e2 Applications

e2 applications are logical units of code that can be started and stopped within an Erlang VM.

For more information on how Erlang uses applications to build complex systems, see Erlang as an Operating System.

An e2 application is implemented as a single e2_application behavior module that defines the application’s top-level processes. These processes are started and supervised when the application is started.

Here’s an e2_application behavior module that defined a single hello service:

-module(hello_app).

-behavior(e2_application).

-export([init/0]).

init() ->
    {ok, [hello]}.

This is the application used in e2 Quick Start – refer to that guide to see how an application is used to automatically start services in an Erlang VM.

Application Supervisor

The e2 application behavior works closely with e2_application_supervisor, which is responsible for supervising all of the top-level processes in your application. The list of children returned by init/0 (e.g. [hello] in the example above) represent those processes that are stated and supervised when the application is started.

Note

This section is under development. For the full specification of a child, see e2_supervisor:start_link/3.

Default Supervisor Options

This init/0 return value:

{ok, [hello]}

is equivalent to this:

{ok, [{{hello, start_link, []}, [{id, hello},
                                 {restart, permanent},
                                 {shutdown, brutal_kill}]}],
     [{strategy, one_for_one}, {max_restart, {1, 1}}]}

Child Spec Examples

Start a hello service using hello:start_link():

{ok, [hello]}

Start the process using hello:start_link("Hello World!"):

{ok, [{hello, start_link, ["Hello World!"]}]}

Give the process 1000 milliseconds to shutdown before killing it:

{ok, [{hello, [{shutdown, 1000}]}]}

Specify an argument to start_link and a shutdown interval:

{ok, [{{hello, start_link, ["Hello World!"]}, [{shutdown, 1000}]}]}

Start two services:

{ok, [hello, goodbye]}

Start two services with a different max restart setting for the supervisor:

{ok, [hello, goodbye], [{max_restart, {5, 5}}]}
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