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 Erlang.mk build system. If you are using Windows, please check the Installation instructions to get your environment setup before you continue.

Bootstrap

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

$ mkdir hello_erlang
$ cd hello_erlang

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

$ 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.

$ 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.

$ make run
...
([email protected]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:

PROJECT = hello_erlang

DEPS = cowboy
dep_cowboy_commit = master

DEP_PLUGINS = cowboy

include erlang.mk

We also tell the build system to load the plugins Cowboy provides. These include predefined templates that we will use soon.

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:

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 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:

$ 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.

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!

Cowboy 2.0 User Guide

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