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<title>Nine Nines: REST handlers</title>
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<h1 class="lined-header"><span>REST handlers</span></h1>
<p>REST is implemented in Cowboy as a sub protocol. The request is handled as a state machine with many optional callbacks describing the resource and modifying the machine's behavior.</p>
<p>The REST handler is the recommended way to handle HTTP requests.</p>
<h2 id="_initialization">Initialization</h2>
<p>First, the <code>init/2</code> callback is called. This callback is common to all handlers. To use REST for the current request, this function must return a <code>cowboy_rest</code> tuple.</p>
<div class="listingblock"><div class="content"><!-- Generator: GNU source-highlight 3.1.9
by Lorenzo Bettini
http://www.lorenzobettini.it
http://www.gnu.org/software/src-highlite -->
<pre><tt><b><font color="#000000">init</font></b>(<font color="#009900">Req</font>, <font color="#009900">State</font>) <font color="#990000">-></font>
{<font color="#FF6600">cowboy_rest</font>, <font color="#009900">Req</font>, <font color="#009900">State</font>}<font color="#990000">.</font></tt></pre>
</div></div>
<p>Cowboy will then switch to the REST protocol and start executing the state machine.</p>
<p>After reaching the end of the flowchart, the <code>terminate/3</code> callback will be called if it is defined.</p>
<h2 id="_methods">Methods</h2>
<p>The REST component has code for handling the following HTTP methods: HEAD, GET, POST, PATCH, PUT, DELETE and OPTIONS.</p>
<p>Other methods can be accepted, however they have no specific callback defined for them at this time.</p>
<h2 id="_callbacks">Callbacks</h2>
<p>All callbacks are optional. Some may become mandatory depending on what other defined callbacks return. The various flowcharts in the next chapter should be a useful to determine which callbacks you need.</p>
<p>All callbacks take two arguments, the Req object and the State, and return a three-element tuple of the form <code>{Value, Req, State}</code>.</p>
<p>Nearly all callbacks can also return <code>{stop, Req, State}</code> to stop execution of the request, and <code>{{switch_handler, Module}, Req, State}</code> or <code>{{switch_handler, Module, Opts}, Req, State}</code> to switch to a different handler type. The exceptions are <code>expires</code> <code>generate_etag</code>, <code>last_modified</code> and <code>variances</code>.</p>
<p>The following table summarizes the callbacks and their default values. If the callback isn't defined, then the default value will be used. Please look at the flowcharts to find out the result of each return value.</p>
<p>In the following table, "skip" means the callback is entirely skipped if it is undefined, moving directly to the next step. Similarly, "none" means there is no default value for this callback.</p>
<table rules="all" width="100%" frame="border"
cellspacing="0" cellpadding="4">
<thead><tr><th>Callback name</th>
<th>Default value</th>
</tr></thead><tbody><tr><td>allowed_methods</td>
<td><code>[<<"GET">>, <<"HEAD">>, <<"OPTIONS">>]</code></td>
</tr>
<tr><td>allow_missing_post</td>
<td><code>true</code></td>
</tr>
<tr><td>charsets_provided</td>
<td>skip</td>
</tr>
<tr><td>content_types_accepted</td>
<td>none</td>
</tr>
<tr><td>content_types_provided</td>
<td><code>[{{ <<"text">>, <<"html">>, '*'}, to_html}]</code></td>
</tr>
<tr><td>delete_completed</td>
<td><code>true</code></td>
</tr>
<tr><td>delete_resource</td>
<td><code>false</code></td>
</tr>
<tr><td>expires</td>
<td><code>undefined</code></td>
</tr>
<tr><td>forbidden</td>
<td><code>false</code></td>
</tr>
<tr><td>generate_etag</td>
<td><code>undefined</code></td>
</tr>
<tr><td>is_authorized</td>
<td><code>true</code></td>
</tr>
<tr><td>is_conflict</td>
<td><code>false</code></td>
</tr>
<tr><td>known_methods</td>
<td><code>[<<"GET">>, <<"HEAD">>, <<"POST">>, <<"PUT">>, <<"PATCH">>, <<"DELETE">>, <<"OPTIONS">>]</code></td>
</tr>
<tr><td>languages_provided</td>
<td>skip</td>
</tr>
<tr><td>last_modified</td>
<td><code>undefined</code></td>
</tr>
<tr><td>malformed_request</td>
<td><code>false</code></td>
</tr>
<tr><td>moved_permanently</td>
<td><code>false</code></td>
</tr>
<tr><td>moved_temporarily</td>
<td><code>false</code></td>
</tr>
<tr><td>multiple_choices</td>
<td><code>false</code></td>
</tr>
<tr><td>options</td>
<td><code>ok</code></td>
</tr>
<tr><td>previously_existed</td>
<td><code>false</code></td>
</tr>
<tr><td>rate_limited</td>
<td><code>false</code></td>
</tr>
<tr><td>resource_exists</td>
<td><code>true</code></td>
</tr>
<tr><td>service_available</td>
<td><code>true</code></td>
</tr>
<tr><td>uri_too_long</td>
<td><code>false</code></td>
</tr>
<tr><td>valid_content_headers</td>
<td><code>true</code></td>
</tr>
<tr><td>valid_entity_length</td>
<td><code>true</code></td>
</tr>
<tr><td>variances</td>
<td><code>[]</code></td>
</tr>
</tbody></table>
<p>As you can see, Cowboy tries to move on with the request whenever possible by using well thought out default values.</p>
<p>In addition to these, there can be any number of user-defined callbacks that are specified through <code>content_types_accepted/2</code> and <code>content_types_provided/2</code>. They can take any name, however it is recommended to use a separate prefix for the callbacks of each function. For example, <code>from_html</code> and <code>to_html</code> indicate in the first case that we're accepting a resource given as HTML, and in the second case that we send one as HTML.</p>
<h2 id="_meta_data">Meta data</h2>
<p>Cowboy will set informative values to the Req object at various points of the execution. You can retrieve them by matching the Req object directly. The values are defined in the following table:</p>
<table rules="all" width="100%" frame="border"
cellspacing="0" cellpadding="4">
<thead><tr><th>Key</th>
<th>Details</th>
</tr></thead><tbody><tr><td>media_type</td>
<td>The content-type negotiated for the response entity.</td>
</tr>
<tr><td>language</td>
<td>The language negotiated for the response entity.</td>
</tr>
<tr><td>charset</td>
<td>The charset negotiated for the response entity.</td>
</tr>
</tbody></table>
<p>They can be used to send a proper body with the response to a request that used a method other than HEAD or GET.</p>
<h2 id="_response_headers">Response headers</h2>
<p>Cowboy will set response headers automatically over the execution of the REST code. They are listed in the following table.</p>
<table rules="all" width="100%" frame="border"
cellspacing="0" cellpadding="4">
<thead><tr><th>Header name</th>
<th>Details</th>
</tr></thead><tbody><tr><td>content-language</td>
<td>Language used in the response body</td>
</tr>
<tr><td>content-type</td>
<td>Media type and charset of the response body</td>
</tr>
<tr><td>etag</td>
<td>Etag of the resource</td>
</tr>
<tr><td>expires</td>
<td>Expiration date of the resource</td>
</tr>
<tr><td>last-modified</td>
<td>Last modification date for the resource</td>
</tr>
<tr><td>location</td>
<td>Relative or absolute URI to the requested resource</td>
</tr>
<tr><td>vary</td>
<td>List of headers that may change the representation of the resource</td>
</tr>
</tbody></table>
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<a style="float:left" href="https://ninenines.eu/docs/en/cowboy/2.10/guide/rest_principles/">
REST principles
</a>
<a style="float:right" href="https://ninenines.eu/docs/en/cowboy/2.10/guide/rest_flowcharts/">
REST flowcharts
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