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## Chapter 11. 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.

## 11.1. 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. Alternatively, the tuple may optionally contain a positive integer to create a `timeout` message when the process has not received messages for too long.

This snippet enables the loop handler:

    init(Req, State) ->
        {cowboy_loop, Req, State}.

This also makes the process hibernate:

    init(Req, State) ->
        {cowboy_loop, Req, State, hibernate}.

This makes the process time out after 1000ms of idle time.

    init(Req, State) ->
        {cowboy_loop, Req, State, 1000}.

## 11.2. 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.

    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.

## 11.3. 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.

    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}.

## 11.4. Cleaning up

Please refer to the Handlers chapter for general instructions about cleaning up.

## 11.5. 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.

## 11.6. Idle timeout

You may activate timeout events by returning a positive integer `N` as part of the `loop` tuple callbacks return. The default value is `infinity`. The `info` callback will be called with the atom `timeout` unless a message is received within `N` milliseconds:

    info(timeout, Req, State) ->
        %% Do something...
        {ok, Req, State, 1000}.

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