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<!DOCTYPE html>
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<title>Nine Nines: Getting started</title>
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<h1 class="lined-header"><span>Getting started</span></h1>
<div class="paragraph"><p>Erlang is more than a language, it is also an operating system
for your applications. Erlang developers rarely write standalone
modules, they write libraries or applications, and then bundle
those into what is called a release. A release contains the
Erlang VM plus all applications required to run the node, so
it can be pushed to production directly.</p></div>
<div class="paragraph"><p>This chapter walks you through all the steps of setting up
Cowboy, writing your first application and generating your first
release. At the end of this chapter you should know everything
you need to push your first Cowboy application to production.</p></div>
<div class="sect1">
<h2 id="_prerequisites">Prerequisites</h2>
<div class="sectionbody">
<div class="paragraph"><p>We are going to use the <a href="https://github.com/ninenines/erlang.mk">Erlang.mk</a>
build system. If you are using Windows, please check the
<a href="http://erlang.mk/guide/installation.html">Installation instructions</a>
to get your environment setup before you continue.</p></div>
</div>
</div>
<div class="sect1">
<h2 id="_bootstrap">Bootstrap</h2>
<div class="sectionbody">
<div class="paragraph"><p>First, let’s create the directory for our application.</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>$ mkdir hello_erlang
$ cd hello_erlang</tt></pre></div></div>
<div class="paragraph"><p>Then we need to download Erlang.mk. Either use the following
command or download it manually.</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>$ wget https<span style="color: #990000">:</span>//erlang<span style="color: #990000">.</span>mk/erlang<span style="color: #990000">.</span>mk</tt></pre></div></div>
<div class="paragraph"><p>We can now bootstrap our application. Since we are going to generate
a release, we will also bootstrap it at the same time.</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>$ make -f erlang<span style="color: #990000">.</span>mk bootstrap bootstrap-rel</tt></pre></div></div>
<div class="paragraph"><p>This creates a Makefile, a base application, and the release files
necessary for creating the release. We can already build and start
this release.</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>$ make run
<span style="color: #990000">...</span>
<span style="color: #990000">(</span>hello_erlang@<span style="color: #993399">127.0</span><span style="color: #990000">.</span><span style="color: #993399">0.1</span><span style="color: #990000">)</span><span style="color: #993399">1</span><span style="color: #990000">></span></tt></pre></div></div>
<div class="paragraph"><p>Entering the command <code>i().</code> will show the running processes, including
one called <code>hello_erlang_sup</code>. This is the supervisor for our
application.</p></div>
<div class="paragraph"><p>The release currently does nothing. In the rest of this chapter we
will add Cowboy as a dependency and write a simple "Hello world!"
handler.</p></div>
</div>
</div>
<div class="sect1">
<h2 id="_cowboy_setup">Cowboy setup</h2>
<div class="sectionbody">
<div class="paragraph"><p>We will modify the <em>Makefile</em> to tell the build system it needs to
fetch and compile Cowboy:</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="color: #009900">PROJECT =</span> hello_erlang
<span style="color: #009900">DEPS =</span> cowboy
<span style="color: #009900">dep_cowboy_commit =</span> 2.2.2
<span style="color: #009900">DEP_PLUGINS =</span> cowboy
include erlang.mk</tt></pre></div></div>
<div class="paragraph"><p>We also tell the build system to load the plugins Cowboy provides.
These include predefined templates that we will use soon.</p></div>
<div class="paragraph"><p>If you do <code>make run</code> now, Cowboy will be included in the release
and started automatically. This is not enough however, as Cowboy
doesn’t do anything by default. We still need to tell Cowboy to
listen for connections.</p></div>
</div>
</div>
<div class="sect1">
<h2 id="_listening_for_connections">Listening for connections</h2>
<div class="sectionbody">
<div class="paragraph"><p>First we define the routes that Cowboy will use to map requests
to handler modules, and then we start the listener. This is best
done at application startup.</p></div>
<div class="paragraph"><p>Open the <em>src/hello_erlang_app.erl</em> file and add the necessary
code to the <code>start/2</code> function to make it look like this:</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">start</span></span>(<span style="color: #009900">_Type</span>, <span style="color: #009900">_Args</span>) <span style="color: #990000">-></span>
<span style="color: #009900">Dispatch</span> <span style="color: #990000">=</span> <span style="font-weight: bold"><span style="color: #000000">cowboy_router:compile</span></span>([
{<span style="color: #FF6600">'_'</span>, [{<span style="color: #FF0000">"/"</span>, <span style="color: #FF6600">hello_handler</span>, []}]}
]),
{<span style="color: #FF6600">ok</span>, <span style="color: #990000">_</span>} <span style="color: #990000">=</span> <span style="font-weight: bold"><span style="color: #000000">cowboy:start_clear</span></span>(<span style="color: #FF6600">my_http_listener</span>,
[{<span style="color: #FF6600">port</span>, <span style="color: #993399">8080</span>}],
#{<span style="color: #0000FF">env</span> <span style="color: #990000">=></span> #{<span style="color: #0000FF">dispatch</span> <span style="color: #990000">=></span> <span style="color: #009900">Dispatch</span>}}
),
<span style="font-weight: bold"><span style="color: #000000">hello_erlang_sup:start_link</span></span>()<span style="color: #990000">.</span></tt></pre></div></div>
<div class="paragraph"><p>Routes are explained in details in the <a href="../routing">Routing</a>
chapter. For this tutorial we map the path <code>/</code> to the handler
module <code>hello_handler</code>. This module doesn’t exist yet.</p></div>
<div class="paragraph"><p>Build and start the release, then open <a href="http://localhost:8080">http://localhost:8080</a>
in your browser. You will get a 500 error because the module is missing.
Any other URL, like <a href="http://localhost:8080/test">http://localhost:8080/test</a>, will result in a
404 error.</p></div>
</div>
</div>
<div class="sect1">
<h2 id="_handling_requests">Handling requests</h2>
<div class="sectionbody">
<div class="paragraph"><p>Cowboy features different kinds of handlers, including REST
and Websocket handlers. For this tutorial we will use a plain
HTTP handler.</p></div>
<div class="paragraph"><p>Generate a handler from a template:</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>$ make new <span style="color: #009900">t</span><span style="color: #990000">=</span>cowboy<span style="color: #990000">.</span>http <span style="color: #009900">n</span><span style="color: #990000">=</span>hello_handler</tt></pre></div></div>
<div class="paragraph"><p>Then, open the <em>src/hello_handler.erl</em> file and modify
the <code>init/2</code> function like this to send a reply.</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">Req0</span>, <span style="color: #009900">State</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/plain"</span><span style="color: #990000">>></span>},
<span style="color: #990000"><<</span><span style="color: #FF0000">"Hello Erlang!"</span><span style="color: #990000">>></span>,
<span style="color: #009900">Req0</span>),
{<span style="color: #FF6600">ok</span>, <span style="color: #009900">Req</span>, <span style="color: #009900">State</span>}<span style="color: #990000">.</span></tt></pre></div></div>
<div class="paragraph"><p>What the above code does is send a 200 OK reply, with the
Content-type header set to <code>text/plain</code> and the response
body set to <code>Hello Erlang!</code>.</p></div>
<div class="paragraph"><p>If you run the release and open <a href="http://localhost:8080">http://localhost:8080</a>
in your browser, you should get a nice <code>Hello Erlang!</code> displayed!</p></div>
</div>
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Introduction
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Flow diagram
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Cowboy
2.2
User Guide
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