Allow placeholder values in configuration files

[FaveroFerreira]

I took a look at the source code, and looks like loco expects us override configurations defined in the config-{env}.yaml with environment variables with an specific APP__ prefix.

Sometimes we do not decide how our environment variables will be named, so we don't get to define them with prefixes like APP__

It would be nice if we could declare placeholders in our configuration files where the values should be taken from our environment variables.

Example:

database:
  uri: ${DB_URI:postgres://loco:loco@localhost:5432/loco_app} #value taken from the DB_URI or uses the default value after ':' 
  enable_logging: false
  connect_timeout: 500
  idle_timeout: 500
  min_connections: 1
  max_connections: 1
  auto_migrate: true
  dangerously_truncate: false
  dangerously_recreate: false

[FaveroFerreira]

I don't think that config-rs supports this btw. If you guys feel that loco should have this then it will be needed to be implemented from scratch.

[kaplanelad]

Hi @FaveroFerreira,

Are you considering setting up an environment variable first directly in the config?

[FaveroFerreira]

Sorry, can you explain it a little further, I don't understand what you mean.

[jondot]

@kaplanelad I can give some more info, in Rails, you can plug environment variables into config, so for us, this is easily translated to:

1. load yaml as String
2. render yaml String (with tera or such), everywhere there is `{{env("var name")}}` will get replaced with a value from env
3. deserialize yaml

Now, config-rs would probably NOT allow for this intermediate -- load + render stage because it wants to read the file itself. This is where we can find a way around it by using something different or finding an API in config-rs that will allow

[jondot]

more here: rust-cli/config-rs#447

[kaplanelad]

Thanks @FaveroFerreira for your feedback.
the updated config was released in version: 0.1.6

[jondot]

@FaveroFerreira FYI your feature request is now implemented. You can do something like {{ get_env(name="ENV_VAR") }} in your config files.