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 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.

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

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

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

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 to your application, you 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:

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.

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:

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 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.

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

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!

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