cowboy/doc/src/guide/loop_handlers.asciidoc

[[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.