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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.
Usage
-----
Loop handlers are used for requests where a response might not
be immediately available, but where you would like to keep the
connection open for a while in case the response arrives. The
most known example of such practice is known as long-polling.
Loop handlers can also be used for requests where a response is
partially available and you need to stream the response body
while the connection is open. The most known example of such
practice is known as server-sent events.
Loop handlers essentially wait for one or more Erlang messages
and feed these messages to the `info/3` callback. It also features
the `init/3` and `terminate/2` callbacks which work the same as
for plain HTTP handlers.
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.
```
|
