From 3edee0ace8f3f12653381e8038461689363c9541 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Lo=C3=AFc=20Hoguin?= <essen@ninenines.eu>
Date: Mon, 19 Nov 2018 13:29:35 +0100
Subject: Cowboy 2.6.0

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+[[loop_handlers]]
+== Loop handlers
+
+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.
+
+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 server-sent events, but it also applies to any
+response that takes a long time to send.
+
+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.
+
+Loop handlers essentially wait for one or more Erlang messages
+and feed these messages to the `info/3` callback. It also features
+the `init/2` and `terminate/3` callbacks which work the same as
+for plain HTTP handlers.
+
+=== Initialization
+
+The `init/2` function must return a `cowboy_loop` tuple to enable
+loop handler behavior. This tuple may optionally contain
+the atom `hibernate` to make the process enter hibernation
+until a message is received.
+
+This snippet enables the loop handler:
+
+[source,erlang]
+----
+init(Req, State) ->
+    {cowboy_loop, Req, State}.
+----
+
+This also makes the process hibernate:
+
+[source,erlang]
+----
+init(Req, State) ->
+    {cowboy_loop, Req, State, hibernate}.
+----
+
+=== Receive loop
+
+Once initialized, Cowboy will wait for messages to arrive
+in the process' mailbox. When a message arrives, Cowboy
+calls the `info/3` function with the message, the Req object
+and the handler's state.
+
+The following snippet sends a reply when it receives a
+`reply` message from another process, or waits for another
+message otherwise.
+
+[source,erlang]
+----
+info({reply, Body}, Req, State) ->
+    cowboy_req:reply(200, #{}, Body, Req),
+    {stop, Req, State};
+info(_Msg, Req, State) ->
+    {ok, Req, State, hibernate}.
+----
+
+Do note that the `reply` tuple here may be any message
+and is simply an example.
+
+This callback may perform any necessary operation including
+sending all or parts of a reply, and will subsequently
+return a tuple indicating if more messages are to be expected.
+
+The callback may also choose to do nothing at all and just
+skip the message received.
+
+If a reply is sent, then the `stop` tuple should be returned.
+This will instruct Cowboy to end the request.
+
+Otherwise an `ok` tuple should be returned.
+
+=== Streaming loop
+
+Another common case well suited for loop handlers is
+streaming data received in the form of Erlang messages.
+This can be done by initiating a chunked reply in the
+`init/2` callback and then using `cowboy_req:chunk/2`
+every time a message is received.
+
+The following snippet does exactly that. As you can see
+a chunk is sent every time an `event` message is received,
+and the loop is stopped by sending an `eof` message.
+
+[source,erlang]
+----
+init(Req, State) ->
+    Req2 = cowboy_req:stream_reply(200, Req),
+    {cowboy_loop, Req2, State}.
+
+info(eof, Req, State) ->
+    {stop, Req, State};
+info({event, Data}, Req, State) ->
+    cowboy_req:stream_body(Data, nofin, Req),
+    {ok, Req, State};
+info(_Msg, Req, State) ->
+    {ok, Req, State}.
+----
+
+=== Cleaning up
+
+Please refer to the xref:handlers[Handlers chapter]
+for general instructions about cleaning up.
+
+=== Hibernate
+
+To save memory, you may hibernate the process in between
+messages received. This is done by returning the atom
+`hibernate` as part of the `loop` tuple callbacks normally
+return. Just add the atom at the end and Cowboy will hibernate
+accordingly.
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