Behavior for writing web applications

[kiran khaladkar]

Hi,

When working on erlang for almost more than 2 years now, I felt using there
could have been more generic behaviours than the standard one’s i.e.
gen_server, gen_fsm, gen_event. I wanted a behaviour that is suited for
writing web applications
Following is the attempt to write such a behaviour


%%%-------------------------------------------------------------------------
%%% Author: Kiran Khaladkar
%%% Description: Behavior for writing web applications.
%%% Created: Sat 5th Dec 2009
%%%-------------------------------------------------------------------------

-module(gen_app).
-export([behavior_info/1, start/3, start/2]).
-behavior(gen_server).

-record(gen_app_state, {app_name :: atom(),         %%stores app name
            cookie :: string(),         %%cookie to be stored to refer to
client
            app_state :: tuple()         %%state maintained by the apps
extending this behavior
                   }).

-export([init/1, handle_call/3, handle_cast/2, handle_info/2,
         terminate/2, code_change/3]).

-export([get_info/1, get_cookie/1, set_cookie/2, handle_sync_request/2,
handle_async_request/2]).

behavior_info(callbacks) ->
    [{init,1}, {handle_request, 2}, {respond, 1},{terminate,1}].

%% Function: start/3
%% Description: starts the application by AppName with Params and Options.
%%        Options are gen_app process options
%% Return: {ok, Pid}
start(AppName, Param, Options) ->
    gen_server:start_link(?MODULE, {AppName,Param}, Options).

%% Function: start/2
%% Description: Starts the application by AppName with params but without
Options
%% Return: {ok, Pid}
start(AppName, Param) ->
    gen_server:start_link(?MODULE, {AppName, Param}, []).

init({AppName, Param}) ->
    {ok, AppInitCookie, AppInitState} = AppName:init(Param),
    {ok, #gen_app_state{app_name = AppName, cookie = AppInitCookie,
app_state = AppInitState}}.

%% Function: get_info/1
%% Return: returns state of the gen_app
get_info(AppId) ->
    gen_app:handle_sync_request(AppId, get_state).

get_cookie(AppId) ->
    handle_sync_request(AppId, get_cookie).

set_cookie(AppId, Cookie) ->
    handle_sync_request(AppId, {set_cookie, Cookie}).

%% Funtion: handle_async_request(/2
%% Description: API to handle asynchronous calls that return immidiately
without results.
%% Rerurn: {noreply, State}
handle_async_request(AppId, Reqest) ->
    gen_server:handle_cast(AppId, {app_req, Reqest}),
    {ok, processing}.

%% Function: handle_sync_request/2
%% Description: API to handle synchronous calls that return the result of
the
%%        request after completion of the processing of that request
%% Return: {reply, Reply, NewState} after completion of the processing
handle_sync_request(AppId, Request) ->
    gen_server:handle_call(AppId, {app_req, Request}).

%% Funtion: handle_cast/2
%% Desciption: handle cast messages to gen_app process
%% Return: {noreply, #state{}}
handle_cast({app_req, Request}, #gen_app_state{app_name=AppName,
app_state=AppState}=State) ->
    {ok, NewAppState, Response}=AppName:handle_request(Request, AppState),
    AppName:respond(Response),
    {noreply, State#gen_app_state{app_state=NewAppState}};

handle_cast(_Msg, _State) ->
    {noreplay, _State}.

%% Funtion: handle_call/3
%% Desciption: handle cast messages to gen_app process
%% Return: {noreply, #state{}}
handle_call(get_state,_From, State) ->
    {reply, app_state, State};

handle_call({app_req, Req},_From, #gen_app_state{app_name = AppName,
app_state = AppState} = State) ->
    {ok, NewAppState, Response}=AppName:handle_request(Req, AppState),
    AppName:respond(Response),
    {reply, ok, State#gen_app_state{app_state=NewAppState}};

handle_call(get_cookie, _From, State) ->
    {reply, State#gen_app_state.cookie, State};

handle_call({set_cookie, Cookie}, _From, State)->
    {reply, ok, State#gen_app_state{cookie = Cookie}};

handle_call(_Msg,_From, _State) ->
    Reply = _Msg,
    {reply, Reply, _State}.

handle_info(_Info, _State) ->
    {noreply, _State}.

code_change(_OldVsn, State, _Extra) ->
    {ok, State}.

terminate(Reason, #gen_app_state{app_name=AppName, app_state=AppState} =
_State) ->
    AppName:terminate(Reason, AppState).

Need your comments on this...
regards,
Kiran