This is an almost verbatem port of Garrett Smith's port_server example for how to use Erlang (or in this case Elixir) to extend Erlang supervision to external apps.

Here's Garrett's description:

port_server is a simple set of example code that illustrates how an Erlang OTP application can be used to start, supervise and stop a set of related external applications.

For background on this code, refer to doc/ceug-dec-2010.svg.

The motivation for the presentation to the Chicago Erlang User Group was to provide a tangible benefit for using Erlang in non-Erlang environments.

The gist:

• Use Erlang ports to run external applications from a single Erlang node

• Use OTP application supervisors to start, supervise, and shutdown the applicable services

• Refactor external applications written in other languages to be smaller, more service oriented, and use a "fail fast and hard" pattern for error handling

Install Erlang and Elixir on your system and run:

$ iex compile

You'll need Python 2.x installed since the external applications are written in Python. The scripts expect the Python interpreter to be /usr/bin/python2, so if that's not the case on your system, edit the scripts to point to the right version of Python. The scripts are in the priv directory.

To start Elixir and run the two services, run:

$ iex -S mix

Now if you run pgrep, you should see the services running. For example,

$ pgrep service_a
9391
$ pgrep service_b
9392

If you kill a service (e.g., to simulate a crash), it will be restarted:

$ pgrep service_a
9391
$ pkill service_a
$ pgrep service_a
9469

The supervisor is setup to allow for a maximum of 2 restarts in 5 seconds, so if you kill a service too many times, too quickly, the application will fail and the service won't be restarted. This is to prevent restarts from overloading the system. However in this case, the threshold is set artificially low to make it possible to trigger manually.

Garrett's repository has even more information on using Erlang to add fault tolerance to non-Erlang applications. Everything applies to Elixir so if you're considering writing an app in Elixir, you don't have to code everything in Elixir in the beginning to take advantage of the Erlang VM.