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Rails applications are usually configured via the encrypted credentials.yml file or via environmental variables.

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Environmental variables

Environment variables are a great way to configure a Rails application that needs to run in multiple environments.

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Secret variables

Environment variables that have sensitive data in them, like a DATABASE_URL that contains a password, can be kept in a secret that can’t be viewed except when the container is running. To set a secret in Fly, run:

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Non-sensitive variables

Variables that don’t have sensitive data can be set in the fly.toml file under the [env] directive. An example file might look like:

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View the variables

To view the environment variables of your Fly Rails app, run:
There you’ll see all of the environment variables in your application that have been set by fly secrets, the [env] directive in the fly.toml file, and the environment directive from your Dockerfile.

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Encrypted credentials file

Another approach to managing credentials in Rails is to use an encrypted credentials file, such as config/credentials.yml.enc or config/credentials/production.yml.enc, which you can learn more about by running the following from the root of your Rails application:
When deploying to production, the RAILS_MASTER_KEY that will decrypt the credentials file can be set via fly secrets set. For example, if your master key is stored in config/master.key, you can run:
For Windows/PowerShell: