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<h1 class="lined-header"><span>Loop handlers</span></h1>
<p>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.</p>
<p>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.</p>
<p>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.</p>
<p>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.</p>
<p>Loop handlers essentially wait for one or more Erlang messages and feed these messages to the <code>info/3</code> callback. It also features the <code>init/3</code> and <code>terminate/3</code> callbacks which work the same as for plain HTTP handlers.</p>
<h2 id="initialization">Initialization</h2>
<p>The <code>init/3</code> function must return a <code>loop</code> tuple to enable loop handler behavior. This tuple may optionally contain a timeout value and/or the atom <code>hibernate</code> to make the process enter hibernation until a message is received.</p>
<p>This snippet enables the loop handler.</p>
<script type="syntaxhighlighter" class="brush: erlang"><![CDATA[
init(_Type, Req, _Opts) ->
{loop, Req, undefined_state}.
]]></script>
<p>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.</p>
<script type="syntaxhighlighter" class="brush: erlang"><![CDATA[
init(_Type, Req, _Opts) ->
{loop, Req, undefined_state, 30000, hibernate}.
]]></script>
<h2 id="receive_loop">Receive loop</h2>
<p>Once initialized, Cowboy will wait for messages to arrive in the process' mailbox. When a message arrives, Cowboy calls the <code>info/3</code> function with the message, the Req object and the handler's state.</p>
<p>The following snippet sends a reply when it receives a <code>reply</code> message from another process, or waits for another message otherwise.</p>
<script type="syntaxhighlighter" class="brush: erlang"><![CDATA[
info({reply, Body}, Req, State) ->
{ok, Req2} = cowboy_req:reply(200, [], Body, Req),
{ok, Req2, State};
info(_Msg, Req, State) ->
{loop, Req, State, hibernate}.
]]></script>
<p>Do note that the <code>reply</code> tuple here may be any message and is simply an example.</p>
<p>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.</p>
<p>The callback may also choose to do nothing at all and just skip the message received.</p>
<p>If a reply is sent, then the <code>ok</code> tuple should be returned. This will instruct Cowboy to end the request.</p>
<p>Otherwise a <code>loop</code> tuple should be returned.</p>
<h2 id="streaming_loop">Streaming loop</h2>
<p>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 <code>init/3</code> callback and then using <code>cowboy_req:chunk/2</code> every time a message is received.</p>
<p>The following snippet does exactly that. As you can see a chunk is sent every time a <code>chunk</code> message is received, and the loop is stopped by sending an <code>eof</code> message.</p>
<script type="syntaxhighlighter" class="brush: erlang"><![CDATA[
init(_Type, Req, _Opts) ->
{ok, Req2} = cowboy_req:chunked_reply(200, [], Req),
{loop, Req2, undefined_state}.
info(eof, Req, State) ->
{ok, Req, State};
info({chunk, Chunk}, Req, State) ->
ok = cowboy_req:chunk(Chunk, Req),
{loop, Req, State};
info(_Msg, Req, State) ->
{loop, Req, State}.
]]></script>
<h2 id="cleaning_up">Cleaning up</h2>
<p>It is recommended that you set the connection header to <code>close</code> when replying, as this process may be reused for a subsequent request.</p>
<p>Please refer to the <a href="/docs/en/cowboy/1.0/guide/http_handlers">HTTP handlers chapter</a> for general instructions about cleaning up.</p>
<h2 id="timeout">Timeout</h2>
<p>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.</p>
<p>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 <code>loop_max_buffer</code> middleware environment value.</p>
<h2 id="hibernate">Hibernate</h2>
<p>To save memory, you may hibernate the process in between messages received. This is done by returning the atom <code>hibernate</code> as part of the <code>loop</code> tuple callbacks normally return. Just add the atom at the end and Cowboy will hibernate accordingly.</p>
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<h3>See also</h3><ul><li><a href="/docs/en/cowboy/1.0/manual/">Function Reference</a></li><li><a href="/docs/en/cowboy/1.0/index.html">README</a></li></ul>
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