Add documentation for Cowboy 2.2

author: Loïc Hoguin <[email protected]> 2017-12-11 11:27:17 +0100
committer: Loïc Hoguin <[email protected]> 2017-12-11 11:27:17 +0100
commit: 1b863c350ba1f9b8ad1cb458df514ab0fe6bb127 (patch)
tree: f3f88acba06cce7095a047da8163ac0f92b1a64a /docs/en/cowboy/2.2/guide/loop_handlers/index.html
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+<!DOCTYPE html>
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+    <title>Nine Nines: Loop handlers</title>
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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 3.1.8
+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">-&gt;</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 3.1.8
+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">-&gt;</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&#8217;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 3.1.8
+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">-&gt;</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">-&gt;</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 3.1.8
+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">-&gt;</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">-&gt;</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">-&gt;</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">-&gt;</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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author	Loïc Hoguin <[email protected]>	2017-12-11 11:27:17 +0100
committer	Loïc Hoguin <[email protected]>	2017-12-11 11:27:17 +0100
commit	1b863c350ba1f9b8ad1cb458df514ab0fe6bb127 (patch)
tree	f3f88acba06cce7095a047da8163ac0f92b1a64a /docs/en/cowboy/2.2/guide/loop_handlers/index.html
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