add a stub examples folder

--HG--
extra : convert_revision : svn%3A583d1f64-3c3e-0410-8459-0f15fcd77d59/trunk%40107

examples/keepalive/keepalive.erl

@@ -0,0 +1,81 @@
+-module(keepalive).
+
+%% your web app can push data to clients using a technique called comet long
+%% polling.  browsers make a request and your server waits to send a
+%% response until data is available.  see wikipedia for a better explanation:
+%% http://en.wikipedia.org/wiki/Comet_(programming)#Ajax_with_long_polling
+%%
+%% since the majority of your http handlers will be idle at any given moment,
+%% you might consider making them hibernate while they wait for more data from
+%% another process.  however, since the execution stack is discarded when a
+%% process hibernates, the handler would usually terminate after your response
+%% code runs.  this means http keep alives wouldn't work; the handler process
+%% would terminate after each response and close its socket rather than
+%% returning to the big @mochiweb_http@ loop and processing another request.
+%%
+%% however, if mochiweb exposes a continuation that encapsulates the return to
+%% the top of the big loop in @mochiweb_http@, we can call that after the
+%% response.  if you do that then control flow returns to the proper place,
+%% and keep alives work like they would if you hadn't hibernated.
+
+-export([ start/1, loop/1
+        ]).
+
+%% internal export (so hibernate can reach it)
+-export([ resume/3
+        ]).
+
+-define(LOOP, {?MODULE, loop}).
+
+start(Options = [{port, _Port}]) ->
+    mochiweb_http:start([{name, ?MODULE}, {loop, ?LOOP} | Options]).
+
+loop(Req) ->
+    Path = Req:get(path),
+    case string:tokens(Path, "/") of
+        ["longpoll" | RestOfPath] ->
+            %% the "reentry" is a continuation -- what @mochiweb_http@
+            %% needs to do to start its loop back at the top
+            Reentry = mochiweb_http:reentry(?LOOP),
+
+            %% here we could send a message to some other process and hope
+            %% to get an interesting message back after a while.  for
+            %% simplicity let's just send ourselves a message after a few
+            %% seconds
+            erlang:send_after(2000, self(), "honk honk"),
+
+            %% since we expect to wait for a long time before getting a
+            %% reply, let's hibernate.  memory usage will be minimized, so
+            %% we won't be wasting memory just sitting in a @receive@
+            proc_lib:hibernate(?MODULE, resume, [Req, RestOfPath, Reentry]),
+
+            %% we'll never reach this point, and this function @loop/1@
+            %% won't ever return control to @mochiweb_http@.  luckily
+            %% @resume/3@ will take care of that.
+            io:format("not gonna happen~n", []);
+
+        _ ->
+            ok(Req, io_lib:format("some other page: ~p", [Path]))
+    end,
+
+    io:format("restarting loop normally in ~p~n", [Path]),
+    ok.
+
+%% this is the function that's called when a message arrives.
+resume(Req, RestOfPath, Reentry) ->
+    receive
+        Msg ->
+            Text = io_lib:format("wake up message: ~p~nrest of path: ~p", [Msg, RestOfPath]),
+            ok(Req, Text)
+    end,
+
+    %% if we didn't call @Reentry@ here then the function would finish and the
+    %% process would exit.  calling @Reentry@ takes care of returning control
+    %% to @mochiweb_http@
+    io:format("reentering loop via continuation in ~p~n", [Req:get(path)]),
+    Reentry(Req).
+
+ok(Req, Response) ->
+    Req:ok({_ContentType = "text/plain",
+            _Headers = [],
+            Response}).