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Envy

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An OTP application typically gets its configuration using the operating system environment (os:getenv/1) and/or application environment (application:get_env/2). Both are used to allow configuration to be easily overridden by the OS environment, falling back to defaults provided by the application environment.

The result of calling os:getenv/1 is always a string() (or false if not present in the environment). For example, running the following:

KERNEL_HTTP_PORT=8080 erl
1> os:getenv("KERNEL_HTTP_PORT").
"8080"

Whereas using the application environment:

erl -kernel http_port 8080
2> application:get_env(kernel, http_port).
{ok,8080}

Envy is a simple wrapper over both os:getenv and application:get_env which does a couple of things:

  1. Convert the different types naturally returned by os:getenv and application:get_env to the type that you are expecting.

  2. Prefix any os_env with the name of the application, preventing any environment clashes.

As an example, starting erlang with envy running:

cd envy
KERNEL_HTTP_PORT=8080 make shell

The following calls to to_integer/3 demonstrate obtaining configuration either from the OS or application environment:

1> application:set_env(kernel, http_port, 1080).
2> envy:to_integer(kernel, http_port, [app_env, os_env, {default, 80}]).
1080
3> envy:to_integer(kernel, http_port, [os_env, app_env, {default, 80}]).
8080

When using the os_env strategy envy automatically prefixes the application name to the environment variable lookup, so that KERNEL_HTTP_PORT is used when http_port is the key.

Envy has helper wrappers for to_integer/3, to_float/3, to_atom/3, to_boolean/3, to_binary/3 and to_list/3.