Fun | Elems ] ->
   % Every module name should begin with 'w'
   dispatch(Req, DocRoot, list_to_atom("w" ++ Path), Fun, Elems);

  [] ->
   launch(Req, DocRoot, wdefault, do, []);

  _ ->
   Req:respond({502, [], []})
   
 end.

dispatch(Req, DocRoot, Module, Fun, Elems) ->
 M = Req:get(method),
 case M of
  'GET' ->
   launch(Req, DocRoot, Module, Fun, Elems);
  'POST' ->
   launch(Req, DocRoot, Module, Fun, Elems);
  'PUT' ->
   launch(Req, DocRoot, Module, Fun, Elems);
  'DELETE' ->
   launch(Req, DocRoot, Module, Fun, Elems);
  'HEAD' ->
   launch(Req, DocRoot, Module, Fun, Elems);
  _Any ->
   launch(Req, DocRoot, wdefault, get, [])
 end.

launch(Req, DocRoot, wcontent, Fun, Args) ->
 case catch wcontent:default(Req, DocRoot, [ Fun | Args] ) of
  {'EXIT', {Type, _Error}} ->
   Req:ok({"text/plain",  
    io_lib:format("GET Error: '~p' for '~p' ~p ~p~n~p~n", [Type, wcontent, Fun, Args, _Error])});
  _ ->
   ok
 end;
 
launch(Req, DocRoot, Module, Fun, Args) ->
 F = list_to_atom(Fun),
 case catch Module:F(Req, DocRoot, Args) of
  {'EXIT', {Type, _Error}} ->
   Req:ok({"text/plain",  
    io_lib:format("~p Error: '~p' for ~p ~p ~p~n~p~n", [Req:get(method), Type, Module, Fun, Args, _Error])});
  _ ->
   ok
 end.

log(Req) ->
 Ip = Req:get(peer),
 spawn(?MODULE, dolog, [Req, Ip]).

dolog(Req, Ip) ->
 stat_logger:log("~p ~p", [Ip, Req:get(path)]).

First you can see that I use "mochiweb:start" and "mochiweb:stop" in the "start" and "stop" funs.
Then parameters are extracted from the URI:

 case string:tokens(Req:get(path), "/") of

I split the URI on the "/" character, and compare the resulting list with various possibilities.

Then the main part of the code:

launch(Req, DocRoot, Module, Fun, Args) ->
 F = list_to_atom(Fun),
 case catch Module:F(Req, DocRoot, Args) of
  {'EXIT', {Type, _Error}} ->
   Req:ok({"text/plain",  
    io_lib:format("~p Error: '~p' for ~p ~p ~p~n~p~n", [Req:get(method), Type, Module, Fun, Args, _Error])});
  _ ->
   ok
 end.

Using "catch" prevent your app from crashing without any information. If a module doesn't exists, you'll be able to see why (the main problem is sometimes the module is not in the code:path...)

Every webservice module name should start with "w", it's some sort of namespace :), now you know the reason for the 'w' prefix:

  [ Path, Fun | Elems ] ->
   % Every module name should begin with 'w'
   dispatch(Req, DocRoot, list_to_atom("w" ++ Path), Fun, Elems);

Now, how can I use it ? It's simple, you just have to build a module with such a template:

-module(wsample).
-export([do/3]).

do(Req, _DocRoot, Args) ->
 Req:ok({"text/plain", [ <<"Hello ">>, hd(Args) , <<", you are authorized :)">>] }).

Then you build it, and store it somewhere in your "code:path". And call

webservice:start().

in your erl shell. Then locate your webrowser to the URI

www.server.tld:8080/sample/do/PUT-ANYTHING-YOU-WANT-HERE

As a sample module, here is a module that is used everyday the "mobile tags" generator:

-module(wbarcode).
-export([do/3]).

-define(PNG, "image/png").

do(Req, _DocRoot, _Args) ->
 barcode(Req).

% the process 'barcode' must be there
barcode(Req) ->
 Options = Req:parse_qs(),
 %io:format("~p~n", [Options]),

 Text = proplists:get_value("text", Options, "http://www.shootit.fr"),
 Res = proplists:get_value("res", Options, "72"),
 Z = proplists:get_value("z", Options, "1"),

 case barcode:generate('barcode', Text) of 
  timeout ->
   Req:ok({"text/plain", [], <<"Timeout">>});
  
  {data, {Width, Height}, Data} ->
   Zoom = list_to_integer(Z),
   Xres = integer_to_list(list_to_integer(Res)  * Zoom),
   Yres = integer_to_list(list_to_integer(Res)  * Zoom),
   Params = [ {"PS", [integer_to_list(Width + 1), integer_to_list(Height + 1)]}, {"HW", [Xres, Yres]} ],
   case gs:draw(gs, Data, Params) of
    {png, Image} ->
     Req:ok({?PNG, Image});

    _E ->
     Req:ok({"text/plain", [], [ <<"Error: ">>, 
      io_lib:format("~s", [_E])]})
   end
 end.