Turning Our Sequence Program into an OTP Application

Introduction

So, here’s the good news. The application in the Supervisors and Workers lesson is already a full-blown OTP application. When mix created the initial project tree, it added a supervisor (which we then modified) and enough information to our mix.exs file to get the application started. In particular, it filled in the application function:

def application do
  [
    mod: {
      Sequence.Application, []
    },
    extra_applications: [:logger],
  ]
end

This says that the top-level module of our application is called Sequence. OTP assumes this module will implement a start function, and it’ll pass that function an empty list as a parameter.

In our previous version of the start function, we ignored the arguments and instead hardwired the call to start_link to pass 123 to our application. Let’s change that to take the value from mix.exs instead. First, change mix.exs to pass an initial value (we’ll use 456):

def application do 
[
  mod: { 
    Sequence.Application, 456
  },
  extra_applications: [:logger], 
]
end

Then, wechange the application.ex ...