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<title>Nine Nines: The Req object</title>
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<h1 class="lined-header"><span>The Req object</span></h1>
<div class="paragraph"><p>The Req object is this variable that you will use to obtain
information about a request, read the body of the request
and send a response.</p></div>
<div class="sect1">
<h2 id="_a_special_variable">A special variable</h2>
<div class="sectionbody">
<div class="paragraph"><p>While we call it an "object", it is not an object in the
OOP sense of the term. In fact it is completely opaque
to you and the only way you can perform operations using
it is by calling the functions from the <code>cowboy_req</code>
module.</p></div>
<div class="paragraph"><p>Almost all the calls to the <code>cowboy_req</code> module will
return an updated request object. Just like you would
keep the updated <code>State</code> variable in a gen_server,
you MUST keep the updated <code>Req</code> variable in a Cowboy
handler. Cowboy will use this object to know whether
a response has been sent when the handler has finished
executing.</p></div>
<div class="paragraph"><p>The Req object allows accessing both immutable and
mutable state. This means that calling some of the
functions twice will not produce the same result.
For example, when streaming the request body, the
function will return the body by chunks, one at a
time, until there is none left.</p></div>
</div>
</div>
<div class="sect1">
<h2 id="_overview_of_the_cowboy_req_interface">Overview of the cowboy_req interface</h2>
<div class="sectionbody">
<div class="paragraph"><p>With the exception of functions manipulating the request
body, all functions return a single value. Depending on
the function this can be the requested value (method,
host, path, …), a boolean (has_body, has_resp_header…)
a new Req object (set_resp_body, set_resp_header…), or
simply the atom <code>ok</code> (chunk, continue, …).</p></div>
<div class="paragraph"><p>The request body reading functions may return <code>{Result, Req}</code>
or <code>{Result, Value, Req}</code>. The functions in this category
are <code>body/{1,2}</code>, <code>body_qs/{1,2}</code>, <code>part/{1,2}</code>, <code>part_body/{1,2}</code>.</p></div>
<div class="paragraph"><p>This chapter covers the access functions mainly. Cookies,
request body and response functions are covered in their
own chapters.</p></div>
</div>
</div>
<div class="sect1">
<h2 id="_request">Request</h2>
<div class="sectionbody">
<div class="paragraph"><p>When a client performs a request, it first sends a few required
values. They are sent differently depending on the protocol
being used, but the intent is the same. They indicate to the
server the type of action it wants to do and how to locate
the resource to perform it on.</p></div>
<div class="paragraph"><p>The method identifies the action. Standard methods include
GET, HEAD, OPTIONS, PATCH, POST, PUT, DELETE. Method names
are case sensitive.</p></div>
<div class="listingblock">
<div class="content"><!-- Generator: GNU source-highlight 3.1.8
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<pre><tt><span style="color: #009900">Method</span> <span style="color: #990000">=</span> <span style="font-weight: bold"><span style="color: #000000">cowboy_req:method</span></span>(<span style="color: #009900">Req</span>)<span style="color: #990000">.</span></tt></pre></div></div>
<div class="paragraph"><p>The host, port and path parts of the URL identify the resource
being accessed. The host and port information may not be
available if the client uses HTTP/1.0.</p></div>
<div class="listingblock">
<div class="content"><!-- Generator: GNU source-highlight 3.1.8
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<pre><tt><span style="color: #009900">Host</span> <span style="color: #990000">=</span> <span style="font-weight: bold"><span style="color: #000000">cowboy_req:host</span></span>(<span style="color: #009900">Req</span>),
<span style="color: #009900">Port</span> <span style="color: #990000">=</span> <span style="font-weight: bold"><span style="color: #000000">cowboy_req:port</span></span>(<span style="color: #009900">Req</span>),
<span style="color: #009900">Path</span> <span style="color: #990000">=</span> <span style="font-weight: bold"><span style="color: #000000">cowboy_req:path</span></span>(<span style="color: #009900">Req</span>)<span style="color: #990000">.</span></tt></pre></div></div>
<div class="paragraph"><p>The version used by the client can of course also be obtained.</p></div>
<div class="listingblock">
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<pre><tt><span style="color: #009900">Version</span> <span style="color: #990000">=</span> <span style="font-weight: bold"><span style="color: #000000">cowboy_req:version</span></span>(<span style="color: #009900">Req</span>)<span style="color: #990000">.</span></tt></pre></div></div>
<div class="paragraph"><p>Do note however that clients claiming to implement one version
of the protocol does not mean they implement it fully, or even
properly.</p></div>
</div>
</div>
<div class="sect1">
<h2 id="_bindings">Bindings</h2>
<div class="sectionbody">
<div class="paragraph"><p>After routing the request, bindings are available. Bindings
are these parts of the host or path that you chose to extract
when defining the routes of your application.</p></div>
<div class="paragraph"><p>You can fetch a single binding. The value will be <code>undefined</code>
if the binding doesn’t exist.</p></div>
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<pre><tt><span style="color: #009900">Binding</span> <span style="color: #990000">=</span> <span style="font-weight: bold"><span style="color: #000000">cowboy_req:binding</span></span>(<span style="color: #FF6600">my_binding</span>, <span style="color: #009900">Req</span>)<span style="color: #990000">.</span></tt></pre></div></div>
<div class="paragraph"><p>If you need a different value when the binding doesn’t exist,
you can change the default.</p></div>
<div class="listingblock">
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<pre><tt><span style="color: #009900">Binding</span> <span style="color: #990000">=</span> <span style="font-weight: bold"><span style="color: #000000">cowboy_req:binding</span></span>(<span style="color: #FF6600">my_binding</span>, <span style="color: #009900">Req</span>, <span style="color: #993399">42</span>)<span style="color: #990000">.</span></tt></pre></div></div>
<div class="paragraph"><p>You can also obtain all bindings in one call. They will be
returned as a list of key/value tuples.</p></div>
<div class="listingblock">
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<pre><tt><span style="color: #009900">AllBindings</span> <span style="color: #990000">=</span> <span style="font-weight: bold"><span style="color: #000000">cowboy_req:bindings</span></span>(<span style="color: #009900">Req</span>)<span style="color: #990000">.</span></tt></pre></div></div>
<div class="paragraph"><p>If you used <code>...</code> at the beginning of the route’s pattern
for the host, you can retrieve the matched part of the host.
The value will be <code>undefined</code> otherwise.</p></div>
<div class="listingblock">
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<pre><tt><span style="color: #009900">HostInfo</span> <span style="color: #990000">=</span> <span style="font-weight: bold"><span style="color: #000000">cowboy_req:host_info</span></span>(<span style="color: #009900">Req</span>)<span style="color: #990000">.</span></tt></pre></div></div>
<div class="paragraph"><p>Similarly, if you used <code>...</code> at the end of the route’s
pattern for the path, you can retrieve the matched part,
or get <code>undefined</code> otherwise.</p></div>
<div class="listingblock">
<div class="content"><!-- Generator: GNU source-highlight 3.1.8
by Lorenzo Bettini
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<pre><tt><span style="color: #009900">PathInfo</span> <span style="color: #990000">=</span> <span style="font-weight: bold"><span style="color: #000000">cowboy_req:path_info</span></span>(<span style="color: #009900">Req</span>)<span style="color: #990000">.</span></tt></pre></div></div>
</div>
</div>
<div class="sect1">
<h2 id="_query_string">Query string</h2>
<div class="sectionbody">
<div class="paragraph"><p>The raw query string can be obtained directly.</p></div>
<div class="listingblock">
<div class="content"><!-- Generator: GNU source-highlight 3.1.8
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<pre><tt><span style="color: #009900">Qs</span> <span style="color: #990000">=</span> <span style="font-weight: bold"><span style="color: #000000">cowboy_req:qs</span></span>(<span style="color: #009900">Req</span>)<span style="color: #990000">.</span></tt></pre></div></div>
<div class="paragraph"><p>You can parse the query string and then use standard library
functions to access individual values.</p></div>
<div class="listingblock">
<div class="content"><!-- Generator: GNU source-highlight 3.1.8
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<pre><tt><span style="color: #009900">QsVals</span> <span style="color: #990000">=</span> <span style="font-weight: bold"><span style="color: #000000">cowboy_req:parse_qs</span></span>(<span style="color: #009900">Req</span>),
{<span style="color: #990000">_</span>, <span style="color: #009900">Lang</span>} <span style="color: #990000">=</span> <span style="font-weight: bold"><span style="color: #000000">lists:keyfind</span></span>(<span style="color: #990000"><<</span><span style="color: #FF0000">"lang"</span><span style="color: #990000">>></span>, <span style="color: #993399">1</span>, <span style="color: #009900">QsVals</span>)<span style="color: #990000">.</span></tt></pre></div></div>
<div class="paragraph"><p>You can match the query string into a map.</p></div>
<div class="listingblock">
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<pre><tt>#{<span style="color: #FF6600">id</span> <span style="color: #990000">:=</span> <span style="color: #009900">ID</span>, <span style="color: #FF6600">lang</span> <span style="color: #990000">:=</span> <span style="color: #009900">Lang</span>} <span style="color: #990000">=</span> <span style="font-weight: bold"><span style="color: #000000">cowboy_req:match_qs</span></span>([<span style="color: #FF6600">id</span>, <span style="color: #FF6600">lang</span>], <span style="color: #009900">Req</span>)<span style="color: #990000">.</span></tt></pre></div></div>
<div class="paragraph"><p>You can use constraints to validate the values while matching
them. The following snippet will crash if the <code>id</code> value is
not an integer number or if the <code>lang</code> value is empty. Additionally
the <code>id</code> value will be converted to an integer term, saving
you a conversion step.</p></div>
<div class="listingblock">
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<pre><tt><span style="color: #009900">QsMap</span> <span style="color: #990000">=</span> <span style="font-weight: bold"><span style="color: #000000">cowboy_req:match_qs</span></span>([{<span style="color: #FF6600">id</span>, <span style="color: #FF6600">int</span>}, {<span style="color: #FF6600">lang</span>, <span style="color: #FF6600">nonempty</span>}], <span style="color: #009900">Req</span>)<span style="color: #990000">.</span></tt></pre></div></div>
<div class="paragraph"><p>Note that in the case of duplicate query string keys, the map
value will become a list of the different values.</p></div>
<div class="paragraph"><p>Read more about <sup>constraints</sup>.</p></div>
<div class="paragraph"><p>A default value can be provided. The default will be used
if the <code>lang</code> key is not found. It will not be used if
the key is found but has an empty value.</p></div>
<div class="listingblock">
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<pre><tt>#{<span style="color: #FF6600">lang</span> <span style="color: #990000">:=</span> <span style="color: #009900">Lang</span>} <span style="color: #990000">=</span> <span style="font-weight: bold"><span style="color: #000000">cowboy_req:match_qs</span></span>([{<span style="color: #FF6600">lang</span>, [], <span style="color: #990000"><<</span><span style="color: #FF0000">"en-US"</span><span style="color: #990000">>></span>}], <span style="color: #009900">Req</span>)<span style="color: #990000">.</span></tt></pre></div></div>
<div class="paragraph"><p>If no default is provided and the value is missing, the
query string is deemed invalid and the process will crash.</p></div>
</div>
</div>
<div class="sect1">
<h2 id="_request_url">Request URL</h2>
<div class="sectionbody">
<div class="paragraph"><p>You can reconstruct the full URL of the resource.</p></div>
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<div class="content"><!-- Generator: GNU source-highlight 3.1.8
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<pre><tt><span style="color: #009900">URL</span> <span style="color: #990000">=</span> <span style="font-weight: bold"><span style="color: #000000">cowboy_req:url</span></span>(<span style="color: #009900">Req</span>)<span style="color: #990000">.</span></tt></pre></div></div>
<div class="paragraph"><p>You can also obtain only the base of the URL, excluding the
path and query string.</p></div>
<div class="listingblock">
<div class="content"><!-- Generator: GNU source-highlight 3.1.8
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<pre><tt><span style="color: #009900">BaseURL</span> <span style="color: #990000">=</span> <span style="font-weight: bold"><span style="color: #000000">cowboy_req:host_url</span></span>(<span style="color: #009900">Req</span>)<span style="color: #990000">.</span></tt></pre></div></div>
</div>
</div>
<div class="sect1">
<h2 id="_headers">Headers</h2>
<div class="sectionbody">
<div class="paragraph"><p>Cowboy allows you to obtain the header values as string,
or parsed into a more meaningful representation.</p></div>
<div class="paragraph"><p>This will get the string value of a header.</p></div>
<div class="listingblock">
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<pre><tt><span style="color: #009900">HeaderVal</span> <span style="color: #990000">=</span> <span style="font-weight: bold"><span style="color: #000000">cowboy_req:header</span></span>(<span style="color: #990000"><<</span><span style="color: #FF0000">"content-type"</span><span style="color: #990000">>></span>, <span style="color: #009900">Req</span>)<span style="color: #990000">.</span></tt></pre></div></div>
<div class="paragraph"><p>You can of course set a default in case the header is missing.</p></div>
<div class="listingblock">
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<pre><tt><span style="color: #009900">HeaderVal</span> <span style="color: #990000">=</span> <span style="font-weight: bold"><span style="color: #000000">cowboy_req:header</span></span>(<span style="color: #990000"><<</span><span style="color: #FF0000">"content-type"</span><span style="color: #990000">>></span>, <span style="color: #009900">Req</span>, <span style="color: #990000"><<</span><span style="color: #FF0000">"text/plain"</span><span style="color: #990000">>></span>)<span style="color: #990000">.</span></tt></pre></div></div>
<div class="paragraph"><p>And also obtain all headers.</p></div>
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<pre><tt><span style="color: #009900">AllHeaders</span> <span style="color: #990000">=</span> <span style="font-weight: bold"><span style="color: #000000">cowboy_req:headers</span></span>(<span style="color: #009900">Req</span>)<span style="color: #990000">.</span></tt></pre></div></div>
<div class="paragraph"><p>To parse the previous header, simply call <code>parse_header/{2,3}</code>
where you would call <code>header/{2,3}</code> otherwise.</p></div>
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http://www.lorenzobettini.it
http://www.gnu.org/software/src-highlite -->
<pre><tt><span style="color: #009900">ParsedVal</span> <span style="color: #990000">=</span> <span style="font-weight: bold"><span style="color: #000000">cowboy_req:parse_header</span></span>(<span style="color: #990000"><<</span><span style="color: #FF0000">"content-type"</span><span style="color: #990000">>></span>, <span style="color: #009900">Req</span>)<span style="color: #990000">.</span></tt></pre></div></div>
<div class="paragraph"><p>Cowboy will crash if it doesn’t know how to parse the given
header, or if the value is invalid.</p></div>
<div class="paragraph"><p>You can of course define a default value. Note that the default
value you specify here is the parsed value you’d like to get
by default.</p></div>
<div class="listingblock">
<div class="content"><!-- Generator: GNU source-highlight 3.1.8
by Lorenzo Bettini
http://www.lorenzobettini.it
http://www.gnu.org/software/src-highlite -->
<pre><tt><span style="color: #009900">ParsedVal</span> <span style="color: #990000">=</span> <span style="font-weight: bold"><span style="color: #000000">cowboy_req:parse_header</span></span>(<span style="color: #990000"><<</span><span style="color: #FF0000">"content-type"</span><span style="color: #990000">>></span>, <span style="color: #009900">Req</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">"plain"</span><span style="color: #990000">>></span>, []})<span style="color: #990000">.</span></tt></pre></div></div>
<div class="paragraph"><p>The list of known headers and default values is defined in the
manual.</p></div>
</div>
</div>
<div class="sect1">
<h2 id="_meta">Meta</h2>
<div class="sectionbody">
<div class="paragraph"><p>Cowboy will sometimes associate some meta information with
the request. Built-in meta values are listed in the manual
for their respective modules.</p></div>
<div class="paragraph"><p>This will get a meta value. The returned value will be <code>undefined</code>
if it isn’t defined.</p></div>
<div class="listingblock">
<div class="content"><!-- Generator: GNU source-highlight 3.1.8
by Lorenzo Bettini
http://www.lorenzobettini.it
http://www.gnu.org/software/src-highlite -->
<pre><tt><span style="color: #009900">MetaVal</span> <span style="color: #990000">=</span> <span style="font-weight: bold"><span style="color: #000000">cowboy_req:meta</span></span>(<span style="color: #FF6600">websocket_version</span>, <span style="color: #009900">Req</span>)<span style="color: #990000">.</span></tt></pre></div></div>
<div class="paragraph"><p>You can change the default value if needed.</p></div>
<div class="listingblock">
<div class="content"><!-- Generator: GNU source-highlight 3.1.8
by Lorenzo Bettini
http://www.lorenzobettini.it
http://www.gnu.org/software/src-highlite -->
<pre><tt><span style="color: #009900">MetaVal</span> <span style="color: #990000">=</span> <span style="font-weight: bold"><span style="color: #000000">cowboy_req:meta</span></span>(<span style="color: #FF6600">websocket_version</span>, <span style="color: #009900">Req</span>, <span style="color: #993399">13</span>)<span style="color: #990000">.</span></tt></pre></div></div>
<div class="paragraph"><p>You can also define your own meta values. The name must be
an <code>atom()</code>.</p></div>
<div class="listingblock">
<div class="content"><!-- Generator: GNU source-highlight 3.1.8
by Lorenzo Bettini
http://www.lorenzobettini.it
http://www.gnu.org/software/src-highlite -->
<pre><tt><span style="color: #009900">Req2</span> <span style="color: #990000">=</span> <span style="font-weight: bold"><span style="color: #000000">cowboy_req:set_meta</span></span>(<span style="color: #FF6600">the_answer</span>, <span style="color: #993399">42</span>, <span style="color: #009900">Req</span>)<span style="color: #990000">.</span></tt></pre></div></div>
</div>
</div>
<div class="sect1">
<h2 id="_peer">Peer</h2>
<div class="sectionbody">
<div class="paragraph"><p>You can obtain the peer address and port number. This is
not necessarily the actual IP and port of the client, but
rather the one of the machine that connected to the server.</p></div>
<div class="listingblock">
<div class="content"><!-- Generator: GNU source-highlight 3.1.8
by Lorenzo Bettini
http://www.lorenzobettini.it
http://www.gnu.org/software/src-highlite -->
<pre><tt>{<span style="color: #009900">IP</span>, <span style="color: #009900">Port</span>} <span style="color: #990000">=</span> <span style="font-weight: bold"><span style="color: #000000">cowboy_req:peer</span></span>(<span style="color: #009900">Req</span>)<span style="color: #990000">.</span></tt></pre></div></div>
</div>
</div>
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<h3>
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