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 a timeout value and/or the atom hibernate to make the process enter hibernation until a message is received.

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}.

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.

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}.

Cleaning up

It is recommended that you set the connection header to close when replying, as this process may be reused for a subsequent request.

Please refer to the 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.