···

       
12
       
 
       
    - programming

     

       
13
       
 
       
---

     

       
14
       
 
       


     

       
15
       
-
       
By reading this article I assume that you already have written some Elixir code.

     

       
16
       
-
       
Even if you haven't (yet) then I think that you can find it interesting as

     

       
17
       
-
       
a story how BEAM handles it's applications. But back on topic, when you start

     

       
18
       
-
       
your new Elixir project via `mix new my_awesome_project` you will end with

     

       
19
       
-
       
something like this in `mix.exs`:

     

       
20
       
 
       


     

       
21
       
 
       
```elixir

     

       
22
       
 
       
defmodule MyAwesomeProject.Mixfile do

     
···

       
45
       
 
       
```

     

       
46
       
 
       


     

       
47
       
 
       
And in most cases you will focus on `deps/0`, sometimes on `config/0`, but the

     

       
48
       
-
       
`application/0` you will almost never touch, and if you do you probably only

     

       
49
       
 
       
need it to add new entry in `:extra_applications` or sometimes

     

       
50
       
-
       
`:included_applications`. Except that this function interns are almost terra

     

       
51
       
-
       
incognita, place where you never look unless you are forced to, just like "Read

     

       
52
       
-
       
it later" list in Safari. And that is sad, as this is quite powerful and useful

     

       
53
       
-
       
piece of code, that hides really useful stuff.

     

       
54
       
 
       


     

       
55
       
 
       
But first things first.

     

       
56
       
 
       


     

       
57
       
 
       
## What `application/0` is for?

     

       
58
       
 
       


     

       
59
       
-
       
In Erlang the running system is built from applications. These applications are

     

       
60
       
-
       
like processes in your system that are managed by your system supervisor (SysV,

     

       
61
       
-
       
systemd, OpenRC, launchd, etc.) and are launched either during VM startup or on

     

       
62
       
-
       
direct user request via `Application.start/1-2` and it's family. How to start

     

       
63
       
-
       
such application is described in `my_awesome_app.app` file which is commonly

     

       
64
       
 
       
generated by the build system from the template. In Rebar3 projects this

     

       
65
       
 
       
template is `src/appname.app.src` and in Elixir it's the return value of the

     

       
66
       
 
       
named `application/0` function. This generated file is known as [Application

     

       
67
       
-
       
Resource File][app file] and is just tuple in form of `{:application,

     

       
68
       
 
       
:my_awesome_app, opts}` where `opts` is a keyword list of additional properties

     

       
69
       
 
       
(to be exact it's output of the `application/0` function).

     

       
70
       
 
       


     

       
71
       
-
       
Two of these optional fields are quite known in the Elixir community:

     

       
72
       
 
       


     

       
73
       
-
       
- `:mod` which is a tuple `{module(), term()}` which contain the starting point

     

       
74
       
-
       
  of our application, in most cases it's module that will return main

     

       
75
       
 
       
  supervisor of the application.

     

       
76
       
-
       
- `:applications` which contain all applications that are required by our

     

       
77
       
-
       
  application. Younger Elixir developers possibly never seen that as since 1.5

     

       
78
       
-
       
  this field is automatically filled by parsing `:deps`, we still can add

     

       
79
       
-
       
  entries there though via `:extra_applications`.

     

       
80
       
 
       


     

       
81
       
-
       
There are also few Elixir specific fields, that are sometimes used in bigger

     

       
82
       
-
       
projects:

     

       
83
       
 
       


     

       
84
       
-
       
- `:extra_applications` - applications that should be included in the release

     

       
85
       
-
       
  and automatically started before running this application but aren't in the

     

       
86
       
 
       
  dependencies list because, for example, are in default distribution, for

     

       
87
       
 
       
  example `logger` or `inets`.

     

       
88
       
-
       
- `:included_applications` - applications that should be included in the

     

       
89
       
 
       
  release, but not automatically started on boot.

     

       
90
       
 
       


     

       
91
       
 
       
But wait, there is more!

     

···

       
12
       
 
       
    - programming

     

       
13
       
 
       
---

     

       
14
       
 
       


     

       
15
       
+
       
When you start your new Elixir project via `mix new my_awesome_project` you will

     

       
16
       
+
       
end with something like this in `mix.exs`:

     

       
0
       
 
       

     

       
0
       
 
       

     

       
0
       
 
       

     

       
17
       
 
       


     

       
18
       
 
       
```elixir

     

       
19
       
 
       
defmodule MyAwesomeProject.Mixfile do

     
···

       
42
       
 
       
```

     

       
43
       
 
       


     

       
44
       
 
       
And in most cases you will focus on `deps/0`, sometimes on `config/0`, but the

     

       
45
       
+
       
`application/0` you will almost never touch, and if you do, you probably will only

     

       
46
       
 
       
need it to add new entry in `:extra_applications` or sometimes

     

       
47
       
+
       
`:included_applications`. Except for, that this function interns are terra

     

       
48
       
+
       
incognita, a place where you never look unless you are forced to, like "Read

     

       
49
       
+
       
it later" list in Safari. This is sad, as it is quite powerful and useful

     

       
50
       
+
       
piece of code.

     

       
51
       
 
       


     

       
52
       
 
       
But first things first.

     

       
53
       
 
       


     

       
54
       
 
       
## What `application/0` is for?

     

       
55
       
 
       


     

       
56
       
+
       
In Erlang the running system is built from applications. Those applications are

     

       
57
       
+
       
like processes in your system managed by your system supervisor (SysV, systemd,

     

       
58
       
+
       
OpenRC, launchd, etc.). They are launched either during VM startup or on

     

       
59
       
+
       
direct user request via `Application.start/1-2` and its family. Starting

     

       
60
       
+
       
this application is described in `my_awesome_app.app` file which is commonly

     

       
61
       
 
       
generated by the build system from the template. In Rebar3 projects this

     

       
62
       
 
       
template is `src/appname.app.src` and in Elixir it's the return value of the

     

       
63
       
 
       
named `application/0` function. This generated file is known as [Application

     

       
64
       
+
       
Resource File][app file]. It is just tuple in form of `{:application,

     

       
65
       
 
       
:my_awesome_app, opts}` where `opts` is a keyword list of additional properties

     

       
66
       
 
       
(to be exact it's output of the `application/0` function).

     

       
67
       
 
       


     

       
68
       
+
       
Two of those optional fields are quite known in the Elixir community:

     

       
69
       
 
       


     

       
70
       
+
       
- `:mod` which is a tuple `{module(), term()}` containing the starting point

     

       
71
       
+
       
  of our application; in most cases this is the module that will return main

     

       
72
       
 
       
  supervisor of the application.

     

       
73
       
+
       
- `:applications` contains all applications required by our application; younger

     

       
74
       
+
       
  Elixir developers possibly never seen that as since version 1.5 this field

     

       
75
       
+
       
  is automatically filled by parsing `:deps`, though we still can add entries

     

       
76
       
+
       
  there via `:extra_applications`

     

       
77
       
 
       


     

       
78
       
+
       
There are also few Elixir specific fields, sometimes used in larger projects:

     

       
0
       
 
       

     

       
79
       
 
       


     

       
80
       
+
       
- `:extra_applications` - those should be included in the release

     

       
81
       
+
       
  and automatically started before running current application; but aren't in the

     

       
82
       
 
       
  dependencies list because, for example, are in default distribution, for

     

       
83
       
 
       
  example `logger` or `inets`.

     

       
84
       
+
       
- `:included_applications` - applications which should be included in the

     

       
85
       
 
       
  release, but not automatically started on boot.

     

       
86
       
 
       


     

       
87
       
 
       
But wait, there is more!