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A release bundles your applications, the Erlang runtime, and configuration. Two files drive runtime behaviour: sys.config for application settings and the boot script for the startup sequence.

Each OTP application has an environment — a set of key/value parameters. You read them at runtime with application:get_env/2.

sys.config is a single Erlang term: a list of {App, [{Key, Value}]} tuples that override application defaults at boot.

Values in sys.config override those in each app .app file. This lets one codebase run differently per environment (dev, staging, prod) just by swapping config.

Inside a gen_server init, pull configured values so behaviour follows the environment.

The boot script (.script compiled to .boot) lists every application and module to load, and the order to start them. The runtime follows it to bring the system up deterministically.

You describe a release in a .rel file. Tools like systools or rebar3 turn it into the boot script.

Applications start in dependency order. If my_app lists kernel and stdlib as dependencies, the boot script guarantees they start first.

In the boot script an application can be permanent (its crash takes down the node), transient, or temporary. Choose permanent for core services.

You can split configuration across files and merge them, or use runtime os:getenv/1 reads for secrets that should not live in sys.config.

At node startup you point to a config file with the -config flag, letting one release run with different settings just by swapping the file.

You learned how releases are configured and started:

• sys.config sets the application environment per deployment
• Read values with application:get_env/2
• The .rel file is compiled into a boot script
• Boot scripts start applications in dependency order
• Use permanent apps for core services and env vars for secrets