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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 known as 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.
[source,erlang]
----
init(Req, _Opts) ->
{cowboy_loop, Req, #state{}}.
----
However it is largely recommended that you set a timeout
value. The next example sets a timeout value of 30s and
also makes the process hibernate.
[source,erlang]
----
init(Req, _Opts) ->
{cowboy_loop, Req, #state{}, 30000, 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) ->
Req2 = cowboy_req:reply(200, [], Body, Req),
{stop, Req2, 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 a `chunk` message is received,
and the loop is stopped by sending an `eof` message.
[source,erlang]
----
init(Req, _Opts) ->
Req2 = cowboy_req:chunked_reply(200, [], Req),
{cowboy_loop, Req2, #state{}}.
info(eof, Req, State) ->
{stop, Req, State};
info({chunk, Chunk}, Req, State) ->
cowboy_req:chunk(Chunk, 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 xref:handlers[Handlers chapter]
for general instructions about cleaning up.
=== Timeout
By default Cowboy will not attempt to close the connection
if there is no activity from the client. This is not always
desirable, which is why you can set a timeout. Cowboy will
close the connection if no data was received from the client
after the configured time. The timeout only needs to be set
once and can't be modified afterwards.
Because the request may have had a body, or may be followed
by another request, Cowboy is forced to buffer all data it
receives. This data may grow to become too large though,
so there is a configurable limit for it. The default buffer
size is of 5000 bytes, but it may be changed by setting the
`loop_max_buffer` middleware environment value.
=== 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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