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<h1 class="lined-header"><span>Loop handlers</span></h1>
<div class="paragraph"><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></div>
<div class="paragraph"><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></div>
<div class="paragraph"><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.</p></div>
<div class="paragraph"><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></div>
<div class="paragraph"><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></div>
<div class="sect1">
<h2 id="_initialization">Initialization</h2>
<div class="sectionbody">
<div class="paragraph"><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
a timeout value and/or the atom <code>hibernate</code> to make the
process enter hibernation until a message is received.</p></div>
<div class="paragraph"><p>This snippet enables the loop handler:</p></div>
<div class="listingblock">
<div class="content"><!-- Generator: GNU source-highlight
by Lorenzo Bettini
http://www.lorenzobettini.it
http://www.gnu.org/software/src-highlite -->
<pre><tt><span style="font-weight: bold"><span style="color: #000000">init</span></span>(<span style="color: #009900">Req</span>, <span style="color: #009900">State</span>) <span style="color: #990000">-></span>
{<span style="color: #FF6600">cowboy_loop</span>, <span style="color: #009900">Req</span>, <span style="color: #009900">State</span>}<span style="color: #990000">.</span></tt></pre></div></div>
<div class="paragraph"><p>This also makes the process hibernate:</p></div>
<div class="listingblock">
<div class="content"><!-- Generator: GNU source-highlight
by Lorenzo Bettini
http://www.lorenzobettini.it
http://www.gnu.org/software/src-highlite -->
<pre><tt><span style="font-weight: bold"><span style="color: #000000">init</span></span>(<span style="color: #009900">Req</span>, <span style="color: #009900">State</span>) <span style="color: #990000">-></span>
{<span style="color: #FF6600">cowboy_loop</span>, <span style="color: #009900">Req</span>, <span style="color: #009900">State</span>, <span style="color: #FF6600">hibernate</span>}<span style="color: #990000">.</span></tt></pre></div></div>
</div>
</div>
<div class="sect1">
<h2 id="_receive_loop">Receive loop</h2>
<div class="sectionbody">
<div class="paragraph"><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></div>
<div class="paragraph"><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>
<div class="listingblock">
<div class="content"><!-- Generator: GNU source-highlight
by Lorenzo Bettini
http://www.lorenzobettini.it
http://www.gnu.org/software/src-highlite -->
<pre><tt><span style="font-weight: bold"><span style="color: #000000">info</span></span>({<span style="color: #FF6600">reply</span>, <span style="color: #009900">Body</span>}, <span style="color: #009900">Req</span>, <span style="color: #009900">State</span>) <span style="color: #990000">-></span>
<span style="font-weight: bold"><span style="color: #000000">cowboy_req:reply</span></span>(<span style="color: #993399">200</span>, #{}, <span style="color: #009900">Body</span>, <span style="color: #009900">Req</span>),
{<span style="color: #FF6600">stop</span>, <span style="color: #009900">Req</span>, <span style="color: #009900">State</span>};
<span style="font-weight: bold"><span style="color: #000000">info</span></span>(<span style="color: #009900">_Msg</span>, <span style="color: #009900">Req</span>, <span style="color: #009900">State</span>) <span style="color: #990000">-></span>
{<span style="color: #FF6600">ok</span>, <span style="color: #009900">Req</span>, <span style="color: #009900">State</span>, <span style="color: #FF6600">hibernate</span>}<span style="color: #990000">.</span></tt></pre></div></div>
<div class="paragraph"><p>Do note that the <code>reply</code> tuple here may be any message
and is simply an example.</p></div>
<div class="paragraph"><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></div>
<div class="paragraph"><p>The callback may also choose to do nothing at all and just
skip the message received.</p></div>
<div class="paragraph"><p>If a reply is sent, then the <code>stop</code> tuple should be returned.
This will instruct Cowboy to end the request.</p></div>
<div class="paragraph"><p>Otherwise an <code>ok</code> tuple should be returned.</p></div>
</div>
</div>
<div class="sect1">
<h2 id="_streaming_loop">Streaming loop</h2>
<div class="sectionbody">
<div class="paragraph"><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></div>
<div class="paragraph"><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>
<div class="listingblock">
<div class="content"><!-- Generator: GNU source-highlight
by Lorenzo Bettini
http://www.lorenzobettini.it
http://www.gnu.org/software/src-highlite -->
<pre><tt><span style="font-weight: bold"><span style="color: #000000">init</span></span>(<span style="color: #009900">Req</span>, <span style="color: #009900">State</span>) <span style="color: #990000">-></span>
<span style="color: #009900">Req2</span> <span style="color: #990000">=</span> <span style="font-weight: bold"><span style="color: #000000">cowboy_req:stream_reply</span></span>(<span style="color: #993399">200</span>, <span style="color: #009900">Req</span>),
{<span style="color: #FF6600">cowboy_loop</span>, <span style="color: #009900">Req2</span>, <span style="color: #009900">State</span>}<span style="color: #990000">.</span>
<span style="font-weight: bold"><span style="color: #000000">info</span></span>(<span style="color: #FF6600">eof</span>, <span style="color: #009900">Req</span>, <span style="color: #009900">State</span>) <span style="color: #990000">-></span>
{<span style="color: #FF6600">stop</span>, <span style="color: #009900">Req</span>, <span style="color: #009900">State</span>};
<span style="font-weight: bold"><span style="color: #000000">info</span></span>({<span style="color: #FF6600">event</span>, <span style="color: #009900">Data</span>}, <span style="color: #009900">Req</span>, <span style="color: #009900">State</span>) <span style="color: #990000">-></span>
<span style="font-weight: bold"><span style="color: #000000">cowboy_req:stream_body</span></span>(<span style="color: #009900">Data</span>, <span style="color: #FF6600">nofin</span>, <span style="color: #009900">Req</span>),
{<span style="color: #FF6600">ok</span>, <span style="color: #009900">Req</span>, <span style="color: #009900">State</span>};
<span style="font-weight: bold"><span style="color: #000000">info</span></span>(<span style="color: #009900">_Msg</span>, <span style="color: #009900">Req</span>, <span style="color: #009900">State</span>) <span style="color: #990000">-></span>
{<span style="color: #FF6600">ok</span>, <span style="color: #009900">Req</span>, <span style="color: #009900">State</span>}<span style="color: #990000">.</span></tt></pre></div></div>
</div>
</div>
<div class="sect1">
<h2 id="_cleaning_up">Cleaning up</h2>
<div class="sectionbody">
<div class="paragraph"><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></div>
<div class="paragraph"><p>Please refer to the <a href="../handlers">Handlers chapter</a>
for general instructions about cleaning up.</p></div>
</div>
</div>
<div class="sect1">
<h2 id="_hibernate">Hibernate</h2>
<div class="sectionbody">
<div class="paragraph"><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></div>
</div>
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