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<!DOCTYPE html>
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<h1 class="lined-header"><span>Sending a response</span></h1>
<div class="paragraph"><p>The response must be sent using the Req object.</p></div>
<div class="paragraph"><p>Cowboy provides two different ways of sending responses:
either directly or by streaming the body. Response headers
and body may be set in advance. The response is sent as
soon as one of the reply or stream reply function is
called.</p></div>
<div class="paragraph"><p>Cowboy also provides a simplified interface for sending
files. It can also send only specific parts of a file.</p></div>
<div class="paragraph"><p>While only one response is allowed for every request,
HTTP/2 introduced a mechanism that allows the server
to push additional resources related to the response.
This chapter also describes how this feature works in
Cowboy.</p></div>
<div class="sect1">
<h2 id="_reply">Reply</h2>
<div class="sectionbody">
<div class="paragraph"><p>Cowboy provides three functions for sending the entire reply,
depending on whether you need to set headers and body. In all
cases, Cowboy will add any headers required by the protocol
(for example the date header will always be sent).</p></div>
<div class="paragraph"><p>When you need to set only the status code,
use <code>cowboy_req:reply/2</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">Req</span> <span style="color: #990000">=</span> <span style="font-weight: bold"><span style="color: #000000">cowboy_req:reply</span></span>(<span style="color: #993399">200</span>, <span style="color: #009900">Req0</span>)<span style="color: #990000">.</span></tt></pre></div></div>
<div class="paragraph"><p>When you need to set response headers at the same time,
use <code>cowboy_req:reply/3</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">Req</span> <span style="color: #990000">=</span> <span style="font-weight: bold"><span style="color: #000000">cowboy_req:reply</span></span>(<span style="color: #993399">303</span>, #{
<span style="color: #990000"><<</span><span style="color: #FF0000">"location"</span><span style="color: #990000">>></span> <span style="color: #990000">=></span> <span style="color: #990000"><<</span><span style="color: #FF0000">"http://ninenines.eu"</span><span style="color: #990000">>></span>
}, <span style="color: #009900">Req0</span>)<span style="color: #990000">.</span></tt></pre></div></div>
<div class="paragraph"><p>Note that the header name must always be a lowercase
binary.</p></div>
<div class="paragraph"><p>When you also need to set the response body,
use <code>cowboy_req:reply/4</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">Req</span> <span style="color: #990000">=</span> <span style="font-weight: bold"><span style="color: #000000">cowboy_req:reply</span></span>(<span style="color: #993399">200</span>, #{
<span style="color: #990000"><<</span><span style="color: #FF0000">"content-type"</span><span style="color: #990000">>></span> <span style="color: #990000">=></span> <span style="color: #990000"><<</span><span style="color: #FF0000">"text/plain"</span><span style="color: #990000">>></span>
}, <span style="color: #FF0000">"Hello world!"</span>, <span style="color: #009900">Req0</span>)<span style="color: #990000">.</span></tt></pre></div></div>
<div class="paragraph"><p>You should always set the content-type header when the
response has a body. There is however no need to set
the content-length header; Cowboy does it automatically.</p></div>
<div class="paragraph"><p>The response body and the header values must be either
a binary or an iolist. An iolist is a list containing
binaries, characters, strings or other iolists. This
allows you to build a response from different parts
without having to do any concatenation:</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">Title</span> <span style="color: #990000">=</span> <span style="color: #FF0000">"Hello world!"</span>,
<span style="color: #009900">Body</span> <span style="color: #990000">=</span> <span style="color: #990000"><<</span><span style="color: #FF0000">"Hats off!"</span><span style="color: #990000">>></span>,
<span style="color: #009900">Req</span> <span style="color: #990000">=</span> <span style="font-weight: bold"><span style="color: #000000">cowboy_req:reply</span></span>(<span style="color: #993399">200</span>, #{
<span style="color: #990000"><<</span><span style="color: #FF0000">"content-type"</span><span style="color: #990000">>></span> <span style="color: #990000">=></span> <span style="color: #990000"><<</span><span style="color: #FF0000">"text/html"</span><span style="color: #990000">>></span>
}, [<span style="color: #FF0000">"<html><head><title>"</span>, <span style="color: #009900">Title</span>, <span style="color: #FF0000">"</title></head>"</span>,
<span style="color: #FF0000">"<body><p>"</span>, <span style="color: #009900">Body</span>, <span style="color: #FF0000">"</p></body></html>"</span>], <span style="color: #009900">Req0</span>)<span style="color: #990000">.</span></tt></pre></div></div>
<div class="paragraph"><p>This method of building responses is more efficient than
concatenating. Behind the scenes, each element of the list
is simply a pointer, and those pointers are used directly
when writing to the socket.</p></div>
</div>
</div>
<div class="sect1">
<h2 id="_stream_reply">Stream reply</h2>
<div class="sectionbody">
<div class="paragraph"><p>Cowboy provides two functions for initiating a response,
and an additional function for streaming the response body.
Cowboy will add any required headers to the response.</p></div>
<div class="paragraph"><p>When you need to set only the status code,
use <code>cowboy_req:stream_reply/2</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">Req</span> <span style="color: #990000">=</span> <span style="font-weight: bold"><span style="color: #000000">cowboy_req:stream_reply</span></span>(<span style="color: #993399">200</span>, <span style="color: #009900">Req0</span>),
<span style="font-weight: bold"><span style="color: #000000">cowboy_req:stream_body</span></span>(<span style="color: #FF0000">"Hello..."</span>, <span style="color: #FF6600">nofin</span>, <span style="color: #009900">Req</span>),
<span style="font-weight: bold"><span style="color: #000000">cowboy_req:stream_body</span></span>(<span style="color: #FF0000">"chunked..."</span>, <span style="color: #FF6600">nofin</span>, <span style="color: #009900">Req</span>),
<span style="font-weight: bold"><span style="color: #000000">cowboy_req:stream_body</span></span>(<span style="color: #FF0000">"world!!"</span>, <span style="color: #FF6600">fin</span>, <span style="color: #009900">Req</span>)<span style="color: #990000">.</span></tt></pre></div></div>
<div class="paragraph"><p>The second argument to <code>cowboy_req:stream_body/3</code> indicates
whether this data terminates the body. Use <code>fin</code> for the
final flag, and <code>nofin</code> otherwise.</p></div>
<div class="paragraph"><p>This snippet does not set a content-type header. This is
not recommended. All responses with a body should have
a content-type. The header can be set beforehand, or
using the <code>cowboy_req:stream_reply/3</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">Req</span> <span style="color: #990000">=</span> <span style="font-weight: bold"><span style="color: #000000">cowboy_req:stream_reply</span></span>(<span style="color: #993399">200</span>, #{
<span style="color: #990000"><<</span><span style="color: #FF0000">"content-type"</span><span style="color: #990000">>></span> <span style="color: #990000">=></span> <span style="color: #990000"><<</span><span style="color: #FF0000">"text/html"</span><span style="color: #990000">>></span>
}, <span style="color: #009900">Req0</span>),
<span style="font-weight: bold"><span style="color: #000000">cowboy_req:stream_body</span></span>(<span style="color: #FF0000">"<html><head>Hello world!</head>"</span>, <span style="color: #FF6600">nofin</span>, <span style="color: #009900">Req</span>),
<span style="font-weight: bold"><span style="color: #000000">cowboy_req:stream_body</span></span>(<span style="color: #FF0000">"<body><p>Hats off!</p></body></html>"</span>, <span style="color: #FF6600">fin</span>, <span style="color: #009900">Req</span>)<span style="color: #990000">.</span></tt></pre></div></div>
<div class="paragraph"><p>HTTP provides a few different ways to stream response bodies.
Cowboy will select the most appropriate one based on the HTTP
version and the request and response headers.</p></div>
<div class="paragraph"><p>While not required by any means, it is recommended that you
set the content-length header in the response if you know it
in advance. This will ensure that the best response method
is selected and help clients understand when the response
is fully received.</p></div>
</div>
</div>
<div class="sect1">
<h2 id="_preset_response_headers">Preset response headers</h2>
<div class="sectionbody">
<div class="paragraph"><p>Cowboy provides functions to set response headers without
immediately sending them. They are stored in the Req object
and sent as part of the response when a reply function is
called.</p></div>
<div class="paragraph"><p>To set response headers:</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">Req</span> <span style="color: #990000">=</span> <span style="font-weight: bold"><span style="color: #000000">cowboy_req:set_resp_header</span></span>(<span style="color: #990000"><<</span><span style="color: #FF0000">"allow"</span><span style="color: #990000">>></span>, <span style="color: #FF0000">"GET"</span>, <span style="color: #009900">Req0</span>)<span style="color: #990000">.</span></tt></pre></div></div>
<div class="paragraph"><p>Header names must be a lowercase binary.</p></div>
<div class="paragraph"><p>Do not use this function for setting cookies. Refer to
the <a href="../cookies">Cookies</a> chapter for more information.</p></div>
<div class="paragraph"><p>To check if a response header has already been set:</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="font-weight: bold"><span style="color: #000000">cowboy_req:has_resp_header</span></span>(<span style="color: #990000"><<</span><span style="color: #FF0000">"allow"</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>It returns <code>true</code> if the header was set, <code>false</code> otherwise.</p></div>
<div class="paragraph"><p>To delete a response header that was set previously:</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">Req</span> <span style="color: #990000">=</span> <span style="font-weight: bold"><span style="color: #000000">cowboy_req:delete_resp_header</span></span>(<span style="color: #990000"><<</span><span style="color: #FF0000">"allow"</span><span style="color: #990000">>></span>, <span style="color: #009900">Req0</span>)<span style="color: #990000">.</span></tt></pre></div></div>
</div>
</div>
<div class="sect1">
<h2 id="_overriding_headers">Overriding headers</h2>
<div class="sectionbody">
<div class="paragraph"><p>As Cowboy provides different ways of setting response
headers and body, clashes may occur, so it’s important
to understand what happens when a header is set twice.</p></div>
<div class="paragraph"><p>Headers come from five different origins:</p></div>
<div class="ulist"><ul>
<li>
<p>
Protocol-specific headers (for example HTTP/1.1’s connection header)
</p>
</li>
<li>
<p>
Other required headers (for example the date header)
</p>
</li>
<li>
<p>
Preset headers
</p>
</li>
<li>
<p>
Headers given to the reply function
</p>
</li>
<li>
<p>
Set-cookie headers
</p>
</li>
</ul></div>
<div class="paragraph"><p>Cowboy does not allow overriding protocol-specific headers.</p></div>
<div class="paragraph"><p>Set-cookie headers will always be appended at the end of
the list of headers before sending the response.</p></div>
<div class="paragraph"><p>Headers given to the reply function will always override
preset headers and required headers. If a header is found
in two or three of these, then the one in the reply function
is picked and the others are dropped.</p></div>
<div class="paragraph"><p>Similarly, preset headers will always override required
headers.</p></div>
<div class="paragraph"><p>To illustrate, look at the following snippet. Cowboy by
default sends the server header with the value "Cowboy".
We can override it:</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">Req</span> <span style="color: #990000">=</span> <span style="font-weight: bold"><span style="color: #000000">cowboy_req:reply</span></span>(<span style="color: #993399">200</span>, #{
<span style="color: #990000"><<</span><span style="color: #FF0000">"server"</span><span style="color: #990000">>></span> <span style="color: #990000">=></span> <span style="color: #990000"><<</span><span style="color: #FF0000">"yaws"</span><span style="color: #990000">>></span>
}, <span style="color: #009900">Req0</span>)<span style="color: #990000">.</span></tt></pre></div></div>
</div>
</div>
<div class="sect1">
<h2 id="_preset_response_body">Preset response body</h2>
<div class="sectionbody">
<div class="paragraph"><p>Cowboy provides functions to set the response body without
immediately sending it. It is stored in the Req object and
sent when the reply function is called.</p></div>
<div class="paragraph"><p>To set the response body:</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">Req</span> <span style="color: #990000">=</span> <span style="font-weight: bold"><span style="color: #000000">cowboy_req:set_resp_body</span></span>(<span style="color: #FF0000">"Hello world!"</span>, <span style="color: #009900">Req0</span>)<span style="color: #990000">.</span></tt></pre></div></div>
<div class="paragraph"><p>To check if a response body has already been set:</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="font-weight: bold"><span style="color: #000000">cowboy_req:has_resp_body</span></span>(<span style="color: #009900">Req</span>)<span style="color: #990000">.</span></tt></pre></div></div>
<div class="paragraph"><p>It returns <code>true</code> if the body was set and is non-empty,
<code>false</code> otherwise.</p></div>
<div class="paragraph"><p>The preset response body is only sent if the reply function
used is <code>cowboy_req:reply/2</code> or <code>cowboy_req:reply/3</code>.</p></div>
</div>
</div>
<div class="sect1">
<h2 id="_sending_files">Sending files</h2>
<div class="sectionbody">
<div class="paragraph"><p>Cowboy provides a shortcut for sending files. When
using <code>cowboy_req:reply/4</code>, or when presetting the
response header, you can give a <code>sendfile</code> tuple to
Cowboy:</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: #FF6600">sendfile</span>, <span style="color: #009900">Offset</span>, <span style="color: #009900">Length</span>, <span style="color: #009900">Filename</span>}</tt></pre></div></div>
<div class="paragraph"><p>Depending on the values for <code>Offset</code> or <code>Length</code>, the
entire file may be sent, or just a part of it.</p></div>
<div class="paragraph"><p>The length is required even for sending the entire file.
Cowboy sends it in the content-length header.</p></div>
<div class="paragraph"><p>To send a file while replying:</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">Req</span> <span style="color: #990000">=</span> <span style="font-weight: bold"><span style="color: #000000">cowboy_req:reply</span></span>(<span style="color: #993399">200</span>, #{
<span style="color: #990000"><<</span><span style="color: #FF0000">"content-type"</span><span style="color: #990000">>></span> <span style="color: #990000">=></span> <span style="color: #FF0000">"image/png"</span>
}, {<span style="color: #FF6600">sendfile</span>, <span style="color: #993399">0</span>, <span style="color: #993399">12345</span>, <span style="color: #FF0000">"path/to/logo.png"</span>}, <span style="color: #009900">Req0</span>)<span style="color: #990000">.</span></tt></pre></div></div>
</div>
</div>
<div class="sect1">
<h2 id="_push">Push</h2>
<div class="sectionbody">
<div class="paragraph"><p>The HTTP/2 protocol introduced the ability to push resources
related to the one sent in the response. Cowboy provides two
functions for that purpose: <code>cowboy_req:push/3,4</code>.</p></div>
<div class="paragraph"><p>Push is only available for HTTP/2. Cowboy will automatically
ignore push requests if the protocol doesn’t support it.</p></div>
<div class="paragraph"><p>The push function must be called before any of the reply
functions. Doing otherwise will result in a crash.</p></div>
<div class="paragraph"><p>To push a resource, you need to provide the same information
as a client performing a request would. This includes the
HTTP method, the URI and any necessary request headers.</p></div>
<div class="paragraph"><p>Cowboy by default only requires you to give the path to
the resource and the request headers. The rest of the URI
is taken from the current request (excluding the query
string, set to empty) and the method is GET by default.</p></div>
<div class="paragraph"><p>The following snippet pushes a CSS file that is linked to
in the response:</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="font-weight: bold"><span style="color: #000000">cowboy_req:push</span></span>(<span style="color: #FF0000">"/static/style.css"</span>, #{
<span style="color: #990000"><<</span><span style="color: #FF0000">"accept"</span><span style="color: #990000">>></span> <span style="color: #990000">=></span> <span style="color: #990000"><<</span><span style="color: #FF0000">"text/css"</span><span style="color: #990000">>></span>
}, <span style="color: #009900">Req0</span>),
<span style="color: #009900">Req</span> <span style="color: #990000">=</span> <span style="font-weight: bold"><span style="color: #000000">cowboy_req:reply</span></span>(<span style="color: #993399">200</span>, #{
<span style="color: #990000"><<</span><span style="color: #FF0000">"content-type"</span><span style="color: #990000">>></span> <span style="color: #990000">=></span> <span style="color: #990000"><<</span><span style="color: #FF0000">"text/html"</span><span style="color: #990000">>></span>
}, [<span style="color: #FF0000">"<html><head><title>My web page</title>"</span>,
<span style="color: #FF0000">"<link rel='stylesheet' type='text/css' href='/static/style.css'>"</span>,
<span style="color: #FF0000">"<body><p>Welcome to Erlang!</p></body></html>"</span>], <span style="color: #009900">Req0</span>)<span style="color: #990000">.</span></tt></pre></div></div>
<div class="paragraph"><p>To override the method, scheme, host, port or query string,
simply pass in a fourth argument. The following snippet
uses a different host name:</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="font-weight: bold"><span style="color: #000000">cowboy_req:push</span></span>(<span style="color: #FF0000">"/static/style.css"</span>, #{
<span style="color: #990000"><<</span><span style="color: #FF0000">"accept"</span><span style="color: #990000">>></span> <span style="color: #990000">=></span> <span style="color: #990000"><<</span><span style="color: #FF0000">"text/css"</span><span style="color: #990000">>></span>
}, #{<span style="color: #0000FF">host</span> <span style="color: #990000">=></span> <span style="color: #990000"><<</span><span style="color: #FF0000">"cdn.example.org"</span><span style="color: #990000">>></span>}, <span style="color: #009900">Req</span>),</tt></pre></div></div>
<div class="paragraph"><p>Pushed resources don’t have to be files. As long as the push
request is cacheable, safe and does not include a body, the
resource can be pushed.</p></div>
<div class="paragraph"><p>Under the hood, Cowboy handles pushed requests the same as
normal requests: a different process is created which will
ultimately send a response to the client.</p></div>
</div>
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