Erlang: when to perform `inets:start()`?

Question

What is the appropriate place for performing inets:start() ?

1. in `applicationname_app\' module?
2. in applicationname_sup superv

Answer 1

In 2019, we use rebar3 to create an application and manage its dependencies. For dependencies that need to be downloaded, you add them to rebar.config, and rebar3 will download the dependencies. For example, if you add hackney (an http client) to rebar.config:

{erl_opts, [debug_info]}.
{deps, [
  {hackney, ".*", {git, "git://github.com/benoitc/hackney.git", {branch, "master"}}}
]}.

{shell, [
  % {config, "config/sys.config"},
    {apps, [http_client]}
]}.

Then do:

../your_app_name$ rebar3 compile

rebar3 will download hackney and compile all the files in the application.

To make sure that all your dependencies get started before your app, you add the names of the dependencies to:

src/your_app_name.app.src

For instance,

{application, http_client,
 [{description, "An OTP application"},
  {vsn, "0.1.0"},
  {registered, []},
  {mod, {http_client_app, []}},
  {applications,
   [kernel,
    stdlib,
    hackney    %%%<=========HERE
   ]},
  {env,[]},
  {modules, []},

  {licenses, ["Apache 2.0"]},
  {links, []}
 ]}.

The actual .app file gets created here:

_build/default/lib/your_app_name/ebin/your_app_name.app

To start your app in the shell along with all its dependencies, you do:

../your_app_name$ rebar3 shell 

The inets application comes with erlang, so it doesn't need to be downloaded, so you don't specify inets as a dependency in rebar.config (you'll get an error when you $ rebar3 compile). You still need to specify inets as a dependency in your application in the file:

src/your_app_name.app.src

But rebar3 itself uses inets (to download your dependencies), so even if you didn't specify inets as a dependency in your application, inets would still get started before your app. You can test that by not specifying inets as a dependency in your application, then doing:

$ rebar3 shell
...
...
1> application:start(inets)
{error,{already_started,inets}}

But, don't rely on that and DO specify inets as a dependency in your application.