$ mkdir hello_erlang $ cd hello_erlang
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.
We are going to use the 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.
$ mkdir hello_erlang $ cd hello_erlang
Then we need to download Erlang.mk. Either use the following command or download it manually.
$ wget https://raw.githubusercontent.com/ninenines/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.
$ make -f erlang.mk bootstrap bootstrap-relThis 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 ... (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.
Modifying the Makefile allows the build system to know it needs to fetch and compile Cowboy. To do that we simply need to add two lines to our Makefile to make it look like this:
PROJECT = hello_erlang DEPS = cowboy dep_cowboy_commit = master include erlang.mk
If you run 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.
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:
start(_Type, _Args) -> Dispatch = cowboy_router:compile([ {'_', [{"/", hello_handler, []}]} ]), {ok, _} = 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 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 and start the release, then open http://localhost:8080 in your browser, you will get an error because the module is missing. Any other URL, like http://localhost:8080/test, will result in a 404 error.
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.
$ make new t=cowboy_http n=hello_handler
You can then open the src/hello_handler.erl file and modify
the init/2 function like this to send a reply.
init(Req, Opts) -> Req2 = cowboy_req:reply(200, [{<<"content-type">>, <<"text/plain">>}], <<"Hello Erlang!">>, Req), {ok, Req2, Opts}.
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!