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<h1 class="lined-header"><span>cowboy_rest(3)</span></h1>
<div class="sect1">
<h2 id="_name">Name</h2>
<div class="sectionbody">
<div class="paragraph"><p>cowboy_rest - REST handlers</p></div>
</div>
</div>
<div class="sect1">
<h2 id="_description">Description</h2>
<div class="sectionbody">
<div class="paragraph"><p>The module <code>cowboy_rest</code> implements the HTTP state machine.</p></div>
<div class="paragraph"><p>Implementing REST handlers is not enough to provide a REST
interface; this interface must also follow the REST
constraints including HATEOAS (hypermedia as the engine
of application state).</p></div>
</div>
</div>
<div class="sect1">
<h2 id="_callbacks">Callbacks</h2>
<div class="sectionbody">
<div class="paragraph"><p>REST handlers implement the following interface:</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_rest</span>, <span style="color: #009900">Req</span>, <span style="color: #009900">State</span>}
<span style="color: #009900">Callback</span>(<span style="color: #009900">Req</span>, <span style="color: #009900">State</span>)
<span style="color: #990000">-></span> {<span style="color: #009900">Result</span>, <span style="color: #009900">Req</span>, <span style="color: #009900">State</span>}
| {<span style="color: #FF6600">stop</span>, <span style="color: #009900">Req</span>, <span style="color: #009900">State</span>}
| {{<span style="color: #FF6600">switch_handler</span>, <span style="color: #009900">Module</span>}, <span style="color: #009900">Req</span>, <span style="color: #009900">State</span>}
| {{<span style="color: #FF6600">switch_handler</span>, <span style="color: #009900">Module</span>, <span style="color: #009900">Opts</span>}, <span style="color: #009900">Req</span>, <span style="color: #009900">State</span>}
<span style="font-weight: bold"><span style="color: #000000">terminate</span></span>(<span style="color: #009900">Reason</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="font-style: italic"><span style="color: #9A1900">%% optional</span></span>
<span style="color: #009900">Req</span> <span style="color: #990000">::</span> <span style="font-weight: bold"><span style="color: #000000">cowboy_req:req</span></span>()
<span style="color: #009900">State</span> <span style="color: #990000">::</span> <span style="font-weight: bold"><span style="color: #000000">any</span></span>()
<span style="color: #009900">Module</span> <span style="color: #990000">::</span> <span style="font-weight: bold"><span style="color: #000000">module</span></span>()
<span style="color: #009900">Opts</span> <span style="color: #990000">::</span> <span style="font-weight: bold"><span style="color: #000000">any</span></span>()
<span style="color: #009900">Reason</span> <span style="color: #990000">::</span> <span style="font-weight: bold"><span style="color: #000080">normal</span></span>
| {<span style="color: #FF6600">crash</span>, <span style="color: #FF6600">error</span> | <span style="font-weight: bold"><span style="color: #000080">exit</span></span> | <span style="font-weight: bold"><span style="color: #000080">throw</span></span>, <span style="font-weight: bold"><span style="color: #000000">any</span></span>()}
<span style="color: #009900">Callback</span> <span style="color: #990000">-</span> <span style="color: #FF6600">see</span> <span style="color: #FF6600">below</span>
<span style="color: #009900">Result</span> <span style="color: #990000">-</span> <span style="color: #FF6600">see</span> <span style="color: #FF6600">below</span>
<span style="color: #009900">Default</span> <span style="color: #990000">-</span> <span style="color: #FF6600">see</span> <span style="color: #FF6600">below</span></tt></pre></div></div>
<div class="paragraph"><p>The <code>init/2</code> callback is common to all handlers. To switch
to the REST handler behavior, it must return <code>cowboy_rest</code>
as the first element of the tuple.</p></div>
<div class="paragraph"><p>The <code>Callback/2</code> above represents all the REST-specific
callbacks. They are described in the following section
of this manual. REST-specific callbacks differ by their
name, semantics, result and default values. The default
value is the one used when the callback has not been
implemented. They otherwise all follow the same interface.</p></div>
<div class="paragraph"><p>The <code>stop</code> tuple can be returned to stop REST processing.
If no response was sent before then, Cowboy will send a
<em>204 No Content</em>. The <code>stop</code> tuple can be returned from
any callback, excluding <code>expires</code>, <code>generate_etag</code>,
<code>last_modified</code> and <code>variances</code>.</p></div>
<div class="paragraph"><p>A <code>switch_handler</code> tuple can be returned from these same
callbacks to stop REST processing and switch to a different
handler type. This is very useful to, for example, to stream
the response body.</p></div>
<div class="paragraph"><p>The optional <code>terminate/3</code> callback will ultimately be called
with the reason for the termination of the handler.
Cowboy will terminate the process right after this. There
is no need to perform any cleanup in this callback.</p></div>
<div class="paragraph"><p>The following terminate reasons are defined for loop handlers:</p></div>
<div class="dlist"><dl>
<dt class="hdlist1">
normal
</dt>
<dd>
<p>
The handler terminated normally.
</p>
</dd>
<dt class="hdlist1">
{crash, Class, Reason}
</dt>
<dd>
<p>
A crash occurred in the handler. <code>Class</code> and <code>Reason</code> can be
used to obtain more information about the crash. The function
<code>erlang:get_stacktrace/0</code> can also be called to obtain the
stacktrace of the process when the crash occurred.
</p>
</dd>
</dl></div>
</div>
</div>
<div class="sect1">
<h2 id="_rest_callbacks">REST callbacks</h2>
<div class="sectionbody">
<div class="sect2">
<h3 id="_acceptcallback">AcceptCallback</h3>
<div class="listingblock">
<div class="content"><!-- Generator: GNU source-highlight
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<pre><tt><span style="color: #009900">AcceptCallback</span>(<span style="color: #009900">Req</span>, <span style="color: #009900">State</span>) <span style="color: #990000">-></span> {<span style="color: #009900">Result</span>, <span style="color: #009900">Req</span>, <span style="color: #009900">State</span>}
<span style="color: #009900">Result</span> <span style="color: #990000">::</span> <span style="color: #000080">true</span> | {<span style="color: #000080">true</span>, <span style="color: #009900">URI</span> <span style="color: #990000">::</span> <span style="font-weight: bold"><span style="color: #000000">iodata</span></span>()} | <span style="color: #000080">false</span>}
<span style="color: #009900">Default</span> <span style="color: #990000">-</span> <span style="color: #FF6600">crash</span></tt></pre></div></div>
<div class="paragraph"><p>Process the request body.</p></div>
<div class="paragraph"><p>This function should create or update the resource using the
request body.</p></div>
<div class="paragraph"><p>For PUT requests, the body is a representation of the resource
that is being created or replaced.</p></div>
<div class="paragraph"><p>For POST requests, the body is typically application-specific
instructions on how to process the request, but it may also
be a representation of the resource. When creating a new
resource with POST at a different location, return <code>{true, URI}</code>
with <code>URI</code> the new location.</p></div>
<div class="paragraph"><p>For PATCH requests, the body is a series of instructions on
how to update the resource. Patch files or JSON Patch are
examples of such media types.</p></div>
<div class="paragraph"><p>A response body may be sent. The appropriate media type, charset
and language for the response can be retrieved from the Req
object using the <code>media_type</code>, <code>charset</code> and <code>language</code> keys,
respectively. The body can be set using
<a href="../cowboy_req.set_resp_body">cowboy_req:set_resp_body(3)</a>.</p></div>
</div>
<div class="sect2">
<h3 id="_allowed_methods">allowed_methods</h3>
<div class="listingblock">
<div class="content"><!-- Generator: GNU source-highlight
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<pre><tt><span style="font-weight: bold"><span style="color: #000000">allowed_methods</span></span>(<span style="color: #009900">Req</span>, <span style="color: #009900">State</span>) <span style="color: #990000">-></span> {<span style="color: #009900">Result</span>, <span style="color: #009900">Req</span>, <span style="color: #009900">State</span>}
<span style="color: #009900">Result</span> <span style="color: #990000">::</span> [<span style="font-weight: bold"><span style="color: #000080">binary</span></span>()] <span style="font-style: italic"><span style="color: #9A1900">%% case sensitive</span></span>
<span style="color: #009900">Default</span> <span style="color: #990000">::</span> [<span style="color: #990000"><<</span><span style="color: #FF0000">"GET"</span><span style="color: #990000">>></span>, <span style="color: #990000"><<</span><span style="color: #FF0000">"HEAD"</span><span style="color: #990000">>></span>, <span style="color: #990000"><<</span><span style="color: #FF0000">"OPTIONS"</span><span style="color: #990000">>></span>]</tt></pre></div></div>
<div class="paragraph"><p>Return the list of allowed methods.</p></div>
</div>
<div class="sect2">
<h3 id="_allow_missing_post">allow_missing_post</h3>
<div class="listingblock">
<div class="content"><!-- Generator: GNU source-highlight
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<pre><tt><span style="font-weight: bold"><span style="color: #000000">allow_missing_post</span></span>(<span style="color: #009900">Req</span>, <span style="color: #009900">State</span>) <span style="color: #990000">-></span> {<span style="color: #009900">Result</span>, <span style="color: #009900">Req</span>, <span style="color: #009900">State</span>}
<span style="color: #009900">Result</span> <span style="color: #990000">::</span> <span style="font-weight: bold"><span style="color: #000000">boolean</span></span>()
<span style="color: #009900">Default</span> <span style="color: #990000">::</span> <span style="color: #000080">true</span></tt></pre></div></div>
<div class="paragraph"><p>Return whether POST is allowed when the resource doesn’t exist.</p></div>
<div class="paragraph"><p>Returning <code>true</code> here means that a new resource will be
created. The URI for the newly created resource should be
returned from the <code>AcceptCallback</code> function.</p></div>
</div>
<div class="sect2">
<h3 id="_charsets_provided">charsets_provided</h3>
<div class="listingblock">
<div class="content"><!-- Generator: GNU source-highlight
by Lorenzo Bettini
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<pre><tt><span style="font-weight: bold"><span style="color: #000000">charsets_provided</span></span>(<span style="color: #009900">Req</span>, <span style="color: #009900">State</span>) <span style="color: #990000">-></span> {<span style="color: #009900">Result</span>, <span style="color: #009900">Req</span>, <span style="color: #009900">State</span>}
<span style="color: #009900">Result</span> <span style="color: #990000">::</span> [<span style="font-weight: bold"><span style="color: #000080">binary</span></span>()] <span style="font-style: italic"><span style="color: #9A1900">%% lowercase; case insensitive</span></span>
<span style="color: #009900">Default</span> <span style="color: #990000">-</span> <span style="color: #FF6600">skip</span> <span style="color: #FF6600">this</span> <span style="color: #FF6600">step</span></tt></pre></div></div>
<div class="paragraph"><p>Return the list of charsets the resource provides in order
of preference.</p></div>
<div class="paragraph"><p>During content negotiation Cowboy will pick the most
appropriate charset for the client. The client advertises
charsets it prefers with the accept-charset header. When
that header is missing, Cowboy picks the first charset
from the resource.</p></div>
<div class="paragraph"><p>Cowboy will add the negotiated <code>charset</code> to the Req object
after this step completes:</p></div>
<div class="listingblock">
<div class="content"><!-- Generator: GNU source-highlight
by Lorenzo Bettini
http://www.lorenzobettini.it
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<pre><tt><span style="font-weight: bold"><span style="color: #000000">req</span></span>() <span style="color: #990000">::</span> #{
<span style="color: #0000FF">charset</span> <span style="color: #990000">=></span> <span style="font-weight: bold"><span style="color: #000080">binary</span></span>() <span style="font-style: italic"><span style="color: #9A1900">%% lowercase; case insensitive</span></span>
}</tt></pre></div></div>
</div>
<div class="sect2">
<h3 id="_content_types_accepted">content_types_accepted</h3>
<div class="listingblock">
<div class="content"><!-- Generator: GNU source-highlight
by Lorenzo Bettini
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<pre><tt><span style="font-weight: bold"><span style="color: #000000">content_types_accepted</span></span>(<span style="color: #009900">Req</span>, <span style="color: #009900">State</span>) <span style="color: #990000">-></span> {<span style="color: #009900">Result</span>, <span style="color: #009900">Req</span>, <span style="color: #009900">State</span>}
<span style="color: #009900">Result</span> <span style="color: #990000">::</span> [{<span style="font-weight: bold"><span style="color: #000080">binary</span></span>() | <span style="color: #009900">ParsedMime</span>, <span style="color: #009900">AcceptCallback</span> <span style="color: #990000">::</span> <span style="font-weight: bold"><span style="color: #000080">atom</span></span>()}]
<span style="color: #009900">ParsedMime</span> <span style="color: #990000">::</span> {<span style="color: #009900">Type</span> <span style="color: #990000">::</span> <span style="font-weight: bold"><span style="color: #000080">binary</span></span>(), <span style="color: #009900">SubType</span> <span style="color: #990000">::</span> <span style="font-weight: bold"><span style="color: #000080">binary</span></span>(), <span style="color: #FF6600">'*'</span> | <span style="color: #009900">Params</span>}
<span style="color: #009900">Params</span> <span style="color: #990000">::</span> [{<span style="color: #009900">Key</span> <span style="color: #990000">::</span> <span style="font-weight: bold"><span style="color: #000080">binary</span></span>(), <span style="color: #009900">Value</span> <span style="color: #990000">::</span> <span style="font-weight: bold"><span style="color: #000080">binary</span></span>()}]
<span style="color: #009900">Default</span> <span style="color: #990000">-</span> <span style="color: #FF6600">crash</span></tt></pre></div></div>
<div class="paragraph"><p>Return the list of media types the resource accepts in
order of preference.</p></div>
<div class="paragraph"><p>A media type is made of different parts. The media type
<code>text/html;charset=utf-8</code> is of type <code>text</code>, subtype <code>html</code>
and has a single parameter <code>charset</code> with value <code>utf-8</code>.</p></div>
<div class="paragraph"><p>Cowboy will match the content-type request header against
the media types the server accepts and select the appropriate
callback. When that header is missing, or when the server does not
accept this media type, the request fails and an error response
is returned. Cowboy will execute the callback immediately otherwise.</p></div>
<div class="paragraph"><p>An empty parameters list <code>[]</code> means that no parameters will be
accepted. When any parameter is acceptable, the tuple form
should be used with parameters as the atom <code>'*'</code>.</p></div>
<div class="paragraph"><p>Cowboy treats all parameters as case sensitive, except for the
<code>charset</code> parameter, which is known to be case insensitive. You
should therefore always provide the charset as a lowercase
binary string.</p></div>
</div>
<div class="sect2">
<h3 id="_content_types_provided">content_types_provided</h3>
<div class="listingblock">
<div class="content"><!-- Generator: GNU source-highlight
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http://www.lorenzobettini.it
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<pre><tt><span style="font-weight: bold"><span style="color: #000000">content_types_provided</span></span>(<span style="color: #009900">Req</span>, <span style="color: #009900">State</span>) <span style="color: #990000">-></span> {<span style="color: #009900">Result</span>, <span style="color: #009900">Req</span>, <span style="color: #009900">State</span>}
<span style="color: #009900">Result</span> <span style="color: #990000">::</span> [{<span style="font-weight: bold"><span style="color: #000080">binary</span></span>() | <span style="color: #009900">ParsedMime</span>, <span style="color: #009900">ProvideCallback</span> <span style="color: #990000">::</span> <span style="font-weight: bold"><span style="color: #000080">atom</span></span>()}]
<span style="color: #009900">ParsedMime</span> <span style="color: #990000">::</span> {<span style="color: #009900">Type</span> <span style="color: #990000">::</span> <span style="font-weight: bold"><span style="color: #000080">binary</span></span>(), <span style="color: #009900">SubType</span> <span style="color: #990000">::</span> <span style="font-weight: bold"><span style="color: #000080">binary</span></span>(), <span style="color: #FF6600">'*'</span> | <span style="color: #009900">Params</span>}
<span style="color: #009900">Params</span> <span style="color: #990000">::</span> [{<span style="color: #009900">Key</span> <span style="color: #990000">::</span> <span style="font-weight: bold"><span style="color: #000080">binary</span></span>(), <span style="color: #009900">Value</span> <span style="color: #990000">::</span> <span style="font-weight: bold"><span style="color: #000080">binary</span></span>()}]
<span style="color: #009900">Default</span> <span style="color: #990000">-</span> [{{ <span style="color: #990000"><<</span><span style="color: #FF0000">"text"</span><span style="color: #990000">>></span>, <span style="color: #990000"><<</span><span style="color: #FF0000">"html"</span><span style="color: #990000">>></span>, <span style="color: #FF6600">'*'</span>}, <span style="color: #FF6600">to_html</span>}]</tt></pre></div></div>
<div class="paragraph"><p>Return the list of media types the resource provides in
order of preference.</p></div>
<div class="paragraph"><p>A media type is made of different parts. The media type
<code>text/html;charset=utf-8</code> is of type <code>text</code>, subtype <code>html</code>
and has a single parameter <code>charset</code> with value <code>utf-8</code>.</p></div>
<div class="paragraph"><p>During content negotiation Cowboy will pick the most appropriate
media type for the client. The client advertises media types it
prefers with the accept header. When that header is missing,
the content negotiation fails and an error response is returned.</p></div>
<div class="paragraph"><p>The callback given for the selected media type will be called
at the end of the execution of GET and HEAD requests when a
representation must be sent to the client.</p></div>
<div class="paragraph"><p>An empty parameters list <code>[]</code> means that no parameters will be
accepted. When any parameter is acceptable, the tuple form
should be used with parameters as the atom <code>'*'</code>.</p></div>
<div class="paragraph"><p>Cowboy treats all parameters as case sensitive, except for the
<code>charset</code> parameter, which is known to be case insensitive. You
should therefore always provide the charset as a lowercase
binary string.</p></div>
<div class="paragraph"><p>Cowboy will add the negotiated <code>media_type</code> to the Req object
after this step completes:</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">req</span></span>() <span style="color: #990000">::</span> #{
<span style="color: #0000FF">media_type</span> <span style="color: #990000">=></span> <span style="color: #009900">ParsedMime</span>
}</tt></pre></div></div>
</div>
<div class="sect2">
<h3 id="_delete_completed">delete_completed</h3>
<div class="listingblock">
<div class="content"><!-- Generator: GNU source-highlight
by Lorenzo Bettini
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<pre><tt><span style="font-weight: bold"><span style="color: #000000">delete_completed</span></span>(<span style="color: #009900">Req</span>, <span style="color: #009900">State</span>) <span style="color: #990000">-></span> {<span style="color: #009900">Result</span>, <span style="color: #009900">Req</span>, <span style="color: #009900">State</span>}
<span style="color: #009900">Result</span> <span style="color: #990000">::</span> <span style="font-weight: bold"><span style="color: #000000">boolean</span></span>()
<span style="color: #009900">Default</span> <span style="color: #990000">::</span> <span style="color: #000080">true</span></tt></pre></div></div>
<div class="paragraph"><p>Return whether the resource has been fully deleted from the
system, including from any internal cache.</p></div>
<div class="paragraph"><p>Returning <code>false</code> will result in a <em>202 Accepted</em> response
being sent instead of a <em>200 OK</em> or <em>204 No Content</em>.</p></div>
</div>
<div class="sect2">
<h3 id="_delete_resource">delete_resource</h3>
<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">delete_resource</span></span>(<span style="color: #009900">Req</span>, <span style="color: #009900">State</span>) <span style="color: #990000">-></span> {<span style="color: #009900">Result</span>, <span style="color: #009900">Req</span>, <span style="color: #009900">State</span>}
<span style="color: #009900">Result</span> <span style="color: #990000">::</span> <span style="font-weight: bold"><span style="color: #000000">boolean</span></span>()
<span style="color: #009900">Default</span> <span style="color: #990000">::</span> <span style="color: #000080">false</span></tt></pre></div></div>
<div class="paragraph"><p>Delete the resource.</p></div>
<div class="paragraph"><p>Cowboy will send an error response when this function
returns <code>false</code>.</p></div>
</div>
<div class="sect2">
<h3 id="_expires">expires</h3>
<div class="listingblock">
<div class="content"><!-- Generator: GNU source-highlight
by Lorenzo Bettini
http://www.lorenzobettini.it
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<pre><tt><span style="font-weight: bold"><span style="color: #000000">expires</span></span>(<span style="color: #009900">Req</span>, <span style="color: #009900">State</span>) <span style="color: #990000">-></span> {<span style="color: #009900">Result</span>, <span style="color: #009900">Req</span>, <span style="color: #009900">State</span>}
<span style="color: #009900">Result</span> <span style="color: #990000">::</span> <span style="font-weight: bold"><span style="color: #000000">calendar:datetime</span></span>() | <span style="font-weight: bold"><span style="color: #000080">binary</span></span>() | <span style="color: #000080">undefined</span>
<span style="color: #009900">Default</span> <span style="color: #990000">::</span> <span style="color: #000080">undefined</span></tt></pre></div></div>
<div class="paragraph"><p>Return the resource’s expiration date.</p></div>
</div>
<div class="sect2">
<h3 id="_forbidden">forbidden</h3>
<div class="listingblock">
<div class="content"><!-- Generator: GNU source-highlight
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http://www.lorenzobettini.it
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<pre><tt><span style="font-weight: bold"><span style="color: #000000">forbidden</span></span>(<span style="color: #009900">Req</span>, <span style="color: #009900">State</span>) <span style="color: #990000">-></span> {<span style="color: #009900">Result</span>, <span style="color: #009900">Req</span>, <span style="color: #009900">State</span>}
<span style="color: #009900">Result</span> <span style="color: #990000">::</span> <span style="font-weight: bold"><span style="color: #000000">boolean</span></span>()
<span style="color: #009900">Default</span> <span style="color: #990000">::</span> <span style="color: #000080">false</span></tt></pre></div></div>
<div class="paragraph"><p>Return whether access to the resource is forbidden.</p></div>
<div class="paragraph"><p>A <em>403 Forbidden</em> response will be sent if this
function returns <code>true</code>. This status code means that
access is forbidden regardless of authentication,
and that the request shouldn’t be repeated.</p></div>
</div>
<div class="sect2">
<h3 id="_generate_etag">generate_etag</h3>
<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">generate_etag</span></span>(<span style="color: #009900">Req</span>, <span style="color: #009900">State</span>) <span style="color: #990000">-></span> {<span style="color: #009900">Result</span>, <span style="color: #009900">Req</span>, <span style="color: #009900">State</span>}
<span style="color: #009900">Result</span> <span style="color: #990000">::</span> <span style="font-weight: bold"><span style="color: #000080">binary</span></span>() | {<span style="color: #FF6600">weak</span> | <span style="color: #FF6600">strong</span>, <span style="font-weight: bold"><span style="color: #000080">binary</span></span>()}
<span style="color: #009900">Default</span> <span style="color: #990000">-</span> <span style="color: #FF6600">no</span> <span style="color: #FF6600">etag</span> <span style="color: #FF6600">value</span></tt></pre></div></div>
<div class="paragraph"><p>Return the entity tag of the resource.</p></div>
<div class="paragraph"><p>When a binary is returned, the value is automatically
parsed to a tuple. The binary must be in the same
format as the etag header, including quotes.</p></div>
</div>
<div class="sect2">
<h3 id="_is_authorized">is_authorized</h3>
<div class="listingblock">
<div class="content"><!-- Generator: GNU source-highlight
by Lorenzo Bettini
http://www.lorenzobettini.it
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<pre><tt><span style="font-weight: bold"><span style="color: #000000">is_authorized</span></span>(<span style="color: #009900">Req</span>, <span style="color: #009900">State</span>) <span style="color: #990000">-></span> {<span style="color: #009900">Result</span>, <span style="color: #009900">Req</span>, <span style="color: #009900">State</span>}
<span style="color: #009900">Result</span> <span style="color: #990000">::</span> <span style="color: #000080">true</span> | {<span style="color: #000080">false</span>, <span style="color: #009900">AuthHeader</span> <span style="color: #990000">::</span> <span style="font-weight: bold"><span style="color: #000000">iodata</span></span>()}
<span style="color: #009900">Default</span> <span style="color: #990000">-</span> <span style="color: #000080">true</span></tt></pre></div></div>
<div class="paragraph"><p>Return whether the user is authorized to perform the action.</p></div>
<div class="paragraph"><p>This function should be used to perform any necessary
authentication of the user before attempting to perform
any action on the resource.</p></div>
<div class="paragraph"><p>When authentication fails, the <code>AuthHeader</code> value will
be sent in the www-authenticate header for the
<em>401 Unauthorized</em> response.</p></div>
</div>
<div class="sect2">
<h3 id="_is_conflict">is_conflict</h3>
<div class="listingblock">
<div class="content"><!-- Generator: GNU source-highlight
by Lorenzo Bettini
http://www.lorenzobettini.it
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<pre><tt><span style="font-weight: bold"><span style="color: #000000">is_conflict</span></span>(<span style="color: #009900">Req</span>, <span style="color: #009900">State</span>) <span style="color: #990000">-></span> {<span style="color: #009900">Result</span>, <span style="color: #009900">Req</span>, <span style="color: #009900">State</span>}
<span style="color: #009900">Result</span> <span style="color: #990000">::</span> <span style="font-weight: bold"><span style="color: #000000">boolean</span></span>()
<span style="color: #009900">Default</span> <span style="color: #990000">::</span> <span style="color: #000080">false</span></tt></pre></div></div>
<div class="paragraph"><p>Return whether the PUT request results in a conflict.</p></div>
<div class="paragraph"><p>A <em>409 Conflict</em> response is sent when <code>true</code>.</p></div>
</div>
<div class="sect2">
<h3 id="_known_methods">known_methods</h3>
<div class="listingblock">
<div class="content"><!-- Generator: GNU source-highlight
by Lorenzo Bettini
http://www.lorenzobettini.it
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<pre><tt><span style="font-weight: bold"><span style="color: #000000">known_methods</span></span>(<span style="color: #009900">Req</span>, <span style="color: #009900">State</span>) <span style="color: #990000">-></span> {<span style="color: #009900">Result</span>, <span style="color: #009900">Req</span>, <span style="color: #009900">State</span>}
<span style="color: #009900">Result</span> <span style="color: #990000">::</span> [<span style="font-weight: bold"><span style="color: #000080">binary</span></span>()] <span style="font-style: italic"><span style="color: #9A1900">%% case sensitive</span></span>
<span style="color: #009900">Default</span> <span style="color: #990000">::</span> [<span style="color: #990000"><<</span><span style="color: #FF0000">"GET"</span><span style="color: #990000">>></span>, <span style="color: #990000"><<</span><span style="color: #FF0000">"HEAD"</span><span style="color: #990000">>></span>, <span style="color: #990000"><<</span><span style="color: #FF0000">"POST"</span><span style="color: #990000">>></span>, <span style="color: #990000"><<</span><span style="color: #FF0000">"PUT"</span><span style="color: #990000">>></span>,
<span style="color: #990000"><<</span><span style="color: #FF0000">"PATCH"</span><span style="color: #990000">>></span>, <span style="color: #990000"><<</span><span style="color: #FF0000">"DELETE"</span><span style="color: #990000">>></span>, <span style="color: #990000"><<</span><span style="color: #FF0000">"OPTIONS"</span><span style="color: #990000">>></span>]</tt></pre></div></div>
<div class="paragraph"><p>Return the list of known methods.</p></div>
<div class="paragraph"><p>The full list of methods known by the server should be
returned, regardless of their use in the resource.</p></div>
<div class="paragraph"><p>The default value lists the methods Cowboy knows and
implement in <code>cowboy_rest</code>.</p></div>
</div>
<div class="sect2">
<h3 id="_languages_provided">languages_provided</h3>
<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">languages_provided</span></span>(<span style="color: #009900">Req</span>, <span style="color: #009900">State</span>) <span style="color: #990000">-></span> {<span style="color: #009900">Result</span>, <span style="color: #009900">Req</span>, <span style="color: #009900">State</span>}
<span style="color: #009900">Result</span> <span style="color: #990000">::</span> [<span style="font-weight: bold"><span style="color: #000080">binary</span></span>()] <span style="font-style: italic"><span style="color: #9A1900">%% lowercase; case insensitive</span></span>
<span style="color: #009900">Default</span> <span style="color: #990000">-</span> <span style="color: #FF6600">skip</span> <span style="color: #FF6600">this</span> <span style="color: #FF6600">step</span></tt></pre></div></div>
<div class="paragraph"><p>Return the list of languages the resource provides in order
of preference.</p></div>
<div class="paragraph"><p>During content negotiation Cowboy will pick the most
appropriate language for the client. The client advertises
languages it prefers with the accept-language header. When
that header is missing, Cowboy picks the first language
from the resource.</p></div>
<div class="paragraph"><p>Cowboy will add the negotiated <code>language</code> to the Req object
after this step completes:</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">req</span></span>() <span style="color: #990000">::</span> #{
<span style="color: #0000FF">language</span> <span style="color: #990000">=></span> <span style="font-weight: bold"><span style="color: #000080">binary</span></span>() <span style="font-style: italic"><span style="color: #9A1900">%% lowercase; case insensitive</span></span>
}</tt></pre></div></div>
</div>
<div class="sect2">
<h3 id="_last_modified">last_modified</h3>
<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">last_modified</span></span>(<span style="color: #009900">Req</span>, <span style="color: #009900">State</span>) <span style="color: #990000">-></span> {<span style="color: #009900">Result</span>, <span style="color: #009900">Req</span>, <span style="color: #009900">State</span>}
<span style="color: #009900">Result</span> <span style="color: #990000">::</span> <span style="font-weight: bold"><span style="color: #000000">calendar:datetime</span></span>()
<span style="color: #009900">Default</span> <span style="color: #990000">-</span> <span style="color: #FF6600">no</span> <span style="color: #FF6600">last</span> <span style="color: #FF6600">modified</span> <span style="color: #FF6600">value</span></tt></pre></div></div>
<div class="paragraph"><p>Return the resource’s last modification date.</p></div>
<div class="paragraph"><p>This date will be used to test against the if-modified-since
and if-unmodified-since headers, and sent as the last-modified
header in the response to GET and HEAD requests.</p></div>
</div>
<div class="sect2">
<h3 id="_malformed_request">malformed_request</h3>
<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">malformed_request</span></span>(<span style="color: #009900">Req</span>, <span style="color: #009900">State</span>) <span style="color: #990000">-></span> {<span style="color: #009900">Result</span>, <span style="color: #009900">Req</span>, <span style="color: #009900">State</span>}
<span style="color: #009900">Result</span> <span style="color: #990000">::</span> <span style="font-weight: bold"><span style="color: #000000">boolean</span></span>()
<span style="color: #009900">Default</span> <span style="color: #990000">::</span> <span style="color: #000080">false</span></tt></pre></div></div>
<div class="paragraph"><p>Return whether the request is malformed.</p></div>
<div class="paragraph"><p>A request is malformed when a component required by the
resource is invalid. This may include the query string
or individual headers. They should be parsed and validated
in this function. The body should not be read at this point.</p></div>
</div>
<div class="sect2">
<h3 id="_moved_permanently">moved_permanently</h3>
<div class="listingblock">
<div class="content"><!-- Generator: GNU source-highlight
by Lorenzo Bettini
http://www.lorenzobettini.it
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<pre><tt><span style="font-weight: bold"><span style="color: #000000">moved_permanently</span></span>(<span style="color: #009900">Req</span>, <span style="color: #009900">State</span>) <span style="color: #990000">-></span> {<span style="color: #009900">Result</span>, <span style="color: #009900">Req</span>, <span style="color: #009900">State</span>}
<span style="color: #009900">Result</span> <span style="color: #990000">::</span> {<span style="color: #000080">true</span>, <span style="color: #009900">URI</span> <span style="color: #990000">::</span> <span style="font-weight: bold"><span style="color: #000000">iodata</span></span>()} | <span style="color: #000080">false</span>
<span style="color: #009900">Default</span> <span style="color: #990000">::</span> <span style="color: #000080">false</span></tt></pre></div></div>
<div class="paragraph"><p>Return whether the resource was permanently moved, and
what its new location is.</p></div>
</div>
<div class="sect2">
<h3 id="_moved_temporarily">moved_temporarily</h3>
<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">moved_temporarily</span></span>(<span style="color: #009900">Req</span>, <span style="color: #009900">State</span>) <span style="color: #990000">-></span> {<span style="color: #009900">Result</span>, <span style="color: #009900">Req</span>, <span style="color: #009900">State</span>}
<span style="color: #009900">Result</span> <span style="color: #990000">::</span> {<span style="color: #000080">true</span>, <span style="color: #009900">URI</span> <span style="color: #990000">::</span> <span style="font-weight: bold"><span style="color: #000000">iodata</span></span>()} | <span style="color: #000080">false</span>
<span style="color: #009900">Default</span> <span style="color: #990000">::</span> <span style="color: #000080">false</span></tt></pre></div></div>
<div class="paragraph"><p>Return whether the resource was temporarily moved, and
what its new location is.</p></div>
</div>
<div class="sect2">
<h3 id="_multiple_choices">multiple_choices</h3>
<div class="listingblock">
<div class="content"><!-- Generator: GNU source-highlight
by Lorenzo Bettini
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http://www.gnu.org/software/src-highlite -->
<pre><tt><span style="font-weight: bold"><span style="color: #000000">multiple_choices</span></span>(<span style="color: #009900">Req</span>, <span style="color: #009900">State</span>) <span style="color: #990000">-></span> {<span style="color: #009900">Result</span>, <span style="color: #009900">Req</span>, <span style="color: #009900">State</span>}
<span style="color: #009900">Result</span> <span style="color: #990000">::</span> <span style="font-weight: bold"><span style="color: #000000">boolean</span></span>()
<span style="color: #009900">Default</span> <span style="color: #990000">::</span> <span style="color: #000080">false</span></tt></pre></div></div>
<div class="paragraph"><p>Return whether the client should engage in reactive
negotiation.</p></div>
<div class="paragraph"><p>Return <code>true</code> when the server has multiple representations
of a resource, each with their specific identifier, but is
unable to determine which is best for the client. For
example an image might have different sizes and the server
is unable to determine the capabilities of the client.</p></div>
<div class="paragraph"><p>When returning <code>true</code> the server should send a body with
links to the different representations. If the server has
a preferred representation it can send its link inside a
location header.</p></div>
</div>
<div class="sect2">
<h3 id="_options">options</h3>
<div class="listingblock">
<div class="content"><!-- Generator: GNU source-highlight
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<pre><tt><span style="font-weight: bold"><span style="color: #000000">options</span></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>}</tt></pre></div></div>
<div class="paragraph"><p>Respond to an OPTIONS request.</p></div>
<div class="paragraph"><p>The response should inform the client the communication
options available for this resource. By default Cowboy
will send a <em>200 OK</em> response with the allow header set.</p></div>
</div>
<div class="sect2">
<h3 id="_previously_existed">previously_existed</h3>
<div class="listingblock">
<div class="content"><!-- Generator: GNU source-highlight
by Lorenzo Bettini
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<pre><tt><span style="font-weight: bold"><span style="color: #000000">previously_existed</span></span>(<span style="color: #009900">Req</span>, <span style="color: #009900">State</span>) <span style="color: #990000">-></span> {<span style="color: #009900">Result</span>, <span style="color: #009900">Req</span>, <span style="color: #009900">State</span>}
<span style="color: #009900">Result</span> <span style="color: #990000">::</span> <span style="font-weight: bold"><span style="color: #000000">boolean</span></span>()
<span style="color: #009900">Default</span> <span style="color: #990000">::</span> <span style="color: #000080">false</span></tt></pre></div></div>
<div class="paragraph"><p>Return whether the resource existed previously.</p></div>
</div>
<div class="sect2">
<h3 id="_providecallback">ProvideCallback</h3>
<div class="listingblock">
<div class="content"><!-- Generator: GNU source-highlight
by Lorenzo Bettini
http://www.lorenzobettini.it
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<pre><tt><span style="color: #009900">ProvideCallback</span>(<span style="color: #009900">Req</span>, <span style="color: #009900">State</span>) <span style="color: #990000">-></span> {<span style="color: #009900">Result</span>, <span style="color: #009900">Req</span>, <span style="color: #009900">State</span>}
<span style="color: #009900">Result</span> <span style="color: #990000">::</span> <span style="font-weight: bold"><span style="color: #000000">cowboy_req:resp_body</span></span>()
<span style="color: #009900">Default</span> <span style="color: #990000">-</span> <span style="color: #FF6600">crash</span></tt></pre></div></div>
<div class="paragraph"><p>Return the response body.</p></div>
<div class="paragraph"><p>The response body can be provided either as the actual data
to be sent or a tuple indicating which file to send.</p></div>
<div class="paragraph"><p>This function is called for both GET and HEAD requests. For
the latter the body is not sent, however.</p></div>
<div class="paragraph"><p>Note that there used to be a way to stream the response body.
It was temporarily removed and will be added back in a later
release.</p></div>
</div>
<div class="sect2">
<h3 id="_resource_exists">resource_exists</h3>
<div class="listingblock">
<div class="content"><!-- Generator: GNU source-highlight
by Lorenzo Bettini
http://www.lorenzobettini.it
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<pre><tt><span style="font-weight: bold"><span style="color: #000000">resource_exists</span></span>(<span style="color: #009900">Req</span>, <span style="color: #009900">State</span>) <span style="color: #990000">-></span> {<span style="color: #009900">Result</span>, <span style="color: #009900">Req</span>, <span style="color: #009900">State</span>}
<span style="color: #009900">Result</span> <span style="color: #990000">::</span> <span style="font-weight: bold"><span style="color: #000000">boolean</span></span>()
<span style="color: #009900">Default</span> <span style="color: #990000">::</span> <span style="color: #000080">true</span></tt></pre></div></div>
<div class="paragraph"><p>Return whether the resource exists.</p></div>
</div>
<div class="sect2">
<h3 id="_service_available">service_available</h3>
<div class="listingblock">
<div class="content"><!-- Generator: GNU source-highlight
by Lorenzo Bettini
http://www.lorenzobettini.it
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<pre><tt><span style="font-weight: bold"><span style="color: #000000">service_available</span></span>(<span style="color: #009900">Req</span>, <span style="color: #009900">State</span>) <span style="color: #990000">-></span> {<span style="color: #009900">Result</span>, <span style="color: #009900">Req</span>, <span style="color: #009900">State</span>}
<span style="color: #009900">Result</span> <span style="color: #990000">::</span> <span style="font-weight: bold"><span style="color: #000000">boolean</span></span>()
<span style="color: #009900">Default</span> <span style="color: #990000">::</span> <span style="color: #000080">true</span></tt></pre></div></div>
<div class="paragraph"><p>Return whether the service is available.</p></div>
<div class="paragraph"><p>A <em>503 Service Unavailable</em> response will be sent when this
function returns <code>false</code>.</p></div>
</div>
<div class="sect2">
<h3 id="_uri_too_long">uri_too_long</h3>
<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">uri_too_long</span></span>(<span style="color: #009900">Req</span>, <span style="color: #009900">State</span>) <span style="color: #990000">-></span> {<span style="color: #009900">Result</span>, <span style="color: #009900">Req</span>, <span style="color: #009900">State</span>}
<span style="color: #009900">Result</span> <span style="color: #990000">::</span> <span style="font-weight: bold"><span style="color: #000000">boolean</span></span>()
<span style="color: #009900">Default</span> <span style="color: #990000">::</span> <span style="color: #000080">false</span></tt></pre></div></div>
<div class="paragraph"><p>Return whether the requested URI is too long.</p></div>
<div class="paragraph"><p>This function can be used to further restrict the length
of the URI for this specific resource.</p></div>
</div>
<div class="sect2">
<h3 id="_valid_content_headers">valid_content_headers</h3>
<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">valid_content_headers</span></span>(<span style="color: #009900">Req</span>, <span style="color: #009900">State</span>) <span style="color: #990000">-></span> {<span style="color: #009900">Result</span>, <span style="color: #009900">Req</span>, <span style="color: #009900">State</span>}
<span style="color: #009900">Result</span> <span style="color: #990000">::</span> <span style="font-weight: bold"><span style="color: #000000">boolean</span></span>()
<span style="color: #009900">Default</span> <span style="color: #990000">::</span> <span style="color: #000080">true</span></tt></pre></div></div>
<div class="paragraph"><p>Return whether the content headers are valid.</p></div>
<div class="paragraph"><p>This callback can be used to reject requests that have
invalid content header values, for example an unsupported
content-encoding.</p></div>
</div>
<div class="sect2">
<h3 id="_valid_entity_length">valid_entity_length</h3>
<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">valid_entity_length</span></span>(<span style="color: #009900">Req</span>, <span style="color: #009900">State</span>) <span style="color: #990000">-></span> {<span style="color: #009900">Result</span>, <span style="color: #009900">Req</span>, <span style="color: #009900">State</span>}
<span style="color: #009900">Result</span> <span style="color: #990000">::</span> <span style="font-weight: bold"><span style="color: #000000">boolean</span></span>()
<span style="color: #009900">Default</span> <span style="color: #990000">::</span> <span style="color: #000080">true</span></tt></pre></div></div>
<div class="paragraph"><p>Return whether the request body length is within acceptable boundaries.</p></div>
<div class="paragraph"><p>A <em>413 Request Entity Too Large</em> response will be sent if this
function returns <code>false</code>.</p></div>
</div>
<div class="sect2">
<h3 id="_variances">variances</h3>
<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">variances</span></span>(<span style="color: #009900">Req</span>, <span style="color: #009900">State</span>) <span style="color: #990000">-></span> {<span style="color: #009900">Result</span>, <span style="color: #009900">Req</span>, <span style="color: #009900">State</span>}
<span style="color: #009900">Result</span> <span style="color: #990000">::</span> [<span style="font-weight: bold"><span style="color: #000080">binary</span></span>()] <span style="font-style: italic"><span style="color: #9A1900">%% case insensitive</span></span>
<span style="color: #009900">Default</span> <span style="color: #990000">::</span> []</tt></pre></div></div>
<div class="paragraph"><p>Return the list of request headers that affect the
representation of the resource.</p></div>
<div class="paragraph"><p>Cowboy automatically adds the accept, accept-charset and
accept-language headers when necessary. It’s also useful
to note that some standard headers also do not need to be
listed here, like the authorization header.</p></div>
</div>
</div>
</div>
<div class="sect1">
<h2 id="_changelog">Changelog</h2>
<div class="sectionbody">
<div class="ulist"><ul>
<li>
<p>
<strong>2.1</strong>: The <code>switch_handler</code> return value was added.
</p>
</li>
<li>
<p>
<strong>1.0</strong>: Behavior introduced.
</p>
</li>
</ul></div>
</div>
</div>
<div class="sect1">
<h2 id="_see_also">See also</h2>
<div class="sectionbody">
<div class="paragraph"><p><a href="..">cowboy(7)</a>,
<a href="../cowboy_handler">cowboy_handler(3)</a></p></div>
</div>
</div>
</div>
<div class="span3 sidecol">
<h3>
Cowboy
2.2
Function Reference
</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/2.4/manual">2.4</a></li>
<li><a href="/docs/en/cowboy/2.3/manual">2.3</a></li>
<li><a href="/docs/en/cowboy/2.2/manual">2.2</a></li>
<li><a href="/docs/en/cowboy/2.1/manual">2.1</a></li>
<li><a href="/docs/en/cowboy/2.0/manual">2.0</a></li>
<li><a href="/docs/en/cowboy/1.0/manual">1.0</a></li>
</ul>
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