Global, environment, and module layers

The earlier incarnations of Hiera (version 3 or earlier) used a single, entirely global hiera.yaml. Since its hierarchy is entirely global, it's not actually possible to change it without changing all environments simultaneously. Environments are usually used to control code changes, so this really makes a single hiera.yaml file quite inappropriate. Hiera 5 uses three layers of configuration and data: 

• Global layer:
  • In Hiera 3, this was the only layer
  • Useful for very temporary overrides, for example, when your operations team must bypass regular change processes
  • The legacy Hiera 3 backends are still supported—so it can be used while migrating to Hiera 5
  • This layer should generally now be avoided. All regular data should now be specified in the environment layer
• Environment layer:
  • The environment layer is now where most...