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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 server-sent events, but it also applies to any response that takes a long time to send.</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/2</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/2</code> function must return a <code>cowboy_loop</code> tuple to enable loop handler behavior. This tuple may optionally contain 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>
<div class="listingblock"><div class="content"><!-- Generator: GNU source-highlight 3.1.9
by Lorenzo Bettini
http://www.lorenzobettini.it
http://www.gnu.org/software/src-highlite -->
<pre><tt><b><font color="#000000">init</font></b>(<font color="#009900">Req</font>, <font color="#009900">State</font>) <font color="#990000">-></font>
{<font color="#FF6600">cowboy_loop</font>, <font color="#009900">Req</font>, <font color="#009900">State</font>}<font color="#990000">.</font></tt></pre>
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<p>This also makes the process hibernate:</p>
<div class="listingblock"><div class="content"><!-- Generator: GNU source-highlight 3.1.9
by Lorenzo Bettini
http://www.lorenzobettini.it
http://www.gnu.org/software/src-highlite -->
<pre><tt><b><font color="#000000">init</font></b>(<font color="#009900">Req</font>, <font color="#009900">State</font>) <font color="#990000">-></font>
{<font color="#FF6600">cowboy_loop</font>, <font color="#009900">Req</font>, <font color="#009900">State</font>, <font color="#FF6600">hibernate</font>}<font color="#990000">.</font></tt></pre>
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<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>
<div class="listingblock"><div class="content"><!-- Generator: GNU source-highlight 3.1.9
by Lorenzo Bettini
http://www.lorenzobettini.it
http://www.gnu.org/software/src-highlite -->
<pre><tt><b><font color="#000000">info</font></b>({<font color="#FF6600">reply</font>, <font color="#009900">Body</font>}, <font color="#009900">Req</font>, <font color="#009900">State</font>) <font color="#990000">-></font>
<b><font color="#000000">cowboy_req:reply</font></b>(<font color="#993399">200</font>, #{}, <font color="#009900">Body</font>, <font color="#009900">Req</font>),
{<font color="#FF6600">stop</font>, <font color="#009900">Req</font>, <font color="#009900">State</font>};
<b><font color="#000000">info</font></b>(<font color="#009900">_Msg</font>, <font color="#009900">Req</font>, <font color="#009900">State</font>) <font color="#990000">-></font>
{<font color="#FF6600">ok</font>, <font color="#009900">Req</font>, <font color="#009900">State</font>, <font color="#FF6600">hibernate</font>}<font color="#990000">.</font></tt></pre>
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<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>stop</code> tuple should be returned. This will instruct Cowboy to end the request.</p>
<p>Otherwise an <code>ok</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/2</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 an <code>event</code> message is received, and the loop is stopped by sending an <code>eof</code> message.</p>
<div class="listingblock"><div class="content"><!-- Generator: GNU source-highlight 3.1.9
by Lorenzo Bettini
http://www.lorenzobettini.it
http://www.gnu.org/software/src-highlite -->
<pre><tt><b><font color="#000000">init</font></b>(<font color="#009900">Req</font>, <font color="#009900">State</font>) <font color="#990000">-></font>
<font color="#009900">Req2</font> <font color="#990000">=</font> <b><font color="#000000">cowboy_req:stream_reply</font></b>(<font color="#993399">200</font>, <font color="#009900">Req</font>),
{<font color="#FF6600">cowboy_loop</font>, <font color="#009900">Req2</font>, <font color="#009900">State</font>}<font color="#990000">.</font>
<b><font color="#000000">info</font></b>(<font color="#FF6600">eof</font>, <font color="#009900">Req</font>, <font color="#009900">State</font>) <font color="#990000">-></font>
{<font color="#FF6600">stop</font>, <font color="#009900">Req</font>, <font color="#009900">State</font>};
<b><font color="#000000">info</font></b>({<font color="#FF6600">event</font>, <font color="#009900">Data</font>}, <font color="#009900">Req</font>, <font color="#009900">State</font>) <font color="#990000">-></font>
<b><font color="#000000">cowboy_req:stream_body</font></b>(<font color="#009900">Data</font>, <font color="#FF6600">nofin</font>, <font color="#009900">Req</font>),
{<font color="#FF6600">ok</font>, <font color="#009900">Req</font>, <font color="#009900">State</font>};
<b><font color="#000000">info</font></b>(<font color="#009900">_Msg</font>, <font color="#009900">Req</font>, <font color="#009900">State</font>) <font color="#990000">-></font>
{<font color="#FF6600">ok</font>, <font color="#009900">Req</font>, <font color="#009900">State</font>}<font color="#990000">.</font></tt></pre>
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<h2 id="_cleaning_up">Cleaning up</h2>
<p>Please refer to the <a href="../handlers">Handlers chapter</a> for general instructions about cleaning up.</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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