cowboy/guide/loop_handlers.md

Loop handlers
=============

Purpose
-------

Loop handlers are a special kind of HTTP handlers used when the
response can not be sent right away. The handler enters instead
a receive loop waiting for the right message before it can send
a response.

They are most useful when performing long-polling operations or
when using server-sent events.

While the same can be accomplished using plain HTTP handlers,
it is recommended to use loop handlers because they are well-tested
and allow using built-in features like hibernation and timeouts.

Callbacks
---------

@todo Describe the callbacks.

Usage
-----

@todo Explain how to use them.

The following handler waits for a message `{reply, Body}` before
sending a response. If this message doesn't arrive within 60
seconds, it gives up and a `204 No Content` will be replied.
It also hibernates the process to save memory while waiting for
this message.

``` erlang
-module(my_loop_handler).
-behaviour(cowboy_loop_handler).

-export([init/3]).
-export([info/3]).
-export([terminate/2]).

init({tcp, http}, Req, Opts) ->
	{loop, Req, undefined_state, 60000, hibernate}.

info({reply, Body}, Req, State) ->
	{ok, Req2} = cowboy_req:reply(200, [], Body, Req),
	{ok, Req2, State};
info(Message, Req, State) ->
	{loop, Req, State, hibernate}.

terminate(Req, State) ->
	ok.
```