Erlang and Webmachine Tres

In my first post, I talked about the basic technologies I'd be using, and how to get an interactive shell started in emacs that started up your webmachine application. In this third post, I'll talk a little about building a session handling module. This will hopefully familiarize you with using redirects and cookies in your webmachine resources and using Couchbeam to store and retrieve documents. The high-level usage for sessions I have currently is:

1. User agent accesses a supported resource (probably a browser accessing the home page initially).
   • I am defining the is\_authorized/2 function in all of my resource files that correspond to web pages, as well as the finish\_request/2 function.

   • The second parameter in the functions is a record, currently defined as

     -record(context, {session}).
     

2. In the is\_authorized/2 function for public facing pages, it returns the tuple:

   is_authorized(RD, Context) ->
       {true, RD, Context#context{session=session:start(RD)}.
   
   

   • Within session:start/1, I extract the session cookie (if it exists) and retrieve the session document from CouchDB. If the session cookie isn't set, I create a new #session{} record with some defaults pre-filled.

   • #+BEGIN_EXAMPLE %% session.hrl -record(session, {'_id', '_rev', user_id, expires, created, storage=[]}).

     %% session.erl start(RD) -> get_sessionrec(RD).

     get_sessionid(RD) -> wrq:get_cookievalue(?COOKIE_NAME, RD).

     %% get_sessiondoc(request()) -> not_found | json_object() get_sessiondoc(RD) -> case get_sessionid(RD) of undefined -> not_found; SessDocId -> couchbeam_db:open_doc(?DB, SessDocId) end.

     %% get_sessionrec(request()) -> session_rec() get_sessionrec(RD) -> case get_sessiondoc(RD) of not_found -> new(); Doc -> from_couchbeam(Doc) end.

     new() -> Now = calendar:datetime_{togregorianseconds}(calendar:local_time()), #session{created=Now, expires=?MAX_AGE, '_id'=couchbeam_util:new_uuid()}.

     #+END_EXAMPLE

   • So first I pull out the session ID from the cookie header in get\_session\_id/1 which will return either undefined or the ID.
   • If the session ID is undefined, get\_session\_doc/1 returns the not\_found atom, which is what Couchbeam returns when it can't find a document. If a session ID is found, I call couchbeam:open\_doc/2 to retrieve the document from CouchDB.
   • Finally, if get\_session\_doc/1 returns not\_found, I create a new session record calling new/0. If a document is returned, I call a function that transforms the proplist to the session record.
   • Note: Couchbeam will auto assign your '\_id' if the document to be saved doesn't have one or it is undefined. While useful, for sessions it causes a problem as the auto-generated UUIDs are fairly sequential (not exactly but usually only the last four or five characters differ). Calling couchbeam\_util:new\_uuid() creates your random uuids.
   • I store the created time in seconds, as well as the expires time (30 minutes worth of seconds right now). This allows me to easily clear ended sessions from CouchDB with a view indexing the session documents by (created+expires).
3. Now, as the resource request is processed, I can always access the session through the Context parameter.

4. When the request is done processing, it is time to save the session and set the cookie. In wm\_resource:finish\_request/2 I have:

   finish_request(RD, Context) ->
       {true, wm_session:finish(Context#context.session, RD), Context}.
   

   And in session.erl, I define the finish/2 process:

   %% finish(#session(), request()) -> request().
   %%  save session and set cookie headers
   finish(S, RD) -> save_session_rec(S, RD).
   
   save_session_rec(S, RD) ->
       DateTime = calendar:local_time(),
       Now = calendar:datetime_to_gregorian_seconds(DateTime),
       case has_expired(S#session.created + S#session.expires) of
           true -> close(S, RD);
           false ->
               D = to_couchbeam(S#session{created=Now,expires=?MAX_AGE}),
               S1 = from_couchbeam(couchbeam_db:save_doc(?DB, D)),
               set_cookie_header(RD, S1, DateTime, ?MAX_AGE)
       end.
   
   close(S, RD) ->
       DateTime = calendar:local_time(),
       SessionDoc = to_couchbeam(S),
       couchbeam_db:delete_doc(?DB, SessionDoc),
       set_cookie_header(RD, S, DateTime, -1).
   
   set_cookie_header(RD, Session, DateTime, MaxAge) ->
       {CookieHeader, CookieValue} = mochiweb_cookies:cookie(?COOKIE_NAME, Session#session.'_id', [{max_age, MaxAge},
                                                                                                   {local_time, DateTime}]),
       wrq:set_resp_header(CookieHeader, CookieValue, RD).
   

   • So to finish a session, I check if the current session has expired, and if so close it down. Otherwise, I update the created and exipres properties of the session and save it to CouchDB, and then set the cookie headers to reflect the new times.

5. For a resource needing valid authentication to be accessible, the is\_authorized/2 is slightly different:

   %% wm_resource.erl
   is_authorized(RD, Context) ->
       S = session:start(RD),
       case session:is_authorized(S) of
       true -> {true, RD, Context#context{session=S}};
           false ->
               RD0 = wrq:do_redirect(true, RD),
               RD1 = wrq:set_resp_header("Location", "/login", RD0),
               {{halt, 307}, RD1, Context#context{session=S}}
       end.
   

   %% session.erl
   is_authorized(S) ->
       S#session.user_id =/= undefined andalso
           not has_expired(S#session.created + S#session.expires).
   

   • Obviously you can set your own redirect header. The {halt, 307} is a temporary redirect (since hopefully you've only forgotten to login or been away too long and your session expired).