[[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.
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.
