::: 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.

:: Bootstrap

We are going to use the ^"erlang.mk^https://github.com/extend/erlang.mk
build system. It also offers bootstrap features allowing us to
quickly get started without having to deal with minute details.

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

``` bash
$ mkdir hello_erlang
$ cd hello_erlang
```

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

``` bash
$ wget https://raw.githubusercontent.com/extend/erlang.mk/master/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.

``` 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.

``` bash
$ make
...
$ ./_rel/hello_erlang_release/bin/hello_erlang_release console
...
(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

To add Cowboy as a dependency of your application, we need to modify
two files: the Makefile and the application resource file.

Modifying the Makefile allows the build system to know it needs to
fetch and compile Cowboy. To do that we simply need to add one line
to our Makefile to make it look like this:

``` Makefile
PROJECT = hello_erlang
DEPS = cowboy
include erlang.mk
```

Modifying the application resource file, `src/hello_erlang.app.src`,
allows the build system to know it needs to include Cowboy in the
release and start it automatically. This is a different step because
some dependencies are only needed during development.

We are simply going to add `cowboy` to the list of `applications`,
right after `stdlib`. Don't forget the comma separator.

``` erlang
{application, hello_erlang, [
	{description, "Hello Erlang!"},
	{vsn, "0.1.0"},
	{modules, []},
	{registered, []},
	{applications, [
		kernel,
		stdlib,
		cowboy
	]},
	{mod, {hello_erlang_app, []}},
	{env, []}
]}.
```

You may want to set a description for the application while you
are editing the file.

If you run `make` now and start the release, Cowboy will be included
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

We will do this when our application starts. It's a two step process.
First we need to define and compile the dispatch list, a list of
routes that Cowboy will use to map requests to handler modules.
Then we tell Cowboy to listen for connections.

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

``` erlang
start(_Type, _Args) ->
	Dispatch = cowboy_router:compile([
		{'_', [{"/", hello_handler, []}]}
	]),
	cowboy:start_http(my_http_listener, 100, [{port, 8080}],
		[{env, [{dispatch, Dispatch}]}]
	),
	hello_erlang_sup:start_link().
```

The dispatch list is explained in great details in the
^"Routing^routing^ chapter. For this tutorial we map the
path `/` to the handler module `hello_handler`. This module
doesn't exist yet, we still have to write it.

If you build the release, start it and open ^http://localhost:8080
now, you will get an 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.

First, let's generate a handler from a template.

``` bash
$ make new t=cowboy_http n=hello_handler
```

You can then open the `src/hello_handler.erl` file and modify
the `handle/2` function like this to send a reply.

``` erlang
handle(Req, State=#state{}) ->
	{ok, Req2} = cowboy_req:reply(200,
		[{<<"content-type">>, <<"text/plain">>}],
		<<"Hello Erlang!">>,
		Req),
	{ok, Req2, 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 build the release, start it and open ^http://localhost:8080
in your browser, you should get a nice `Hello Erlang!` displayed!