[[getting_started]]
== Getting started

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.

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.

=== Prerequisites

We are going to use the https://github.com/ninenines/erlang.mk[Erlang.mk]
build system. If you are using Windows, please check the
http://erlang.mk/guide/installation.html[Installation instructions]
to get your environment setup before you continue.

=== Bootstrap

First, let's create the directory for our application.

[source,bash]
$ mkdir hello_erlang
$ cd hello_erlang

Then we need to download Erlang.mk. Either use the following
command or download it manually.

[source,bash]
$ wget https://erlang.mk/erlang.mk

We can now bootstrap our application. Since we are going to generate
a release, we will also bootstrap it at the same time.

[source,bash]
$ make -f erlang.mk bootstrap bootstrap-rel

This creates a Makefile, a base application, and the release files
necessary for creating the release. We can already build and start
this release.

[source,bash]
----
$ make run
...
(hello_erlang@127.0.0.1)1>
----

Entering the command `i().` will show the running processes, including
one called `hello_erlang_sup`. This is the supervisor for our
application.

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.

=== Cowboy setup

We will modify the 'Makefile' to tell the build system it needs to
fetch and compile Cowboy, and that we will use releases:

[source,makefile]
----
PROJECT = hello_erlang

DEPS = cowboy
dep_cowboy_commit = 2.16.1

REL_DEPS = relx

DEP_PLUGINS = cowboy

include erlang.mk
----

The `DEP_PLUGINS` line tells the build system to load the plugins
Cowboy provides. These include predefined templates that we will
use soon.

The `REL_DEPS` line tells the build system to fetch and build
`relx`, the library that will create the release.

If you do `make run` 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.

=== Listening for connections

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.

Open the 'src/hello_erlang_app.erl' file and add the necessary
code to the `start/2` function to make it look like this:

[source,erlang]
----
start(_Type, _Args) ->
    Dispatch = cowboy_router:compile([
        {'_', [{"/", hello_handler, []}]}
    ]),
    {ok, _} = cowboy:start_clear(my_http_listener,
        [{port, 8080}],
        #{env => #{dispatch => Dispatch}}
    ),
    hello_erlang_sup:start_link().
----

Routes are explained in details in the xref:routing[Routing]
chapter. For this tutorial we map the path `/` to the handler
module `hello_handler`. This module doesn't exist yet.

Build and start the release, then open http://localhost:8080
in your browser. You will get a 500 error because the module is missing.
Any other URL, like http://localhost:8080/test, will result in a
404 error.

=== Handling requests

Cowboy features different kinds of handlers, including REST
and Websocket handlers. For this tutorial we will use a plain
HTTP handler.

Generate a handler from a template:

[source,bash]
$ make new t=cowboy.http n=hello_handler

Then, open the 'src/hello_handler.erl' file and modify
the `init/2` function like this to send a reply.

[source,erlang]
----
init(Req0, State) ->
    Req = cowboy_req:reply(200,
        #{<<"content-type">> => <<"text/plain">>},
        <<"Hello Erlang!">>,
        Req0),
    {ok, Req, State}.
----

What the above code does is send a 200 OK reply, with the
Content-type header set to `text/plain` and the response
body set to `Hello Erlang!`.

If you run the release and open http://localhost:8080
in your browser, you should get a nice `Hello Erlang!` displayed!
