1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
|
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="utf-8">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<meta name="description" content="">
<meta name="author" content="Loïc Hoguin based on a design from (Soft10) Pol Cámara">
<meta name="generator" content="Hugo 0.37.1" />
<title>Nine Nines: Loop handlers</title>
<link href='https://fonts.googleapis.com/css?family=Open+Sans:400,700,400italic' rel='stylesheet' type='text/css'>
<link href="/css/99s.css?r=1" rel="stylesheet">
<link rel="shortcut icon" href="/img/ico/favicon.ico">
<link rel="apple-touch-icon-precomposed" sizes="114x114" href="/img/ico/apple-touch-icon-114.png">
<link rel="apple-touch-icon-precomposed" sizes="72x72" href="/img/ico/apple-touch-icon-72.png">
<link rel="apple-touch-icon-precomposed" href="/img/ico/apple-touch-icon-57.png">
</head>
<body class="">
<header id="page-head">
<div id="topbar" class="container">
<div class="row">
<div class="span2">
<h1 id="logo"><a href="/" title="99s">99s</a></h1>
</div>
<div class="span10">
<div id="side-header">
<nav>
<ul>
<li><a title="Hear my thoughts" href="/articles">Articles</a></li>
<li><a title="Watch my talks" href="/talks">Talks</a></li>
<li class="active"><a title="Read the docs" href="/docs">Documentation</a></li>
<li><a title="Request my services" href="/services">Consulting & Training</a></li>
</ul>
</nav>
<ul id="social">
<li>
<a href="https://github.com/ninenines" title="Check my Github repositories"><img src="/img/ico_github.png" data-hover="/img/ico_github_alt.png" alt="Github"></a>
</li>
<li>
<a title="Keep in touch!" href="http://twitter.com/lhoguin"><img src="/img/ico_microblog.png" data-hover="/img/ico_microblog_alt.png"></a>
</li>
<li>
<a title="Contact me" href="mailto:[email protected]"><img src="/img/ico_mail.png" data-hover="/img/ico_mail_alt.png"></a>
</li>
</ul>
</div>
</div>
</div>
</div>
</header>
<div id="contents" class="two_col">
<div class="container">
<div class="row">
<div id="docs" class="span9 maincol">
<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>
</div>
<nav style="margin:1em 0">
<a style="float:left" href="https://ninenines.eu/docs/en/cowboy/2.2/guide/handlers/">
Handlers
</a>
<a style="float:right" href="https://ninenines.eu/docs/en/cowboy/2.2/guide/static_files/">
Static files
</a>
</nav>
</div>
<div class="span3 sidecol">
<h3>
Cowboy
2.2
User Guide
</h3>
<ul>
<li><a href="/docs/en/cowboy/2.2/guide">User Guide</a></li>
<li><a href="/docs/en/cowboy/2.2/manual">Function Reference</a></li>
</ul>
<h4 id="docs-nav">Navigation</h4>
<h4>Version select</h4>
<ul>
<li><a href="/docs/en/cowboy/2.3/guide">2.3</a></li>
<li><a href="/docs/en/cowboy/2.2/guide">2.2</a></li>
<li><a href="/docs/en/cowboy/2.1/guide">2.1</a></li>
<li><a href="/docs/en/cowboy/2.0/guide">2.0</a></li>
<li><a href="/docs/en/cowboy/1.0/guide">1.0</a></li>
</ul>
</div>
</div>
</div>
</div>
<footer>
<div class="container">
<div class="row">
<div class="span6">
<p id="scroll-top"><a href="#">↑ Scroll to top</a></p>
<nav>
<ul>
<li><a href="mailto:[email protected]" title="Contact us">Contact us</a></li><li><a href="https://github.com/ninenines/ninenines.github.io" title="Github repository">Contribute to this site</a></li>
</ul>
</nav>
</div>
<div class="span6 credits">
<p><img src="/img/footer_logo.png"></p>
<p>Copyright © Loïc Hoguin 2012-2016</p>
</div>
</div>
</div>
</footer>
<script src="/js/custom.js"></script>
</body>
</html>
|
