Environment variables and command-line arguments are useful to add temporary change parameters for a single run. They are a more convenient way to configure servers to use configuration files. This approach doesn't conform to The Twelve-Factor App methodology, but it's useful in cases when you need to set long parameters.

There are many formats that can be used for configuration files. The popular ones include TOML, YAML, and JSON. We will use TOML, because it is widely used with the Rust programming language.