We've already alluded to the fact that you can set configuration for environ to consume from either environment variables or from Java system properties. This covers about 100% of real-world usage. However, during development, we can also set environment configuration using the lein-environ plugin (which is also packaged as part of a Luminus-generated application).

When running the development server using lein ring server, the lein-environ plugin will fetch environment configuration from the Leiningen project map, as well as an optional profiles.clj file, and merge the two together into the .lein-env file. This .lein-env file is the first go-to place for environment settings, but only when running the development server.