···
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By reading this article I assume that you already have written some Elixir code.
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Even if you haven't (yet) then I think that you can find it interesting as
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a story how BEAM handles it's applications. But back on topic, when you start
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your new Elixir project via `mix new my_awesome_project` you will end with
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something like this in `mix.exs`:
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When you start your new Elixir project via `mix new my_awesome_project` you will
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end with something like this in `mix.exs`:
defmodule MyAwesomeProject.Mixfile do
···
And in most cases you will focus on `deps/0`, sometimes on `config/0`, but the
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`application/0` you will almost never touch, and if you do you probably only
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`application/0` you will almost never touch, and if you do, you probably will only
need it to add new entry in `:extra_applications` or sometimes
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`:included_applications`. Except that this function interns are almost terra
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incognita, place where you never look unless you are forced to, just like "Read
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it later" list in Safari. And that is sad, as this is quite powerful and useful
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piece of code, that hides really useful stuff.
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`:included_applications`. Except for, that this function interns are terra
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incognita, a place where you never look unless you are forced to, like "Read
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it later" list in Safari. This is sad, as it is quite powerful and useful
## What `application/0` is for?
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In Erlang the running system is built from applications. These applications are
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like processes in your system that are managed by your system supervisor (SysV,
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systemd, OpenRC, launchd, etc.) and are launched either during VM startup or on
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direct user request via `Application.start/1-2` and it's family. How to start
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such application is described in `my_awesome_app.app` file which is commonly
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In Erlang the running system is built from applications. Those applications are
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+
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
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+
this application is described in `my_awesome_app.app` file which is commonly
generated by the build system from the template. In Rebar3 projects this
template is `src/appname.app.src` and in Elixir it's the return value of the
named `application/0` function. This generated file is known as [Application
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Resource File][app file] and is just tuple in form of `{:application,
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Resource File][app file]. It is just tuple in form of `{:application,
:my_awesome_app, opts}` where `opts` is a keyword list of additional properties
(to be exact it's output of the `application/0` function).
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Two of these optional fields are quite known in the Elixir community:
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Two of those optional fields are quite known in the Elixir community:
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- `:mod` which is a tuple `{module(), term()}` which contain the starting point
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of our application, in most cases it's module that will return main
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- `:mod` which is a tuple `{module(), term()}` containing the starting point
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of our application; in most cases this is the module that will return main
supervisor of the application.
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- `:applications` which contain all applications that are required by our
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application. Younger Elixir developers possibly never seen that as since 1.5
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this field is automatically filled by parsing `:deps`, we still can add
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entries there though via `:extra_applications`.
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- `:applications` contains all applications required by our application; younger
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Elixir developers possibly never seen that as since version 1.5 this field
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is automatically filled by parsing `:deps`, though we still can add entries
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there via `:extra_applications`
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There are also few Elixir specific fields, that are sometimes used in bigger
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There are also few Elixir specific fields, sometimes used in larger projects:
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- `:extra_applications` - applications that should be included in the release
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and automatically started before running this application but aren't in the
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- `:extra_applications` - those should be included in the release
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and automatically started before running current application; but aren't in the
dependencies list because, for example, are in default distribution, for
example `logger` or `inets`.
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- `:included_applications` - applications that should be included in the
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- `:included_applications` - applications which should be included in the
release, but not automatically started on boot.
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