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<!DOCTYPE html>
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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 <code>cowboy_rest</code> module implements REST semantics on top of
the HTTP protocol.</p></div>
<div class="paragraph"><p>This module is a sub protocol that defines many callbacks
be implemented by handlers. The <code>init/2</code> and <code>terminate/3</code>
callbacks are common to all handler types and are documented
in the manual for the <a href="cowboy_handler.asciidoc">cowboy_handler</a> module.</p></div>
<div class="paragraph"><p>All other callbacks are optional, though some may become
required depending on the return value of previous callbacks.</p></div>
</div>
</div>
<div class="sect1">
<h2 id="_meta_values">Meta values</h2>
<div class="sectionbody">
<div class="dlist"><dl>
<dt class="hdlist1">
charset = binary()
</dt>
<dd>
<p>
Negotiated charset.
<br />
This value may not be defined if no charset was negotiated.
</p>
</dd>
<dt class="hdlist1">
language = binary()
</dt>
<dd>
<p>
Negotiated language.
<br />
This value may not be defined if no language was negotiated.
</p>
</dd>
<dt class="hdlist1">
media_type = {binary(), binary(), <em>*</em> | [{binary(), binary()}]}
</dt>
<dd>
<p>
Negotiated media-type.
<br />
The media-type is the content-type, excluding the charset.
<br />
This value is always defined after the call to
<code>content_types_provided/2</code>.
</p>
</dd>
</dl></div>
</div>
</div>
<div class="sect1">
<h2 id="_terminate_reasons">Terminate reasons</h2>
<div class="sectionbody">
<div class="paragraph"><p>The following values may be received as the terminate reason
in the optional <code>terminate/3</code> callback.</p></div>
<div class="dlist"><dl>
<dt class="hdlist1">
normal
</dt>
<dd>
<p>
The connection was closed 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="_callbacks">Callbacks</h2>
<div class="sectionbody">
<div class="sect2">
<h3 id="_callback_req_state_8594_value_req_state_stop_req_state">Callback(Req, State) → {Value, Req, State} | {stop, Req, State}</h3>
<div class="dlist"><dl>
<dt class="hdlist1">
Callback
</dt>
<dd>
<p>
One of the REST callbacks described below.
</p>
</dd>
<dt class="hdlist1">
Req = cowboy_req:req()
</dt>
<dd>
<p>
The Req object.
</p>
</dd>
<dt class="hdlist1">
State = any()
</dt>
<dd>
<p>
Handler state.
</p>
</dd>
<dt class="hdlist1">
Value
</dt>
<dd>
<p>
See the REST callbacks description below.
</p>
</dd>
</dl></div>
<div class="paragraph"><p>Please see the REST callbacks description below for details
on the <code>Value</code> type, the default value if the callback is
not defined, and more general information on when the
callback is called and what its intended use is.</p></div>
<div class="paragraph"><p>The <code>stop</code> tuple can be returned to stop REST processing.
It is up to the resource code to send a reply before that,
otherwise a <code>204 No Content</code> will be sent.</p></div>
</div>
</div>
</div>
<div class="sect1">
<h2 id="_rest_callbacks_description">REST callbacks description</h2>
<div class="sectionbody">
<div class="sect2">
<h3 id="_allowed_methods">allowed_methods</h3>
<div class="dlist"><dl>
<dt class="hdlist1">
Methods
</dt>
<dd>
<p>
all
</p>
</dd>
<dt class="hdlist1">
Value type
</dt>
<dd>
<p>
[binary()]
</p>
</dd>
<dt class="hdlist1">
Default value
</dt>
<dd>
<p>
<code>[<<"GET">>, <<"HEAD">>, <<"OPTIONS">>]</code>
</p>
</dd>
</dl></div>
<div class="paragraph"><p>Return the list of allowed methods.</p></div>
<div class="paragraph"><p>Methods are case sensitive. Standard methods are always uppercase.</p></div>
</div>
<div class="sect2">
<h3 id="_allow_missing_post">allow_missing_post</h3>
<div class="dlist"><dl>
<dt class="hdlist1">
Methods
</dt>
<dd>
<p>
POST
</p>
</dd>
<dt class="hdlist1">
Value type
</dt>
<dd>
<p>
boolean()
</p>
</dd>
<dt class="hdlist1">
Default value
</dt>
<dd>
<p>
true
</p>
</dd>
</dl></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 URL to the created resource should also be
returned from the <code>AcceptResource</code> callback.</p></div>
</div>
<div class="sect2">
<h3 id="_charsets_provided">charsets_provided</h3>
<div class="dlist"><dl>
<dt class="hdlist1">
Methods
</dt>
<dd>
<p>
GET, HEAD, POST, PUT, PATCH, DELETE
</p>
</dd>
<dt class="hdlist1">
Value type
</dt>
<dd>
<p>
[binary()]
</p>
</dd>
<dt class="hdlist1">
Default behavior
</dt>
<dd>
<p>
Skip to the next step if undefined.
</p>
</dd>
</dl></div>
<div class="paragraph"><p>Return the list of charsets the resource provides.</p></div>
<div class="paragraph"><p>The list must be ordered in order of preference.</p></div>
<div class="paragraph"><p>If the accept-charset header was not sent, the first charset
in the list will be selected. Otherwise Cowboy will select
the most appropriate charset from the list.</p></div>
<div class="paragraph"><p>The chosen charset will be set in the <code>Req</code> object as the meta
value <code>charset</code>.</p></div>
<div class="paragraph"><p>While charsets are case insensitive, this callback is expected
to return them as lowercase binary.</p></div>
</div>
<div class="sect2">
<h3 id="_content_types_accepted">content_types_accepted</h3>
<div class="dlist"><dl>
<dt class="hdlist1">
Methods
</dt>
<dd>
<p>
POST, PUT, PATCH
</p>
</dd>
<dt class="hdlist1">
Value type
</dt>
<dd>
<p>
[{binary() | {Type, SubType, Params}, AcceptResource}]
</p>
</dd>
<dt class="hdlist1">
Default behavior
</dt>
<dd>
<p>
Crash if undefined.
</p>
</dd>
</dl></div>
<div class="paragraph"><p>With types:</p></div>
<div class="ulist"><ul>
<li>
<p>
Type = SubType = binary()
</p>
</li>
<li>
<p>
Params = <em>*</em> | [{binary(), binary()}]
</p>
</li>
<li>
<p>
AcceptResource = atom()
</p>
</li>
</ul></div>
<div class="paragraph"><p>Return the list of content-types the resource accepts.</p></div>
<div class="paragraph"><p>The list must be ordered in order of preference.</p></div>
<div class="paragraph"><p>Each content-type can be given either as a binary string or as
a tuple containing the type, subtype and parameters.</p></div>
<div class="paragraph"><p>Cowboy will select the most appropriate content-type from the list.
If any parameter is acceptable, then the tuple form should be used
with parameters set to <code>'*'</code>. If the parameters value is set to <code>[]</code>
only content-type values with no parameters will be accepted. All
parameter values are treated in a case sensitive manner except the
<code>charset</code> parameter, if present, which is case insensitive.</p></div>
<div class="paragraph"><p>This function will be called for POST, PUT and PATCH requests.
It is entirely possible to define different callbacks for different
methods if the handling of the request differs. Simply verify
what the method is with <code>cowboy_req:method/1</code> and return a
different list for each methods.</p></div>
<div class="paragraph"><p>The <code>AcceptResource</code> value is the name of the callback that will
be called if the content-type matches. It is defined as follows.</p></div>
<div class="dlist"><dl>
<dt class="hdlist1">
Value type
</dt>
<dd>
<p>
true | {true, URL} | false
</p>
</dd>
<dt class="hdlist1">
Default behavior
</dt>
<dd>
<p>
Crash if undefined.
</p>
</dd>
</dl></div>
<div class="paragraph"><p>Process the request body.</p></div>
<div class="paragraph"><p>This function should create or update the resource with the
information contained in the request body. This information
may be full or partial depending on the request method.</p></div>
<div class="paragraph"><p>If the request body was processed successfully, <code>true</code> must
be returned. If the request method is POST, <code>{true, URL}</code> may
be returned instead, and Cowboy will redirect the client to
the location of the newly created resource.</p></div>
<div class="paragraph"><p>If a response body must be sent, the appropriate media-type, charset
and language can be retrieved using the <code>cowboy_req:meta/{2,3}</code>
functions. The respective keys are <code>media_type</code>, <code>charset</code>
and <code>language</code>. The body can be set using <code>cowboy_req:set_resp_body/2</code>.</p></div>
</div>
<div class="sect2">
<h3 id="_content_types_provided">content_types_provided</h3>
<div class="dlist"><dl>
<dt class="hdlist1">
Methods
</dt>
<dd>
<p>
GET, HEAD, POST, PUT, PATCH, DELETE
</p>
</dd>
<dt class="hdlist1">
Value type
</dt>
<dd>
<p>
[{binary() | {Type, SubType, Params}, ProvideResource}]
</p>
</dd>
<dt class="hdlist1">
Default value
</dt>
<dd>
<p>
<code>[{{ <<"text">>, <<"html">>, '*'}, to_html}]</code>
</p>
</dd>
</dl></div>
<div class="paragraph"><p>With types:</p></div>
<div class="ulist"><ul>
<li>
<p>
Type = SubType = binary()
</p>
</li>
<li>
<p>
Params = <em>*</em> | [{binary(), binary()}]
</p>
</li>
<li>
<p>
ProvideResource = atom()
</p>
</li>
</ul></div>
<div class="paragraph"><p>Return the list of content-types the resource provides.</p></div>
<div class="paragraph"><p>The list must be ordered in order of preference.</p></div>
<div class="paragraph"><p>Each content-type can be given either as a binary string or as
a tuple containing the type, subtype and parameters.</p></div>
<div class="paragraph"><p>Cowboy will select the most appropriate content-type from the list.
If any parameter is acceptable, then the tuple form should be used
with parameters set to <code>'*'</code>. If the parameters value is set to <code>[]</code>
only content-type values with no parameters will be accepted. All
parameter values are treated in a case sensitive manner except the
<code>charset</code> parameter, if present, which is case insensitive.</p></div>
<div class="paragraph"><p>The <code>ProvideResource</code> value is the name of the callback that will
be called if the content-type matches. It will only be called when
a representation of the resource needs to be returned. It is defined
as follow.</p></div>
<div class="dlist"><dl>
<dt class="hdlist1">
Methods
</dt>
<dd>
<p>
GET, HEAD
</p>
</dd>
<dt class="hdlist1">
Value type
</dt>
<dd>
<p>
iodata() | {stream, Fun} | {stream, Len, Fun} | {chunked, ChunkedFun}
</p>
</dd>
<dt class="hdlist1">
Default behavior
</dt>
<dd>
<p>
Crash if undefined.
</p>
</dd>
</dl></div>
<div class="paragraph"><p>Return the response body.</p></div>
<div class="paragraph"><p>The response body may be provided directly or through a fun.
If a fun tuple is returned, the appropriate <code>set_resp_body_fun</code>
function will be called. Please refer to the documentation for
these functions for more information about the types.</p></div>
<div class="paragraph"><p>The call to this callback happens a good time after the call to
<code>content_types_provided/2</code>, when it is time to start rendering
the response body.</p></div>
</div>
<div class="sect2">
<h3 id="_delete_completed">delete_completed</h3>
<div class="dlist"><dl>
<dt class="hdlist1">
Methods
</dt>
<dd>
<p>
DELETE
</p>
</dd>
<dt class="hdlist1">
Value type
</dt>
<dd>
<p>
boolean()
</p>
</dd>
<dt class="hdlist1">
Default value
</dt>
<dd>
<p>
true
</p>
</dd>
</dl></div>
<div class="paragraph"><p>Return whether the delete action has been completed.</p></div>
<div class="paragraph"><p>This function should return <code>false</code> if there is no guarantee
that the resource gets deleted immediately from the system,
including from any internal cache.</p></div>
<div class="paragraph"><p>When this function returns <code>false</code>, a <code>202 Accepted</code>
response will be sent instead of a <code>200 OK</code> or <code>204 No Content</code>.</p></div>
</div>
<div class="sect2">
<h3 id="_delete_resource">delete_resource</h3>
<div class="dlist"><dl>
<dt class="hdlist1">
Methods
</dt>
<dd>
<p>
DELETE
</p>
</dd>
<dt class="hdlist1">
Value type
</dt>
<dd>
<p>
boolean()
</p>
</dd>
<dt class="hdlist1">
Default value
</dt>
<dd>
<p>
false
</p>
</dd>
</dl></div>
<div class="paragraph"><p>Delete the resource.</p></div>
<div class="paragraph"><p>The value returned indicates if the action was successful,
regardless of whether the resource is immediately deleted
from the system.</p></div>
</div>
<div class="sect2">
<h3 id="_expires">expires</h3>
<div class="dlist"><dl>
<dt class="hdlist1">
Methods
</dt>
<dd>
<p>
GET, HEAD
</p>
</dd>
<dt class="hdlist1">
Value type
</dt>
<dd>
<p>
calendar:datetime() | binary() | undefined
</p>
</dd>
<dt class="hdlist1">
Default value
</dt>
<dd>
<p>
undefined
</p>
</dd>
</dl></div>
<div class="paragraph"><p>Return the date of expiration of the resource.</p></div>
<div class="paragraph"><p>This date will be sent as the value of the expires header.</p></div>
</div>
<div class="sect2">
<h3 id="_forbidden">forbidden</h3>
<div class="dlist"><dl>
<dt class="hdlist1">
Methods
</dt>
<dd>
<p>
all
</p>
</dd>
<dt class="hdlist1">
Value type
</dt>
<dd>
<p>
boolean()
</p>
</dd>
<dt class="hdlist1">
Default value
</dt>
<dd>
<p>
false
</p>
</dd>
</dl></div>
<div class="paragraph"><p>Return whether access to the resource is forbidden.</p></div>
<div class="paragraph"><p>A <code>403 Forbidden</code> 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="dlist"><dl>
<dt class="hdlist1">
Methods
</dt>
<dd>
<p>
GET, HEAD, POST, PUT, PATCH, DELETE
</p>
</dd>
<dt class="hdlist1">
Value type
</dt>
<dd>
<p>
binary() | {weak | strong, binary()}
</p>
</dd>
<dt class="hdlist1">
Default value
</dt>
<dd>
<p>
undefined
</p>
</dd>
</dl></div>
<div class="paragraph"><p>Return the entity tag of the resource.</p></div>
<div class="paragraph"><p>This value will be sent as the value of the etag header.</p></div>
<div class="paragraph"><p>If a binary is returned, then the value will be parsed
to the tuple form automatically. The value 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="dlist"><dl>
<dt class="hdlist1">
Methods
</dt>
<dd>
<p>
all
</p>
</dd>
<dt class="hdlist1">
Value type
</dt>
<dd>
<p>
true | {false, AuthHeader}
</p>
</dd>
<dt class="hdlist1">
Default value
</dt>
<dd>
<p>
true
</p>
</dd>
</dl></div>
<div class="paragraph"><p>With types:</p></div>
<div class="ulist"><ul>
<li>
<p>
AuthHead = iodata()
</p>
</li>
</ul></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>If the authentication fails, the value returned will be sent
as the value for the www-authenticate header in the
<code>401 Unauthorized</code> response.</p></div>
</div>
<div class="sect2">
<h3 id="_is_conflict">is_conflict</h3>
<div class="dlist"><dl>
<dt class="hdlist1">
Methods
</dt>
<dd>
<p>
PUT
</p>
</dd>
<dt class="hdlist1">
Value type
</dt>
<dd>
<p>
boolean()
</p>
</dd>
<dt class="hdlist1">
Default value
</dt>
<dd>
<p>
false
</p>
</dd>
</dl></div>
<div class="paragraph"><p>Return whether the put action results in a conflict.</p></div>
<div class="paragraph"><p>A <code>409 Conflict</code> response will be sent if this function
returns <code>true</code>.</p></div>
</div>
<div class="sect2">
<h3 id="_known_methods">known_methods</h3>
<div class="dlist"><dl>
<dt class="hdlist1">
Methods
</dt>
<dd>
<p>
all
</p>
</dd>
<dt class="hdlist1">
Value type
</dt>
<dd>
<p>
[binary()]
</p>
</dd>
<dt class="hdlist1">
Default value
</dt>
<dd>
<p>
<code>[<<"GET">>, <<"HEAD">>, <<"POST">>, <<"PUT">>, <<"PATCH">>, <<"DELETE">>, <<"OPTIONS">>]</code>
</p>
</dd>
</dl></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 class="paragraph"><p>Methods are case sensitive. Standard methods are always uppercase.</p></div>
</div>
<div class="sect2">
<h3 id="_languages_provided">languages_provided</h3>
<div class="dlist"><dl>
<dt class="hdlist1">
Methods
</dt>
<dd>
<p>
GET, HEAD, POST, PUT, PATCH, DELETE
</p>
</dd>
<dt class="hdlist1">
Value type
</dt>
<dd>
<p>
[binary()]
</p>
</dd>
<dt class="hdlist1">
Default behavior
</dt>
<dd>
<p>
Skip to the next step if undefined.
</p>
</dd>
</dl></div>
<div class="paragraph"><p>Return the list of languages the resource provides.</p></div>
<div class="paragraph"><p>The list must be ordered in order of preference.</p></div>
<div class="paragraph"><p>If the accept-language header was not sent, the first language
in the list will be selected. Otherwise Cowboy will select
the most appropriate language from the list.</p></div>
<div class="paragraph"><p>The chosen language will be set in the <code>Req</code> object as the meta
value <code>language</code>.</p></div>
<div class="paragraph"><p>While languages are case insensitive, this callback is expected
to return them as lowercase binary.</p></div>
</div>
<div class="sect2">
<h3 id="_last_modified">last_modified</h3>
<div class="dlist"><dl>
<dt class="hdlist1">
Methods
</dt>
<dd>
<p>
GET, HEAD, POST, PUT, PATCH, DELETE
</p>
</dd>
<dt class="hdlist1">
Value type
</dt>
<dd>
<p>
calendar:datetime()
</p>
</dd>
<dt class="hdlist1">
Default value
</dt>
<dd>
<p>
undefined
</p>
</dd>
</dl></div>
<div class="paragraph"><p>Return the date of last modification of the resource.</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 of GET and HEAD requests.</p></div>
</div>
<div class="sect2">
<h3 id="_malformed_request">malformed_request</h3>
<div class="dlist"><dl>
<dt class="hdlist1">
Methods
</dt>
<dd>
<p>
all
</p>
</dd>
<dt class="hdlist1">
Value type
</dt>
<dd>
<p>
boolean()
</p>
</dd>
<dt class="hdlist1">
Default value
</dt>
<dd>
<p>
false
</p>
</dd>
</dl></div>
<div class="paragraph"><p>Return whether the request is malformed.</p></div>
<div class="paragraph"><p>Cowboy has already performed all the necessary checks
by the time this function is called, so few resources
are expected to implement it.</p></div>
<div class="paragraph"><p>The check is to be done on the request itself, not on
the request body, which is processed later.</p></div>
</div>
<div class="sect2">
<h3 id="_moved_permanently">moved_permanently</h3>
<div class="dlist"><dl>
<dt class="hdlist1">
Methods
</dt>
<dd>
<p>
GET, HEAD, POST, PUT, PATCH, DELETE
</p>
</dd>
<dt class="hdlist1">
Value type
</dt>
<dd>
<p>
{true, URL} | false
</p>
</dd>
<dt class="hdlist1">
Default value
</dt>
<dd>
<p>
false
</p>
</dd>
</dl></div>
<div class="paragraph"><p>With types:</p></div>
<div class="ulist"><ul>
<li>
<p>
URL = iodata()
</p>
</li>
</ul></div>
<div class="paragraph"><p>Return whether the resource was permanently moved.</p></div>
<div class="paragraph"><p>If it was, its new URL is also returned and sent in the
location header in the response.</p></div>
</div>
<div class="sect2">
<h3 id="_moved_temporarily">moved_temporarily</h3>
<div class="dlist"><dl>
<dt class="hdlist1">
Methods
</dt>
<dd>
<p>
GET, HEAD, POST, PATCH, DELETE
</p>
</dd>
<dt class="hdlist1">
Value type
</dt>
<dd>
<p>
{true, URL} | false
</p>
</dd>
<dt class="hdlist1">
Default value
</dt>
<dd>
<p>
false
</p>
</dd>
</dl></div>
<div class="paragraph"><p>With types:</p></div>
<div class="ulist"><ul>
<li>
<p>
URL = iodata()
</p>
</li>
</ul></div>
<div class="paragraph"><p>Return whether the resource was temporarily moved.</p></div>
<div class="paragraph"><p>If it was, its new URL is also returned and sent in the
location header in the response.</p></div>
</div>
<div class="sect2">
<h3 id="_multiple_choices">multiple_choices</h3>
<div class="dlist"><dl>
<dt class="hdlist1">
Methods
</dt>
<dd>
<p>
GET, HEAD, POST, PUT, PATCH, DELETE
</p>
</dd>
<dt class="hdlist1">
Value type
</dt>
<dd>
<p>
boolean()
</p>
</dd>
<dt class="hdlist1">
Default value
</dt>
<dd>
<p>
false
</p>
</dd>
</dl></div>
<div class="paragraph"><p>Return whether there are multiple representations of the resource.</p></div>
<div class="paragraph"><p>This function should be used to inform the client if there
are different representations of the resource, for example
different content-type. If this function returns <code>true</code>,
the response body should include information about these
different representations using <code>cowboy_req:set_resp_body/2</code>.
The content-type of the response should be the one previously
negociated and that can be obtained by calling
<code>cowboy_req:meta(media_type, Req)</code>.</p></div>
</div>
<div class="sect2">
<h3 id="_options">options</h3>
<div class="dlist"><dl>
<dt class="hdlist1">
Methods
</dt>
<dd>
<p>
OPTIONS
</p>
</dd>
<dt class="hdlist1">
Value type
</dt>
<dd>
<p>
ok
</p>
</dd>
<dt class="hdlist1">
Default value
</dt>
<dd>
<p>
ok
</p>
</dd>
</dl></div>
<div class="paragraph"><p>Handle a request for information.</p></div>
<div class="paragraph"><p>The response should inform the client the communication
options available for this resource.</p></div>
<div class="paragraph"><p>By default, Cowboy will send a <code>200 OK</code> response with the
allow header set.</p></div>
</div>
<div class="sect2">
<h3 id="_previously_existed">previously_existed</h3>
<div class="dlist"><dl>
<dt class="hdlist1">
Methods
</dt>
<dd>
<p>
GET, HEAD, POST, PATCH, DELETE
</p>
</dd>
<dt class="hdlist1">
Value type
</dt>
<dd>
<p>
boolean()
</p>
</dd>
<dt class="hdlist1">
Default value
</dt>
<dd>
<p>
false
</p>
</dd>
</dl></div>
<div class="paragraph"><p>Return whether the resource existed previously.</p></div>
</div>
<div class="sect2">
<h3 id="_resource_exists">resource_exists</h3>
<div class="dlist"><dl>
<dt class="hdlist1">
Methods
</dt>
<dd>
<p>
GET, HEAD, POST, PUT, PATCH, DELETE
</p>
</dd>
<dt class="hdlist1">
Value type
</dt>
<dd>
<p>
boolean()
</p>
</dd>
<dt class="hdlist1">
Default value
</dt>
<dd>
<p>
true
</p>
</dd>
</dl></div>
<div class="paragraph"><p>Return whether the resource exists.</p></div>
<div class="paragraph"><p>If it exists, conditional headers will be tested before
attempting to perform the action. Otherwise, Cowboy will
check if the resource previously existed first.</p></div>
</div>
<div class="sect2">
<h3 id="_service_available">service_available</h3>
<div class="dlist"><dl>
<dt class="hdlist1">
Methods
</dt>
<dd>
<p>
all
</p>
</dd>
<dt class="hdlist1">
Value type
</dt>
<dd>
<p>
boolean()
</p>
</dd>
<dt class="hdlist1">
Default value
</dt>
<dd>
<p>
true
</p>
</dd>
</dl></div>
<div class="paragraph"><p>Return whether the service is available.</p></div>
<div class="paragraph"><p>This function can be used to test that all relevant backend
systems are up and able to handle requests.</p></div>
<div class="paragraph"><p>A <code>503 Service Unavailable</code> response will be sent if this
function returns <code>false</code>.</p></div>
</div>
<div class="sect2">
<h3 id="_uri_too_long">uri_too_long</h3>
<div class="dlist"><dl>
<dt class="hdlist1">
Methods
</dt>
<dd>
<p>
all
</p>
</dd>
<dt class="hdlist1">
Value type
</dt>
<dd>
<p>
boolean()
</p>
</dd>
<dt class="hdlist1">
Default value
</dt>
<dd>
<p>
false
</p>
</dd>
</dl></div>
<div class="paragraph"><p>Return whether the requested URI is too long.</p></div>
<div class="paragraph"><p>Cowboy has already performed all the necessary checks
by the time this function is called, so few resources
are expected to implement it.</p></div>
<div class="paragraph"><p>A <code>414 Request-URI Too Long</code> response will be sent if this
function returns <code>true</code>.</p></div>
</div>
<div class="sect2">
<h3 id="_valid_content_headers">valid_content_headers</h3>
<div class="dlist"><dl>
<dt class="hdlist1">
Methods
</dt>
<dd>
<p>
all
</p>
</dd>
<dt class="hdlist1">
Value type
</dt>
<dd>
<p>
boolean()
</p>
</dd>
<dt class="hdlist1">
Default value
</dt>
<dd>
<p>
true
</p>
</dd>
</dl></div>
<div class="paragraph"><p>Return whether the content-* headers are valid.</p></div>
<div class="paragraph"><p>This also applies to the transfer-encoding header. This
function must return <code>false</code> for any unknown content-*
headers, or if the headers can’t be understood. The
function <code>cowboy_req:parse_header/2</code> can be used to
quickly check the headers can be parsed.</p></div>
<div class="paragraph"><p>A <code>501 Not Implemented</code> response will be sent if this
function returns <code>false</code>.</p></div>
</div>
<div class="sect2">
<h3 id="_valid_entity_length">valid_entity_length</h3>
<div class="dlist"><dl>
<dt class="hdlist1">
Methods
</dt>
<dd>
<p>
all
</p>
</dd>
<dt class="hdlist1">
Value type
</dt>
<dd>
<p>
boolean()
</p>
</dd>
<dt class="hdlist1">
Default value
</dt>
<dd>
<p>
true
</p>
</dd>
</dl></div>
<div class="paragraph"><p>Return whether the request body length is within acceptable boundaries.</p></div>
<div class="paragraph"><p>A <code>413 Request Entity Too Large</code> response will be sent if this
function returns <code>false</code>.</p></div>
</div>
<div class="sect2">
<h3 id="_variances">variances</h3>
<div class="dlist"><dl>
<dt class="hdlist1">
Methods
</dt>
<dd>
<p>
GET, HEAD, POST, PUT, PATCH, DELETE
</p>
</dd>
<dt class="hdlist1">
Value type
</dt>
<dd>
<p>
[binary()]
</p>
</dd>
<dt class="hdlist1">
Default value
</dt>
<dd>
<p>
[]
</p>
</dd>
</dl></div>
<div class="paragraph"><p>Return the list of headers that affect the representation of the resource.</p></div>
<div class="paragraph"><p>These request headers return the same resource but with different
parameters, like another language or a different content-type.</p></div>
<div class="paragraph"><p>Cowboy will automatically add the accept, accept-language and
accept-charset headers to the list if the respective functions
were defined in the resource.</p></div>
<div class="paragraph"><p>This operation is performed right before the <code>resource_exists/2</code>
callback. All responses past that point will contain the vary
header which holds this list.</p></div>
</div>
</div>
</div>
</div>
<div class="span3 sidecol">
<h3>
Cowboy
2.0
Function Reference
</h3>
<ul>
<li><a href="/docs/en/cowboy/2.0/guide">User Guide</a></li>
<li><a href="/docs/en/cowboy/2.0/manual">Function Reference</a></li>
</ul>
<h4 id="docs-nav">Navigation</h4>
<h4>Version select</h4>
<ul>
<li><a href="/docs/en/cowboy/1.0/manual">1.0</a></li>
<li><a href="/docs/en/cowboy/2.0/manual">2.0</a></li>
</ul>
</div>
</div>
</div>
</div>
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